The physics of alloy solidification during additive manufacturing (AM) in methods such as laser powder bed fusion (LPBF), electron beam powder bed fusion (EPBF), and laser directed energy deposition (LDED) is distinct due to the combination of (a) the rapid solidification conditions that often prevail in AM, (b) adjoining scan tracks that result in the overlap of the adjacent melt pools, and (c) layer-wise fabrication that causes the pre-deposited layer to influence the subsequent layer’s microstructural evolution. The complex interplay between these and each alloy’s distinct solidification characteristics results in a wide spectrum of hierarchical microstructures that span multiple length scales, with diverse grain morphologies and non-equilibrium phases. Consequently, a detailed understanding of the solidification phenomena that occur during LPBF, EPBF, and LDED is necessary for controlling the microstructural evolution, which ensures repeatable and predictable mechanical response of the built part and, hence, structural reliability of it in service. Keeping this in view, substantial efforts have been made to develop a detailed understanding of the solidification during AM of alloys, which are summarised in this review. From the local interfacial equilibrium applicable to a range of rapid solidification conditions to non-equilibrium conditions that prevail during ultra-fast solidification are reviewed. Numerical efforts ranging from the atomic scale to the macro-scale have been reviewed to highlight the phenomena of dislocation evolution, grain growth, and phase formation during solidification. Specific challenges, such as solidification cracking in non-weldable alloys and porosity-cracking dilemmas, are discussed. Unique opportunities for tailoring microstructures, such as in-situ alloying, are presented.

中文翻译：

---

金属增材制造中的凝固：挑战、解决方案和机遇


在激光粉末床熔融 （LPBF）、电子束粉末床熔融 （EPBF） 和激光定向能量沉积 （LDED） 等方法中，增材制造 （AM） 过程中合金凝固的物理特性是不同的，因为 （a） 增材制造中经常存在的快速凝固条件，（b） 导致相邻熔池重叠的相邻扫描轨迹， （c） 导致预沉积层影响后续层微观结构演变的逐层制造。这些与每种合金独特的凝固特性之间的复杂相互作用导致了跨越多个长度尺度的广泛分层微观结构，具有不同的晶粒形态和非平衡相。因此，详细了解 LPBF、EPBF 和 LDED 过程中发生的凝固现象对于控制微观结构演变是必要的，这确保了构建部件的可重复和可预测的机械响应，从而确保其在使用中的结构可靠性。考虑到这一点，已经做出了大量努力来详细了解合金增材制造过程中的凝固，这些内容在本综述中进行了总结。从适用于一系列快速凝固条件的局部界面平衡到超快速凝固期间普遍存在的非平衡条件，本文综述了这些条件。已经回顾了从原子尺度到宏尺度的数值工作，以突出凝固过程中位错演变、晶粒生长和相形成的现象。讨论了具体挑战，例如不可焊接合金的凝固开裂和孔隙开裂困境。 提出了定制微观结构的独特机会，例如原位合金化。