Study On Microstructure And Machanical Properties Of Refill Friction Stir Spot Welded Aluminum Alloy

Refill friction Stir Welding（RFSSW）is a solid-state joining technology that offers significant advantages in joining dissimilar materials of the same kind and dissimilarity.It successfully eliminates keyhole defects and avoids hot cracks,pores and inclusions in resistance spot welds Etc.,while effectively solving the problem of increasing the weight of the structure.In this paper,the effects of process parameters on the microstructure and properties of RFSSW joints were investigated using 5083 aluminum and AZ31 magnesium.Firstly,using orthogonal test to study the influence of process parameters on the joint performance.Rotational speed is the dominant factor influencing the performance of aluminum-magnesium joints,followed by the plunge depth and joining time.Joining time is the main factor affecting the magnesium-aluminum joint,followed by the plunge depth and the rotational speed.The optimum parameters for aluminum-magnesium and magnesium-aluminum spot welding are:2400 rpm,2.4 mm,2 s and 2200 rpm,2.2 mm,2 s respectively.Secondly,the influence of joining time,plunge depth and speed on the microstructure and mechanical properties of the joints were studied by using the control variable method.Surface formation defects such as material adhesion,dents,and solder joints falling off;inner defects such as incomplete,exit lines,voids,unwelded joints and transition layers.The transition layer consists of Al₁₂Mg₁₇ and Al₃Mg₂ and is an important factor in the performance of the joint.For the aluminum-magnesium joint,with the increase of welding time,the hardness of the two plates boundary increased,the joint load first increased and then decreased.2s average hardness and maximum load of 270Hv and 3.6 KN.The load of the magnesium-aluminum joint gradually decreased with time,and the maximum value was3.475 KN at a time of 1 s.With the increase of plunge depth and rotation speed,the load of aluminum-magnesium and magnesium-aluminum joint firstly increases and then decreases,taking both the maximum of 3.6 KN and 3.51 KN at 2.4mm;the maximum of 3.6 KN at 2400rpm and 2200 rpm and 3.125 KN.Finally,the influence of process parameters on the fracture mechanism of the joint was studied.The results show that the fracture modes of aluminum-magnesium or magnesium-aluminum lap joints are all shear fractures,and the formation of intermetallic compounds is the main cause of joint fracture.