Effect of Cryogenic, Solution and Aging Combined Heat Treatment on Friction and Wear Properties of Vacuum Die-Cast Al Alloy

New energy vehicles are an essential part of a low-carbon and circular economy. To reduce energy consumption, increase range and improve vehicle handling and safety, aluminium alloy with lightweight and high strength is an ideal material to replace the traditional materials used in automobiles. Especially the current new energy vehicle die casting is towards the direction of large-scale integration, thin-walled development, the performance of aluminium alloy materials put forward extremely high requirements. In recent years, vacuum die-casting technology has rapidly developed, greatly reducing the casting in the gas content, effectively improves the traditional die-casting process production castings heat treatment appear porosity and deformation problems, and provided a strong support for the heat treatment of die-cast aluminum alloys. Due to the many factors affecting the strengthening effect of the heat treatment process of vacuum die-casting aluminum alloy, there are few relevant studies and a lack of reasonable process design optimization methods. To achieve the goal of improving the mechanical properties of vacuum die-casting aluminum alloy, the cryogenic, solid solution, aging and their composite treatment processes matching the vacuum die-casting process were systematically studied in the early stage, and the optimal process scheme of cryogenic + low temperature short-term solid solution + low-temperature aging was obtained. Based on the previous research results, aiming at the friction and wear problems prone to occur in the service process of vacuum die casting aluminum alloy, with the help of friction and wear testing machine, field emission scanning electron microscopy, electronic universal tensile testing machine and other testing and analysis methods, In this paper, the effects of four different processes on microstructure, mechanical properties and friction and wear properties of vacuum die casting aluminum alloy were studied. The results indicated that the mechanical properties of the alloy were significantly improved after cryogenic solution aging treatment. The hardness of the alloy was increased to 124.6 HV, which was 38.0% higher than that of the as-cast, the tensile strength was increased to 249.5 MPa, which was 35.0% higher than that of the as-cast, and the elongation was increased to 7.72%, which was 22.5% higher than that of the as-cast. The eutectic Si phases of the as-cast alloys were mostly sheet-like and stripy, which were densely distributed and seriously agglomerated. There were polygonal block Al₂Cu phase and long acicular Al₅FeSi phase, which seriously affected the mechanical properties of the alloys. After deep cooling treatment, the eutectic Si phase and Al₅FeSi phase started to become shorter and finer. After solid solution aging treatment, the eutectic Si phase transformed to granular, and the Al₅FeSi phase was still dominated by long needles. After deep-cooling solid solution aging treatment, the roundness of the Si phase was further improved, and the corners of the Al₅FeSi phase were blunted, becoming more rounded and smaller, which greatly reduced the cutting effect on the aluminium matrix. At the same time, the organisational structure of the alloy was more uniform, which was conducive to improving the mechanical properties of vacuum die-casting aluminium alloy. Under different loads, the friction coefficient of the deep-cooled solid solution aging alloy was the smallest and the lowest degree of fluctuation, and the time required to reach a stable friction state was the shortest, mainly showing the transformation process of abrasive wear → adhesive wear, even under higher loads, no serious damage behaviours such as delamination and oxidative wear, the alloy's abrasive debris was mostly in the form of particles, and flaky abrasive debris was rare, with a lower content of oxygen elements. The excellent wear resistance of the alloy in the deep-cooled solution-ageing state was on the one hand attributed to the finer grain size of the organisation, and the morphology of the eutectic silica phase had became fine and rounded, which made the interference resistance of the micro-convex body greatly weakened. On the other hand, the hardness of the alloy was significantly improved, the deformation resistance was stronger in the friction process, the growth rate of cracks between the Si phase and the matrix was slowed down, the micro-convex body was not easy to fall off and form a spalling zone, which effectively slowed down the occurrence of spalling wear and oxidative wear, and the abrasion resistance was optimal.