Mechanical Anisotropy Investigated in the Complex SLM-Processed Sc- and Zr-Modified Al-Mg Alloy Microstructure

Additive manufactured Sc-modified aluminum alloys hold great promise for aerospace components under high and/or complex loads. While the thermal patterns formed in the melt pool during selective laser melting (SLM) of the powdered material result in a complex microstructure featuring a bimodal grain size distribution, such alloys show an isotropic mechanical response during mm-scale testing. To understand the role of microstructure on the mechanical response, nanoindentation is performed on SLM-processed Scalmalloy((R)) for both the untreated and precipitate-hardened alloy, studied both parallel and orthogonal to the build direction. The elastic modulus of the coarse and fine grained regions is significantly greater than the macro-scale modulus of the material, suggesting macroscopic control over the elastic performance. With negligible influence over the elastic properties, there is a significant correlation between the grain size distribution and the nanoindentation hardness. This weak but significant grain boundary strengthening is lost when applying load orthogonal to the build direction for the precipitate-hardened material. Hardness anisotropy is moreover observed in fine-grained regions for the as-processed sample. In contrast to the isotropy reported from larger-scale mechanical tests, the discrete microstructural zones deviate from ideal isotropic behavior, and this may have consequences for simulation and design of this alloy.