高压凝固Al-Cu合金组织演变及力学性能研究

Al-Cu alloys possess both excellent mechanical properties and machinability, which exhibit extensive application prospects in aerospace and auto industry. However, due to the wide crystallization temperature, the Al-Cu alloys display poor casting performance and lead to segregation as well as hot cracks, which restrict their applications severely. Conventional studies are mainly focused on reducing the segregation and hot cracking tendency by addition of alloying elements or solution treatment. The significant influence of high pressure on inhibiting segregation has been ignored. Alloys solidified under high pressure can be characterized by increased solid solubility, refined microstructure, improved uniformity of the microstructure distribution and elimination of segregation. Therefore, Al-Cu alloys solidified under high pressure (GPa level) are chosen as the research subject. The microstructure evolution and the phase election mechanism are investigated. In addition, the correlation between heat treatment and the high pressure solidification microstructure will be established. The microstructure variation, the crack initiation and propagation as well as dislocation migration are clarified by situ tensile tests of Al-Cu alloys, the relationships between the microstructure and the mechanical properties are clarified. This study has an importmant meaning to understand the non-equilibrium solidification theory of Al-Cu alloys, and also have directive significance for the applications of Al-Cu alloys.

Al-Cu 合金具有优异的力学性能和切削加工性能使其在航空、航天、汽车等领域得到广泛应用。但由于其结晶温度范围宽，铸造性能差，易偏析产生热裂纹，严重制约了该合金的发展。现有的方法主要是通过添加合金元素、固溶处理等方式来减小偏析降低合金的热裂倾向，而忽略了高压对偏析的抑制作用。高压凝固作为一种新的制备方法，能显著增大固溶度、细化组织、改善组织均匀性、有效消除偏析。因此，本项目以GPa级高压凝固Al-Cu二元合金为对象展开研究，揭示高压对合金相选择机制及组织演变规律的影响；建立热处理与高压凝固组织热稳定性的内在联系。结合原位拉伸测试结果，阐明合金微观结构变化与裂纹萌生、扩展及位错迁移规律机理，揭示合金的微观断裂机制，明确组织与力学性能的内在关系。因此本项目的研究对丰富Al-Cu合金的非平衡凝固理论，进一步推动Al-Cu合金的应用具有理论指导意义。

项目采用六面顶液压机在GPa级高压下凝固Al-Cu合金，课题围绕高压凝固Al-Cu合金的相及组织演变规律、高压凝固组织热稳定性以及合金力学性能展开研究。.研究发现，随着凝固压力增加，Al-5Cu合金中Al2Cu相全部固溶进a-Al基体中；Al-15Cu合金二次枝晶臂间距由常压凝固时的10μm减小至3μm，共晶Al2Cu相形貌转变为球形。常压凝固Al-33Cu合金中共晶Αl2Cu呈现板片状以及不规则的颗粒状，而高压凝固该合金组织中Αl2Cu相变为纤维状，且二次枝晶臂间距逐渐减小。常压凝固时Al-40Cu合金共晶Αl2Cu相逐渐向颗粒状转变，初生Αl2Cu相尺寸由29 µm降低至19 µm。热力学过冷、成分过冷和动力过冷均随凝固压力的增加而增大。在高压凝固条件下，热力学过冷度和动力学过冷度的增加促进凝固过程中的形核，而凝固速率的增加抑制晶粒的生长。合金维氏硬度随凝固压力及Cu含量增加而增加；185℃时效8h后，过饱和固溶的Αl2Cu相弥散均匀析出，合金的硬度进一步增大。.结合纳米压痕测试，获得高压凝固后合金的强度和塑性以及蠕变性能。研究发现，高压凝固Al-Cu合金的纳米硬度增加的同时，其蠕变抗力也随之增大。结合应变速率敏感指数m分析并计算了其蠕变阶段中的自由体积V*。研究发现，对于亚共晶和共晶成分的Al-Cu合金，高压凝固后的自由体积V*减小，而对于过共晶成分Al-Cu合金，其蠕变阶段自由体积的增加降低了其塑性。该项目研究为优化Al-Cu合金组织及力学性能，扩展其应用范围具有十分重要的意义。