A quasi-2D model of Micro-selective laser melting (μ-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 μ-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.

建立了基于分子动力学的微选择激光熔化（μ-SLM）过程的准二维模型，以研究随机分布的铝纳米粉末床的局部熔化和凝固。对μ-SLM过程建模的最大挑战之一是对熔池中晶核形成和生长的计算处理。当前的工作克服了使用分子动力学模拟的挑战，因为它能够显式地模拟熔池内部晶粒的成核和生长。通过直接控制熔池中的温度在空间和时间上模拟熔池的局部加热和快速凝固。熔池中的快速凝固表明，冷却速率取决于熔池中心等轴晶粒的均匀形核。另外，沿着最高的热流方向观察到了从相邻的激光径迹，先前的层以及部分熔化的纳米粉末到固化熔池的外延晶粒生长。如果前一层重熔过程中的渗透深度小于等轴晶粒的深度，则将阻止长柱状晶粒向顶层的生长。在三层铝纳米粉末颗粒上经过十次激光束照射后，在最终凝固的纳米结构中观察到了长柱状晶粒，这些晶粒遍布三层，等轴晶粒，纳米孔，孪晶边界和堆积缺陷。最终凝固的纳米结构的热等静压（HIP）用于消除纳米孔，该纳米孔在拉伸载荷过程中充当裂纹引发的来源。