Microstructure-property relationships in a new Al Alloy designed for Laser Powder Bed Fusion

Additive manufacturing enables the fabrication of sophisticated 3D-structures with unprecedented degrees of freedom. However, most common alloys are susceptible to hot cracking. Herein, we present a novel Al-Cr-Fe-Zr aluminium alloy specifically designed for laser powder bed fusion (LPBF), with mechanical properties exceeding those of Al alloys from the 6XXX series. Emphasis is placed on the multi-scale characterisation of the as-printed microstructure and its ageing response during a post-fabrication heat treatment. EBSD maps reveal the typical bimodal grain structure of Zr-containing Al alloys processed by LB-PBF. Micron-scale equiaxed grains are found near the melt pool boundaries, while coarser columnar grains are observed in the melt pool interiors. The intermetallic population is characterised using a combination of SEM, XRD, and TEM. The nature and spatial distribution of the different intermetallic particles are investigated and differences are found depending on the position within the melt pool. Finally, atom probe tomography experiments are carried out in different regions of the melt pool to investigate the solid solution composition and its evolution upon ageing. These atomic scale measurements show that the solid solution is highly supersaturated due to solute trapping. This supersaturation of the solid solution in the as-printed state provides an opportunity to enhance the mechanical properties via direct ageing. The microstructural evolutions responsible for the increase in strength of this novel alloy during direct ageing are clarified.