In the laser powder bed fusion additive manufacturing of metals, extreme thermal conditions create many highly dynamic physical phenomena, such as vaporization and recoil, Marangoni convection, and protrusion and keyhole instability. Collectively, however, the full set of phenomena is too complicated for practical applications and, in reality, the melting modes are used as a guideline for printing. With an increasing local material temperature beyond the boiling point, the mode can change from conduction to keyhole. These mode designations ignore laser-matter interaction details but in many cases are adequate to determine the approximate microstructures, and hence the properties of the build. To date no consistent, common, and coherent definitions have been agreed upon because of historic limitations in melt pool and vapor depression morphology measurements. In this review, process-based definitions of different melting modes are distinguished from those based on postmortem evidence. The latter are derived mainly from the transverse cross sections of the fusion zone, whereas the former come directly from time-resolved x-ray imaging of melt pool and vapor depression morphologies. These process-based definitions are more strict and physically sound, and they offer new guidelines for laser additive manufacturing practices and create new research directions. The significance of the keyhole, which substantially enhances the laser energy absorption by the melt pool, is highlighted. Recent studies strongly suggest that stable-keyhole laser melting enables efficient, sustainable, and robust additive manufacturing. The realization of this scenario demands the development of multiphysics models, signal translations from morphology to other feasible signals, and in-process metrology across platforms and scales.

中文翻译：

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金属粉末床熔融增材制造中的激光熔化模式

在金属的激光粉末床熔融增材制造中，极端的热条件会产生许多高度动态的物理现象，例如汽化和反冲、马兰戈尼对流以及突出和锁孔不稳定性。然而，总的来说，全套现象对于实际应用来说过于复杂，并且实际上，熔化模式被用作打印指南。随着局部材料温度升高超过沸点，模式可以从传导模式变为小孔模式。这些模式名称忽略了激光与物质相互作用的细节，但在许多情况下足以确定近似的微观结构，从而确定构建的属性。迄今为止，由于熔池和蒸气凹陷形貌测量的历史局限性，尚未就一致、通用和连贯的定义达成一致。在这篇综述中，不同熔化模式的基于过程的定义与基于事后证据的定义有所不同。后者主要来自熔合区的横截面，而前者直接来自熔池和蒸气凹陷形态的时间分辨X射线成像。这些基于过程的定义更加严格且物理上合理，它们为激光增材制造实践提供了新的指导方针并创造了新的研究方向。强调了小孔的重要性，它大大增强了熔池对激光能量的吸收。最近的研究强烈表明，稳定的小孔激光熔化能够实现高效、可持续和稳健的增材制造。这一场景的实现需要开发多物理场模型、从形态到其他可行信号的信号转换，以及跨平台和尺度的过程计量。