Smelting of die casting aluminum alloy is an important process in die casting production process. If smelting process is not strictly controlled, slag content and gas content of aluminum (abbreviated as: Al) liquid will increase, and chemical composition will change, resulting in pinholes, oxidized slag, shrinkage and unqualified chemical composition in casting, affecting quality of casting. Die casting aluminum alloy is easy to absorb air and oxidize during smelting process. During continuous melting process, oxidized slag will be generated on the surface and inside of Al liquid. Smelting raw materials are divided into Al ingots and recycled materials. Slag content of recycled materials is higher than that of Al ingots. After smelting, oxidized slag in Al liquid will increase accordingly. Oxidized slag is easily adsorbed by gas in Al liquid, making it difficult for gas to precipitate from Al liquid. Due to presence of gas and oxide inclusions in Al liquid, defects such as shrinkage, pores, and slag inclusions are generated in die castings. High-quality Al liquid should have characteristics of qualified chemical composition, low content of gas and oxide inclusions, good casting performance, and mechanical properties of castings that meet requirements. Starting from main links that affect quality of Al liquid in smelting process, effects of processes in each link are tested to determine optimal process parameters for each smelting link, effectively improve quality of Al liquid, thus meet production needs of die castings.

There are many types of inclusions in Al liquid. In addition to shedding of furnace body itself, most of them come from recycled materials. During placement and melting process, recycled materials will form oxide inclusions such as aluminum oxide and magnesium oxide. Since recycled materials cannot be absolutely clean, and surfaces of gates, unqualified die castings, etc. will adhere to oily substances and are relatively moist, a large amount of smoke and water vapor will be generated during melting process, which is main source of hydrogen. After density testing and metallographic analysis of Al ingots before smelting and recycled materials after casting in same furnace (see Table 1), it was found that hydrogen content and inclusions in recycled materials increased significantly.

Table 1 Comparison of impurities in Al ingots and recycled materials

Table 2 Effect of Al liquid refining and degassing time on Mg content

Table 3 Chemical composition of Al ingots and recycled materials (%)

Table 4 Reference table for classification and control of recycled materials
During die-casting production process of aluminum alloy, injection punch needs to be lubricated, and lubricating medium is punch lubricating oil or lubricating particles. After die-casting, most of punch lubricating residues accumulate on the surface of material cake to form inclusions. Some of recycled materials are waste products after processing, and their surfaces contain residual components of chip liquid and oil. During melting process, residual chip liquid and oil are heated at high temperature to produce a large amount of smoke and water vapor, which accelerates suction of Al liquid and generation of oxidized inclusions. To ensure quality of Al liquid, recommended proportion of recycled materials is shown in Table 4. According to above test data analysis, recycled materials not only lead to an increase in inclusions and changes in chemical composition in Al liquid, but also affect mechanical properties of castings due to increase in inclusions; it also affects fluidity of Al liquid during die casting filling, resulting in many die casting defects. Therefore, in daily melting process, both smelting cost and quality of Al liquid must be considered. According to product requirements, recycled materials must be graded and used in required proportions.

 

Figure 1 Relationship between hydrogen content in Al liquid and melting temperature and placement time

 

Figure 2 Relationship between refining and degassing time and density of aluminum alloy sample

 

Figure 3 Comparison of aluminum alloy samples with different densities (g·cm-3)

 

Figure 4 Example of K mold

 

Figure 5 Example of K mold size

(1) K-mold preparation. K-mold cleaning once/shift, and evenly spraying mold agent coating for primer; K-mold preheating for 30-60 minutes; angle between its bottom surface and end of ground is 30°-45°, which is convenient for Al liquid introduction and gas discharge. (2) K-mold size (see Figure 5) and sample pouring: ① Use a spoon to scoop Al liquid. When pouring spoon into K-mold, do not pour too fast to avoid blocking pouring port, resulting in inability of gas in K-mold to be discharged from pouring port. Keep a constant speed until it is full; ② After cooling for 8-10 seconds, take out K-mold sample, and pour 5 molds of samples in this way continuously; ③ Before K-mold pouring: Al liquid must be deslagging and degassing by a rotary degasser before pouring, and slag in ladle must be cleaned; ④ Wait for K-mold sample to cool naturally or water-cooled, wipe off residual moisture on the surface of sample, evenly knock each K-mold sample into 4 small sections according to break line position, and stack them neatly together, a total of 5 groups.

 

Figure 6 Schematic diagram of slag content

Table 5 K value standard comparison table
Sample is placed in a closed space in a semi-solidified state, and then vacuumed. After sample is completely solidified in vacuum container, obtained sample is observed by appearance and compared with standard sample to obtain hydrogen content in Al liquid. During test, a small amount of Al liquid was poured into preheated crucible, immediately (less than 10 seconds) placed in a sealed vacuum chamber, then vacuum pump of hydrogen detector was turned on to form a vacuum state in vacuum chamber.

 

Figure 7 Comparison of appearance of vacuum samples under different refining and degassing times

 

Figure 8 Comparison of cut samples

 

Figure 9 Density test and comparison samples

Table 6 Relationship between casting density and mechanical properties

 

Figure 10 Effect of Fe content on mechanical properties of die-cast ADC12 aluminum alloy

(1) In process of aluminum alloy smelting, return materials need to be clearly classified; for castings with dense internal structure and airtightness requirements, proportion of return materials should be controlled within 30%, and proportion of return materials for ordinary castings can be relaxed to 40%.
(2) Keeping melting temperature of Al liquid at 720-750℃ can reduce absorption of Al liquid.
(3) To ensure refining and degassing effect, refining and degassing time of aluminum alloy shall not be less than 8 minutes.
(4) Fe content in ADC12 aluminum alloy should be controlled at 0.70% to 0.85%.