Effect of Cryogenic, Solution and Aging Combined Heat Treatment on Friction and Wear Properties of Vacuum Die-Cast Al Alloy
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摘要:
为提升真空压铸铝合金的力学性能及耐磨性，借助场发射扫描电镜和电子万能拉伸试验机等技术，系统研究了深冷+低温短时固溶+低温时效的处理工艺对真空压铸铝合金微观组织和摩擦磨损性能的影响. 结果表明：经过深冷+低温固溶时效处理后，合金的组织结构相更加均匀，共晶Si相圆整度明显提高，长针状的Al5FeSi相棱角钝化，大大降低了对铝基体的割裂作用. 此时，合金的硬度提高至124.6 HV，抗拉强度提升至249.5 MPa，伸长率提高至7.72%. 深冷+低温固溶时效态合金的摩擦磨损性能得到显著提高，在不同的载荷下，深冷固溶时效态试样的犁沟最浅，剥落区相对最少，表现出更优异的耐磨性能.
Abstract: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.
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图 1 不同处理工艺真空压铸铝合金的力学性能
Figure 1. Mechanical properties of vacuum die-cast aluminum alloys with different treatment processes
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图 2 不同处理工艺下压铸铝合金组织的SEM照片：(a)铸态；(b)深冷态；(c)固溶时效态；(d)深冷固溶时效态
Figure 2. SEM micrographs of die cast aluminum alloy under different treatment processes: (a) As-cast; (b) Cry; (c) T6; (d) Cry-Sol-Age
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图 4 不同载荷的摩擦系数曲线：(a) 3 N；(b) 5 N；(c) 7 N；(d) 9 N；(e)平均摩擦系数
Figure 4. Friction coefficient curves of different loads: (a) 3 N; (b) 5 N; (c) 7 N; (d) 9 N; (e) mean friction coefficient
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图 5 不同状态的压铸铝合金在不同载荷下磨损后的磨面形貌的SEM照片：(a)铸态，3 N；(b)铸态，5 N；(c) 铸态，7 N；(d) 铸态，9 N；(e)深冷态，3 N；(f)深冷态，5 N；(g)深冷态，7 N；(h)深冷态，9 N；(i)固溶时效态，3 N；(j)固溶时效态，5 N；(k)固溶时效态，7 N；(l)固溶时效态，9 N；(m)深冷固溶时效态，3 N；(n)深冷固溶时效态，5 N；(o)深冷固溶时效态，7 N；(p)深冷固溶时效态，9 N
Figure 5. SEM micrographs of wear surface morphology of die-cast aluminum alloys in different states after wear under different loads: (a) As cast, 3 N; (b) As cast, 5 N; (c) As cast, 7 N; (d) As cast, 9 N; (e) Cry, 3 N; (f) Cry, 5 N; (g) Cry, 7 N; (h) Cry, 9 N; (i) T6, 3 N; (j) T6, 5 N; (k) T6, 7 N; (l) T6, 9 N; (m) Cry-Sol-Age, 3 N; (n) Cry-Sol-Age, 5 N; (o) Cry-Sol-Age, 7 N; (p) Cry-Sol-Age, 9 N
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图 6 磨损剖面的SEM照片：(a)铸态，3 N；(b)铸态，9 N；(c)深冷固溶时效态，3 N；(d)深冷固溶时效态，9 N
Figure 6. SEM micrographs of cross-section of wear profiles: (a) As cast, 3 N; (b) As cast, 9 N; (c) Cry-Sol-Age, 3 N; (d) Cry-Sol-Age, 9 N
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图 7 不同状态合金磨屑的SEM照片及EDS图：(a)铸态-3 N；(b)铸态-3 N磨屑能谱图；(c)铸态-9 N；(d) 铸态-9 N磨屑能谱图；(e)深冷固溶时效态-3 N；(f)深冷固溶时效态-3 N磨屑能谱图；(g)深冷固溶时效态-9 N；(h)深冷固溶时效态-9 N磨屑能谱图
Figure 7. SEM micrographs and EDS plots of different states of alloy abrasive chips: (a) As-cast, 3 N; (b) As-cast -3 N abrasive energy spectra;(c) As-cast 9 N; (d) As-cast 9 N abrasive energy spectra; (e) Cry-Sol-Age 3 N; (f) Cry-Sol-Age 3 N abrasive energy spectra; (g) Cry-Sol-Age 9 N; (h) Cry-Sol-Age 9 N abrasive energy spectra
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图 8 真空压铸铝合金在不同载荷工况下摩擦磨损组织演变及其强化作用示意图：(a)铸态；(b)深冷固溶时效态
Figure 8. Schematic diagram of friction and wear microstructure evolution and strengthening effect of vacuum die-casting Al alloy under different loads: (a) As-cast; (b) Cry-Sol-Age
表 1 合金化学成分
Table 1 Alloy chemical composition
Element Al Si Cu Zn Fe Mg Mn Ni Cr Other elements Mass fraction/% 84.90 10.42 2.10 0.99 0.93 0.23 0.20 0.06 0.05 Bal. 
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表 2 真空压铸铝合金的处理工艺
Table 2 Treatment process of vacuum die-cast Al alloy
Abbreviation Processing As-cast None Cry Cryogenic (−196 ℃ for 12 h) T6 Solution (476 ℃ for 22 min)+aging (159 ℃ for 403 min) Cry-Sol-Age Cryogenic (−196 ℃ for 12 h)+solution (476 ℃ for 22 min)+aging (159 ℃ for 403 min)
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