The impact of macro/microstructure features on the mechanical properties of refill friction stir spot–welded joints of AA2219 alloy with a large thickness ratio

AA2219 aluminium alloy plates of large thickness ratios were successfully produced by refill friction stir spot welding (RFSSW) method. In addition to common defects, such as holes and hook, insufficient refill and weak bonding also appeared in the joints with large thickness ratios. Besides, the stir zone of the typical RFSSWed joint exhibited refined and equiaxed grains because of the dynamic recrystallization, while the elongated grains with a large proportion of the low angle grain boundaries were the main microstructure features in the thermomechanically affected zone due to insufficient heat input and deformation. Moreover, in all cases of large thickness ratio, the micro-hardness distribution of the joints presented an M-shaped profile. The maximum tensile-shear strength reached 7.2 kN, corresponding to the rotation speed of 2000 rpm, welding time of 3.6 s and plunge depth of 2.2 mm, which displayed a close correlation with the defect morphology, such as the weak bonding length and hook height. Also, two types of fracture modes were observed in the tensile-shear specimens, plug fracture mode and mixed fracture mode.