Fabrication of High Strength-Plastic Synergistic Mg-Al-Zn Alloy by Creating Ultrafine Twinned Microstructures During Multi-Pass Submerged Friction Stir Processing

Abstract

Traditional multipass 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 causes the inability to create large high-performance areas. In this paper, magnesium alloy with ultrafine grains (UFGs) was correctly prepared by multipass 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 texture 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 leads to more uniform distribution of second phase particles and introduces high-density dislocations in the grain, while the dislocation density decreases for AFSP. This distinctive microstructure led to the synergistic enhancement of tensile strength and ductility at room temperature.

Keywords: Magnesium alloy, Friction stir processing, microstructure, Mechanical properties, Electron backscatter diffraction (EBSD), Texture