Heat treatment of die-castings is a crucial process that involves heating, holding, and cooling the parts to alter their internal structure, thereby improving mechanical properties (such as strength, hardness, and toughness) or eliminating internal stress. Due to the rapid cooling of the molten metal during the die-casting process, supersaturated solid solutions, coarse grains, and internal stress concentrations can easily form within the die-casting. These problems can be effectively addressed through appropriate heat treatment processes. Different die-casting alloys (such as aluminum alloys, zinc alloys, and magnesium alloys) differ significantly in their composition and phase transformation characteristics, requiring significantly different heat treatment processes. Heat treatment of aluminum alloy die-castings is the most widely used, while zinc alloy die-castings generally do not require complex heat treatment due to their lower melting points.
The heat treatment process for aluminum alloy die-castings primarily includes annealing, solution treatment, and aging. The core of these processes is to control temperature and holding time to achieve the dissolution and precipitation of alloying elements, thereby strengthening the material. Annealing (such as low-temperature annealing) is used to eliminate internal stresses generated during the die-casting process. The process parameters typically involve heating to 200-300 °C, holding for 2-4 hours, and then slowly cooling. This effectively reduces deformation in die-castings and is particularly suitable for thin-walled parts requiring high precision. Solution treatment involves heating the die-casting to 500-550 °C (near the melting point of the aluminum alloy) for 1-3 hours to fully dissolve the strengthening phases (such as Mg₂Si ) in the alloy into the aluminum matrix, forming a uniform, supersaturated solid solution. This is followed by rapid water cooling, achieving ” quench strengthening.” Aging treatment involves heating the solution-treated die-casting to 120-180°C for 4-12 hours, promoting the uniform precipitation of the strengthening phases in the form of fine particles, further enhancing the material’s strength and hardness. For example, after T6 heat treatment (solid solution + artificial aging) of 6061 aluminum alloy die castings, the tensile strength can be increased from 200MPa in the die-cast state to more than 300MPa, and the hardness (HB) can be increased from 60 to more than 90.
The heat treatment of magnesium alloy die-castings must balance strengthening effects with corrosion resistance. Common processes include solution treatment, aging treatment, and annealing. Because magnesium alloys are susceptible to oxidation, heat treatment must be performed under inert gas or in a vacuum environment to prevent surface oxidation and burning. Solution treatment temperatures are typically 380-420°C for 1-4 hours, followed by water quenching or air cooling. Aging treatments are typically performed at 120-200°C for 4-24 hours, which can increase tensile strength by 15%-25%. For example, AZ91D magnesium alloy die-castings, after T6 treatment, can achieve a tensile strength of 250 MPa and a yield strength of 160 MPa, making them suitable for load-bearing components such as automotive transmission systems. Annealing (200-250°C for 2-6 hours) is primarily used to eliminate internal stresses and reduce deformation during subsequent processing.
Heat treatment of zinc alloy die-castings is rarely used due to the alloy’s low melting point (approximately 380-420°C). Excessive temperatures can easily lead to coarsening of the grains or surface oxidation, which in turn reduces mechanical properties. Low-temperature annealing (100-150°C for 1-2 hours) is only used for certain zinc alloy die-castings that require internal stress relief to reduce dimensional changes caused by internal stress. For example, low-temperature annealing can improve the dimensional stability of zinc alloy die-castings used in precision instrument housings by over 30%, effectively preventing sticking during assembly.
Heat treatment quality control requires strict control of temperature uniformity, holding time, and cooling rate. These parameters directly impact the performance stability of die-castings. Heating equipment (such as box furnaces and vacuum furnaces) must be equipped with precise temperature control systems, and temperature fluctuations should be controlled within ±5°C to ensure uniform heating of all parts of the die-casting. Insufficient holding time will result in insufficient dissolution or precipitation of strengthening phases, while excessive holding time may cause coarse grains. Excessively slow cooling rates (especially after solution treatment) can cause premature precipitation of strengthening phases, reducing the quenching effect. For example, if the water cooling rate of aluminum alloys after solution treatment is less than 100°C/s, the tensile strength can drop by more than 20%. Furthermore, die-castings undergo mechanical property testing (such as tensile testing and hardness testing) and deformation measurement after heat treatment to ensure they meet design requirements. Scientifically formulated heat treatment processes can significantly improve the performance of die-castings and expand their application in high-strength, high-reliability applications.
