Die-casting aluminum alloy smelting is an important process in die-casting production process. If smelting process is not strictly controlled, it will cause Al liquid slag and gas content to increase, and will change chemical composition, resulting in pinholes, oxidized slag inclusions, shrinkage porosity and unqualified chemical composition in castings, affecting quality of castings. Die-cast aluminum alloys are prone to inhalation and oxidation during melting process. During continuous melting process, oxidized slag inclusions 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. Oxide 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 oxidized inclusions in Al liquid, defects such as shrinkage cavities, pores, and slag inclusions occur in die castings. High-quality Al liquid should have characteristics of qualified chemical composition, low content of gas and oxidized 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, by testing effects of each process, optimal process parameters of each smelting link are determined to effectively improve quality of Al liquid and meet production needs of die castings.

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

Table 1 Comparison of impurities in Al ingots and recycled materials

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

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

Table 4 Reference table for classification control of recycled materials
In die-casting production process of aluminum alloy, since injection punch needs to be lubricated, lubricating medium is punch lubricating oil or lubricating particles. After die casting is completed, most of punch lubrication residue accumulates on the surface of material cake, forming inclusions. Some of recycled materials are processed waste products, and their surfaces contain residual components of cutting fluid and oil. During melting process, residual cutting fluid and oil are heated at high temperatures, producing a large amount of smoke and water vapor, which accelerates suction of Al liquid and generation of oxidation inclusions. In order to ensure quality of Al liquid, recommended proportion of recycled materials is shown in Table 4. According to analysis of above test data, recycled material not only causes an increase in inclusions and changes in chemical composition of Al liquid, but also affects mechanical properties of casting due to increase in inclusions; it also affects fluidity of Al liquid during die-casting filling process, resulting in many die-casting defects. Therefore, in daily melting process, both melting cost and quality of Al liquid must be taken into consideration. Recycled materials must be graded according to product requirements and used in a reasonable proportion.

 

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

 

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

 

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

 

Figure 4 K-mode example diagram

 

Figure 5 K-die size example

(1) K-mold mold preparation: Clean K-mold once per shift, and evenly spray mold agent paint as a base; preheat K-mold for 30 to 60 minutes; angle between bottom surface and the end of ground is 30° to 45° to facilitate introduction of Al liquid , gas discharge. (2) K mold size (see Figure 5) and sample pouring: ① Scoop Al solution with a spoon. When pouring spoon into K mold, do not pour it too fast to avoid blocking pouring port, causing gas in K mold to be unable to be discharged from pouring opening. Keep pouring speed until it is full; ② After cooling for 8 to 10 seconds, take out K mold sample, and pour 5 mold samples continuously in this way; ③ Before pouring K mold: Al liquid must be deslaged and degassed by a rotary degassing machine, and scum in pouring ladle must be cleaned; ④ Wait for K-mold sample to cool naturally or water-cool, then wipe away remaining moisture on the surface of sample. Knock each K-mold sample evenly into 4 small sections according to break line position, and stack them neatly together, for a total of 5 groups.

 

Figure 6 Schematic diagram of slag content

Table 5 K value standard comparison table
Note: The smaller K20 value, the better cleanliness.
Sample is placed in a closed space in a semi-solidified state, and then evacuated. After sample is completely solidified in vacuum container, hydrogen content in Al solution is obtained by observing appearance of sample and comparing it with standard sample. During test, pour a small amount of Al solution into preheated crucible, immediately (less than 10 seconds) put it into a sealed vacuum chamber, then open vacuum pump of hydrogen detector, and vacuum chamber forms a vacuum state.

 

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

 

Figure 8 Comparison chart 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) During aluminum alloy smelting process, recycled materials need to be clearly classified; for castings that require dense internal structures and air tightness requirements, proportion of recycled materials should be controlled within 30%, while proportion of recycled materials for ordinary castings can be relaxed to 40%.
(2) Keeping Al liquid melting temperature at 720~750℃ can reduce suction of Al liquid.
(3) In order to ensure refining and degassing effect, aluminum alloy refining and degassing time shall not be shorter than 8 minutes.
(4) Fe content in ADC12 aluminum alloy should be controlled between 0.70% and 0.85%.