Microstructures and tensile properties of submerged friction stir processed AZ91 magnesium alloy

6 mm thick AZ91 casting alloy plates were subjected to normal friction stir processing (NFSP, in air) and submerged friction stir processing (SFSP, under water), and microstructures and tensile properties of the experimental materials were investigated. After FSP, the coarse microstructures in the as-cast condition are replaced by fine and equiaxed grains and the network-like eutectic β-Mg17Al12 phases disappear and are changed into particles pinned at the grain boundaries. SFSP results in further grain refinement in comparison with NFSP, and the average grain sizes of the NFSP and SFSP alloys are 8.4 ± 1.3 and 2.8 ± 0.8 µm, respectively. XRD results reveal that the intensity of β-Mg17Al12 diffraction peaks in the SFSP specimen decreases compared with NFSP. Due to significant grain refinement, the tensile strength and elongation of the SFSP AZ91 alloy are increased from 262 MPa and 18.9% for the NFSP material to 282 MPa and 25.4%, and the tensile strength (282 MPa) is nearly three times that of the BM (105 MPa). SFSP is an effective approach to refine the grain size and enhance the tensile properties of AZ91 casting alloy.