Multi-pass submerged friction stir processing of AZ61 magnesium alloy: strengthening mechanisms and fracture behavior

Multi-pass submerged friction stir processing (M-SFSP) with an overlapping ratio of 50% was applied on as-cast AZ61 magnesium plate. The relationship between the microstructure and tensile behavior of the M-SFSP plate was investigated. With the cooling enhancement during processing, the grain size is significantly refined to be ~ 3.7 ± 1.7 μm in the stir zone. The initial network-like β-Mg17Al12 second phase in the cast alloy is effectively fragmented and dissolved into magnesium matrix, as revealed by a consistent right shift of magnesium peaks in the XRD patterns due to the lattice shortening caused by the smaller aluminum atoms getting into magnesium matrix to form substitutional solid solution. Texture is modified with a lower intensity in the center of stir zone than at the top surface. Compared with the base material, the strength and ductility of the M-SFSP Mg plate are significantly improved, mainly due to grain refinement along with dislocation strengthening and solid solution strengthening. Despite texture weakening, the M-SFSP plate still exhibits mechanical anisotropy to a certain extent, depending on the initial texture and loading direction. Cracking is observed to initiate from the transitional zone and propagate along the microstructural band due to the presence of high local residual stresses.