Effect of Annealing on Anisotropic Tensile Properties of Al-12%Si Alloy Fabricated by Laser Powder Bed Fusion

In this study, we systematically investigated microstructures and tensile properties of an Al-12mass%Si alloy additive-manufactured by laser powder bed fusion (LPBF) process and subsequently annealed at various temperatures. Microstructure of the as-fabricated sample was characterized by a number of melt pools consisting of alpha-Al phases surrounded by Si eutectic phases. Fine Si precipitates were observed in the alpha-Al phase in the sample annealed at 200 degrees C. The eutectic Si phase appears to agglutinate, resulting in a coarsened Si phase formed at high temperatures above 300 degrees C. The initial cellular microstructure completely disappears and a number of coarsened Si phases and plate-shaped intermetallic phases (beta-AlFeSi) were formed in the sample annealed at 530 degrees C. However, the grain morphology of the alpha-Al matrix slightly changed after the annealing at high temperatures. The as-fabricated specimen showed a high strength above 400 MPa and a low ductility of below 10% in total elongation. The tensile ductility varied depending on the tensile direction. The annealed specimens exhibited a lower tensile strength and larger elongation, whereas the direction dependence of the tensile properties was less pronounced in the specimens annealed at higher temperatures. The anisotropic tensile ductility can be rationalized by preferential fractures occurred around melt pool boundaries.