Tailoring the microstructure and mechanical properties of multi-layer and multi-pass wire and arc additive manufactured AZ91 magnesium alloy via multi-pass friction stir processing

Wire arc additive manufacturing (WAAM) is a commonly used additive manufacturing technique for producing magnesium alloy components with complex structures. However, the effect of friction stir processing (FSP) on WAAM magnesium alloy samples has not been properly examined. In this study, a bulk sample of the AZ91 magnesium alloy was prepared via multi-layer and multi-pass WAAM technology, and its microstructure was modified by FSP to improve its mechanical properties. The effect of FSP on the microstructure and mechanical properties of the WAAM sample was investigated. The obtained results revealed that the lack of fusion defects in the lapped area between the adjacent deposited passes were significantly eliminated after FSP, while recrystallization occurred in the stirred zone (SZ) and thermos-mechanically affected zone (TMAZ). Compared with that of the unprocessed sample, the grain size of the sample after FSP became more uniform, and the amount of the β-Mg17Al12 phase decreased significantly. In addition, a short lamellar β-Mg17Al12 phase precipitated within some grains in the TMAZ. The average ultimate tensile strength and elongation of the sample after FSP along the pass overlap direction reached 237.9 MPa and 21.7 %, which were more than 80 % higher than those before FSP (127.1 MPa and 11.6 %, respectively) owing to the more uniform microstructure and defect elimination. The anisotropy of the tensile properties of the WAAM sample was significantly reduced by the introduction of FSP.