通过透射X射线成像和离散元模拟对薄金属粉末层进行定量分析：基于叶片的散布方法
Quantitative analysis of thin metal powder layers via transmission X-ray imaging and discrete element simulation: Blade-based spreading approaches
摘要：
铺展均匀致密的层对于使用粉末床增材制造（AM）制造高质量部件至关重要。刀片状工具通常用于散布粉末金属原料，特别是在激光粉末床熔合和电子束熔化中，其中粉末的特征在于30微米或更大的D50。随着由打印部件的逐层几何形状和表面形貌引入的边界条件的变化，铺展工具和粉末之间的随机相互作用导致层质量的空间变化，这仍然没有得到很好的理解。在这里，为了研究在代表粉末床AM的条件下的粉末铺展，我们采用模块化的机械化装置，用选择的刀片状铺展工具从中等和高度粘性的粉末中产生粉末层。粉末层有效深度的空间映射使用透射X射线成像，并通过统计方法的均匀性进行量化。我们首先比较层密度，或粉末层的有效深度，并表明，叶片几何形状与曲线轮廓导致增加材料沉积。其次，这种方法能够量化局部波动或层缺陷严重程度。例如，我们观察到，与45度刚性刀片相比，V形橡胶刀片的主要益处在于使刀片边缘能够局部偏转以消除大颗粒的条纹，同时还增加沉积。此外，我们采用自定义DEM模拟来阐明颗粒密度和表面能与伪材料方法的相反作用，其中惯性力和cohesion内聚力的平衡决定了宏观尺度的粉末流动性。对于特定的合金密度，我们发现了一个临界表面能，超过该临界表面能，当使用刀片进行粉末铺展时，层密度会大大受损。
英文摘要：
Spreading uniform and dense layers is of paramount importance to creating high-quality components using powder bed additive manufacturing (AM). Blade-like tools are often employed for spreading powder metal feedstocks, especially in laser powder bed fusion and electron beam melting, where powders are characterized by a D50 of 30 microns or greater. Along with variations in boundary conditions introduced by the layer-wise geometry and surface topography of the printed component, stochastic interactions between the spreading tool and powder result in spatial variations of layer quality that are still not well understood. Here, to study powder spreading under conditions representative of powder bed AM, we employ a modular, mechanized apparatus to create powder layers from moderately and highly cohesive powders with a selection of blade-like spreading tools. Powder layer effective depth is spatially mapped using transmission X-ray imaging, and uniformity is quantified via a statistical approach. We first compare layer density, or the effective depth of powder layer, and show that blade geometries with a curved profile lead to increased material deposition. Second, this approach enables quantification of local fluctuations, or layer defect severity. For example, we observe that the primary benefit of a V-shaped rubber blade, as compared to a 45 degree rigid blade, lies in enabling local deflection of the blade edge to eliminate streaking from large particles, while also increasing deposition. Additionally, we employ a custom DEM simulation to elucidate the opposing roles of particle density and surface energy with a pseudo-material approach, where the balance of inertial and cohesive forces determine macro-scale powder flowability. For specific alloy densities, we find a critical surface energy beyond which layer density is greatly impaired when powder spreading is performed using a blade.
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