Microstructure characterization of SLM-processed Al-Mg-Sc-Zr alloy in the heat treated and HIPed condition

Abstract

Sc- Zr-modified Al-Mg alloy, processed by selective laser melting, offers excellent properties in the as processed condition, due to the formation of a desirable microstructure. As in conventional processing, such alloys are age hardenable, thereby precipitating a high fraction of finely dispersed coherent Al3(Scx Zr1-x) intermetallics, which serve for the improvement of the mechanical strength. Electron backscatter diffraction measurements and transmission electron microscopy were used to determine the effects of heat treatment and HIP on the microstructures of SLM processed specimens. In addition, the chemistry and number density of Al3Sc particles was analysed by atom probe tomography. The results show that the bi-modal grain size distribution observed in the as-processed condition can be maintained even after a heat treatment, due to a high density of intragranular Al3(ScxZr1-x) precipitates, and various other particles pinning the grain boundaries. A HIP post-processing can lead to grain growth in certain coarser grained areas, probably due to a local imbalance between driving and dragging forces, hence higher defect density and fewer pinning precipitates. Applying a heat treatment results in an increase of the density of ≤5 nm sized intragranular Al3(Scx Zr1-x) particles by a factor of 4–6, reaching 3·1023 m−3 to 5·1023 m−3.