Effect of self-reacting friction stir welding on microstructure and mechanical properties of Mg-Al-Zn alloy joints

The 5 mm-thick Mg-6Al-1Zn alloy was self-reacting friction stir welded (SR-FSW) at various welding speeds. Sound joints were achieved with welding speed ranging from 300 to 400 mm/min. At lower welding speed, pore defect formed in the weld nugget zone (WNZ) due to the inadequate mixing of the plastic materials. The thermo-mechanically affected zone (TMAZ) consisted of fine equiaxed dynamic recrystallized grains instead of the elongated and rotated grains formed in the TMAZs of FSWed Mg alloys and SR-FSWed Al alloys. Finer grains were obtained in the WNZ, TMAZ and heat affected zone (HAZ) at higher welding speed due to the lower heat input. Both the TMAZ and WNZ have grains with a high density of dislocations due to the straining exerted by the tool. Hardness profiles show slight variation throughout the weld with the hardness values fluctuated between 50 to 68 HV. Results of tensile tests revealed that the tensile strength increased with increasing welding speed with the maximal strength efficiency reached 77%. Fracture elongations were lower than that of the base metal due to the inhomogeneous microstructure throughout the joint. The impact energy of both the WNZ and HAZ increased with increasing welding speed.