Laser powder bed fusion of ultrahigh strength Fe-Cu alloys using elemental powders

Abstract

Laser powder bed fusion (LPBF) was employed to produce Fe and Fe-(10–50 vol%) Cu immiscible alloys using elemental powders. The addition of 10 vol% Cu to Fe resulted in ultrahigh compressive yield and ultimate strengths of >~ 1000 and ~ 1400 MPa (more than 2 times greater than those in LPBF Fe and 4–6 times than in conventionally processed Fe), respectively, with fracture strain of ~ 20%. However, a further increase in the amount of Cu gave rise to significantly more heterogeneous microstructures with non-uniform distribution of Cu, leading to considerable liquation cracking, large voids and reduced strength and plasticity. Strengthening was primarily attributable to the dispersion of nano-Cu particles of 2–5 nm with spacings of < 10 nm and ultrafine grains (<~ 50–300 nm). The substantial grain refinement in Fe-Cu alloys during LPBF was mainly a result of the confinement of Fe crystals by Cu films, hindering their growth. By analysing microstructures in single laser tracks, single layers, top and middle sections of the LPBF Fe, it was revealed that constitutional undercooling during solidification or the subsequent transformations of γ → α were responsible for the formation of equiaxed, rather than columnar, grains in pure Fe.