Abstract: A detailed comparison of the microstructures formed during selective laser melting (SLM) of an Al-Zn-Mg alloy with and without (Sc,Zr) additions has been carried out. A key observation is that during SLM, additions of Sc and Zr not only act directly to promote grain nucleation, but also act indirectly by stimulating the formation of η' phase precipitates. Depth-dependent analysis of the microstructure in the final printed layers shows that a columnar/equiaxed-grain bimodal microstructure forms during melt-pool solidification in the alloy without (Sc,Zr) addition. However, subsequent thermal cycling from overprinted layers results in significant grain growth, leading to the development of coarse columnar grains with 〈001〉 parallel to the build direction. In contrast, for the alloy with (Sc,Zr) additions, a fine-scale bimodal grain structure forms during melt pool solidification, and moreover remains stable throughout the subsequent thermal cycling, due to the presence of η' phase precipitates at grain boundaries with a density one order of magnitude higher than in the (Sc,Zr)-free alloy. The investigation highlights the importance of understanding the interaction between solidification and in-situ annealing with regard to microstructural control during the SLM process.

Key words: Selective laser melting, 7xxx series Al alloys, Sc-modification, Microstructure, Precipitation behavior