What is the role of heat treatment in electromechanical aluminum die castings?

Heat treatment plays a vital role in the production of aluminum alloy die castings, mainly in terms of eliminating casting stress and structural defects. Aluminum alloys are prone to residual stress during rapid cooling, which not only causes dimensional deformation, but is also likely to cause serious problems such as cracking. In order to solve this problem, T2 annealing (keeping at 280-300℃ for 2-4 hours) is widely used. This process effectively eliminates internal stress and ensures the dimensional stability of castings through the decomposition of solid solution and the precipitation of second-phase particles. For example, a certain automobile manufacturer's engine cylinder showed a 0.3mm warpage deformation during subsequent machining without annealing, which seriously affected the assembly accuracy. This case fully illustrates the importance of heat treatment. In addition, heat treatment can also promote the homogenization of intergranular segregation, redistribute solute atoms through diffusion mechanism, thereby eliminating defects such as microporosity and improving the density of castings.

Another core value of heat treatment is to significantly improve the mechanical properties of materials. Taking AlSi10Mg alloy as an example, after T6 solution and aging treatment (solution at 535℃ for 2-6 hours, followed by water cooling, and then aging at 175-185℃ for 5-24 hours), its tensile strength can exceed 320MPa and its elongation can reach 8%. In this process, the synergistic effect of solution strengthening and precipitation strengthening is the key: the high-temperature solution stage fully dissolves alloy elements such as silicon and magnesium to form a supersaturated solid solution; and the subsequent aging treatment promotes the precipitation of β'' phase (Mg?Si) at the nanoscale, producing a significant dislocation pinning effect. A new energy vehicle company successfully improved the impact resistance of the battery tray by 40% by optimizing the heat treatment process, and successfully passed the 150kJ drop hammer impact test, further verifying the effectiveness of heat treatment in improving material performance.

In addition to mechanical properties, heat treatment also makes important contributions to improving corrosion resistance and fatigue performance. Aluminum alloys are prone to pitting and intergranular corrosion in the natural environment, while T7 aging treatment (keeping at 190-230℃ for 4-9 hours) can form a stable θ'' phase, significantly hindering the diffusion path of the corrosive medium, and extending the corrosion life of the casting in the salt spray test by more than two times. In terms of fatigue performance, heat treatment significantly improves the crack propagation resistance of the material by refining the grains and regulating the morphology of the precipitated phase. For example, an aviation company uses a two-stage aging process to increase the fatigue limit of aircraft landing gear castings from 120MPa to 160MPa, successfully meeting the stringent requirements of 200,000 take-off and landing cycles.

In order to ensure the stability of the heat treatment effect, precise control of process parameters is essential. The solution temperature must be strictly controlled within the range of ±5℃. Too high a temperature may cause overburning, while too low a temperature will not allow the solute atoms to be completely dissolved. For example, in the solution treatment of AlSi7Mg alloy, the solubility of silicon phase can reach 95% at 535℃, while only 70% can be dissolved at 520℃, which will significantly affect the subsequent aging strengthening effect. At the same time, the matching of aging time and temperature is also extremely critical. When aged at 175℃ for 5 hours, the size of β'' phase can achieve the best strengthening effect (8-12nm), while too long aging time may lead to coarsening of β phase, thereby reducing strength. A company once had an aging temperature fluctuation of ±10℃, which caused the hardness of the casting to fluctuate by 15HRB, seriously affecting the stability of product quality.