Hypereutectic Al-Si alloy upgraded by repetition of local melting and rapid solidification in laser powder bed fusion

Hypereutectic Al–Si alloys are widely used for their hardness and wear resistance; however, the coarse primary Si phase formed during conventional casting limits strength and ductility. Rapid solidification inherent to laser powder bed fusion (L-PBF) can upgrade a classical hypereutectic Al–20Si (mass%) alloy to achieve a superior strength–ductility balance. This study focuses on microstructural features formed under L-PBF, including those associated with the asymmetric coupled zone of the Al–Si binary system and the heterogeneous hard/soft microstructure induced by repetitive local melting. The effective eutectic composition was experimentally shown to shift from the equilibrium value of 12.6 mass% to ∼ 16.3 mass% under L-PBF, increasing the fraction of fine eutectic Si (∼30 nm, 24% area fraction). In addition, the alloy exhibited wide melt-pool boundaries (∼7 μm), which produced a characteristic hard/soft heterostructure. These microstructural modifications resulted in a tensile strength of ∼ 600 MPa and an elongation of ∼ 7.5%, comparable to high-strength aluminum alloys despite the simple hypereutectic composition. Overall, this work highlights how additive manufacturing can access non-equilibrium solidification pathways and upgrade classical casting alloys without compositional modification.