Role of inclination angle on columnar-to-equiaxed transition in the eutectic Al-5Mg-2Si alloy fabricated by laser powder bed fusion

In this study, the effect of inclination angles relative to the building direction in the additively manufactured eutectic Al-5Mg-2Si alloy was investigated through the laser powder bed fusion (LPBF). The microstructures and mechanical properties of the Al-5Mg-2Si alloy manufactured with different inclination angles (0°, 30°, 45°, 60° and 90°) were reported and discussed. It is found that the “semicircular” melt pool (MP) in the load bearing face of 0° sample was eventually transformed into “stripe-like” MP in the 90° sample, accompanied by an increased fraction of melt pool boundaries (MPBs). Moreover, the microstructural analysis revealed that the columnar-to-equiaxed transition (CET) of the α-Al grains and eutectic Mg₂Si was completed in the 90° sample, which were significantly refined with the average size of 9.5 µm and 0.44 µm, respectively. It is also found that the 90° sample exhibited good combination of strength and elongation (i. e. yield strength (YS) of 393 MPa, ultimate tensile strength (UTS) of 483 MPa and elongation (El) of 8.1%). The anisotropic mechanical properties were highly associated with the refined microstructures, thermal stress, and density of MPBs. Additionally, the CET driven by inclination angles were attributed to the variation of thermal conditions inside the local MPs.