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材料，被广泛应用于制造轻质结构以节约能源和降低燃料消耗。然而，铝合金由于熔点低、导热系数大、热膨胀系数高而难以焊接。搅拌摩擦焊（FSW）是一种用于连接高强度铝合金的首选固态工艺。为了获得无缺陷、结构良好和可靠的焊缝，必须充分了解搅拌摩擦焊工艺的工作机理。尽管人们为理解搅拌摩擦焊过程做了大量的工作，但搅拌摩擦焊过程的基本物理过程还没有完全被描述。在本计画中，建立了一个完整的物理模型，以数值模拟搅拌摩擦焊制程的热产生、塑性材料流动与热机械行为，并进行实验校正与验证。热产生的机制被揭示，以适当地描述之间的分区摩擦效应（滑动）和与材料流动（粘附）相关联的塑性变形。采用顺序迭代法求解了热历史、材料流动和热生成现象的耦合问题。基于材料流场和温度场耦合的高保真模型，分析了工艺参数和搅拌头设计对焊缝质量的影响，预测了搅拌摩擦焊焊缝的残余应力和性能分布。对搅拌摩擦焊接头的组织和力学性能进行了表征。为指导高强铝合金的高质量、高效率连接建立了知识库。这将为高强铝合金在制造重要的轻型结构，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.