Blown-powder directed energy deposition (DED) additive manufacturing is impeded for novel alloys processing by perceivable and detrimental porosity. During multi-layer depositions, however, mechanisms of pore formation and evolution remain elusive for developing pore mitigation strategies. Here, conduction-mode multi-layer DED process of an exemplary high-entropy Cantor alloy have been investigated in-situ by high-energy high-speed synchrotron X-ray imaging. Three new pore formation mechanisms are unveiled when depositing first layer and successive layers: gas pore induced by high-velocity powder injection into melt pool, pore generated from swirl shear of turbulent melt flow, and pore trapped by surface wave. Three pore formation mechanisms are reconfirmed: pore inheritance from feedstock powder, pore generation when laser remelting defect-sensitive locations of existing pore from previous layer or unmelted powder attached on the melt pool surface, and pore formation as cooling of melt pool. A unique mechanism for pore elimination is proposed: a counter-Marangoni melt flow is experimentally found in the stable melt pool and contributes to the prolonged pore lifetime at tens of milliseconds scale; pores are prone to coalesce into larger sizes in laser interaction zone and the adjacent location with circulation zone; coalesced larger pores driven by combined effect of Marangoni and buoyant forces easily get eliminated from melt pool. The results of pore formation and evolution dynamics revealed in Cantor alloy provide quantified experimental data for high-fidelity computational modeling and in-depth insights of porosity control for high-entropy alloy printing down to melt pool scale.

中文翻译：

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原位同步加速器X射线成像多层定向能量沉积增材制造过程中孔隙形成和演化的动力学：以高熵康托合金为例


吹粉定向能量沉积（DED）增材制造由于可察觉的有害孔隙率而阻碍了新型合金的加工。然而，在多层沉积过程中，孔隙形成和演化的机制对于制定孔隙缓解策略仍然难以捉摸。在这里，通过高能高速同步加速器 X 射线成像对示例性高熵康托合金的传导模式多层 DED 过程进行了原位研究。在沉积第一层和后续层时，揭示了三种新的孔隙形成机制：高速粉末注入熔池引起的气孔、湍流熔体流的旋涡剪切产生的孔隙以及表面波捕获的孔隙。重新确认了三种孔隙形成机制：来自原料粉末的孔隙遗传、激光重熔前一层现有孔隙的缺陷敏感位置或熔池表面附着的未熔化粉末时产生的孔隙、以及熔池冷却时孔隙的形成。提出了一种独特的孔隙消除机制：在稳定熔池中实验发现了反马兰戈尼熔体流动，有助于在数十毫秒尺度上延长孔隙寿命；在激光相互作用区和与循环区相邻的位置，孔隙容易合并成较大尺寸；由马兰戈尼和浮力的共同作用驱动的聚结的较大孔隙很容易从熔池中消除。坎托合金中揭示的孔隙形成和演化动力学结果为高保真计算建模提供了量化的实验数据，并为高熵合金打印到熔池规模的孔隙率控制提供了深入的见解。