Rapid Solidification and Non-equilibrium Phase Constitution in Laser Powder Bed Fusion (LPBF) of AlSi10Mg Alloy: Analysis of Nano-precipitates, Eutectic Phases, and Hardness Evolution

The non-equilibrium phase evolution during laser powder bed fusion (LPBF) of AlSi10Mg alloy is thoroughly characterized and analyzed by means of advanced electron microscopy and analytical simulation of rapid solidification phenomenon. The evolution of microstructural strengthening agents such as inter-cellular eutectic phase and intra-cellular precipitates is presented in correspondence with the local variation of cellular/dendritic solidification patterns within a typical melt pool. The eutectic phase exhibits two different morphologies: lamellar and fibrous. As with the cell size variation, the overall volume fraction of eutectic phase and the lamella spacing is shown to gradually decrease by moving away from the melt-pool boundary (MPB), i.e., through crossing over from a coarse to a fine cellular zone. The eutectic-free regions within the alpha-Al cells contain a large number density of nano-sized precipitates that are predominantly Si-rich and are either fully or semi-coherent with the Al matrix. The formation of nano-precipitates is linked to the increased (non-equilibrium) solubility limits of alpha-Al cells due to the rapid solidification effect. For the first time, we identify such nano-precipitates with non-equilibrium crystal structures and morphologies: Spheres and Ellipsoids with Face Centered Cubic (FCC), and Plates and Needles with a Diamond Cubic (DC) superlattice structure that emerges from within the Al matrix. The microstructure in the heat-affected zone (HAZ) right underneath the MPB exhibits an absence of cell boundaries and eutectic phases while consisting primarily of large Si-rich and Mg-rich precipitates. Finally, the local variation of nano-hardness across a solidified melt pool is shown to correlate well with the corresponding profile of microstructural refinement, i.e., exhibiting a minimum at the HAZ and a peak at around the melt-pool centerline. The findings here can significantly advance the state of knowledge for the strengthening behavior in an as-built LPBF-processed AlSi10Mg alloy.