Effects of process parameters on strength of Mg alloy AZ61 friction stir spot welds

Extruded AZ61 magnesium alloy sheets were lap-joined by friction stir spot weld (FSSW) under different welding conditions. Tool rotational speed, plunge rate, and dwell time were varied to determine their effects on the microstructure, fracture behavior, and mechanical properties of welds. Metallographic analytical results indicate that two stir zones formed in regions near the upper side and lower side of the keyhole, separately. Dynamic recrystallization occurred during the FSSW process, generating equiaxed grains in the weld region. Experiment results demonstrate that the strength of friction stir spot welds increased as bond width increased, which depended on tool speed and dwell time. An optimal combination of tool speed and dwell time can maximize the strength of joints. Under the tensile shear test, cracking started at fissure tip and extended through the partial bonding interface to the keyhole. Moreover, fracture behavior in the tensile shear test was governed mainly by bond width.

► Extruded AZ61 Mg alloy sheets were lap-joined by FSSW under different welding conditions.
► Failure behavior was governed by bond width and material strength.
► Grain growth was not the most important factor for FSSW failure.
► FSSW strength decreased, as dwell time exceeded a value of 6 s.