Material Flow Behavior And Mechanical Properties Of Refill Friction Stir Spot Welded Aluminum Alloy

As one of the main weight-reducing materials, aluminum alloy has been increasing used. However, welding defects, which always appear during spot joining technologies such as resistance spot welding and riveting, are the main reasons limiting the large-scale application of aluminum alloys. Being a solid-state joining technology and without keyhole after welding, Refill friction stir spot welding(RFSSW) has become the main technology to spot joining aluminum. The concentration of the present study was placed on material flow behavior and mechanical properties of 2024-T4 aluminum alloy RFSSW joints.Firstly, material flow behavior during RFSSW was investigated. Result showed that high-flow area appears at inner, outer and bottom of sleeve and pin. Based on the simulation results of altering the welding parameters, it can be seen that both increasing the rotating speed and decreasing the refilling time can increase high-flow area, in which increasing the rotating speed was more effective. Based on the simulation results of altering the tool geometries, thread can accelerate the flow velocity during welding. From the viewpoint of decreasing the bonding ligament thickness, the inner-thread tool was more effective.Secondly, effects of welding parameters on microstructure and mechanical properties were investigated. Surface formation defects such as flash, annular groove and material adhesion were observed. Inner defect such as hook, incomplete refilling, tearing and void, which always appear at stir zone/thermal-mechanically affected zone(SZ/TMAZ) interface and lap interface, can be observed. Bonding ligament was one of the main features. With increasing the rotating speed, thickness of the bonding ligament thickness reduced, lap shear failure load of the joint decreased, hardness of the stir zone(SZ) decreased. Maximum lap shear failure load of 9023 N and hardness of 159.7HV were observed using 1000 rpm. Bonding ligament became thicker and lap shear failure load firstly increased and then decreased with increasing the plunge depth and refilling time. The maximum values were observed using 1.8mm and 3.0s. By changing the welding parameter, variation law of the lap shear failure load was simple, while cross tension failure load variation law was rather complicated. Results of the impact experiment showed that: impact strength of RFSSW joint was higher than the base material(7.69J). The maximum value of 17.15 J was attained when using 2000 rpm.Thirdly, fracture mechanisms of the joints were investigated. Three fracture modes: shear fracture, shear-plug fracture and plug fracture modes can be observed during the lap shear test. Different fracture paths were mainly determined by the bonding strengths of the SZ/TMAZ interface and the bonding ligament, which were further determined by heat input and sleeve plunge depth during welding. Cross-tension properties of RFSSW joints were also mainly related to bonding strengths of the SZ/TMAZ interface and the bonding ligament.