Laser powder bed fusion of WE43 magnesium alloy with superior balance of strength and ductility

Abstract

WE43 is a high-strength magnesium alloy containing rare-earth elements such as Y, Gd and Nd. Nevertheless, how to further obtain the balance of strength and ductility, as well as the manufacture of complex structures is still a dilemma for its engineering application. In this study, WE43 alloy samples with fine microstructures, high densification and excellent mechanical properties were successfully prepared by laser powder bed fusion (LPBF) additive manufacturing. The optimal process window was established, and the formation mechanisms of three types of porosity defects were revealed, namely lack-of-fusion pores, melt fluctuation-induced pores, and keyhole-induced pores. With the combined process of laser power of 200 W and scanning speed of 600 mm/s, samples with a high density of 99.89% were obtained. Furthermore, periodic heterogeneous microstructure was prepared along the build direction, i.e., fine grains (∼4.1 µm) at melt pool boundaries and coarse grain (∼23.6 µm) inside melt pool. This was mainly due to the preferential precipitation of Zr and Mg₃(Gd, Nd) nano-precipitates at the melt pool boundaries providing nucleation sites for the grains. This special feature could provide an extra hetero-deformation induced (HDI) strengthening and retard fracture. The optimal tensile yield strength, ultimate tensile strength and elongation at break were 276 ± 1 MPa, 292 ± 1 MPa and 6.1 ± 0.2%, respectively. The obtained tensile properties were superior to those of other magnesium alloys and those fabricated by other processes. The solid solution strengthening (∼24.5%), grain boundary strengthening (∼14.4%) and HDI strengthening (∼32.2%) were the main sources of high yield strength. This work provides a guidance on studying the pore defect suppression and strengthening mechanisms of WE43 alloy and other magnesium alloys produced by LPBF.