Hetero-microstructure and non-uniform properties of additive friction stir deposition for Mg-Gd-Y-Zr rare-earth magnesium alloy

In this study, utilizing the extruded feedstock of the Mg-8.94Gd-3.17Y-0.46Zr(wt.%) rare-earth magnesium alloy (GW83), fully dense deposited builds of 16–18 layers were successfully fabricated using additive friction stir deposition (AFSD) with different tool rotation speeds of 250 rpm and 300 rpm. The effects of tool rotation speed on the microstructure and mechanical properties of the deposited builds were investigated in detail. The results indicated that both 250 rpm and 300 rpm deposited builds exhibited significant mixed grain microstructure and the grain size in different regions showed the same size relationship, i.e., advancing side (AS) > middle > retreating side (RS). As the tool rotation speed increased from 250 rpm to 300 rpm, the grain size in different regions of the deposited builds increased. The deposited builds all exhibited a prominent basal plane 〈0001〉 texture and possessed the same second phases of α-Mg, Zr and (Gd, Y)H 2 phases, but the β-Mg 24 (Gd, Y) 5 phase was only found in the middle and RS regions. The tensile properties of the deposited builds for the GW83 Mg alloy exhibited significant heterogeneity. The ultimate tensile strengths (UTS) in the longitudinal direction were all obtained over 92% of the feedstock’s strength (285 MPa), while the UTS in the build direction were only ∼83% of the feedstock’s strength.