Evolution Mechanisms of Microstructure and Performance of Aluminum Alloy Thin-Walled Components Repaired by Friction Stir Spot Welding

Abstract

Taking the repair of prefabricated hole defects in 2024 aluminum alloy thin-walled components by friction stir spot welding (FSSW) as the research object, the evolution laws of microstructure and mechanical properties of FSSW-repaired joints of thin-walled components were clarified through process experiments and numerical simulations. The collaborative effect of the temperature field and material flow field during the FSSW repair process and their regulation laws on the microstructure and properties were revealed. The results show that as the repair speed increases, the macroscopic surface quality of the FSSW joint improves. When the repair speed reaches 2000 r/min, a high-quality repaired joint with a smooth and flat surface and no porosity defects can be obtained. Meanwhile, within the repair speed range of 800 to 2000 r/min, the grains undergo dynamic recrystallization (DRX) due to the combined effect of heat and mechanical forces, eventually forming a uniform equiaxed grain structure in the weld core area. ABAQUS 2023 simulation verifies the temperature distribution during the FSSW repair process. When the repair speed is 2000 r/min, the maximum temperature obtained from the simulation is 431.1 °C, which agrees with the measured value from the experiment. The simulation results further reveal that when the repair speed increases from 1200 r/min to 2000 r/min, the material fluidity significantly enhances, and the flow velocity on the advancing side is always higher than that in other areas. At the rotational speed of 2000 r/min, the plastic material flows continuously from the periphery and eventually fills the defect area completely. The fracture mode of the FSSW-repaired joint is mainly ductile fracture. With the increase in the repair speed, the number of dimples at the fracture surface increases significantly. When the rotational speed reaches 2000 r/min, the joint achieves the best mechanical properties, and the FSSW-repaired joint reaches the maximum tensile strength of 169 MPa.