活塞用Al-Si-Cu-Mg-Ni合金凝固析出Si相和富Ni相的调控

发布时间：2018-04-22 08:19

  本文选题：活塞 + 铝硅合金　； 参考：《西安工业大学》2017年硕士论文

【摘要】：活塞长期工作在高温高压的恶劣环境中,承受着极高的周期性热机耦合疲劳载荷,活塞合金中粗大的形状不规则的初生硅和共晶硅是高周疲劳裂纹萌生的主要源头,研究Si相的细化方法是提升活塞合金综合性能的有效途径。本文分别研究了Si含量、稀土元素Nd、混合稀土La-Ce和超声场对活塞用Al-Si-Cu-Mg-Ni系合金的组织演变和力学性能的影响。通过光学显微镜(OM)、扫描电子显微镜(SEM)等仪器,分析了上述因素对该合金凝固过程中析出相的形貌和分布的影响。并用电子万能试验机对其力学性能进行了测试研究。确定了稀土的最佳添加量,也确定了超声的最佳超声功率和超声温度。主要的研究成果如下:(1)随着Si含量的增加,活塞合金的热膨胀系数在减小,其流动性在增加,合金显微组织中初生Si的等积圆直径和所占的面积分数都在增加,合金的高温抗拉强度(350℃、425℃)和延伸率都在减小。(2)当Nd含量≤0.3%时,α-Al基体得以细化。在Nd含量为0.3%时,初生硅的等积圆直径和未添加Nd时的相比,减小了73%,共晶硅明显细化。当Nd含量增加至0.8%及以上时,合金中出现了富稀土相,其力学性能显著降低。Nd含量为0.3%时合金的350℃抗拉强度较未添加Nd时提高了29.8%。Nd元素聚集在AlCuNi相周围,造成了成分过冷,抑制了AlCuNi相的长大,AlCuNi相由粗大的骨骼状转变为细小的棒状。(3)混合稀土La-Ce对铝硅合金中的Si相具有良好的变质效果。当混合稀土La-Ce含量为0.5%时,初生Si的等积圆直径和共晶Si的平均长度均最小,分别为16.7μm和22.12μm。但是混合稀土加入量超过0.5%时,会产生一种长针状的富稀土相,这种富稀土相的出现,会严重损害活塞合金的力学性能。当混合稀土La-Ce添加含量为0.35%时,合金的室温抗拉强度和350℃抗拉强度均达到最大值,分别为318MPa和124MPa,较未添加混合稀土时,分别增加了10.0%和30.5%。(4)超声场促进了铝硅合金内等轴状晶粒的形成,细化了初生Si、共晶Si。在处理温度为720℃,超声功率为0~1400W处理条件下,初生Si的平均等积圆直径从40.73μm减小到23.35μm;共晶Si的平均长度从39.6μm减小到27.1μm;其他第二相的形态未发生改变;合金的室温抗拉强度和350℃抗拉强度比未超声时分别提高了21.9%和22.8%。熔体超声处理细化了硅相,提升了合金性能。
[Abstract]:The piston works in the harsh environment of high temperature and high pressure for a long time and bears extremely high cyclic thermo-mechanical coupling fatigue load. The coarse and irregular primary silicon and eutectic silicon in the piston alloy are the main source of high cycle fatigue crack initiation. It is an effective way to improve the comprehensive properties of piston alloy by studying the refining method of Si phase. The effects of Si content, rare earth element ND, mixed rare earth La-Ce and ultrasonic field on the microstructure evolution and mechanical properties of Al-Si-Cu-Mg-Ni series alloy for piston were studied in this paper. The influence of the above factors on the morphology and distribution of precipitated phases in the solidification process of the alloy was analyzed by means of optical microscope and scanning electron microscope (SEM). The mechanical properties are tested and studied by electronic universal testing machine. The optimum addition amount of rare earth, the optimum ultrasonic power and ultrasonic temperature were also determined. The main results are as follows: (1) with the increase of Si content, the coefficient of thermal expansion of the piston alloy decreases, its fluidity increases, and the diameter and area fraction of the primary Si in the alloy microstructure increase. The high temperature tensile strength of the alloy is 350 鈩，

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