Melting of die-casting aluminum alloy is an important process in the die-casting production process. If the melting process is not strictly controlled, it can cause an increase in the slag and gas content of the aluminum (referred to as Al) liquid, as well as changes in the chemical composition, resulting in pinholes, oxidation slag inclusion, shrinkage, and unqualified chemical composition of the casting, which affects the quality of the casting. Die cast aluminum alloys are prone to aspiration oxidation during the melting process, and during continuous melting, oxidation inclusions will be generated on the surface and inside of the Al liquid. The smelting raw materials are divided into Al ingots and recycled materials. The recycled materials have a higher slag content than the Al ingots, and the oxidation slag in the Al liquid will increase accordingly after smelting. Oxidized slag is easily adsorbed by gases in the Al liquid, making it difficult for gases to precipitate from the Al liquid. Due to the presence of gas and oxide inclusions in Al liquid, defects such as shrinkage, porosity, and slag inclusion occur in die castings. High quality Al liquid should have qualified chemical composition, low content of gas and oxide inclusions, good casting performance, and mechanical properties of castings that meet the requirements. Starting from the main links that affect the quality of Al liquid in the smelting process, through experiments on the effects of each link's process, the optimal process parameters for each smelting link are determined to effectively improve the quality of Al liquid and meet the production needs of die-casting parts.

Graphic and textual results
There are many types of inclusions present in Al liquid, most of which come from the recycled materials in addition to the material detachment of the furnace body itself. During the placement and melting process, recycled materials will form oxide inclusions such as alumina and magnesia. Due to the inability to achieve absolute cleanliness of the recycled materials, and the adhesion of oily substances and moisture on the surfaces of gates, unqualified die castings, etc., a large amount of smoke and water vapor will be generated during the melting process, which is the main source of hydrogen gas. Through density testing and metallographic analysis of Al ingots before melting and recycled materials after casting in the same furnace (see Table 1), it was found that the hydrogen content and inclusions in the recycled materials increased significantly.

During the die-casting production process of aluminum alloy, lubrication of the injection punch is required, and the lubricating medium is punch lubricating oil or lubricating particles. After the die-casting is completed, most of the lubricating residue of the punch accumulates on the surface of the material cake, forming inclusions. A portion of the recycled material is processed waste, which contains residual components of chip liquid and oil stains on its surface. During the melting process, the residual chip liquid and oil stains are heated at high temperatures, producing a large amount of flue gas and water vapor, accelerating the suction of Al liquid and the production of oxidation inclusions. To ensure the quality of Al liquid, it is recommended to refer to Table 4 for the proportion of recycled materials. According to the above experimental data analysis, the recycled material not only leads to an increase in inclusions and changes in chemical composition in the Al liquid, but also affects the mechanical properties of the castings due to the increase in inclusions; During the filling process of die-casting, it also affects the fluidity of Al liquid, resulting in many die-casting defects. Therefore, in the daily melting process, it is necessary to consider both the melting cost and the quality of the Al liquid. According to the product requirements, the recycled materials must be graded and used reasonably in the required proportion.

(1) K模模具准备K模清洁1次/班, 并均匀喷涂模剂涂料打底;K模预热30～60min;其底面与地面末端夹角为30°～45°, 方便Al液导入、气体排出。(2) K模尺寸 (见图5) 及试样浇注：①用勺子舀Al液, 勺子倒入K模时不得浇注过快以免堵塞浇料口, 导致K模内气体无法从浇料口排出, 保持匀速直至浇满; ②待冷却8～10s将K模试样取出, 按照此方式连续浇注5模试样; ③K模浇注前:Al液必须经过旋转除气机除渣除气完成后进行, 浇包内浮渣清理干净; ④等待K模试样自然冷却或者水冷后擦干试样表面残留水分, 将每根K模试样按照折断线位置均匀敲成4小段, 并整齐码放一起, 共计5组。

The sample is placed in a sealed space in a semi-solid state, and then evacuated until it is completely solidified in the vacuum container. The obtained sample is observed for its appearance and compared with the standard sample to determine the hydrogen content in the Al liquid. During the experiment, take a small amount of Al liquid and pour it into a preheated crucible. Immediately (less than 10 seconds) place it in a sealed vacuum chamber, and then turn on the hydrogen analyzer vacuum pump to create a vacuum state in the vacuum chamber.

conclusion
(1) Clear classification of recycled materials is required during the aluminum alloy melting process; For castings that require dense internal organization and airtightness, the proportion of recycled materials should be controlled within 30%, while for ordinary castings, the proportion of recycled materials can be relaxed to 40%.
(2) Maintaining the melting temperature of Al liquid at 720-750 ℃ can reduce the suction of Al liquid.
(3) To ensure the effectiveness of refining degassing, the degassing time for aluminum alloy refining shall not be less than 8 minutes.
(4) The Fe content in ADC12 aluminum alloy should be controlled between 0.70% and 0.85%.

author
Wang Longsheng, Xue Dongqing, Zhang Qingchang
Shandong Yinguang Yuyuan Light Metal Precision Forming Co., Ltd
This article comes from: "Special Casting and Nonferrous Alloys" magazine, "Die Casting Weekly" strategic partner