Microstructure evolution and mechanical properties of a high-strength Mg-10Gd-3Y–1Zn-0.4Zr alloy fabricated by laser powder bed fusion

Abstract

A high-strength Mg-10Gd-3Y–1Zn-0.4Zr (GWZ1031K, wt%) alloy was prepared by laser powder bed fusion (LPBF), and the microstructure and mechanical properties of the as built, LPBF-T5, LPBF-T4, and LPBF-T6 states were systematically studied. The as built alloy is composed of fine equiaxed ɑ-Mg grains with an average grain size of 4.1 ± 0.5 µm, reticular β-(Mg,Zn)3(Gd,Y) eutectic phase and flaky Y2O3 oxide phase, and exhibits yield strength (YS) of 310 ± 8 MPa, ultimate tensile strength (UTS) of 347 ± 6 MPa and elongation (EL) of 4.1 ± 0.8%. A simple direct aging heat treatment at 175 ℃ for 64 h after LPBF leads to an ultra-high YS of 365 ± 12 MPa but a low EL of 0.8 ± 0.3% in the LPBF-T5 alloy. The solution heat treatment improves ductility by transforming the hard and brittle eutectic phase into the relatively soft and deformable lamellar long period stacking ordered (LPSO) structure inside grains and X phase at grain boundaries without obvious grain growth. Moreover, the LPBF-T4 alloy solution-treated at 450 ℃ for 12 h exhibits YS of 255 ± 8 MPa, UTS of 328 ± 9 MPa, and EL of 10.3 ± 0.5%. Aging heat treatment after solution introduces numerous prismatic β′ and β1 precipitates, which help to increase tensile strength. The YS, UTS, and EL of the LPBF-T6 alloy are 316 ± 5 MPa, 400 ± 7 MPa, and 2.2 ± 0.3%, respectively. It can be concluded that the LPBF process when combined with specially designed post-processing holds great promise for the manufacturing of high-performance components of the Mg-rare earth alloys with significantly higher YS for various applications.