Melt-pool solidification dynamics and microstructural heterogeneity dictating local mechanical response in additively manufactured AlNiFe eutectic alloy

While structural heterogeneity in additively manufactured metals is well-documented, solidification dynamics and mechanical variations at the melt pool (MP) scale remain insufficiently understood. This study investigates an Al-6.4Ni-1.4Fe eutectic alloy produced by laser powder bed fusion using single-track melting experiments, multi-physics simulations, advanced characterization, and multi-scale mechanical testing. Three processing regimes are defined by the MP depth-to-width ratio: keyhole mode susceptible to vapor-depression and bubble entrapment, stable conduction mode for defect-free parts, and balling mode caused by inadequate fusion. Optimized parameters yielded a 99.9% relative density, with a room-temperature ultimate tensile strength (UTS) of 550.6 MPa and 6.5% elongation. Spatial variations in thermal gradients and cyclic reheating partition the MP into three distinct morphological zones: dispersed Al 3 Ni and Al 9 FeNi particles at MP boundaries, lamellar eutectics in transition zones, and interconnected rod-like precipitates in MP interiors. Accordingly, nanoindentation reveals peak hardness (3.0 GPa) at fine-structured MP boundaries, whereas micropillar compression identifies a higher yield strength (860 MPa) in the MP interior where continuous eutectic pathways facilitate load-bearing. Thermal exposure at 400 °C follows two-stage coarsening kinetics, moving from rapid supersaturation depletion to diffusion-limited ripening, allowing the alloy to retain a UTS of 162 MPa at 350 °C. Rather than viewing MP heterogeneity as a flaw, this work demonstrates how solidification gradients drive localized deformation, offering a blueprint for utilizing intrinsic AM heterogeneity in heat-resistant Al alloy design.