Recycled aluminum is made from recycled scrap aluminum parts or leftovers from production process as main raw materials. After composition adjustment, it is smelted and formulated to produce aluminum ingots that meet various standards. ADC12ZS studied in this project is a recycled aluminum alloy. Due to addition of waste products during smelting process, too many impurity elements, such as Zn, may be introduced. Through analysis, compared with ADC12 alloy, ADC12ZS has an increased tendency of high-temperature thermal brittleness and a greater tendency of hot cracking; during smelting process, burning loss is serious and loss rate of molten aluminum is high. Aim is to relax Zn content in aluminum ingot composition without affecting material’s mechanical properties and casting performance, thereby increasing proportion of scrap aluminum in aluminum ingot preparation process, thereby reducing raw material costs. Al-Si-Cu alloys have excellent casting properties, such as low shrinkage, strong fluidity, and low tendency to hot crack. They are one of the most commonly used alloy series in cast aluminum alloys. Its performance mainly depends on shape, size and distribution of primary α-Al, eutectic Si and primary Si, secondary phases (intermetallic compounds) and casting defects. Among them, w (Zn) in ADC12 alloy is ≤1.0%, and in ADC12ZS studied in this topic, it is relaxed to ≤2.5%, which is an increase of 150%. Existing studies have shown that Zn can increase fluidity of alloy and improve mechanical properties of alloy, but it will increase its high-temperature brittleness and cracking tendency. Therefore, when using high-Zn aluminum alloys, focus should be placed on its casting performance, mechanical properties and cost.
Graphical results
For aluminum ingots used for testing, die-casting companies must conduct relevant performance evaluations. ADC12ZS evaluation process is roughly: (1) Formulate chemical composition requirements of ADC12ZS aluminum ingots, see Table 1; (2) Make aluminum ingot (3) samples for evaluation; (3) Evaluate aluminum ingot samples; (4) After evaluation of aluminum ingot samples, small batch aluminum ingots (10t) are produced; (5) Comprehensive evaluation of productivity, including casting performance, mechanical properties and coating performance of ADC12ZS aluminum alloy; (6) Make a judgment based on evaluation results.
wB
Si
Fe
Cu
Zn
Mg
Ni
Mn
Sn
Ca
Pb
Al
9.0-12.0
0.70-0.95
1.5-3.0
2.05
≤0.30
≤0.50
≤0.50
≤0.10
≤0.005
≤0.10
margin
Table 1 Chemical composition requirements of ADC12ZS aluminum alloy (%)
Sample
wB
Si
Fe
Cu
Zn
Mn
Mg
Ni
Sn
Ca
Pb
Al
Aluminum ingot
10.4
0.92
1.7
1.77
0.15
0.16
0.05
0.04
0.001
0.05
Margin
Utilization test
10.4
0.82
1.6
1.78
0.15
0.15
0.05
0.04
0.001
0.05
Margin
Shrinkage test
10.2
0.85
1.6
1.79
0.14
0.12
0.04
0.04
0.001
0.05
Margin
Table 2 Chemical composition of ADC12ZS alloy at different stages Actual measurement value (%)
Hydrogen content/(mL*kg-1)
1
2
3
Average
2.8
3.0
3.2
3.0
Table 3 Hydrogen content of ADC12ZS alloy Note: Hydrogen content refers to hydrogen content in 1kg aluminum (mL)
Figure 2 Microstructure of ADC12ZS alloy at different locations Alloy microstructure is mainly composed of α-Al phase and eutectic. α-Al phase is irregular and evenly distributed; eutectic Si is dot-shaped or worm-shaped, with a small amount; there are gray strip-shaped second phase precipitates in grains and at grain boundaries. Analysis using an energy spectrum analyzer shows that second phase contains Zn, Cu, Fe, and Mg elements, of which Zn content is 5.33%; there is a small amount of slag phase around α-Al phase. Generally speaking, alloy structure is basically normal, but it contains unknown second phase precipitates; and dendritic eutectic Si content is low, so attention needs to be paid during testing.
Figure 4 Aluminum sticking to wall of smelting furnace
Weight/kg
Attrition rate/%
Input
Die Casting
Pouring risers and cold mold parts
Scrap
Losses (slag and burning losses)
12391.9
6067.745
5385.3
416.4
522.45
4.22
Table 7 Actual loss rate of ADC12ZS alloy
Aluminum ingot marking
Polishing/kg
Heat treatment/kg
5VLCC1
5VLCC2
5VLCC1
5VLCC2
ADC12ZS
2.335
1.78
2.29
1.785
ADC12
2.305
1.765
2.26
1.77
Deviation/%
1.3
0.85
1.3
0.85
Table 8 Effect of increasing Zn content on unit quality After production evaluation, there are three main problems in casting process of ADC12ZS alloy: (1) Tendency of casting cracks is serious; (2) Aluminum sticking to furnace wall is serious during smelting process of aluminum ingots (current aluminum materials also have aluminum sticking, but it is easy to remove); ( 3) Aluminum loss rate of ADC12ZS alloy is relatively high, 4.22%. In view of problems and result analysis during production evaluation stage of ADC12ZS alloy, corresponding countermeasures were taken on original smelting process. (1) It is stipulated that aluminum ingot manufacturers cannot add pure Zn when adjusting alloy composition during production of aluminum ingots to avoid damaging grain structure; (2) Amount of slag removal agent during refining is increased from original 0.2% to 0.3%, which reduces wettability of slag and allows more molten metal wrapped in slag bag to flow back to melt; (3) In order to reduce burning loss of metal elements, use a moderate bulkiness of charge and reduce specific surface area of charge; roasting method is used to treat surface coating of charge to reduce heat generation; ensure that charging port is closed during smelting, and control O2 concentration in furnace; appropriately increase heating speed and shorten smelting time.
Figure 5 Microstructure of ADC12ZS alloy after taking measures After taking above measures, actual measured Zn content was 1.77%, which met standard of ≤2.5%. Microstructure is shown in Figure 5. It can be seen that α-Al phase and eutectic Si are distributed evenly, dendritic eutectic Si increases, slag phase decreases significantly, and there are no strip-shaped second phase precipitates. Casting production evaluation was carried out again, and it was found that cracks in original defective parts disappeared; aluminum adhesion on walls of smelting furnace was significantly improved, and it was easy to remove; loss rate of ADC12ZS alloy dropped to 3.5%, plus impact of increased quality of castings, comprehensive loss rate was about 4.5%, close to industry standard.
Lightweighting automobiles is one of the most effective measures to reduce energy consumption and pollutant emissions. Due to its high specific strength and high cost performance, aluminum alloy is preferred material for lightweighting automobiles. It is used in structural parts and components such as bodies, engines, wheels, etc., and its strength increases. High-pressure die casting has characteristics of high product dimensional accuracy, high production efficiency and considerable economic benefits, and is suitable for production of various aluminum alloy complex castings. Based on cold chamber die-casting process, researchers proposed theory of externally solidified crystals (ESCs) in pressure chamber. It is believed that when melt stays in pressure chamber, especially when it is in slow injection stage, superheat of melt dissipates quickly due to contact with colder pressure chamber wall and punch, nucleation and growth conditions of primary phase have been reached. Studies believe that a large part of coarse dendrite structure formed near core of die casting comes from ESCs in pressure chamber, and as melt fills cavity, fluid force is eventually distributed in core area of casting. At the same time, effects of different pressure chamber sizes, pressure chamber residence times, addition of refiners on content and distribution of ESCs in castings were studied for magnesium-aluminum alloy and aluminum alloy die-casting. It was found that the greater heat transfer coefficient of pressure chamber and the longer residence time of pressure chamber, the higher ESCs content in casting. Adding a refiner will increase number of rounded ESCs in casting. Studying Mg-Al alloy die-casting process found that the lower pouring temperature, the higher ESCs content in die-casting parts; the higher speed, the rounder and finer ESCs in die-casting parts. Some researchers have studied effect of pouring temperature on microstructure, casting performance and mechanical properties of aluminum alloys, found that fluidity of molten metal is poor at lower pouring temperatures, possibility of eddy currents and air entrapment during injection process is reduced. Intrinsic quality of die castings is improved, but structure of die castings is coarsened. In actual production process of cold chamber high-pressure casting, ESCs are inevitable due to mutual constraints between process parameters, volume fraction of ESCs can only be reduced and their morphology controlled by optimizing process parameters. In summary, there are currently few research reports on ESCs. In actual production process of high-pressure casting, there are few reports on systematic study of impact of die-casting process on microstructure morphology and mechanical properties of Al-Si-Mg alloys. When testing body performance of aluminum alloy structural castings produced by high-pressure casting, it was found that when there is a considerable amount of coarse pre-crystallized structure in microstructure, yield strength and hardness of casting body will be significantly reduced, making it impossible to meet customer standards [yield strength] ≥120MPa, hardness (HB)≥75]. In order to improve performance of casting body and meet customer standards, effects of pouring temperature, injection waiting time and chemical composition on volume fraction, size and morphology of pre-crystallized structure in microstructure, as well as on mechanical properties were studied, aiming to provide a reference for its application.
Graphic and text results
An aluminum alloy beam (structural casting) was selected as sample, an IDRA-1600 die-casting machine equipped with a vacuum system and an aluminum alloy quantitative pouring furnace was used for test. By changing process parameters such as pouring temperature, injection waiting time, and chemical composition of aluminum liquid (pouring temperature is 670 and 680℃; injection waiting time is 3, 4, 6 and 8 s; chemical composition of aluminum liquid is Al-7Si-0.25Mg, Al-7.5Si-0.25Mg), samples under different process conditions were obtained. Samples were cut from body, processed into tensile and hardness samples. After rough grinding, fine grinding, and polishing, metallographic samples were corroded using HF aqueous solution with a volume fraction of 0.5%. ZEISSAxio Observer 7m automatic inverted metallographic microscope was used for microstructural observation. ZWICK Z100 tensile testing machine was used for mechanical property testing. Tensile rate was 0.08%/s. Sample dimensions are shown in Figure 1. XHB-3000Z digital display Brinell hardness tester was used hardness testing. Load was 62.5kN and held for 15s. Test conditions were HB 2.5/62.5. A DSC0901 differential thermal analyzer was used to analyze solidification behavior of alloys with different components. Cooling rates were 10℃/min and 30℃/min respectively.
wB
Si
Fe
Mn
Mg
Ti
Ca
Sr
Al
6.5-7.5
≤0.15
0.50-0.90
0.20-0.30
≤0.14
≤0.001
0.015-0.025
margin
Table 1 Chemical composition of Al-Si-Mg alloy (%)
Figure 1 Tensile specimen dimensions
Figure 2 Microstructure of Al-7Si-0.25Mg alloy at different pouring temperatures
Pouring temperature/℃
ESCs area fraction/%
ESCs average diameter/um
Yield strength/MPa
Tensile strength/MPa
Elongation/%
Hardness(HB)
670
15.8
122.37
115
264
14.5
72.2
680
6.3
60.21
118
270
13.3
74.1
Table 2 ESCs area fraction, average diameter and mechanical properties of casting body at different pouring temperatures
Figure 3 Microstructure of Al-7Si-0.25Mg at 670℃ under different injection waiting times.
Figure 4 Area fraction and average diameter of ESCs in Al-7Si-0.25Mg alloy under different injection waiting times It can be seen that as injection waiting time increases, pre-crystallized structure in microstructure gradually becomes coarser and dendrites grow fully. Figure 4 shows changes in area fraction and average diameter of ESCs of Al-7Si-0.25Mg alloy under different injection waiting times. It can be seen that area fraction of ESCs increased from 15.8% to 30.2%, and average diameter increased from 122.37 μm to 158.32 μm. Figure 5 shows mechanical properties of casting body under different injection waiting times. It can be seen that yield strength, tensile strength and Brinell hardness of castings decrease as injection waiting time increases, while elongation shows an upward trend. Therefore, in order to improve yield strength and hardness of castings, injection waiting time should be reduced. However, when injection waiting time is in the range of 3 to 8 seconds, yield strength and hardness of casting still cannot meet requirements.
Figure 5 Mechanical properties of Al-7Si-0.25Mg casting body with different injection waiting times
Figure 6 DSC curves of Al-7Si-0.25Mg and Al-7.5Si-0.25Mg alloys at different solidification rates On the basis of adjusting pouring temperature and injection waiting time, influence of chemical composition on performance of casting body was further analyzed. Si content in alloy was adjusted from 7.0% to 7.5%. First, DSC was used to test solidification behavior of Al-7Si-0.25Mg and Al-7.5Si-0.25Mg alloys under different cooling rates. Figure 6 shows DSC curves of Al-7Si-0.25Mg and Al-7.5Si-0.25Mg alloys at cooling rates of 10℃/min and 30℃/min. It can be seen that under solidification condition of 10℃/min, liquidus temperature of two alloys differs by about 8℃. As cooling rate increases to 30℃/min, liquidus temperature of two alloys differs by about 13℃.
Figure 7 Microstructure of alloy under different Si contents
w(Si)/%
ESCs area fraction/%
ESCs average diameter/μm
Yield strength/MPa
Tensile strength/MPa
Elongation/%
Hardness(HB)
7.0
15.8
122.37
115
264
14.5
72.2
7.5
2.1
40.23
123
286
12.5
76.9
Table 3 ESCs area fraction, average diameter and mechanical properties of casting body under different Si contents
In conclusion
(1) Pre-crystallized structure existing in microstructure is main reason for reducing yield strength and hardness of Al-7Si-0.25Mg castings. (2) Increasing pouring temperature can significantly reduce ESCs in microstructure of Al-7Si-0.25Mg alloy, promote transformation of its morphology from dendritic to spherical, and improve bulk performance of casting. (3) As injection waiting time increases, area fraction and average size of ESCs in Al-7Si-0.25Mg alloy microstructure show an increasing trend, yield strength and hardness show a decreasing trend. (4) Increasing Si content, under appropriate cooling rate conditions, can significantly reduce liquidus temperature of alloy, thereby inhibiting precipitation of pre-crystallized structures (ESCs) and significantly improving mechanical properties of casting body.
Aluminum die castings have advantages of high production efficiency, low processing cost, easy mechanical automation in production process, high dimensional accuracy of castings, good surface quality, and good overall mechanical properties; however, defects such as pores, flow marks, scratches, depressions, cracks, and undercasting are prone to occur during casting process, these defects reduce appearance quality and mechanical properties of die-casting parts. degrade appearance quality and mechanical properties of die-casting parts. In order to avoid above problems in die-casting process, structural designer needs to evaluate plan in advance in structural design of die-casting part, make a reasonable layout in structural design of part, and minimize defects by optimizing structure.
1 Forming principle of aluminum alloy die castings
Aluminum alloy die-casting parts must have a process of mold forming, combined with die-casting machine and aluminum alloy for comprehensive use. Principle of die-casting process is to use high pressure to flow molten metal into a precision metal mold cavity at high speed, molten metal is cooled and solidified under pressure to form a casting. Cold and hot chamber die casting are two basic methods of die casting process. In cold chamber die casting, metal liquid is poured into pressure chamber by manual or automatic pouring device, then injection punch advances to hydraulically inject metal into cavity. In hot chamber die-casting process, pressure chamber is perpendicular to crucible, molten metal flows into pressure chamber automatically through feed port on pressure chamber. Injection punch moves downward to push molten metal into cavity through gooseneck. After molten metal solidifies, die-casting mold is opened and casting is taken out to complete the entire die-casting forming process.
Relation between rib thickness and die casting wall thickness (mm)
Die casting wall thickness
Rib Thickness
0.8-2.5
1.5-2.5
2.0-3.5
2.5-3.5
2 Design points of aluminum alloy die castings
Rationality of die-casting design is related to the entire die-casting process. When designing die-castings, structural characteristics of die-castings and process requirements of die-casting should be fully considered, occurrence of defects in designed die castings during die casting molding process should be minimized, and quality of die-casting parts should be improved to the greatest extent with optimal design scheme.
Die casting minimum draft angle
Alloy type
Zinc alloy
Aluminum alloy
Copper alloy
Casting inner cavity
0°20’
0°32’
0°45’
Casting outer cavity
0°10’
0°15’
0°30’
2.1 Reasonable design of die casting wall thickness
Structural design of aluminum alloy die castings should fully consider wall thickness. Wall thickness is a factor with special significance in die casting process. Wall thickness has a close relationship with the entire process specification, such as calculation of filling time, selection of ingate speed, calculation of solidification time, analysis of mold temperature gradient, effect of pressure (final specific pressure), length of mold retention time, level of casting ejection temperature and operating efficiency; if design wall thickness is too thick, surface defects such as shrinkage cavities, blisters, pores, and coarse internal grains will appear, which will reduce mechanical properties and increase quality of parts, resulting in an increase in cost; If designed wall thickness is too thin, it will lead to poor filling of molten aluminum, difficulty in forming, poor dissolution of aluminum alloy, defects such as difficulty in filling casting surface and lack of material, which will bring difficulties to die-casting process; With increase of pores in die castings, defects such as internal pores and shrinkage cavities increase. Therefore, under premise of ensuring sufficient strength and rigidity of castings, wall thickness of castings should be reduced as much as possible and thickness of section should be kept uniform.
2.2 Rational design of die casting ribs
For die-castings with large planes or thin walls, their strength and rigidity are poor, and they are easy to deform. At this time, use of reinforcing ribs can effectively prevent shrinkage and fracture of die-castings parts, eliminate deformation, enhance strength and rigidity of die-castings parts. For structures such as columns and platforms that are too high, reinforcing ribs can be used to improve stress distribution and prevent root fractures. At the same time, reinforcing ribs can assist flow of molten metal and improve filling performance of castings. Thickness of root of reinforcing rib is not greater than thickness of wall here, and general thickness is designed to be 0.8-2.0mm; demoulding slope of reinforcing rib is generally designed to be 1°-3°, the higher height, the smaller design demoulding slope; Fillets need to be added to the root of ribs to avoid sharp changes in cross-section of part, at the same time assist flow of molten metal, reduce stress concentration of part, and improve strength of part. Fillet is generally close to wall thickness here; height of rib is generally not more than 5 times its thickness, and thickness of rib is generally required to be uniform. If design is too thin, rib itself is easy to break, and if it is too thick, it is easy to produce defects such as depressions and pores. Table 1 shows relationship between rib thickness and die casting wall thickness.
2.3 Reasonable design of die casting slope
Role of draft angle of die casting is to reduce friction between casting and mold cavity, making it easy to take out casting; ensuring that die casting surface is not strained, and at the same time prolonging life of mold. Draft angle is related to height of die casting, the larger height, the smaller draft angle. In general, draft slope of outer surface of die casting is about 1/2 of draft slope of inner cavity, but in actual design, draft slope of inner and outer surfaces of die casting can be designed to keep wall thickness uniform and simplify structural design. For example, Table 2 shows minimum draft slope reference values of various alloy die castings, and Table 3 shows relationship between cavity draft slope and depth of each die casting.
Relationship between cavity draft slope and depth
Casting Cavity Depth / Casting Material
6mm
6-8mm
8-10mm
10-15mm
15-20mm
20-30mm
30-60mm
Zinc alloy
2°30’
2°
1°45’
1°30’
1°15’
1°
0°45’
Aluminum alloy
4°
3°30’
3°
2°30’
2°
1°30’
1°15’
Copper alloy
5°
4°
3°30’
3°
2°30’
2°
1°30’
2.4 Reasonable design of machining allowance
When designing die-casting parts, machining should be avoided as much as possible. Machining will destroy dense layer on the surface of part and affect mechanical properties of part; it will expose internal pores of die-casting part, affecting surface quality and increasing cost of part. When machining cannot be avoided for die-casting parts, designs with large cutting volume should be avoided as far as possible, structural design should be as easy as possible for machining or reduce machining area and reduce machining costs. Die-casting parts have higher dimensional accuracy requirements, or some plane surface roughness requirements, and die-casting process is difficult to meet requirements. At this time, follow-up processing is required. For this part of structure, processing allowance should be reserved as much as possible during design. Strength and hardness of surface of die-casting parts are higher than those of interior. Care should be taken to preserve surface density during machining, so machining allowance should not be excessive. Too much machining may cause pores and outer surface defects. Table 4 is a reference for machining margin reservation.
Machining margin reference (mm)
Nominal size
0-30
30-50
50-80
80-120
120-180
180-260
Margin per side
0.3
0.4
0.5
0.6
0.7
0.8
2.5 Spraying design of aluminum alloy die castings
Surface spraying design of die castings generally adopts powder spraying process, and its principle is electrostatic powder spraying: coating is mainly polarized through electrodes, then object to be sprayed is charged with opposite charge, and powder is evenly attached to surface of object under action of electric field force. Features of powder spraying process: powder electrostatic spraying will not cause air pollution, powder can be recycled to reduce material consumption costs, coating performance is good in acid resistance, alkali resistance and corrosion resistance.
Structural characteristics of a two-in-one casing of a new energy vehicle are introduced. An initial die-casting process was designed and trial production was carried out. With the help of CAE software, we conducted in-depth research on local defects of shell specimen, analyzed their causes, determined direction of optimization and improvement of gating system, solved defects of internal air holes and slag holes in castings. In recent years, China’s new energy vehicle industry has developed rapidly, and new energy vehicles are becoming a “new force” in automotive industry. Drive systems such as motors and gearboxes are core components of new energy vehicles. Products designed by integrating power systems and drive systems such as motors and gearboxes are increasingly favored by new energy vehicle industry. High-pressure casting production can significantly reduce product wall thickness while maintaining structural strength. Moreover, die casting is close to net shape, cost is significantly reduced, and production efficiency is greatly improved. High-pressure casting production of aluminum alloy two-in-one housings consisting of motors and gearboxes has been recognized by major companies. Since die-casting process uses high-speed, high-pressure filling, gas is easily involved during die-casting filling process, resulting in problems such as pores and oxidized inclusions in die-casting parts.
1. Product analysis
Aluminum alloy two-in-one housing of a new energy vehicle is shown in Figure 1. Structure mainly consists of two parts, one part is motor housing and the other part is gearbox housing. Outline dimensions of two-in-one housing are 468 mm * 312 mm * 286 mm, the thickest part is 29.8 mm, basic wall thickness of motor housing part is 7 mm, and basic wall thickness of gearbox housing part is 5 mm. Structure is relatively complex, housing volume is 4 251 cm³, weight is 11.8 kg, and planned output is 100,000 pieces/year. Material is Al-Si-Cu alloy, implementation standard is JISH5302-2000, brand is ADC12, its liquidus temperature is 592 ℃ and solidus temperature is 539 ℃. Due to need for motor temperature control, a cooling water channel is designed in housing, so there is a limit on leakage. Test at a pressure of 300 kPa at normal temperature, and leakage amount should be within 0.4 cm³/min after maintaining pressure for 40 s. In addition, shell needs to be welded, it is friction stir welded with water jacket ring and water jacket.
Figure 1 A certain two-in-one housing
2. Die-casting process design
Two-in-one shell casting is large in size, and process after aluminum liquid passes through pouring system is long. It is necessary to choose a filling scheme with a shorter process. After program demonstration, the overall “Y” type pouring scheme was determined, as shown in Figure 2a. In order to reduce energy loss at sprue during filling process, an olecranon-shaped sprue was adopted. Set slag bags at ends respectively, connect slag bags through exhaust duct, and collect them into one exhaust port for vacuuming.
Figure 2 Pouring and cooling system
2.1 Watering and drainage system design
Pouring and drainage system can ensure reasonable arrangement of various areas during casting filling, smooth exhaust, and minimize involvement of gas. Basic wall thickness of the two parts of shell is 7 mm and 5 mm respectively, gate speed is 28~35 m/s, gate cross-sectional area is 1824 mm2, gate thickness is 5 mm, and punch diameter is 150 mm, filling degree of material cylinder is 42%, cross-sectional area of gate and piston is 1:9.7. According to Bernoulli’s principle, when flow velocity in gate is 35 m/s, punch speed is 3.6 m/s. DCC2500 horizontal cold chamber die-casting machine of Lijin Group was selected, with a clamping force of 25 000 kN. Overflow system uses a slag bag and vacuum exhaust. Use of vacuum reduces contact oxidation between aluminum liquid and air in cavity during filling process. Slag bag helps to discharge release agent, lubricating particles, oxidized slag in contact with air, gas involved in flow front end that are mixed into aluminum liquid during die casting from mold cavity and store them in slag bag to ensure quality of casting. Vacuum system uses Haiwang Company’s HVY800-100SM V5 vacuum machine, which has a vacuum capacity of 100 m³/h and is equipped with a hydraulic vacuum valve. During production process, vacuum gauge shows 100 mbar. Table 1 lists process parameters of pouring and drainage system.
Casting weight/kg
Sprue weight/kg
Slag bag exhaust weight/kg
Total orthographic projection area/cm2
Total weight/kg
11.8
5.3
2.3
2195
19.4
Table 1 Pouring and drainage system parameters
2.2 Cooling system design
Set up cooling water in wall thickness area of casting to ensure cooling effect in wall thickness area, avoid shrinkage and shrinkage holes in wall thickness area. Design of cooling system is affected by position of ejector pin and core, and it is difficult to completely take into account all wall thickness areas of casting. However, it is still necessary to take into account thermal balance of mold and cooling of thick wall area of casting as much as possible. Cooling system is shown in Figure 2b. Blue is normal pressure cooling water circuit, green is mold temperature machine oil circuit, red and magenta are high-pressure point cooling.
3. Numerical simulation and casting defect analysis
Anycasting software was used to numerically simulate designed die-casting pouring process plan to analyze filling effect of aluminum alloy liquid and whether design caused internal defects in casting.
3.1 Numerical simulation analysis
According to initial die-casting process design, calculation conditions are set in numerical preprocessing: pouring temperature 670 ℃; piston diameter 150 mm; low injection speed is 0.8 m/s, high speed is 4.1 m/s; mold material is SKD61 steel, preheating temperature is 180 ℃; cooling medium is set to water, and inlet water temperature is controlled to 25 ℃; default setting of Anycasting is selected for oxidation slag inclusion, 515 ℃ corresponds to dimension 0 and pouring temperature of 670 ℃ corresponds to dimension 1; vacuum setting is 50 mbar. Numerical simulation results of casting filling process are shown in Figure 3. It can be seen from figure that aluminum liquid enters mold cavity from left and right inner runner at the same time. During process, aluminum liquid first fills gearbox part below runner, then fills motor shell part. When motor housing part is basically filled, gearbox part without sprues is filled. Circumference of movable mold side of motor shell is filling end, and gearbox shell part without sprues is also filling end. Filling process was smooth, with less air entrainment. Filling sequence was basically consistent with expectations. Mold cavity was completely filled, and there was no under-pouring.
Figure 3 Filling sequence
3.2 Casting defect analysis
Based on above analysis, trial production was carried out on Lijin DCC2500 die-casting machine. After die-casting trial production, castings were first subjected to X-ray flaw detection, all areas were scanned and enhanced to detect internal quality of die-castings. After multiple rounds of debugging, it was found that internal defects of two-in-one motor housing were mainly concentrated at a suspension hole below sprue, as shown in Figure 4, which could not be significantly reduced and failed to meet customer’s acceptance standards.
Figure 4 Defects of pores and slag holes inside castings Particle tracking function of Anycasting software during mold filling process is used to obtain information such as streamlines and vortices in flow field, as shown in Figure 5. It can be seen from figure that during mold filling process, aluminum liquid is filled downward at a very high speed through runner, then fills in reverse direction after encountering resistance below suspension hole, and filled along thick wall of shell, forming a rewinding vortex. Gas in mold cavity cannot be discharged. In addition, granular aluminum slag cooled at the front end of aluminum liquid cannot be discharged from mold cavity. It mixes with undischarged gas to cause local pores and slag holes in casting.
Figure 5 Particle tracking analysis of mold filling process
4. Structural optimization and numerical simulation analysis of pouring system
Use UG software to optimize two-in-one motor housing gating system. As shown in Figure 6, first cut off sprue near suspension hole and change it to a slag bag to discharge defects caused by eddy current entrainment during filling process of suspension hole to outside of casting, first option; due to blocking of one runner, the overall runner area is reduced, so sprues on both sides of cut runner are enlarged as second option; runner on the left side of cut runner is not enlarged as the third option to verify mold filling effect.
Figure 6 Improvement plan for gating system Above three pouring system solutions were used, and Anycasting software was used to perform numerical simulation analysis to check filling effect of suspended hole below sprue. As shown in Figure 7, in the first option, aluminum liquid enters slag bag on the left side of slag bag opening and then squeezes into mold cavity on the right side; in second option, aluminum liquid enters slag bag on the right side of slag bag opening, then is squeezed in large quantities on the left side and discharged into mold cavity; in third plan, after aluminum liquid was introduced into slag package, no backflow into cavity was found.
Figure 7 Numerical simulation results for different gating system improvement plans In summary, pouring system of Plan 3 is the most ideal; based on Plan 3, increase volume of slag bag and improve mold filling effect, as shown in Figure 8a as final pouring system improvement plan.
Figure 8 Final pouring system scheme and test mold verification results
5. Optimization effect
In the process of this research, firstly, by analyzing structural characteristics of castings, initial die-casting process was designed and trial production of castings was carried out; secondly, by analyzing defects of trial production castings and adjusting design of gating system multiple times, the overall scheme of gating system was determined; thirdly, local defects in suspension holes of castings were further analyzed and an improvement plan was determined. Finally, based on improvement plan, volume of slag bag was increased to determine final optimized pouring system structure. In above process, numerical simulation technology was used many times to analyze causes of defects and find direction for process improvement. Mold trial verified that gating system was effective, gas and slag inclusions entered slag bag, eliminating air holes and slag hole defects in suspension holes, as shown in Figure 8b.
6. Conclusion
In response to forming requirements of a new energy car’s aluminum alloy two-in-one shell, Anycasting software and process experiments were repeatedly used to improve die-casting process. Causes of casting defects caused by local eddy currents and air entrainment during casting filling process were analyzed and discovered. Finally, an optimized gating system design was obtained. After mold testing, feasibility of optimized plan was proved.
Hard anodic oxide films generally require a thickness of 25-150um. Most hard anodized films have a thickness of 50-80um. Hard anodized films with a film thickness less than 25um are used for parts such as tooth keys and spirals. Thickness of anodic oxide film used for wear resistance or insulation is about 50um. Under certain special process conditions, it is required to produce a hard anodized film with a thickness of more than 125um. However, it must be noted that the thicker anodized film, the lower microhardness of its outer layer can be, and roughness of film surface increases. Bath liquid for hard anodizing is usually a sulfuric acid solution and sulfuric acid added with organic acids, such as oxalic acid, sulfamic acid, etc. In addition, hard anodizing can be achieved by lowering anodizing temperature or reducing sulfuric acid concentration. For deformed aluminum alloys with copper content greater than 5% or silicon content greater than 8%, or high silicon die-cast aluminum alloys, some special measures for anodizing may also be considered. For example: for 2XXX series aluminum alloy, in order to prevent aluminum alloy from being burned during anodization process, 385g/L sulfuric acid plus 15g/L oxalic acid can be used as electrolytic bath liquid, and current density should also be increased to above 2.5A/dm.
Process method
There are many electrolysis methods for hard anodizing, such as: sulfuric acid, oxalic acid, propylene glycol, sulfosalicylic acid and other inorganic salts and organic acids. Power supply used can be divided into DC, AC, AC-DC superposition, pulse and superposition pulse power supply, etc. Currently, following types of hard anodization are widely used. (1) Sulfuric acid hard anodizing method; (2) Oxalic acid hard anodizing method. (3) Mixed acid hard anodizing Among them, sulfuric acid method is a hard oxidation method that is currently widely used.
Principle
1 Principle of hard anodizing Principle of pure sulfuric acid type aluminum alloy hard anodizing is not essentially different from ordinary anodizing. If it is mixed acid type hard anodizing, there will be some side reactions. reaction nature: 1 Cathodic reaction: 4H+ +4e=2H2↑ 2 Anode reaction: 4OH－－4e=2H2O+O2↑ 3 Aluminum oxidation: Oxygen precipitated on anode is in atomic state, which is more active than molecular oxygen and is more likely to react with aluminum: 2A1+3O→A12O3 4 Dynamic balance of oxidation and dissolution of anode film: As energization time increases, current increases and oxide film thickens. At the same time, due to dual nature of chemical properties of (Al2O3), that is, it appears as an alkaline oxide in an acidic solution and an acidic oxide in an alkaline solution. There is no doubt that oxide film liquid dissolves in sulfuric acid solution. Only when formation rate of oxide film is greater than its dissolution rate, can oxide film be thickened. When dissolution rate is equal to formation rate, oxide film will no longer thicken. When oxidation rate is too much greater than dissolution rate, powdery oxide films are likely to form on the surfaces of aluminum and aluminum alloy parts.
Process requirements
In order to obtain a good quality hard anodized film and ensure required size of parts, processing must be carried out according to following requirements.
Acute angle rounding
Parts to be processed are not allowed to have sharp corners, burrs and other sharp edges. Because of hard oxidation, anodizing time is generally very long, and oxidation process (A1+O2→A12O3+ Q) itself is an exothermic reaction. And because edges and corners of general parts are often where current is concentrated, these parts are most likely to cause local overheating of parts and cause parts to be burned. Therefore, all edges and corners of aluminum and aluminum alloys should be chamfered, and radius of chamfering circle should not be less than 0.5 mm. Surface finish
Surface finish
After hard anodizing, surface finish of parts has changed. For rougher surfaces, they can appear smoother than before after this treatment. For parts with higher original smoothness, they often go through this process. After treatment, brightness of displayed surface is reduced, and degree of reduction is about 1 to 2 levels.
Allowance for part size
Due to high thickness of hard oxide film, if aluminum parts require further processing or parts that need to be assembled later, a certain processing allowance should be left in advance and a designated clamping location should be reserved. Since size of part needs to be changed during hard anodizing, it is necessary to predict possible thickness and dimensional tolerance of oxide film in advance during machining, then determine actual size of part before anodizing so that it meets specified tolerance range after processing. Generally speaking, increased size of part is roughly half thickness of resulting oxide film.
Special fixture
Because hard anodized parts have to withstand high voltages and high currents during oxidation process, it is important to maintain excellent contact between fixture and parts, otherwise poor contact will cause breakdown or burns to contact parts of parts. Therefore, it is required to design and manufacture special fixtures for parts with different shapes and specific requirements after oxidation of parts. Local protection For example, if there are both ordinary anodized and hard anodized parts on same part, specific process will be arranged according to smoothness and precision of part. Usually, ordinary anodizing is performed first, then hard anodizing is performed. Surface that does not require hard anodizing is insulated. Insulation method is to use a spray gun or brush. Apply prepared nitro glue or hydrogenated vinyl glue to the surface that does not need to be treated. Insulation layer should be applied thinly and evenly. Each layer should be dried at low temperature for 30 to 60 minutes, and a total of 2 to 4 layers can be applied.
Process characteristics
Electrolyte for hard anodization is electrolyzed at a temperature of about -10℃ to +5℃. Since oxide film layer generated by hard anodization has high resistance, it will directly affect oxidation effect of current intensity. In order to obtain a thicker oxide film, it is necessary to increase external voltage. Purpose is to eliminate influence of high resistance and keep current density constant. However, when current is large, intense heating will occur. In addition, a large amount of heat will be released when oxide film is formed, causing temperature of electrolyte around parts to rise sharply. Temperature increase will accelerate dissolution of oxide film and prevent oxide film from becoming thicker. In addition, heating phenomenon is most serious at contact point between film layer and metal. If not solved in time, local surface of processed parts will be burned due to temperature rise. Solution is to use a combination of cooling equipment and stirring.Cooling equipment forcibly cools down electrolyte, and stirring is to make temperature of electrolyte in the entire tank uniform, so as to obtain a higher-quality hard oxide film.
Process influencing factors
Influence of various factors on hardness and growth rate of oxide film Whether a high-quality hard oxide film layer can be formed on the surface of aluminum and aluminum alloys mainly depends on component concentration, temperature, current density of electrolyte, and composition of its raw materials. Electrolyte concentration When using sulfuric acid electrolyte for hard anodization, it is generally in concentration range of 10% to 30%. When concentration is low, hardness of oxide film is high, especially for pure aluminum, except for aluminum alloys with high copper content (CY12). Because aluminum alloys with higher copper content are prone to generate CuAl2 compounds, this compound dissolves quickly during oxidation and can easily burn aluminum parts. Therefore, it is generally not suitable to use low-concentration sulfuric acid electrolyte. It must be oxidized in high concentration (H2SO4 at 300~400g/L) or treated by AC and DC superposition method. Effect of temperature on film layer Temperature of electrolyte has a great influence on wear resistance of oxide film. Generally speaking, if temperature decreases, wear resistance of anodized film of aluminum and aluminum alloys increases. This is due to decrease in dissolution rate of electrolyte to the film, in order to obtain a higher hardness oxide film. We must control temperature within range of ±2℃ for hard anodizing treatment.
Compared with sand casting, die casting production has characteristics of dense casting structure and high production efficiency, and is widely used in automobiles, communications and other fields. As core component of water pump, water pump casing needs to have certain anti-leakage and anti-corrosion capabilities, can meet mechanical properties and low-temperature impact resistance under specific conditions in water. Therefore, it has high requirements for air tightness and mechanical properties. This requirement can be met by using die castings to deal with defects such as shrinkage cavities and shrinkage porosity. Combining computer numerical simulation technology with actual production can greatly reduce production costs. Taking ADC12 aluminum alloy shell die-casting as research object, die-casting process design is carried out based on its structural characteristics. Determine location and form of casting parting surface, gating system, and overflow system as well as die-casting process parameters, and initially formulate a die-casting process plan. Use Flow-3D software to numerically simulate filling process and filling results of initial plan, determine location and causes of casting defects based on pressure, temperature, air entrainment, and surface quality. Based on analysis results, original process plan is optimized, optimized plan is simulated and analyzed again to obtain a process plan that meets production requirements.
GraphicsResults
Research object is a water pump shell die-cast with a volume of 185cm3, a maximum wall thickness of 10mm, an average wall thickness of 3.27mm, and a weight of about 450g. It is selected to have good fluidity, medium air tightness and good thermal crack resistance. In particular, ADC12 aluminum alloy with high wear resistance and low thermal expansion coefficient is used as a die casting material. Figure 1 is a three-dimensional structural diagram of water pump housing. According to selection principle of parting surface and combined with structural characteristics of research object, parting method is shown in Figure 2.
Figure 1 Solid 3D model of die casting
Figure 2 Solid parting method of shell castings Figure 3 shows size of inner gate. Two inner gates are set up, and width of each inner gate is 20mm. Cross-sectional area Ar of cross runner is 3 to 4 times that of inner gate, and Ar is determined to be 180mm2. Appropriateness of overflow system affects quality of castings to a great extent. Shape of overflow tank mainly used in this plan is shown in Figure 4. Among them, b, a, h, and A are the width, length, thickness, and length of overflow port respectively. B and H are width and thickness of overflow tank respectively.
Figure 3 Inner gate thickness, width and length
Figure 4 Overflow tank shape
Figure 5 Original process plan Original die-casting plan was imported into Flow-3D software for numerical simulation. Figure 6 shows flow field simulation results of original plan. According to simulation results, maximum temperature of die casting is 680℃ and minimum temperature is about 603℃. Temperature of molten metal in mold cavity generally decreases gradually as distance from inner gate increases. Locations with lower temperatures solidify first, and locations with higher temperatures solidify more slowly. It can be seen from Figure 6 that filling process is not stable, and droplet splashing occurs at t=0.027s. Comprehensive analysis shows that during simulation process, solidification method of die casting is layer by layer, and order of solidification in direction away from inner gate is followed, so shrinkage can be obtained in time, so serious shrinkage and shrinkage holes will not occur.
a. t=0.015s
b. t=0.027s
c. t=0.030s
(b)t=0.038s Figure 6 Flow field distribution simulation
a. Side A view
(b) Side B view Figure 7 Simulation results of air entrainment distribution in initial scheme
a. Side A view
(b) Side B view Figure 8 Surface defect simulation results and analysis For castings with complex shapes, multi-strand ingates are often used, and three ingates are set up. Widths of ingates are 8, 10, and 20mm respectively, and length is mm. Cross-sectional area of inner gate is increased to increase injection speed. In order to improve exhaust conditions, three annular overflow grooves are added on both sides of casting. Dimensions of overflow groove I are A=5mm, H=8mm, B=15mm; dimensions of overflow groove II are A=10mm, H=10mm. B=15mm, overflow tank III dimensions are A=5mm, H=8mm, B=15mm; an overflow tank is added at lifting lug, dimensions are A=15mm, H=10mm, B=20mm, and a cylindrical riser is also installed, R=10mm, H=15mm. In order to increase exhaust effect and improve air entrainment, an exhaust slot is added to the end cover of casting. Exhaust slot is used to exhaust gas generated from air and paint volatilization in cavity. In order to exhaust as much gas as possible in mold cavity during injection, exhaust slot is set at last filled position of molten metal. Optimization plan is shown in Figure 9.
Figure 9 Three-dimensional diagram of optimization plan
a. t=0.016s
b. t=0.033s
c. t=0.044s
(b)t=0.045s Figure 10 Flow field simulation of optimization plan
Figure 11 Air entrainment simulation results of optimization scheme
Figure 12 Defect simulation results of optimization plan
Figure 13 Actual production castings
#in conclusion #
Based on Flow-3D software, water pump housing die-casting process was numerically simulated, causes of air entrainment and defects were analyzed, and process was optimized. Multiple internal gates were added, overflow grooves and exhaust grooves were added. Simulation showed that there were no defects. Casting begins to solidify from inner gate. During filling process of casting, molten metal is filled relatively smoothly, and no splashing of molten metal occurs, indicating that optimized solution has a good filling effect. Places with serious air entrainment are concentrated in overflow groove and exhaust groove, which shows that overflow groove and exhaust groove set up have a good effect. There is no air entrainment on casting, and no defects appear in casting. Maximum surface defect is 2 % or less, serious surface defects appear on overflow groove, and overflow groove can be removed after die-casting is completed. Optimization plan is reasonable and feasible, and castings that meet requirements are obtained after production verification.
There are two main types of oxidation treatment methods for aluminum and aluminum alloys:
Chemical oxidation
Oxide film is thin, with a thickness of about 0.5 to 4 microns, is porous, soft, has good adsorption properties. It can be used as bottom layer of organic coatings, but its wear resistance and corrosion resistance are not as good as anodized films;
Electrochemical oxidation
Thickness of oxide film is about 5 to 20 microns (thickness of hard anodized film can reach 60 to 200 microns). It has high hardness, good heat resistance and insulation, and its corrosion resistance is higher than that of chemical oxide film. It is porous and has good adsorption capacity.
Chemical oxidation
Chemical oxidation treatment equipment for aluminum and aluminum alloys is simple, easy to operate, has high production efficiency, does not consume electricity, has a wide range of applications, is not limited by size and shape of parts. Chemical oxidation process of aluminum and aluminum alloys can be divided into two categories: alkaline oxidation method and acidic oxidation method according to properties of solution. According to properties of film layer, it can be divided into: oxide film, phosphate film, chromate film, and chromic acid-phosphate film.
Akaline oxidation
Mass concentration of composition/g·L
Recipe number
1
2
3
Sodium carbonate
40～60
50～60
40～50
Sodium chromate
15～25
15～20
10～20
Sodium hydroxide
2～5
Trisodium phosphate
1.5～2
Sodium silicate
0.6～1.0
Temperature/℃
85～100
95～100
90～95
Time/min
5～8
8～10
8～10
Note: ① Formulas 1 and 2 are suitable for chemical oxidation of pure aluminum, aluminum-magnesium alloy, aluminum-manganese alloy and aluminum-silicon alloy. Color of film is golden yellow, but color of oxide film obtained on the latter two alloys is darker. Film layer obtained in alkaline oxidizing solution is soft, has poor corrosion resistance, high porosity and good adsorption, and is suitable as a coating base layer. ② Add sodium silicate to formula 3, and oxide film obtained is colorless, with slightly higher hardness and corrosion resistance, slightly lower porosity and adsorption. It can be sealed in a solution with a mass fraction of 2% sodium silicate. Used alone as a protective layer, suitable for oxidation of aluminum alloys containing heavy metals. ③ In order to improve corrosion resistance of workpiece after oxidation treatment, it can be passivated in 20g/L CrO3 solution at room temperature for 5 to 15 seconds, and then dried at a temperature below 50℃.
Acidic oxidation
Mass concentration of composition/g·L
Recipe number
1
2
3
4
5
Phosphoric acid
10～15
50～60
22
Chromic anhydride
1～2
20～25
2～4
4～5
3.5～5
Sodium fluoride
3～5
5
1～1.2
0.8
Ammonia hydrogen fluoride
3～3.5
Diamine hydrogen phosphate
2～2.5
Boric acid
0.6～1.2
2
Potassium ferricyanide
0.5～0.7
Potassium dichromate
3～3.5
Temperature/℃
20～25
30～40
Room temperature
25～35
25～30
Time/min
8～15
2～8
15～60s
0.5～1.0
3
Note: ① Oxide film obtained by formula 1 is thin, has good toughness and good corrosion resistance. It is suitable for aluminum and aluminum alloys that need to be deformed after oxidation. It can also be used for surface protection of aluminum castings. It does not require passivation or filling treatment after oxidation. ②pH value of solution in Formula 2 is 1.5 to 2.2. Resulting oxide film is thicker, about 1 to 3 microns, with good density and corrosion resistance. There is no change in size of parts after oxidation, and color of oxide film is colorless to light blue. It is suitable for oxidation treatment of various aluminum and aluminum alloys. After oxidation treatment in formula 2 solution, parts should be cleaned immediately with cold water, then filled with potassium dichromate 40~50g/L solution (when pH=4.5~6.5, use sodium carbonate to adjust), temperature is 90~95℃, time 5 to 10 minutes, wash and dry at 70℃. ③Oxide film obtained in solution of Formula 3 is colorless and transparent, with a thickness of about 0.3 to 0.5 microns. Film layer has good conductivity and is mainly used for deformed aluminum electrical parts. ④Formula 4 is suitable for pure aluminum, rust-proof aluminum, cast aluminum and other alloys. Oxide film is very thin, has good conductivity and corrosion resistance, low hardness, and is not wear-resistant. It can be spot welded or argon arc welded, but cannot be soldered. It is mainly used for aluminum alloy parts that require certain conductive properties. ⑤Oxide film obtained by Formula 5 is thin, about 0.5 microns, has good conductivity and corrosion resistance, has few pores, and can be used as a separate protective layer.
Anodizing
Aluminum is a relatively active metal with a standard potential of -1.66v. It can naturally form an oxide film with a thickness of about 0.01 to 0.1 microns in air. This oxide film is amorphous, thin and porous, and has poor corrosion resistance. However, if aluminum and its alloys are placed in an appropriate electrolyte, aluminum product is used as anode, and an oxide film is formed on the surface under action of an external current. This method is called anodizing. By selecting different types and concentrations of electrolytes, and controlling process conditions during oxidation, anodized films with different properties and thicknesses of about tens to hundreds of microns can be obtained, their corrosion resistance, wear resistance and decorative properties have been significantly improved.
Form
Electrolyte used in anodizing of Al and aluminum alloys is generally an acidic solution with medium solubility, lead serves as cathode and only plays a conductive role. When aluminum and its alloys are anodized, following reactions occur at anode: 2Al —> 6e-+ 2Al3+ Following reactions occur at cathode: 6H2O +6e—-> 3H2 + 6OH- At the same time, acid chemically dissolves aluminum and resulting oxide film, and reaction is: 2Al + 6H+—> 2Al3+ +3H2 Al2O3 + 6H+—> 2Al3+ + 3H2O Growth process of oxide film is process of continuous generation and dissolution of oxide film. The first section a (curve section ab): non-porous layer is formed. Within a few seconds to tens of seconds after power is turned on, a dense, highly insulating oxide film immediately forms on aluminum surface, with a thickness of about 0.01 to 0.1 microns. It is a continuous, non-porous film layer, called a non-porous layer or barrier layer. Appearance of this film hinders passage of current and continued thickening of film layer. Thickness of nonporous layer is directly proportional to formation voltage and inversely proportional to dissolution rate of oxide film in electrolyte. Therefore, voltage in section ab of curve shows a sharp increase from zero to maximum value. The second section b (curve bc section): porous layer is formed. With formation of oxide film, dissolution of film by electrolyte begins. Since generated oxide film is not uniform, holes will be dissolved first in the thinnest part of film. Electrolyte can reach fresh surface of aluminum through these holes, electrochemical reaction can continue, resistance decreases, voltage decreases (decrease is 10 to 15% of maximum value), and a porous layer appears on membrane. The third section c (curve section cd): porous layer thickens. After anodizing for about 20 seconds, voltage enters a relatively stable and slow rising stage. It shows that while non-porous layer is continuously being dissolved to form a porous layer, a new non-porous layer is growing. That is to say, formation speed and dissolution speed of non-porous layer in oxide film have basically reached a balance, so thickness of non-porous layer no longer increases and voltage change is also very small. However, formation and dissolution of oxide film at the bottom of hole did not stop at this time. They were still continuing, causing bottom of hole to gradually move toward inside of metal matrix. As oxidation time continues, holes deepen to form pores, and film layer with pores gradually thickens. When film formation rate and dissolution rate reach a dynamic balance, thickness of oxide film will not increase even if oxidation time is extended, and anodizing process should be stopped at this time. Anodizing characteristic curve and oxide film growth process are shown in figure below.
Craftsmanship
There are many methods for anodizing aluminum and its aluminum alloys. Commonly used ones include sulfuric acid anodizing, chromic acid anodizing, oxalic acid anodizing, hard anodizing and porcelain anodizing.
Sulfuric acid
Aluminum and its alloys are anodized by direct current and alternating current in dilute sulfuric acid electrolyte, a colorless and transparent oxide film with a thickness of 5 to 20 microns and good adsorption can be obtained. Sulfuric acid anodizing process is simple, solution is stable, easy to operate, allows a wide range of impurity content, consumes less electricity, has low cost, can be applied to processing of almost all aluminum and various aluminum alloys, so it has been widely used in China. Following table shows several typical anodizing processes:
Formula and process conditions
DC method
AC method
1
2
3
Sulfuric acid (g/L)
50～200
160～170
100～150
Aluminum ion Al3+ (g/L)
<20
<15
<25
Temperature (℃)
15～25
0～3
15～25
Anode current density (A/dm2)
0.8～1.5
0.4～6
2～4
Voltage(V)
18～25
16～20
18～30
Time(min)
20～40
60
20～40
Stir
Compressed air
Compressed air
Compressed air
Cathode area/anode area
1.5:1
1.5:1
1:1
Main factors affecting quality of oxide film are: ① Sulfuric acid concentration: usually 15% to 20%. As concentration increases, dissolution rate of membrane increases, growth rate of membrane decreases, membrane has high porosity, strong adsorption, elasticity, good dyeability (easy to dye dark colors), but slightly poor hardness and wear resistance; when concentration of sulfuric acid is reduced, growth rate of oxide film is accelerated, film has fewer pores, high hardness, and good wear resistance. Therefore, when used for protection, decoration and pure decorative processing, upper limit of allowable concentration, that is, 20% concentration of sulfuric acid is often used as electrolyte. ② Electrolyte temperature: Electrolyte temperature has a great influence on quality of oxide film. As temperature increases, dissolution rate of film increases and film thickness decreases. When temperature is 22 to 30℃, resulting membrane is soft and has good adsorption capacity, but poor wear resistance; when temperature is greater than 30℃, film becomes loose and uneven, sometimes even discontinuous, and has low hardness, thus losing its use value; when temperature is between 10 and 20℃, oxide film generated is porous, has strong adsorption capacity, is elastic, suitable for dyeing, but hardness of film is low and wear resistance is poor; when temperature is lower than 10℃, thickness of oxide film increases, hardness is high, and wear resistance is good, but porosity is low. Therefore, temperature of electrolyte must be strictly controlled during production. To produce a thick and hard oxide film, operating temperature must be lowered. Compressed air stirring and relatively low temperature are used during oxidation process, usually around zero for hard oxidation.
③Current density: Within a certain limit, when current density increases, film growth rate increases, oxidation time shortens, resulting film has many pores, is easy to color, hardness and wear resistance increase; if current density is too high, surface of part will be overheated and local solution temperature will increase due to influence of Joule heat, dissolution rate of film will increase, and there is a possibility of burning part; If current density is too low, film growth rate will be slow, but resulting film will be denser, hardness and wear resistance will be reduced. ④ Oxidation time: Selection of oxidation time depends on electrolyte concentration, temperature, anode current density and required film thickness. Under same conditions, when current density is constant, growth rate of film is proportional to oxidation time; but when film grows to a certain thickness, film resistance increases, which affects conductivity, film’s dissolution rate increases due to temperature rise, so film growth rate will gradually decrease and will no longer increase in the end. ⑤ Stirring and moving: It can promote convection of electrolyte, strengthen cooling effect, ensure uniformity of solution temperature, and will not cause quality of oxide film to decrease due to local heating of metal. ⑥Impurities in electrolyte: Impurities that may exist in electrolyte used for aluminum anodization include Clˉ, Fˉ, NO3ˉ, Cu2+, Al3+, Fe2+, etc. Among them, Clˉ, Fˉ, NO3ˉ increase porosity of membrane, make surface rough and loose. If its content exceeds limit value, it may even cause corrosion and perforation of workpiece (Clˉ should be less than 0.05g/L, Fˉ should be less than 0.01g/L); when Al3+ content in electrolyte exceeds a certain value, white spots or patchy white patches often appear on the surface of workpiece, which reduces adsorption performance of film and makes dyeing difficult (Al3+ should be less than 20g/L); when Cu2+ content reaches 0.02g/L, dark stripes or black spots will appear on oxide film; Si2+ often exists in a suspended state in electrolyte, making electrolyte slightly turbid and adsorbed on film as brown powder. ⑦ Aluminum alloy composition: Generally speaking, other elements in aluminum metal reduce quality of film, obtained oxide film is not as thick as that obtained on pure aluminum, and hardness is also low. When anodizing aluminum alloys with different compositions, care should be taken not to perform them in same tank.
Chromic acid
Chromic acid anodizing refers to technology of anodizing aluminum and its alloys using 5 to 10% chromic acid electrolyte. Oxide film obtained by this method has following characteristics: ① Thinner (compared to sulfuric acid and oxalic acid oxide films), about 2 to 5 microns, which can maintain original accuracy and roughness of workpiece; ② Soft and highly elastic, with almost no pores, and stronger corrosion resistance than sulfuric acid anodized films; ③ Opaque, color ranges from gray to dark gray, or even rainbow, so it is not easy to dye; ④ Due to small number of pores, film layer can be used without sealing treatment; ⑤ It has good binding force with organic matter, so it is often used as base layer of paint; ⑥ Compared with sulfuric acid anodization, cost is higher, and its use is subject to certain restrictions. Following table shows several chromic acid anodizing processes:
Formula and process conditions
1
2
3
Chromic acid (g/L)
90～100
50～55
30～35
Temperature (℃)
37±2
39±2
40±2
Current density (A/dm2)
0.3～2.5
0.3～0.7
0.2～0.6
Voltage(V)
0～40
0～40
0～40
Oxidation time (min)
35
60
60
Cathode material
Aluminum plate and graphite
Oxalic acid
Oxalic acid anodizing is an oxidation process that uses 2% to 10% oxalic acid electrolyte and passes through direct current or alternating current. When using direct current for anodizing, hardness and corrosion resistance of resulting film are no less than those of H2SO4 anodized films. Moreover, since solubility of oxalic acid solution to aluminum and oxide films is small, a thicker oxide film layer can be obtained than in sulfuric acid solution; If AC current is used for oxidation, a softer and more elastic film layer can be obtained. Film layer of oxalic acid anodization is generally 8 to 20 microns, and maximum thickness can be 60 microns.
During oxidation process, as long as process conditions (such as oxalic acid concentration, temperature, current density, waveform, etc.) are changed, decorative film layers such as silvery white, golden yellow to brown, etc. can be obtained without need for dyeing. Oxalic acid anodizing electrolyte is very sensitive to chloride ions, and if its mass concentration exceeds 0.04g/L, corrosion spots will appear on film layer. Mass concentration of trivalent aluminum ions is also not allowed to exceed 3g/L. However, oxalic acid anodizing costs more and consumes more energy (because resistance of oxalic acid electrolyte is greater than that of sulfuric acid and chromic acid), solution is toxic, and stability of electrolyte is poor. Several oxalic acid anodizing processes are shown in table below.
Formula and process conditions
1
2
3
Oxalic acid (g/L)
30±3
50±5
50±10
Temperature (℃)
18±3
30±3
30±3
Current density (A/dm2)
1～2
1～2
2～3
Voltage(V)
110～120
30～35
40～60
Oxidation time (min)
120
30～60
30～60
Cathode material
Carbon rod
Carbon rod
Power supply
DC
DC
AC
Porcelain
Certain substances are added to electrolyte so that they are adsorbed in film layer while forming an oxide film, thereby obtaining a smooth, shiny, uniform and opaque oxide film similar to porcelain glaze and enamel color, which is called “porcelain anodic oxidation film” or “porcelain oxide film”. This kind of oxide film has good elasticity and good corrosion resistance, and can have a plastic appearance after dyeing. Resulting film thickness is approximately 6 to 25 microns. Following are two methods of porcelain oxidation: ① Add salts of certain rare metal elements (such as titanium, thorium, etc.) to sulfuric acid or oxalic acid solutions: During oxidation process, due to hydrolysis of these salts, chromogenic substances are deposited in pores of oxide film, forming a glaze-like glaze. Film layer has high hardness, can maintain high precision and smoothness of parts, but it is expensive, solution has a short service life, and process conditions are strict. ② Use a mixture of chromic anhydride and boric acid as anodizing liquid: simple ingredients, low cost, good elasticity of the oxide film, but lower hardness than previous one, and can be used for general decorative porcelain oxidation surface treatment. Porcelain anodizing solution and process conditions are shown in table below.
Recipe number
Electrolyte composition
Mass concentration/g·L
Temperature/℃
Current density /A/dm2
Voltage /V
Time/min
Illustrate
1
Chromic anhydride
35～40
45～55
0.5～1.0
25～40
40～50
1. Film layer is milky white and can be dyed. 2. Film thickness is 10~16μm. 3. Suitable for general decorative parts.
Oxalic acid
5～12
Boric acid
5～7
2
Chromic anhydride
30～40
40～50
Start: 2～3 Termination: 0.1～0.6
Gradually rise 90～110 Keep 40～80
Boost time <5 Hold: 35～55 Total time: 40～60
1. Good solution stability and easy operation, 2. Film layer is gray 3. Thickness is 10~15μm 4. Suitable for general decorative parts, low cost.
Boric acid
1～3
3
Potassium titanium oxalate
35～45
24～28
Starting: 2～3 Termination: 0.6～1.2
Gradually rise 90～110 Keep
90～110
Boost time 5～10 Maintain: 25～30 Total time: 40～60
1. Film layer is off-white and has high hardness. 2. Film thickness is 8~16μm. 3. Suitable for wear-resistant high-precision parts decoration. 4. High cost and short service life of solution
Boric acid
8～10
Oxalic acid
2～5
Citric acid
1～1.5
Note: Cathode material can be pure aluminum, lead plate or stainless steel plate. In oxidation solution, changes in various components will determine color of oxide film: for example, as amount of chromic anhydride increases, color of film changes to opaque gray; as amount of boric acid increases, color of film changes to milky white; With increase of oxalic acid, color of film layer changes to yellow.
Die-casting machine is basic equipment for pressure casting of non-ferrous metals and their alloys and has a complex structure. Mold clamping mechanism consists of a mold plate and a machine hinge, and is the key mechanism of die-casting machine. Each die-casting production cycle is accompanied by an opening and closing action of mold closing mechanism. Mold clamping and mold opening of mold closing mechanism mainly uses oil cylinder to push hinge system. Structure is shown in Figure 1. Hinge system rapidly expands thrust of oil cylinder to promote movement of mold plate. Machine hinge system is a typical multi-link mechanism. Unreasonable design of toggle rod (connecting rod) size will lead to insufficient expansion ratio of mold closing mechanism, large impact during mold closing process, low mold life, long mold opening and clamping time, and low die-casting efficiency. To achieve a large clamping force, it is necessary to increase cylinder thrust, which requires high energy consumption. At present, design of die-casting machine hinges is mainly based on theoretical calculation methods. Since other complex structural parts in mold clamping mechanism and deformation effects of various parts of hinge system itself cannot be taken into account, calculation results have large errors, resulting in an unreasonable hinge structure design. Repeated design and manufacturing lead to long R&D cycles and high costs. Collaborative application of digital modeling, finite element method and kinematic simulation technology provides new solutions for design of die-casting machine hinges. Through numerical simulation technology, not only quantitative design of performance can be achieved, but also optimal design can be achieved, significantly shortening research and development cycle of hinge system and saving costs. In response to actual problems reported by enterprise that large die-casting machine with a clamping force of 25,000kN has a large oil cylinder, high energy consumption, is accompanied by high abnormal noise during each mold opening and closing process, we carried out defect cause analysis and optimized design of hinge system based on numerical simulation methods machine hinge analysis based on numerical simulation methods to increase stroke of movable mold base plate, while minimizing stroke of oil cylinder, achieve smooth mold plate movement without pauses and noise during mold opening and closing process.
Graphical results
Establish a 3D assembly model of die-casting machine’s mold clamping mechanism with a clamping force of 25000kN. Materials of components are QT500 and No. 45 steel respectively. Physical property parameters of materials are shown in Table 1. Discretize components in assembly model respectively, define contact relationship and friction factor and other parameters between connecting parts, apply corresponding load of 25000kN, and constrain displacement freedom of fixed mold base plate. Established mold clamping mechanism model is shown in Figure 2.
Figure 1 Schematic diagram of machine hinge system
1. Mold clamping cylinder 2. Tail plate 3. Hook hinge 4. Cross head 5. Small hinge 6. Long hinge 7. Moving mold base plate
Material
Density/(kg*m-3)
Elastic modulus/MPa
Poisson’s ratio
Yield strength/MPa
Tensile strength/MPa
QT500
7000
1.62*100000
0.292
320
500
45 # steel
7890
2.09*100000
0.269
355
600
Table 1 Material physical performance parameters
Figure 2 Mold clamping mechanism model
Figure 3 Displacement cloud diagram of mold clamping mechanism It can be seen that maximum displacement of mold clamping mechanism is 3.22mm, which is located at the end of Gorin column near tail plate. Ignoring local stress at unit contact boundary, maximum stress of mold clamping mechanism is 118MPa, located inside connection between hook hinge and tail plate, which is far less than yield strength of material (320MPa), so the overall structure of mold clamping mechanism can meet strength design requirements.
Figure 4 Stress cloud diagram of mold clamping mechanism
Component/Subsystem
Tailgate
Hinge system
Fixed and movable mold base plates and molds
Corinthians
horizontal
vertical
Vertical 1
horizontal
Vertical 2
horizontal
vertical
Load/kN
25000
910
910
25000
875
25000
875
25000
Displacement/mm
0.298
0.345
0.923
0.986
3.42
0.961
0.178
2.63
StiffnesskN/mm
83893
2638
986
25355
256
26015
4916
9506
Table 2 Stiffness of mold clamping mechanism components and subsystems Figure 5 shows hinge system of a 25000kN die-casting machine. It is a typical double-bent toggle structure with 5 hinges. Cross head, small hinge, hook hinge and long hinge are used as toggle levers respectively. Toggle levers are connected by rotating shafts. A, B, C, D and E are hinge points between hook hinge and tail plate, long hinge and hook hinge, movable mold base plate and hook hinge, hook hinge and small hinge, small hinge and cross head in upper half hinge. According to structure and working mechanism of hinge system, its motion geometric relationship is established, as shown in Figure 5.
Figure 5 Schematic diagram of motion relationship of machine hinge system
Figure 6 Mold clamping force calculation flow chart
Figure 7 Kinematics simulation model of machine hinge system
Figure 8 Kinematics simulation results of the machine hinge system Expansion ratio, stroke ratio and speed ratio are all key parameters of hinge system. Goal of optimized design of machine hinge system is to have a large expansion ratio and stroke ratio in clamping state. During expansion of machine hinge system, speed in the middle of stroke is relatively high, speed in initial and final stages of stroke (close to mold clamping state) is relatively small. Machine hinge system is a typical multi-link mechanism. Without changing number of toggle levers, hinged positions of toggle levers and movable seat plate and tail plate, main influencing factors of system performance are length and stiffness of each toggle lever. Therefore, plane coordinate values of the three toggle lever hinge points in Figure 5, B, D, and E, are used as design variables, as shown in Table 3. Length of each toggle lever is controlled by design variables to optimize design of machine hinge system.
Variable name
DV1
DV2
DV3
DV4
DV5
DV6
Variable coordinates
Bx
By
Dx
Dy
Ex
Ey
Original value
1169.840
275.968
1031.287
552.829
662.800
460.000
Table 3 Design variable table
Design variable
Initial value
Stroke ratio
Expansion multiple
Speed ratio
DV1
1169.840
-0.000083
0.11
-0.001
DV2
275.968
0
-40.28
-0.0019
DV3
1031.287
0.00082
-0.079
0.00097
DV4
552.829
0.0021
-349.81
0.00035
DV5
662.800
0.000049
48.99
0.00018
DV6
460.000
-0.00032
-150.16
0.0012
Table 4 Sensitivity of design variables to design goals
Variable name
DV1
DV2
DV3
DV4
DV5
DV6
Expansion multiple
Stroke ratio
Original value
1169.840
275.968
1031.287
552.829
662.800
460.000
21.45
1.03
Optimization value
1146.443
264.929
1072.539
536.244
696.000
446.200
24.57
1.08
Table 5 Coordinates of toggle joint hinge points
Figure 9 Comparison curve of performance indicators of machine hinge system before and after optimization
In conclusion
(1) Deformation of toggle bar has a great impact on performance of machine hinge system. Influence of toggle bar deformation should be considered in design process of machine hinge system. (2) Calculate stiffness and strength of mold clamping mechanism through finite element method. When mold clamping force is 25000kN, local maximum stress of mold clamping mechanism is 118MPa, and the overall structure meets strength design requirements. (3) Considering influence of toggle deformation, based on rigid-flexible multi-body dynamics modeling and kinematic simulation calculations, force expansion multiple of hinge system is 21.45, which is close to actual test result of 21.40, and numerical model is reliable. (4) By optimizing spatial position of toggle joint hinge point of die-casting machine, hinge system can be optimized. After optimized design, force expansion factor of hinge system reaches 24.57, and stroke ratio is 1.08, which are 14.5% and 4.85% higher than before optimization respectively. During mold clamping process, impact on driving cylinder, hinge system, mold plate and mold is smaller.
For die-casting sites, automated and intelligent equipment have been used in production. However, how to ensure long-term stable production of these equipments, main task is to “maintain stability”, find out fluctuation factors in each link, use feasible and reliable methods to restore them to a stable state, “seeking stability first, then seeking improvement”, this work is repeated over and over again, there is no one-and-done solution, and there are no shortcuts. Product quality problems encountered at die-casting site may not be caused by one factor. When solving problem, multiple factors need to be integrated to deal with it, so that product can be produced efficiently, with high quality and stably. This article uses Six Management Factors (5M1E) to find out factors that may cause instability on die-casting process site, controls and improves these factors, proposes a strategy for maintaining stability of die-casting process. Die casting is a technology that produces parts with complex shapes with high automation and high efficiency. Parts produced by this technology have advantages of good density, high precision, small machining allowance, and excellent mechanical properties. They have been widely used in automobiles, mechanical equipment and other fields. In high-pressure casting industry, problems of pores, shrinkage cavities, and internal cold isolation in die-casting parts have always troubled die-casting people. Although there are prescribed standards for requirements for porosity and shrinkage, quality of die-casting parts is often good and bad due to various uncertain fluctuation factors in production process, which leads to unstable quality of die-casting parts. Due to rapid filling and forming, a certain amount of gas will remain inside die casting due to various factors. With emergence of advanced process technologies such as vacuum die-casting, oxygenated die-casting, and local extrusion, internal quality of die-casting parts has been improved to a certain extent, but existence of pores, shrinkage cavities, and cold insulation is still unavoidable. Even so, problems such as pores, shrinkage cavities, and cold insulation can be controlled within a certain range. Die-casting actually refers to collective name of the three elements of die-casting machine, die-casting alloy and die-casting mold. Die-casting process integrates and applies these three elements to production. Hoda Dini et al. studied effect of die-casting pressure on deformation and residual stress of die-cast AZ91D alloy castings. Research results show that die-casting pressure has a greater impact on deformation and residual stress of castings. Increasing die-casting pressure will reduce deformation of casting, but residual stress on the surface will increase. Many scholars have studied issues such as temperature field distribution and temperature gradient of molds and die-casting parts during die-casting process. P.Sharifi et al. conducted experiments to study influence of process parameters on castings during die-casting process. Results show that among many process parameters, punch speed has the greatest impact on porosity of castings. However, for some special alloys, slow injection speed has a greater impact. Nowadays, more and more companies are using Internet big data to monitor die-casting process in real time and adjust die-casting parameters. Formulation of die-casting process plan is a very important step. Rationality of process directly affects quality of castings and subsequent processing and production links. Quality of die-casting parts is often related to many factors. Reasonable and effective control of quality of die-casting parts requires comprehensive consideration of many factors. Long-term stability of die-casting quality is very important to company’s costs and profits. However, complexity of die-casting site makes it more difficult to control and manage quality of die-casting parts. Based on this, a die-casting process stability maintenance strategy based on six factors of on-site management (5M1E) was proposed to systematically solve problem of quality management of die-casting parts at die-casting site.
1. Die-casting site problems and measures
When quality fluctuations occur in die-casting parts, on-site technicians often first choose to adjust process parameters instead of finding root cause. For example: when internal defects occur, change high-speed switching point, increase fast injection speed, and increase pressure opening; ② When mold is pulled, increase release agent spray volume or lengthen spray time; ③ When punch is stuck, increase amount of lubricant; ④ When temperature of aluminum liquid in holding furnace is not enough, use a flamethrower to bake on liquid surface of pouring port. Each of above inappropriate adjustments may bring about a 3% defective rate, and more than three inappropriate adjustments may bring about a defective rate of more than 10%. Therefore, we should not think too deeply about problem, but find cause in detail for each problem, use appropriate methods to fundamentally solve problem and maintain it. That’s kind of conviction our field technicians need to have.
2. Die-casting site stability maintenance factors
When equipment, molds, and process technology conditions have met quality requirements of die-casting parts, the most critical thing at die-casting site is to control stability of comprehensive factors. For example: comprehensive condition of equipment and mold, quality of compressed air and range of pressure fluctuations, insulation effect and temperature fluctuation range of alloy liquid, mold temperature and ambient temperature. Only when comprehensive factors are stable can quality of die castings be relatively stable.
2.1 Six factors of on-site management (5M1E)
Six factors of on-site management refer to Man, Machine, Material, Method, Measurement and Environment, referred to as 5M1E. As shown in Figure 1 fishbone diagram.
Figure 1 Fishbone diagram
2.1.1 Human factors
People are leader in die-casting scene and are the first of all factors. Including production safety, all work surrounding production activities is decided by people. Product quality will always fluctuate. Use management tools to analyze and solve problems, control fluctuations in each link to a minimum, and reduce the rate of defective products. All of this still relies on people.
2.1.2 Machine factors
Machine refers to working machine, mold and supporting auxiliary equipment. Due to importance of mold in die-casting process, it will be analyzed as a separate factor below. Capability of machinery and equipment is decisive factor in production and manufacturing capabilities, so the higher high-end equipment, the higher maintenance requirements. During production arrangement process, machines and equipment suitable for different products are selected. Carry out daily inspections so that problems can be discovered and repaired promptly. Carry out regular maintenance or scheduled maintenance to ensure that equipment is in good condition and has stable performance. It is strictly prohibited to dismantle one thing to make up the other, and keep equipment intact. Problems that are easily overlooked at die-casting production site: ① Impact of poor matching of injection rod connection part on quality of die-casting parts; ② Impact of unstable feeding of feeder on quality of die-casting parts. As shown in Table 1 (density of liquid aluminum 2.64g/cm³).
Injection head diameter/mm
Cross-sectional area of injection head/cm2
10mm/thickness handle weight/g
Allowable fluctuation range product of material handle thickness/mm (empirical data)
60
28.26
74.61
1.Ф80+3 2.Ф90 soil 4 3. It also depends on weight of die casting. The lighter casting, the worse its ability to resist fluctuations.
70
38.47
101.56
80
50.24
132.63
90
63.59
167.88
100
78.5
207.24
Table 1 Relationship between material handle thickness and weight As mentioned in Table 1, because most of our current die-casting machines use position triggering to set high-speed switching point, if pouring machine is unstable (thickness of material handle is too different), it can directly lead to a change in actual starting point of high-speed filling, affecting stability of die casting quality. Assume that when high-speed switching point has been set at a certain position value, as shown in Figure 2, Figure 3, and Figure 4, flow channel is full and aluminum liquid just reaches gate. This is ideal high-speed switching point, as shown in Figure 2.
Figure 2 Normal state when pouring volume is stable Some molten aluminum has entered mold cavity through inner runner during slow injection process, which can easily form a cold isolation inside casting, as shown in Figure 3.
Figure 3 Abnormal state when pouring amount is too much Liquid aluminum failed to fill flow channel, and gas was trapped in the front end, causing pores inside casting, as shown in Figure 4.
Figure 4 Abnormal state when pouring amount is too small Unstable spray volume and atomization effect of sprayer can also affect quality of die castings. Large fluctuations in compressed air pressure, normal air pressure should be controlled at 5.5 ~ 6.5 kg/c㎡, which is very important; large fluctuations in air pressure cause release agent pressure to also fluctuate significantly. Large fluctuations in air pressure and water pressure cause release agent spray volume and atomization effect to change with changes in air pressure. Mold temperature and release lubrication effect also change accordingly, and product quality also fluctuates. Air humidity in most areas of southern my country is relatively high in summer, and a large amount of emulsified water will be produced in compressed air. This emulsified water will be sprayed onto mold cavity with air flow, which will increase number of pores and cold insulation inside die casting, and will also cause spots and other undesirable phenomena to appear on the surface of die casting. This is an undesirable factor that many people ignore. Therefore, automatic drain valves should be installed at the bottom of gas storage tank and the lowest point of main pipe to automatically drain water regularly to minimize moisture in compressed air.
2.1.3 Mold factors
On the premise that equipment meets on-site conditions, condition of die-casting mold has main influence on quality of die-casting parts. During die-casting production process, on-site personnel often spend the most time and energy on mold. Therefore, daily maintenance of mold is very important. Following points should be paid attention to. (1) Ensure that parting surface is intact. If parting surface is not well matched, air edges or flying materials may easily occur. Flying materials will cause pressure loss in cavity and no material feeding, causing large pores and large areas of looseness inside die casting. For quality and safety, strict control is required. (2) Maintenance of size and shape of gate. At die-casting production site, it is very important to control thickness, width and extension of gate. During production process, as gate continues to be eroded, width and thickness of gate continue to increase, extension of gate becomes shorter and shorter, internal defect rate of die casting will become higher and higher. Above phenomenon is called runner erosion, which is an inevitable phenomenon during die-casting production process. Adverse consequences caused by sprue erosion are common and often ignored. Due to erosion, width and thickness of gate are increased, which increases cross-sectional area of gate. Consequences of changes in process parameters are as follows: ① Filling resistance decreases, flow rate of ingate changes accordingly, which also changes filling time; ② Filling time is shortened, which increases pressure of gas in cavity and increases exhaust resistance; ③ Exhaust speed changes accordingly, part of gas is not discharged in time and remains inside die casting; ④ Gas that is not discharged in time remains to form pores inside die casting, distribution gradually evolves from regular to irregular. As erosion continues, cold gaps will gradually appear inside casting. Therefore, it is necessary to control dimensional changes in gate width, thickness and gate extension within a reasonable range. Judging from empirical data, extension of gate should be 1 to 2 times thickness of gate. Its main function is to crush gas in molten aluminum in flow channel during high-speed filling to avoid formation of large pores in casting. It is also helpful in reducing cold shut, as shown in Figure 5.
Figure 5 Gate size and gate extension (3) Cooling water path should be kept clean and smooth and water pressure should be stable. Mold temperature control is an effective means to stabilize quality of die castings. Without cooperation of a mold temperature controller, integrity and effectiveness of mold cooling system is also a link that must be controlled in die-casting production process. If cooling effect is not good and mold temperature is too high (more than 250℃), release agent will be difficult to effectively adhere due to vapor rebound, resulting in shrinkage holes inside die casting and localized aluminum sticking on the surface, etc., which will also affect production efficiency. Excessive cooling will cause mold temperature to be low (below 180 ℃), which will cause release agent to be unable to effectively adhere, affecting demoulding effect, water will not evaporate normally, which will cause undesirable phenomena such as pores, cold insulation, and local cold material accumulation inside die casting. There are several problems that are easily overlooked at die-casting production site. (1) Impact of poor coordination between sprue sleeve and melting cup on quality of die castings. (2) Impact of clogging of exhaust groove of exhaust block on quality of die castings. At production site, especially in old molds, this situation often occurs: there are many places on parting surface that can be vented, but only vent grooves and vent blocks are blocked and cannot be vented normally. This phenomenon is often overlooked, but it actually changes exhaust sequence. Filling sequence and exhaust sequence should go in opposite directions, must not go opposite to each other. This is also one of factors that mistakenly believe that mold exhaust effect is very good, but porosity of die castings is always high. (3) Impact of cooling water dripping on quality of die castings. Cooling water dripping is a common problem at die-casting site. It may seem like a small matter, but in fact it hides major hidden dangers: ① Water leaking into mold cavity and melting cup not only causes a local temperature drop, but also causes a drop in local temperature when water encounters molten aluminum. It will quickly vaporize to produce a large amount of hydrogen, which will be absorbed by aluminum liquid and filled into mold cavity. This is also one of factors causing high porosity of die castings; ② Water leaking from cooling holes, centralized distributors, etc. to mold base cannot be underestimated. When amount of water leakage is large, it will lower mold base temperature and make it difficult to maintain mold core temperature. It will also enter mold cavity from fitting gaps. This is also one of factors causing high porosity rate of die castings. (4) Impact of cylinder oil leakage on quality of die castings. Oil leakage from oil cylinder will not only cause same pore problems as water leakage, but also cause greater oil consumption, directly increase production costs, leave hidden dangers such as safety and environmental protection.
2.1.4 Material factors
Materials mainly refer to raw materials and auxiliary materials. Raw materials that affect quality of die castings (mainly pores) mainly include following factors.
(1) Internal density of aluminum ingots. According to experience, aluminum ingots with neat fractures and fine and uniform cross-section crystals can be judged as good materials, while those with coarse fracture surface crystals can easily cause internal pores in die casting, as shown in Figure 6. Cross-section of aluminum ingot on the left has thick crystals, while cross-section of right part has fine and uniform crystals.
Figure 6 Internal quality of parts (2) Melting temperature: Temperature of molten aluminum in insulation chamber of central melting furnace should be controlled at 720~740℃. If temperature is too high, it will easily cause material crystals to become coarse, causing pores inside die casting. If temperature is too low, it will be difficult to maintain accumulated temperature drop in each link during turnover process. (3) Insulation temperature: Insulation temperature in holding furnace should be maintained at 650 ~ 680 ℃. For small die castings, when material temperature is lower than 650 ℃ (especially lower than 640 ℃), pores and cold insulation are likely to occur inside die castings. . Problems that are easily overlooked at die-casting production site: whether temperature of melting furnace insulation pool reaches above 720℃; whether transfer process causes too much heat loss; whether insulation furnace has sufficient insulation and heating capabilities. Whether holding furnace has sufficient insulation and heating capabilities, pay attention to following points: whether seal between furnace body and furnace cover is intact; whether feeding trough has a cover seal; whether sealing between feeding trough and furnace body is intact. Negligence in each of above steps will cause heat energy loss and result in low aluminum pouring temperature. (4) Auxiliary materials: Auxiliary materials include refining agents, slag breakers, release agents, punch oil (particles), etc. Quality of these auxiliary materials, or whether they are used timely and effectively, will also directly affect quality of die castings.
2.1.5 Legal factors
Dharma refers to decision-making, methods and techniques. This is one of management tools that runs through the entire process of production activities. If there is a problem in each of four factors above, you can use this tool to analyze it, find out reasons, formulate improvement plans and implementation measures, then follow up and summarize effects. This cycle continues, but you need to pay attention to following points. (1) For production safety, equipment management and maintenance work, daily inspection process and production process must meet response speed and countermeasures for discovered problems, ensure that equipment operating conditions meet needs of production technology and technology. This is an unshakable premise. (2) For use, maintenance and upkeep of molds before and after production, during production process, analyze potential failure modes (FMEA), formulate a storage plan for wearing parts, formulate and implement a spare mold plan based on purchase volume. (3) Formulate, implement and review die-casting process, and conduct daily inspections to ensure seriousness of process card. (4) Control of temperature of molten aluminum is an unavoidable issue, and lower limit of 650℃ is a red line that cannot be exceeded. Most technicians at production site only focus on data of fast injection and boosting parts. In fact, improper setting of slow injection speed has a great impact on quality of castings. Problems that are easily overlooked at die-casting production site: impact of slow injection speed on quality of die-casting parts; difference between slow-speed filling state theory and practice. Slow injection is front part of fast injection. Theoretically, function of slow injection is to fill flow channel with molten aluminum and push it to vicinity of gate. As can be seen from Figure 7, this is not the case. In fact, when a small amount of front peak of aluminum liquid has crossed inner runner, runner is not completely filled and is also mixed with gas. This phenomenon becomes more serious the faster injection speed, the more serious it is. Therefore, it is important to choose a suitable slow injection speed.
Figure 7 Slow filling theoretical calculation and actual state Air blowing is an effective method to remove moisture residue, but it is not a good thing to blow too much air from sprayer and for too long. Over-reliance on air blowing not only loses mold temperature, but also wastes resources. The lower mold temperature, the more moisture remains, and the more it needs to be solved by blowing air, and so on. Therefore, concentration of release agent should be as high as possible to enhance lubrication of mold while reducing amount of water sprayed out.
2.1.6 Environmental factors
Environmental factors refer to environmental factors, which can affect all above links, mainly safety, which is also a factor that cannot be ignored. Changes in ambient temperature will affect personnel’s emotions, pose safety risks, also affect equipment accuracy and performance. Equipment failure rates are always higher in summer. For precision cutting processes and inspection processes, ambient temperature has a great impact on dimensional accuracy, and some small details are often not paid attention to. Problems that are easily overlooked at die-casting production sites: For production sites without mold temperature machines, release agent concentration, spray time, and cooling water flow must be adjusted immediately according to ambient temperature changes in summer and winter to keep mold temperature within a suitable and relatively stable temperature difference range. For example, if ambient temperature is high in summer, release agent can be used with a lower concentration and an appropriately long spray time. When ambient temperature is low in winter, a higher concentration of release agent can be used and spraying time should be shortened appropriately. According to ambient temperature changes in summer and winter, cooling water flow rate is increased in summer and reduced in winter. Clean cooling water channels regularly to maintain cooling effect. For areas with high relative air humidity, when relative air humidity reaches 90% or higher, in addition to fact that die castings are prone to blackening and moldiness, gas generated by air compressor will also contain a large amount of emulsified water, which can cause pores in die castings. High humidity seasons can easily cause electrical failures. Equipment that has been suspended from production should be kept powered on regularly. In particular, weak current control system can be powered on to keep it dry by self-heating.
2.1.7 Measurement factors
Measurement is quality inspection, which mainly refers to measurement tools, measurement methods, trained and authorized measurers. In modern production process, testing is an important factor. Without testing and control, it is difficult to ensure product quality. When measuring and controlling, you need to pay attention to: ① Whether responsible person is designated; ② Whether prescribed measuring tools are used; ③ Whether it is at designated measuring point; ④ whether correct measurement method is used; ⑤ Whether measurements are carried out at a certain frequency; ⑥ whether there are records. Improving ease of operation of measuring instruments and ensuring measurement accuracy is very helpful for product quality control.
3. Application examples of die-casting process stability maintenance strategy
3.1 Release agent maintains stability
Application examples of separately proportioned release agents. For valve body shown in Figures 8 and 9, due to small draft angle of individual holes, only 5027 release agent can be used, and proportion concentration is too high (1:160 ~180), so that product quality is better and stability of on-site production can be achieved.
Figure 8 Draft angle of individual holes in valve body
Figure 9 Valve body and valve port surface quality requirements are high
3.2 Maintaining stable dimensions of gate
3.8JB flame distributor is shown in Figure 10. Product has a diameter of 140 mm and a height of 28 mm. It is required that there should be no pores, micro-spots, flow marks, chromatic aberration, etc. after large-area mirror processing on the surface. Gate size: The two points in the middle are 18mm * 1.2mm (limit 1.4mm), the two points on both sides are 16mm * 1.1mm (limit 1.3 mm), punch diameter is 55 mm, injection head area ratio of gate size is 1:30.3, and fast injection speed is set to 2.3m/s (actual speed 1.9~2m/s). By adopting such a gate size, product will have better quality and achieve stable production.
Figure 10 3.8JB flame distributor Under premise that set conditions remain unchanged, when actual speed of fast injection reaches more than 2m/s, pores and micro-spots will appear on processed surface.
3.3 Maintain stability of mold parting surface
368B type flame distributor is shown in Figure 11. This piece has 7 core-pulling die-casting parts. It is required that there are no pores, micro-spots, flow marks, chromatic aberration, etc. after large-area mirror processing on the surface. During die-casting production process of this part, if cracks appear on parting surface, pores will appear on machined surface of die-casting part, and this has been verified many times. Therefore, keeping parting surface intact, die-casting parts without gaps, and keeping size of ingate within established range are focus of maintenance work for this set of molds.
Figure 11 Type 368B flame distributor
4 Conclusion
Among six major factors, according to analysis results of last five major factors, factors affecting production are wide-ranging, diverse, changing at any time, and unpredictable. But when undesirable factors occur, key is whether problem can be discovered or taken seriously in time, whether it can be dealt with in time, how to deal with it, who will deal with it, what results will be after treatment, etc. All spearheads here point to people, so talent is core element among six elements. For die-casting site, main job is to “maintain stability”, find out fluctuation factors in each link, use feasible and reliable methods to restore it to a stable state. “Seek stability first, then seek improvement.” This work is repeated, there is no one-time solution, and there are no shortcuts either. A product problem at die-casting site may be caused by joint action of several of six elements. When solving problem, product problem is not solved by just solving one of elements, but by integrating six elements and solving it comprehensively. Therefore, product problems can be solved efficiently and with high quality according to “stability maintenance” strategy of die-casting process.
Effect of heat treatment on mechanical and thermal conductivity properties of die-cast aluminum alloy ZL102 was studied using microstructure characterization, mechanical and thermal conductivity testing. Results show that physical phases in room temperature structure of die-cast aluminum alloy ZL102 include primary α (Al), aluminum-silicon eutectic structure, primary crystalline silicon and a small amount of intermetallic compounds. After solid solution treatment, silicon phase in die-cast aluminum alloy ZL102 fuses and spheroidizes; after aging treatment, fine point-like second phase precipitates on α (Al) matrix, and sphericity of silicon phase at grain boundary is further improved. Among three heat treatment states, aluminum alloy ZL102 has the highest mechanical properties after solution treatment, but the lowest thermal conductivity. In summary, aging treatment takes into account mechanical and thermal conductivity properties of alloy. At this time, tensile strength of alloy is 212MPa, elongation is 3.9%, and room temperature thermal conductivity is 142.7W/(m·K). With advent of 5G communication era, integration of electronic communication equipment and products is gradually increasing, and amount of heat generated per unit volume is also increasing. At this time, relevant materials and structures are required to have good thermal conductivity properties to ensure normal operation of equipment and products and extend their service life. Take 5G communication filter as an example. It has high power and high integration. In order to improve heat dissipation capacity, filter housing structure is usually designed with many irregular thin-walled heat sinks. For mass forming and manufacturing of this type of structural shells, die-casting process has significant efficiency and cost advantages. Density of metallic aluminum is only 1/3 of steel and iron, and it has huge potential for lightweighting. In recent years, it has been widely used in automobiles, communications, aerospace and other fields. Room temperature thermal conductivity of pure aluminum is approximately 237 W/(m·K), and it has excellent thermal conductivity. However, strength of pure aluminum is low. In actual production, some alloying elements are often added to improve its mechanical properties, and addition of alloying elements will have a certain impact on its thermal conductivity properties. Usually, alloying elements strengthen aluminum alloys in the form of solid solution atoms, intermediate phases or precipitation strengthening phases. However, whether it exists in the form of solid solution atoms or intermediate phases, it will bring a large number of vacancies, dislocations and other crystal defects to alloy, and precipitated phase will also cause lattice distortion in alloy. Existence of these defects increases probability of free electron scattering in alloy and reduces number of electrons for effective heat conduction, resulting in a reduction in thermal conductivity of alloy. In order to take into account mechanical and thermal conductivity properties of aluminum alloys, researchers have conducted in-depth research. Wen Cheng studied effects of 22 alloy elements on electrical and thermal conductivity of industrial pure aluminum and found that different elements have different effects. Addition of transition elements such as Mn, Cr, etc. will cause electrical and thermal conductivity of pure Al to decline rapidly, while Zn, Sr and rare earth metamorphic elements have less impact. Li Linjun found that different magnesium to silicon ratios have different effects on thermal conductivity of aluminum alloy 6063. When magnesium to silicon ratio is 1.5, alloy has the best thermal conductivity. Lumley et al. studied effects of alloy composition and heat treatment on thermal conductivity of Al-Si-Cu aluminum alloy die castings. Study showed that thermal conductivity of alloys with certain compositions can be increased by more than 60% through use of heat treatment. Kim et al. tested thermal diffusivity of Al-1Si and Al-9Si alloys under different heat treatment conditions, studied relationship between thermal diffusivity and silicon phase solid solution and precipitation, concluded that re-precipitation of dissolved silicon in solution-treated samples will increase thermal diffusivity of alloy. Choi et al. studied effect of mold temperature on thermal and mechanical properties of aluminum alloys and concluded that the higher mold temperature, the slower solidification rate of alloy. At this time, the larger silicon particles are, the better thermal properties of alloy are. After aging treatment, mechanical strength of alloys at different mold temperatures becomes similar. This article takes a 5G communication filter housing as an actual die-cast aluminum alloy ZL102 as research object, uses flash method to test thermal conductivity of alloy at different temperatures, changes microstructure of alloy through heat treatment to explore impact of alloy microstructure changes on mechanical and thermal conductivity properties, with a view to providing a reference for improving mechanical and thermal conductivity properties of aluminum alloy die castings in actual production.
01 Experimental procedure
1.1 Sample preparation
An actual die-cast product of a 5G communication filter housing was used as test piece. As shown in Figure 1a, die-cast piece weighs about 5.4kg and has an overall size of about 539mm * 410mm * 45mm. Die-casting part has a complex structure and shape. Wall thickness of main body is only about 2mm, while wall thickness at mounting lugs and other locations reaches 6mm. Sampling position for microstructure characterization, mechanical and thermal conductivity testing in test is shown in Figure 1b. Wall thickness at this position is wall thickness of main body of die casting, which is well representative and convenient for sampling. Chemical composition of aluminum alloy ZL102 measured by X-ray fluorescence spectrometer (XRF) is shown in Table 1. Mechanical properties tensile specimens were prepared by wire cutting, and dimensions were determined in accordance with national standard GB/T228.1-2010. At the same time, Φ4mm*1mm disc samples were wire-cut for testing thermal conductivity of alloy. Heat treatment of samples is divided into three control groups. One group is in die-cast state without heat treatment; second group undergoes solution treatment at 500℃*4h; and third group undergoes aging treatment at 200℃*3h based on second group.
Figure 1 Actual die-cast product of a 5G communication filter housing
1.2 Test methods
Metallographic samples were mechanically ground with 240#, 600#, 1200#, 1500#, and 2000# sandpaper in sequence, polished with diamond polishing agent, then used 95%H2O+2.5%HNO3+1.5%HCl+1%HF Keller’s reagent to corrode for 10~20s. Structural morphology was observed under a Leica DM2700M optical microscope (OM) and a JEOL 6301F scanning electron microscope (SEM). Instron 5967 electronic universal testing machine was used to conduct room temperature tensile test. Tensile speed was 2mm/min. Mechanical property test data was average of 5 effective specimens. According to physical laws of heat conduction, temperature field inside object will change with time during heat conduction, which makes it difficult to directly measure thermal conductivity. In this paper, based on measuring density, specific heat capacity and thermal diffusion coefficient of alloy, thermal conductivity of alloy is calculated using equation (1) λ=αρCρ (1) In formula: λ is thermal conductivity of sample to be measured, unit is W/(m·K); α is thermal diffusion coefficient of sample to be measured, unit is mm2/s; ρ is density of sample to be measured, unit is g /cm3; Cρ is specific heat capacity of sample being tested, in J/(g·K). LFA457 laser thermal conductivity meter was used to measure thermal diffusion coefficient of sample, specific heat capacity was measured using TA-DSC2500 differential scanning calorimetry, and Archimedean drainage method was used to measure density of sample.
02 Aluminum alloy ZL102 die casting structure
Filter housing is die-cast from aluminum alloy ZL102. According to Table 1, silicon content in alloy is 12.8% (mass fraction, same below), which is close to eutectic composition of aluminum-silicon binary alloy (Si content of eutectic composition point in Al-Si binary equilibrium phase diagram is 12.6%). When alloy solidifies, eutectic reaction L→α(Al) + β(Si) mainly occurs, forming a large amount of α(Al) + Si eutectic structure. However, due to high cooling rate of alloy during die-casting process and solidification process away from equilibrium, phases present in die-casting structure of aluminum alloy ZL102 at room temperature include primary α (Al), aluminum-silicon eutectic structure, primary crystal silicon and a small amount of intermetallic compounds.. Figure 2 shows surface and core microstructure of die-casting obtained by OM. Gray-white matrix is primary α (Al), and there are a large number of gray-black alternating aluminum-silicon eutectic structures distributed between matrix, as well as needle-shaped / lath-shaped primary crystal silicon. Comparing Figure 2a and Figure 2b, surface structure is finer and more uniform. This is because surface layer of die casting is in direct contact with mold wall during forming. Alloy has a high cooling rate and a high degree of supercooling, making grains in surface structure finer and more uniform. In addition, a small amount of large gray polygonal structures were found in core structure of die casting, as shown in Figure 2b. Since iron content in alloy composition is slightly higher, it is initially judged to be an iron-containing phase. Alloy phase composition was tested using X-ray diffraction (XRD), with a scanning range of 10°~90° and a scanning speed of 4°/min. Test results were imported into MDI Jade 6 for analysis. Results are shown in Figure 3. It can be seen from XRD test results that there are only α (Al) and Si phases in alloy, and no iron-containing intermetallic compounds were detected. Reason for above situation may be that content of intermetallic compounds in alloy is low, resulting in XRD not detecting detected.
Si
Mg
Fe
Mn
Cu
Zn
Ti
Al
12.8
0.019
0.915
0.184
0.224
0.127
0.001
magin
Table 1 Chemical composition of aluminum alloy ZL102 wB/%
Figure 2 Microstructure of aluminum alloy ZL102 die casting
Figure 3 XRD pattern of die-cast aluminum alloy ZL102 SEM was used to further analyze the above polygonal phases, as shown in Figure 4. Based on energy spectrometer (EDS) results (Table 2), it is speculated that it is a complex AiSiFeMn quaternary intermetallic compound. Based on some literature such as Yuan, Wang, etc., it was determined that phase is α-Al 15 (Mn, Fe)3Si2. This phase is further evolved by adding Mn element to β-Al5FeSi phase. When β-Al5FeSi phase is formed, some Mn atoms take away positions occupied by Fe atoms in β phase, which is equivalent to Mn atoms partially replacing Fe atoms, thus forming AlSiFeMn quaternary composite phase.
Figure 4 SEM image of polygonal phase
Element
Al
Si
Fe
Mn
Cr
Quality score
54.63
13.38
21.79
6.52
3.67
Atomic fraction
65.72
15.47
12.67
3.85
2.29
Table 2 EDS analysis results of point P in Figure 4 %
03 Effect of heat treatment on die casting structure
Figure 5 compares and analyzes aluminum alloy ZL102 die-casting structure before and after heat treatment. It can be seen that primary α (Al) changes little before and after heat treatment, and dendrite orientation has no obvious rules, while morphology and distribution of silicon phase have changed significantly. Silicon phase in die-cast structure shows a needle-like/lath-like distribution, which seriously splits α(Al) matrix; after solid solution treatment, silicon element originally dissolved into α(Al) matrix in solid solution stage precipitates, showing a fine point-like shape on α-Al matrix, while silicon phase at grain boundary is more rounded and sphericity is further improved. For polygonal AiSiFeMn quaternary intermetallic compound that appears in die-casting structure, its morphology does not change significantly during solution and aging treatment processes. It is speculated that reason is that solution treatment temperature in this article is not sufficient to dissolve it into α(Al) matrix. It can also be seen from Figure 5 that there are a certain number of pores distributed in die-casting structure. Pores in structure have a tendency to increase after solution treatment, but pores do not expand further with continued aging treatment.
Figure 5 Comparative analysis of die-cast, solid solution and aging structures
04 Effect of heat treatment on mechanical properties and thermal conductivity
Mechanical properties of die-cast aluminum alloy ZL102 under different heat treatment processes are shown in Figure 6. It can be seen that in terms of strength and elongation, solution-treated sample is better than die-cast and aging-treated sample. Its tensile strength and elongation are 222.8MPa and 6.1% respectively. Compared with die-cast state, tensile strength has increased by 9.2%, and elongation has been significantly improved, increasing by 205 %, while mechanical properties of aged specimens are in the middle. Analyzing reasons, although there have been studies that ordinary die castings are not suitable for heat treatment due to existence of pores, and aluminum alloy ZL102 is a non-heat treatment strengthened alloy. However, fusing and spheroidization of silicon phase during solid solution treatment process significantly improves its splitting effect on α (Al) matrix, reduces stress concentration generated around alloy when it is loaded, is conducive to improvement of alloy strength and elongation. After further aging treatment, mechanical properties of alloy have declined to a certain extent compared to solid solution state. Reason is that silicon phase precipitated in α (Al) grain boundaries and solid solution further aggregates and grows, causing coarsening, which damages mechanical properties of alloy, no aging strengthening phase precipitates during aging process of alloy.
Figure 6 Effect of heat treatment on mechanical properties of die-cast aluminum alloy ZL102 Figure 7 shows influence of different temperatures and heat treatment processes on thermal conductivity of die-cast aluminum alloy ZL102 measured in experiment. First of all, it can be seen that thermal conductivity of alloy in three states of die-casting, solid solution and aging increases with increase of temperature. From room temperature to 300℃, it shows a trend of rapid growth first and then slow growth. This changing trend is mainly related to thermal conductivity mechanism of alloys. For general metals, thermal conductivity consists of electronic thermal conductivity and phonon thermal conductivity. Correspondingly, thermal resistance is also divided into electronic thermal resistance and phonon thermal resistance. Electronic thermal resistance is caused by electrons being scattered by various media and consists of electron-phonon scattering and electron-defect scattering. Phonon thermal resistance is also determined by two processes: one is collision between phonons caused by nonlinear vibration of crystal lattice; the other is collision between phonons and defects in solid. When temperature is low, vibration amplitude of atoms on crystal lattice is very small, and contribution of phonons to thermal conductivity is small. Thermal conductivity is dominated by electrons. At this time, thermal conductivity of alloy is mainly determined by interaction between electrons and defects. Number of internal defects in alloy basically does not change with temperature changes in low-temperature region, and movement rate of electrons basically remains unchanged whether in the high-temperature or low-temperature region. At this time, mean free path of electrons can be approximately considered to be a constant. At this time, thermal conductivity of alloy is mainly determined by specific heat, and specific heat has a linear relationship with temperature. Therefore, thermal conductivity of alloy in three heat treatment states increases rapidly with increase of temperature at the beginning. As temperature continues to rise, vibration of atoms in alloy under three heat treatment states slowly intensifies. At this time, scattering effect of phonons on electrons begins to increase, probability of electrons being scattered increases, and mean free path of electrons begins to decrease, which causes thermal conductivity of alloy to slowly increase as temperature continues to increase.
Figure 7 Effect of temperature and heat treatment process on die-cast aluminum alloy ZL102 Effect of thermal conductivity Comparing thermal conductivity of alloys in three heat treatment states in Figure 7, it can be seen that die-cast alloy has the highest thermal conductivity at different temperatures. That is, solution and aging treatment processes in this article cannot improve thermal conductivity of die-cast aluminum alloy ZL102, but instead reduce thermal conductivity of alloy. Analysis of reasons shows that there are a large number of solid solution atoms in the sample after solution treatment, which causes lattice distortion, resulting in an increase in crystal defects in alloy, an increase in probability of electrons being scattered, a decrease in mean free path of electrons, and a significant reduction in thermal conductivity of alloy. After aging treatment, thermal conductivity of alloy recovers somewhat compared to solid solution state. This is because second phase precipitates during aging treatment, which reduces solid solubility of alloy and reduces degree of lattice distortion. However, it can be seen that thermal conductivity of aged alloy is still lower than that of die-cast state. This is because second phase precipitated during aging treatment will increase phase interface of alloy, which will in turn increase probability of electrons being scattered, reducing thermal conductivity of alloy. Generally speaking, among negative effects of alloying elements on thermal conductivity of aluminum alloy ZL102, solid solution form is greater than precipitated phase form. In addition to solid solution and precipitation of alloy elements, growth of grains and pores in structure of die-cast aluminum alloy ZL102 during heat treatment process will also have a certain impact on thermal conductivity of alloy. Relevant studies have shown that grain growth reduces grain interface of alloy and can improve thermal conductivity of alloy to a certain extent, while existence of holes will obviously weaken thermal conductivity of alloy.
05 in conclusion
(1) Phases in die-casting structure of aluminum alloy ZL102 at room temperature include primary α (Al), aluminum-silicon eutectic structure, primary crystal silicon and a small amount of intermetallic compounds. Polygonal phase is AlSiFeMn quaternary composite phase. (2) After solution treatment, silicon phase in die-casting structure of aluminum alloy ZL102 has fused and spheroidized; after aging treatment, fine point-like second phase precipitates on α(Al) matrix, and silicon phase balls at grain boundaries degree further increased. (3) Among the three heat treatment states, aluminum alloy ZL102 has the highest mechanical properties after solution treatment. Tensile strength and elongation are 222.8MPa and 6.1% respectively, which are respectively increased by 9.2% and 205% compared with die-cast state. However, At this time, thermal conductivity of alloy is the lowest, and thermal conductivity at room temperature drops from 155.8 W/(m·K) in die-cast state to 127.8 W/(m·K). In summary, aging treatment takes into account mechanical and thermal conductivity properties of alloy. At this time, tensile strength of alloy is 212MPa, elongation is 3.9%, and room temperature thermal conductivity is 142.7 W/(m·K).
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