Influences of welding parameters on mechanical properties of AZ31 friction stir spot welds

In this study, the influence of welding parameters, including tool rotational speed, plunge rate and dwell time, on the overlap tensile shear properties of AZ31 friction stir spot welds was investigated. The microstructures in stir zones and fracture surfaces were observed using optical microscope and scanning electron microscope respectively. The bonded width and h value (the distance from the tip of the partially bonded region to the top of the weld surface) were measured. The results indicated that larger bonded width and higher h value of the AZ31 weld result in better mechanical property. It is proposed that high tensile shear loads are produced when the tool rotational speed range of 1500-2250 rev min(-1) and 1 s dwell time are applied during the friction stir spot welding of AZ31. The plunge rate range from 2.5 to 10 mm s(-1) has insignificant influence on the tensile shear load of AZ31 joints under the present conditions. The failure mode changes from interfacial to pullout when the tool rotational speed is >2250 rev min(-1). The fracture feature of AZ31 welds is brittle fracture.