Effect of Welding Conditions on Weld Zone Temperature and Mechanical Properties of Friction Stir Welded AZ61 Magnesium Alloy Sheet

The peak temperature range of the weld zones of the AZ61 alloy sheet during FSW varied from 450.1 to 569.4 degrees C depending on the welding conditions. The evaluation of FSW integrity suggests that both void defects and tunneling cavities observed at the conditions of 400 rpm-100 mm/min and 400 rpm-300 mm/min are caused by insufficient material fluidity because of the low temperature inside the welds. However, tunnel- and surface defects at the condition of 1600 rpm-100 mm/min are caused by a high peak temperature of 569.4 degrees C, which leads to mass loss at the outer boundary of the stir zone because of excessive flash and local melting during the rotation of the tool. To enhance the integrity and mechanical properties of the weld, the welding parameters should be controlled such that the peak temperature in the weld zones is in the range of 493.2-508.9 degrees C. The microstructure of the FSWed AZ61 joints was analyzed in terms of grain size and micro-texture by optical microscopy and Electron back-scattered diffraction. The fracture behaviors and mechanical properties of the FSWed AZ61 joints were closely related to the microstructure of the thermomechanical affected zone (TMAZ), and it was discussed based on the grain size and average Schmid factor in the TMAZ.