On the enhanced strength-ductility synergy of magnesium alloy fabricated by laser powder bed fusion with a static magnetic field

Owing to its inherent poor weldability and high susceptibility to evaporation, it is particularly challenging to fabricate a magnesium (Mg) alloy with superior mechanical properties using laser powder bed fusion (L-PBF). Herein, we describe an external static magnetic field (SMF) assisted L-PBF hybrid process to tailor the internal defects, microstructure, and properties of L-PBF AZ91D Mg alloy. The experimental results show that the electromagnetic braking force and the thermo-electromagnetic convection (TEMC) act together to realize the elimination of vortex flow and the establishment of ordered flow, thus enhancing the surface quality and densification of samples fabricated by this hybrid process. Besides, the thermoelectric current induced by the Seebeck effect creates a thermo-electromagnetic force, which is strong enough for grain refinement and second-phase fragmentation. Meanwhile, the stirring effect of the TEMC around the solid-liquid interface facilitates the migration of solute atoms, thus promoting the precipitation of intragranular nanoparticles. Eventually, the L-PBF AZ91D Mg alloy prepared with a SMF of 0.45 T exhibits an ultimate tensile strength (UTS) of 375 MPa and elongation (EL) up to 7.2 %, representing a 17 % and an 89 % enhancement in the UTS and EL, respectively, as compared to those of the counterparts prepared without SMF. On one hand, the risk of crack nucleation is dramatically minimized by the increased densification and dispersed eutectic-phases. On the other hand, the stable strain hardening rate by dislocation obstruction of dense nanoprecipitates and stress relief of the deformable β-Mg₁₇(Al, Zn)₁₂ intragranular nanoparticles is supposed to account for the enhanced strength-ductility synergy. Our work thus provides a novel insight into defect suppression, microstructure modulation, and performance enhancement of the external SMF assisted L-PBF Mg alloy.