Developing Ultrafine Twinned Microstructure Enabled Excellent Strength–Ductility Synergy in Mg–Al–Zn Alloy by Submerged Friction Stir Processing

Traditional multi-pass friction stir processing (FSP) can homogenize the microstructure and enhance the mechanical properties of magnesium alloy, but the thermal cycle between adjacent passes will lead to a large heat-affected zone (HAZ), which will cause the inability to create large high-performance areas. In this paper, magnesium alloy with ultrafine grains was correctly prepared by multi-pass submerged FSP (SFSP), and a large number of fine tensile twins were introduced into the stirring zone (SZ). The microstructure and properties of the SZ under different processing conditions were characterized by scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. The effects of microstructure and substructure on mechanical properties were systematically studied. The results show that the thermal cycle was seriously weakened and the HAZ was significantly reduced compared with air-cooling FSP (AFSP). In addition, SFSP led to more uniform distribution of second-phase particles and introduced high-density dislocations, while the dislocation density decreased after AFSP. Although the high dislocation density tended to lead to plasticity loss, this unique microstructure allows the material to achieve satisfactory mechanical properties.