Microstructures and mechanical properties of aluminium alloy refilled friction stir spot welded joint at low rotational speed

Refill friction stir spot welding was used to weld 5182-O aluminium alloy of 1. 5 mm in thickness was prepared at low rotation speed. The comprehensive properties of 5182-O aluminium alloy joint at low rotation speed were studied through the analysis of its appearance,shear tensile,cross tensile and fatigue properties by using orthogonal test. The results show that the optimum parameters are the welding time of 6. 5 s,the rotation speed of 1 000 r/min and the depth of 1. 8 mm. The shear tensile force and cross tensile strength of the joint are 6. 62 k N and 3. 67 k N,respectively. The fatigue load is 1 108 N at the fatigue life of 1×106. SEM and OM analysis show that the joint fails in TMAZ between the upper and lower plates during shear tensile test,and the cracks expend along the thickness of the upper plate. The fatigue microcracks initiates from both upper and lower plates near the load side at the same time. The main reason for fatigue micro-cracks source is the dispersion of oxide inhomogeneous under low heat input at lower rotation speed.