Under the impetus of industry, China’s economy has developed rapidly and successfully promoted the “Made in China” brand to the world. Under the influence of globalization, if China wants to further improve its economic level, it needs to start from the perspective of improving the quality of industrial products, expand marketing channels with high-quality products, and achieve all-round development. Based on this guiding ideology, this article focuses on the quality defects caused by the production of aluminum alloys using low-pressure casting technology, and provides a detailed analysis of its heat treatment process, aiming to contribute to China’s industrial production.
Defects in low-pressure cast aluminum alloy
(1) Casting cracks. If aluminum alloy castings have complex shapes and uneven thickness at various positions, incomplete matching cooling rates will occur in different parts during solidification, resulting in internal stress. Once the internal stress exceeds the maximum tensile strength of the material, it will cause the casting to crack. There are two common types of casting cracks: hot cracks that crack along the grain, and black oxide is observed in the cracks, which are mostly serrated in shape; Cold cracks occur along the interior of the crystal, and the fracture surface appears shiny silver because there is no oxidation reaction. Solution: When casting alloys, it is necessary to clarify the chemical composition and strictly control impurities within the standard range. At the same time, it is also necessary to avoid the influence of high temperature on the melt as much as possible. When the maximum temperature it can bear is exceeded, the residence time of the melt in the furnace should be controlled. Scientific methods should be used to control the temperature and cooling rate of the casting mold, maintain a uniform and stable flow and cooling rate of the liquid metal, and prevent impurities from accidentally falling into the melt. The increase in iron content and needle shaped β – AlFeSi in the composition of A356 aluminum alloy increases its effect on the fracture of the alloy structure and the tendency towards hot cracking. The iron content in low-pressure casting of strength parts such as aluminum alloy wheels and frames should be controlled within 0.15%, and the maximum should not exceed 0.2%.
(2) The boundary grains will have wavy or serrated edges, which are similar in shape to casting twins and mostly feather shaped. When observed under a microscope, it can be found that the tissue is mostly thin and maintains a parallel state. This situation often occurs when the composition is not adjusted properly before casting, or when the melt overheats and stays in the furnace for a long time, similar situations can also occur. However, if the filter tube holes do not have a relatively large diameter, and the crystallizer is too hot or too short during casting, or if the refining agent deteriorates and loses its effect, lace tissue will appear. Solution: Strictly control the chemical composition of the alloy to ensure that its impurity content is within the standard range, and use scientific methods to design the filtration system reasonably and optimize the crystallization device.
(3) Pinholes are a common defect in low-pressure casting, mainly caused by excessive hydrogen content in the aluminum liquid (with a test block density of less than 2), which forms densely distributed needle shaped pores during mold solidification. They are generally prone to occur on the machined surface, especially when thick and overheated. There are many reasons for the excessive hydrogen content in aluminum liquid, such as excessive return to furnace material, inadequate refining of aluminum liquid, excessive water content in low-pressure compressed air, and high pouring temperature. Solution: If there is too much recycled material, control it within 20%. The aluminum liquid is not refined properly, and argon or nitrogen gas with a purity of 99.999% is used for degassing. The degassing parameters (speed, gas flow rate, degassing time) are adjusted based on the test results of the vacuum pumping test block, and the density of the test block is required to be no less than 2.3g/Cm3. The water content of low-pressure compressed air exceeds the standard, and the water content is generally expressed as atmospheric dew point, with a requirement of -60 ℃~-70 ℃. When the pouring temperature is too high, it is generally required not to exceed 740 ℃.
(4) Casting products cannot guarantee a complete state because the temperature is low during pouring, resulting in a faster solidification rate of the casting. The casting has already solidified and formed before it is fully poured, which constitutes a production defect of insufficient pouring. This is because the pouring system lacks a wide channel, which cannot guarantee a large flow rate of aluminum liquid, resulting in bottleneck problems, or poor mold exhaust causing air resistance and insufficient pouring (cold insulation). When the pouring flow rate cannot match the production speed of aluminum liquid filling, insufficient pouring may occur, resulting in the product being treated as waste. Solution: Design the pouring system to optimize the aluminum flow rate and optimize the mold exhaust system. During production, it is necessary to do a good job of preheating the mold to effectively avoid the problem of rapid cooling of the aluminum liquid during pouring. The coating should be distributed reasonably, and the coating inside the cavity should be uniformly thick. Ensure that the mold is compatible There are good exhaust conditions to avoid waste of resources caused by insufficient pouring. When designing the mold, it is required that the meat thickness of the casting be greater than 3mm, and the exhaust plug should be designed reasonably.
If the heating temperature during quenching is gradually increased in casting aluminum alloys without causing overheating of the structure, it can assist in accelerating the dissolution of strengthening phases with Al based solid solutions, thereby reducing the formation time of saturation state and improving the strengthening effect of aluminum alloys. When applying heat treatment technology to treat quality defects in aluminum alloys, it is necessary to strictly control the temperature and treatment process to ensure that the aluminum alloy can achieve high strength and hardness, and improve product quality. At the same time, it is necessary to continuously update the production process, check whether there are any faults in the current production equipment, study the causes of problems, and use scientific methods to solve them. Enterprises should also regularly provide professional training to frontline production workers, cultivate safety production awareness, comprehensively improve production level, ensure that aluminum alloy products can gain market recognition, thereby enhancing the company’s reputation in the industry and making preparations for expanding the market.