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.

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.

Table 1 Chemical composition requirements of ADC12ZS aluminum alloy (%)

Table 2 Chemical composition of ADC12ZS alloy at different stages
Actual measurement value (%)

Table 3 Hydrogen content of ADC12ZS alloy
Note: Hydrogen content refers to hydrogen content in 1kg aluminum (mL)

Table 4 ADC12ZS alloy aluminum ingot slag content and utilization rate

 

Figure 1 Shrinkage state of ADC12ZS alloy

 

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.

Table 5 ADC12ZS alloy productivity verification pass rate

Table 6 List of ADC12ZS alloy die-casting defects

 

Figure 3 Crack defects in crankcase 2

 

Figure 4 Aluminum sticking to wall of smelting furnace

Table 7 Actual loss rate of ADC12ZS alloy

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.