Simulation of the Coupled Plastic Material Flow-Thermal-Mechanical Phenomena in Friction Stir Welding of High Strength Aluminium Alloys

高强铝合金作为一种重要的轻金属材料，被广泛应用于制造轻量化结构，以达到节能降耗的目的。但铝合金熔点低、导热系数大、热膨胀系数大，焊接难度大。搅拌摩擦焊(FSW)是连接高强铝合金的首选固态工艺。为了获得无缺陷、结构合理、可靠的焊缝，必须充分了解搅拌摩擦焊工艺的工作机理。尽管人们对搅拌摩擦焊过程进行了大量的研究，但对搅拌摩擦焊过程的物理基础还不够全面。在本项目中，建立了一个完整的底层物理模型，对搅拌摩擦焊过程中的热产生、塑性材料流动和热力学行为进行了数值模拟，并进行了实验标定和验证。揭示了产生热的机理，较好地描述了摩擦效应(滑动)和与材料流动(粘结)相关的塑性变形之间的分割。采用逐次迭代的方法，解决了热历史、物质流动和产热现象的耦合。在高保真的材料流动和温度场耦合模型的基础上，阐述了工艺参数和工具设计对焊接质量的影响，预测了搅拌摩擦焊焊接过程中的残余应力和性能分布。对搅拌摩擦焊接件的组织和力学性能进行了表征。为指导高强铝合金的优质高效连接建立知识库。这将为高强度铝合金在制造重要的轻量化结构中的广泛应用奠定坚实的基础，例如航空航天、汽车或造船应用。

High strength aluminium alloys, as one of the most important light metals, are widely used to manufacture lightweight structures to save energy and lower fuel consumption. However, aluminium alloys are difficult to weld because of the low melting point, large thermal conductivity, and high thermal expansion coefficient. Friction stir welding (FSW) is a preferred solid-state process for the joining of high strength aluminium alloys. To obtain defect free, structurally sound and reliable welds, it is essential to fully understand the operating mechanisms of FSW process. Although significant effort has been made for understanding FSW process, the underlying physics of FSW process are not fully characterized. In this project, an integrated model of the underlying physics is developed to numerically simulate the heat generation, plastic material flow, and thermomechanical behaviors of the FSW process, and experimental calibration and validation are carried out. The mechanism of heat generation is revealed to appropriately describe the partition between the frictional effect (sliding) and the plastic deformation associated with material flow (sticking). The coupling of thermal history, material flow and heat generation phenomena are solved via sequential iterative approaches. Based on the high-fidelity modeling of coupled material flow and temperature field, the influences of the process parameters and tool design on the weld quality are elucidated, the residual stresses and property distributions in friction stir welds are predicted. The microstructure and mechanical properties of friction stir weldment are characterized. A knowledge base is going to be established for guiding high-quality and high-efficiency joining of high strength aluminium alloys. It will lay solid foundation for wide applications of high strength aluminium alloys in manufacturing important lightweight structures, e.g., aerospace, automotive or shipbuilding applications.