Selective laser melting of aluminum nano-powder particles, a molecular dynamics study
S Kurian and R Mirzaeifar, ADDITIVE MANUFACTURING, 35, 101272 (2020).
A quasi-2D model of Micro-selective laser melting (mu-SLM) process using molecular dynamics is developed to investigate the localized melting and solidification of a randomly-distributed Aluminum nano-powder bed. One of the biggest challenges in modeling the mu-SLM process is the computational treatment of the formation and growth of crystal nuclei in the meltpool. The present work overcomes this challenge using molecular dynamics simulation because of its capability to explicitly model the nucleation and growth of grains inside the meltpool. The localized heating and rapid solidification of meltpool is simulated by the direct control of the temperature in the meltpool both spatially and temporally. The rapid solidification in the meltpool reveals the cooling rate dependent homogeneous nucleation of equiaxed grains at the center of the meltpool. Additionally, the epitaxial grain growth from the adjacent laser tracks, previous layers, and partially melted nano- powders into the solidifying meltpool is observed along the highest heat flow directions. The growth of the long columnar grains into the top layer is inhibited if the penetration depth during the remelting of a previous layer is less than the depth of the equiaxed grains. Long columnar grains that spread across three layers, equiaxed grains, nano- pores, twin boundaries, and stacking faults are observed in the final solidified nanostructure obtained after ten passes of the laser beam on three layers of Aluminum nano-powder particles. Hot isostatic pressing (HIP) of the final solidified nanostructure is employed to eliminate the nano-pores, which act as sources of crack initiation during tensile loading. Higher applied stress is required for the crack initiation and propagation in the hot isostatically pressed nanostructure compared to the as-built Aluminum nanostructure owing to the absence of nano-pores.
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