Direct ageing of LPBF Al-1Fe-1Zr for high conductivity and mechanical performance

Abstract

The novel Al-1Fe-1Zr alloy leverages the non-equilibrium solidification conditions characteristic of the laser powder bed fusion (LPBF) process, namely a high thermal gradient (G) and solidification front velocity (R). The multiscale characterization of its microstructure in as-built and peak-aged conditions by SEM, TEM with EDS and automated crystal orientation mapping (ACOM), brings new insights on alloy design for LPBF. The solidification conditions permit the solute supersaturation of Fe and Zr, with the latter precipitating as L12-Al3Zr nanoparticles following ageing, bringing a high strengthening contribution. In conventional Al alloy ingots, Fe tends to create coarse particles which are detrimental to ductility. Here the Fe particles show a fine globular morphology in the as-built condition, and fine faceted Fe-rich intermetallics are obtained upon ageing. Nanoindentation and in situ tensile tests in the SEM coupled with microscale strain measurements shed light on the relation between microstructural and strain heterogeneities in the as-built condition. Our characterization results for this new Al-1Fe-1Zr grade are synthesized into a simple model for its solidification upon LPBF. In particular, the origin of melt pool boundaries is revealed. These are associated with Fe depletion caused by an early planar growth front, and the evolution of these zones upon ageing is presented. With a high potential for industrial applications, the novel alloy with the two elements Zr and Fe exhibiting low solubility at equilibrium and diffusivity in the Al matrix permit to achieve high conductivity (27 MS/m) and yield strength (330MPa) after a simple direct ageing step at 400°C/4h.