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

高强度铝合金作为一种重要的轻金属，被广泛用于制造轻量化结构，以节省能源和降低燃料消耗。但铝合金熔点低，导热系数大，热膨胀系数高，焊接困难。搅拌摩擦焊（FSW）是连接高强度铝合金的首选固态焊接工艺。为了获得无缺陷、结构健全和可靠的焊缝，必须充分了解FSW工艺的工作机理。尽管人们已经为理解FSW过程做出了巨大的努力，但FSW过程的基本物理特性并没有得到充分的表征。在本项目中，建立了基础物理的集成模型，以数值模拟FSW过程的热量产生，塑性材料流动和热力学行为，并进行了实验校准和验证。揭示了热的产生机制，以适当地描述摩擦效应（滑动）和与材料流动相关的塑性变形（粘着）之间的分配。采用序贯迭代法求解了热历史、物质流动和热现象的耦合问题。基于高保真的材料流和温度场耦合建模，阐述了工艺参数和工具设计对焊接质量的影响，预测了搅拌摩擦焊缝的残余应力和性能分布。对搅拌摩擦焊件的显微组织和力学性能进行了表征。为指导高强度铝合金的高质量、高效率连接建立知识库。它将为高强度铝合金在制造重要轻量化结构，如航空航天，汽车或造船应用中的广泛应用奠定坚实的基础。

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.