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

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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.