Effects of rotation rate on microstructure and mechanical properties of friction stir-welded Mg-5Al-1Sn magnesium alloy

Mg-Al-Sn magnesium alloys have bright future in application because of good properties without rare earth elements. To study the effects of rotation rate on microstructures and mechanical properties of Mg-5Al-1Sn alloy after friction stir welding (FSW), extruded plates were butt welded by FSW with various rotation rates at constant other parameters. The results showed that after FSW, Mg17Al12 phase with poor thermal stability was dissolved, while Mg2Sn phase remained due to high dissolution point and good thermal stability. The hole-type defects were observed at 600 rpm, and the weld joint was with no defects at 800 similar to 1100 rpm. The weld joint at 800 rpm gave a maximum ultimate tensile strength (UTS), which was 91% of the base material (BM). After FSW, both UTS and elongation of weld joints decreased compared with the BM, caused by the soften region between nugget zone (NZ) and thermo-mechanically affected zone (TMAZ), the dissolution of Mg17Al12 phases, the residual stress and dislocation content in the TMAZ, and the textural variation. With increasing rotation rate from 600 to 1100 rpm, the UTS of weld joints first increased and then decreased, while the elongation kept mostly unchangeable due to the common action of multifactors.