In situ alloying of AlSi10Mg-5 wt% Ni through laser powder bed fusion and subsequent heat treatment

In the current study, the effect of in-situ alloying of AlSi10Mg with 5 wt% Ni (pseudoeutectic composition) through the LPBF additive manufacturing technique was investigated. Fabricated samples underwent supplementary annealing treatment at 300 °C for 15 and 120 min, eventually. During the thermal treatment, the fish-scale grains created after printing inside the melt pools after printing vanished gradually. In the as-built sample, molten Ni particles settled as big chunks of Al3Ni in the center and a thin strip of tiny Ni-rich masses in the borders of the melt pools. FIB/SEM and AFM images revealed that after 15 min annealing, silicon cellular dendrites fragment into fine Si particles, and during annealing for 120 min, the aluminum matrix expels out the supersaturated solute silicon atoms. Consequently, Si atoms leave the substrate and diffuse to pre-existing Si particles in cell walls and the triple junctions. Such a collective diffusion of Si atoms leads to the formation of coarse Si particles widespread in the Al matrix. Based on the XRD outputs, annealing for 15 min had not any major effect on Ni-rich phases. however after 120 min, a more brittle intermetallic shell of a Ni-rich phase formed on pre-exist coarse phases. The comparison of the mechanical properties of the Ni-reinforced AlSi10Mg alloy with those reinforced via the addition of other elements/compounds revealed that in a size range close to AlSi10Mg particles, Ni could not be an appropriate candidate for in-situ alloying through the LPBF method. Furthermore, spheroidization of the silicon particles and formation of fragile Ni-rich intermetallic shells having weak interfacial bonding to the Al matrix during the long-term annealing results in a significant reduction in mechanical strength of the specimens.