Compared with steels, Al alloys possess lots of advantages, such as light specific gravity, good plasticity, excellent thermal conductivity, corrosion resistant, etc. Nowadays, more and more products are designed by welding structure of Al alloys and used in the shipbuilding, automobile manufacturing, aerospace and petrochemical industry, presenting unique advantages and significant economic benefits. The welding problems of Al alloys have always been a focus of attention, such as cracks, pores, overburning, etc. As a result, the safety problems severely restrict the industrial application of Al alloys. Friction stir welding (FSW) is a solid-state bonding process. FSW achieves the joining between materials through the generation of frictional heat and strong plastic deformation. At the same time, the stirring head shoulder applies a certain compressive stress to the material to ensure a high-density and defect-free weld. Compared with the traditional fusion welding process, it has the advantages of good joint quality, refined microstructure, excellent fatigue performance, low peak temperature, energy saving, etc. Therefore, FSW is exactly suitable for the welding of Al alloys. Because of the working principle of FSW, when the probe is retracted, the defect such as terminal lock hole will be formed, resulting in the degeneration of mechanical properties. In order to obtain enough heat and enough forging force, the shoulder of the stirring head needs to be embedded into the top surface of the workpiece to a certain depth. In addition, high shoulder embedment depth will cause the shoulder height below the substrate metal, resulting in thinning of the weld. For the components with hollow or curved structures, fixed and rigid supports are required, which will increase the welding difficulty and the manufacturing cost. Moreover, the applications of FSW in thick Al alloy plates are limited by the narrow parameter range for obtaining good joint quality. Researchers usually solve these problems through the process optimization. It has been found that the keyhole defects can be solved by friction plug welding, filled friction stir welding and drawback friction stir welded. The defects of weld thinning can be minimized by stationary shoulder friction stir welding and deposition friction stir welding. The bobbin tool friction stir welding and non-dip angle friction stir welding can solve the back support problem well and provide support for welding complex structures of Al alloys components. With the increasing researches on the microstructures and mechanical properties of thick Al alloy plates, the welding of thick Al alloy plates is becoming mature. The key problems of the FSW application in Al alloys were summarized. The applicable fields of each kind of FSW process were overviewed, including friction plug welding, filled friction stir welding, drawback friction stir welding, stationary shoulder friction stir welded, deposition friction stir welding, bobbin tool friction stir welding and non-dip angle friction stir welding. The principle and mechanism of each process were discussed in detail, and the advantages and disadvantages of each process were expounded. The effects of the process parameters optimization, auxiliary equipment addition and process improvement on the microstructure and mechanical properties of the FSW joints were emphasized. The review and summary of Al alloy friction stir welding will provide reference for obtaining high-quality friction stir welding joints and welding complex structural parts. On this basis, the future application of FSW techniques in Al alloys is prospected. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.
