Heterogeneous structure formation of Mg alloy during direct laser deposition with dissimilar alloy

Dissimilar metal deposition of Al-Si-Mg powders on Mg-Al-Zn plate promote the formation of a heterogeneous "necklace" morphology formation via direct energy deposition surface alloying with Al-Si-Mg powder structure consisting of fine grains (FGs) at the top while columnar grains (CGs) surrounding FGs at the bottom. Al-rich distribution within FGs, which are characterized by random texture as well, indicates that the Al-Si-Mg powders promote heterogeneous nucleation of the Mg matrix at the upper melt region (MR). Similar with the strong epitaxial growth in non-dissimilar-metal-deposited Mg-Al-Zn plate, the CGs dominate at the MR bottom and possess grain orientations close to those of the base material. By computational thermal-fluid dynamics simulation, a higher ratio of Al-Si-Mg at higher temperatures and a higher ratio of Mg-Al-Zn at lower temperatures together results in a relatively weak thermal convection, causing difficult mass transportation of Al-Si-Mg towards the MR bottom and thus the formation of residual bottom CGs.