Inhomogeneous α-Al/Mg2Si two-phase microstructures with chemical fluctuation produced by laser-beam powder bed fusion

• PBF-LB process of Al-Mg-Si alloy produced inhomogeneous α-Al/Mg 2 Si microstructure. • Laser-induced Mg vaporization caused chemical fluctuation in melt-pool structure. • Local Mg contents correlated microstructural morphology and nano-hardness. • Mg alloying achieved high-strength by solid solution and nano-precipitate hardening. This study investigated the additive manufacturing of high-strength Al–Mg–Si ternary alloy with refined α-Al/Mg 2 Si two-phase microstructures by laser-beam powder bed fusion (PBF-LB). Although PBF-LB processing produced crack-free samples with high relative densities (>99.5 %), the scanning laser irradiation caused significant Mg vaporization, reducing the Mg content of the sample from 11.3 to 7.6 %. Laser-induced vaporization caused micron-scale chemical fluctuations in the melt-pool structure, resulting in the development of inhomogeneous microstructures. Refined cellular microstructures with many columnar α-Al phases surrounded by numerous Mg 2 Si nano-particles were observed in most parts of the melt-pool structure, corresponding to hypo-eutectic compositions in the Al–Mg–Si ternary system. However, primary solidified Mg 2 Si phases were observed in some parts of the melt-pool boundaries with local Mg-rich compositions (hyper-eutectic compositions). The PBF-LB manufactured alloy specimens with a total composition of Al–7.6 Mg–5.4Si (wt%) exhibited a high tensile strength, which reduced significantly with increasing testing temperature, and low ductility (529 MPa and < 2 %, respectively) at room temperature. Moreover, the specimens underwent mechanical deterioration at elevated temperatures, owing to a significant coarsening of metastable microstructural factors (nanoscale Mg 2 Si precipitates or their related metastable phases and atomistic clusters) that contribute towards the high room-temperature strength.