Effect of heat treatment on microstructure evolution and mechanical properties of selective laser melted Mg-11Gd-2Zn-0.4Zr alloy

Abstract

The Mg-11Gd-2Zn-0.4Zr (GZ112K, wt.%) alloys fabricated by selective laser melting (SLM) still contain some hard and brittle β-(Mg,Zn)3Gd eutectic phase with an area fraction of 4.86% despite the ultra-fast cooling rate of SLM process, so post heat treatment is necessary. The effects of different solution conditions and aging heat treatment on microstructure and mechanical properties of the SLMed GZ112K alloys were systematically analyzed. The results show that the optimized solution condition (400 °C × 12 h) for the SLMed GZ112K alloys can not only transform hard and brittle β-(Mg,Zn)3Gd eutectic phase into relatively softer lamellar 14H type long period stacking order (LPSO) structure leading to the improved elongation (EL) of the SLM-T4 alloys, but also retain the fine grains of the SLMed GZ112K alloys to a large extent with a small average grain size of 3.1 μm in the SLM-T4 state contributing to the high yield strength (YS) of the SLM-T4 and SLM-T6 GZ112K alloys. After peak aging heat treatment, basal γ′ precipitates or lamellar 14H-LPSO structure and dense prismatic elliptical β′ precipitates coexist in the SLM-T6 GZ112K alloys. As a result, the SLM-T6 GZ112K alloys exhibit YS of 343 ± 9 MPa, ultimate tensile strength (UTS) of 371 ± 4 MPa and EL of 4.0 ± 0.05%, while those of the SLMed GZ112K alloys are 332 ± 3 MPa, 351 ± 5 MPa and 8.6 ± 1.0%. The ultra-high YS of SLM-T6 GZ112K alloys results from the fine grain strengthening, the precipitation strengthening from β′ aging precipitates, the secondary phase strengthening from LPSO structure and X phase and the extra composite strengthening from the coexistence of basal and prismatic precipitates. The findings indicate that appropriate post heat treatment is an effective scheme for modifying microstructure and mechanical properties of the SLMed alloys.