EAGER: Reduction of Melt Pool Balling in Metal Additive Manufacturing

EARLY概念探索性研究资助（EAGER）将探索使用合金金属粉末进行增材制造的可行性。增材制造正在改变金属零件的制造方式。 这项研究的工作重点是一种特定的增材制造工艺，其中激光束在金属粉末层上跟踪零件形状。 当激光离开时，光束下的粉末熔化并固化。 当描迹完成时，新的粉末被散布并且描迹被重复。 最终，一个坚固的部分被建造出来。 为了构建可用的部件，迹线应该是平坦的、连续的层。 这取决于激光束的功率和其行进的速度。 在高功率和高速度下，熔化的材料固化为一系列液滴，而不是连续的层。这就是所谓的球化，它会导致不完全坚固的零件无法使用。 球化限制了可用部件的制造速度，因为在高功率和高速度下，制造速度更快。 令人惊讶的是，用于增材制造的大多数金属粉末组合物是基于减材制造工艺（例如切割或铣削）中常用的那些，并且没有针对增材应用进行优化。该团队将通过向粉末中添加合金元素来改变熔融金属的行为，并通过展示一种新的、具有成本效益的方法来评估不同的合金成分来探索控制球化的可行性，从而解决这个问题。EAGER奖项的两个研究组成部分都有可能在增材制造和部件制造的粉末定制的速度和成本效益方面做出重大改进。 本研究将探讨选择性激光熔化增材制造中的球化缺陷及表面活性元素的影响。 随着表面活性元素的加入，表面能的降低预计会导致球化的减少，因为它与Plateau-Rayleigh不稳定性和润湿性差有关。由于硫降低表面张力，并强烈影响激光焊接中的熔池形状，本研究将研究硫合金化含量对不锈钢合金熔池的影响。为了克服与用于研究目的的小批量定制合金金属粉末的制造相关的过高成本，EAGER奖将评估固体测试样品的测试适用性。固体试样将通过电弧熔炼和机械加工工艺制造。最初，固体样品的本体材料组成将与商业金属添加剂粉末相同。将在一定范围的光束功率和行进速度下在这些样品上制作激光重熔材料的轨迹，重点关注功率和速度组合，其中预期过渡到球化。还将对样品进行不同粉末层厚度的测试，然后对具有不同硫含量的样品进行测试。将对熔池形状和微观结构进行表征和比较。这项工作的潜在贡献是验证了一种具有成本效益的测试方法，以评估使用固体样品进行选择性激光熔化增材制造的合金成分，并证明了通过调整合金成分来减少与工艺相关的缺陷。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This EArly-concept Grant for Exploratory Research (EAGER) grant will explore the feasibility of using alloy metal powder for additive manufacturing. Additive manufacturing is changing how metal parts are built. The work in this research focuses on a particular additive manufacturing process where a laser beam traces a part shape over a layer of metal powder. The powder under the beam melts and solidifies as the laser moves away. When the trace is finished, new powder is spread and the trace repeated. Eventually a solid part is built. To build usable parts, the traces should be flat, continuous layers. This is determined by the power of the laser beam and the speed at which it travels. At the high values of power and speed, the melted material solidifies as a series of droplets instead of a continuous layer. This is called balling and it leads to unusable parts that are not completely solid. Balling limits how fast usable parts can be built because building is faster at high power and speed. Surprisingly, most metal powder compositions for additive manufacturing are based on those commonly used in subtractive manufacturing processes (e.g. cutting or milling) and are not optimized for additive applications. The team will tackle this problem by exploring the feasibility of controlling balling by adding alloying elements to the powders that change the behavior of the molten metal, and by demonstrating a new, cost effective method to evaluate different alloy compositions. Both research components of this EAGER award have the potential to make significant improvements in the speed and cost effectiveness of powder customization for additive manufacturing and component fabrication. The research work will investigate the balling defect in selective laser melting additive manufacturing and the effect of surface active elements. The reduction in surface energy with addition of surface active elements is expected to lead to reduce balling, since it has been associated with Plateau-Rayleigh instabilities and poor wettability. As sulfur lowers surface tension and strongly influences melt pool shape in laser welding, this research will investigate the effect of sulfur alloying content on stainless steel alloy melt pools. To overcome the prohibitively high costs associated with the fabrication of small batches of customized alloy metal powders for research purposes, this EAGER award will evaluate the suitability of testing on solid test samples. Solid test samples will be fabricated by arc-melting and machining processes. Initially the bulk material composition of the solid samples will be the same as commercial metal additive powders. Tracks of laser re-melted material will be made on these samples over a range of beam power and travel speeds with a focus on power and speed combinations where the transition to balling is expected. Tests will also be conducted with different powder layer thicknesses on the samples, and then on samples with varying sulfur content. Melt pool shapes and microstructures will be characterized and compared. The potential contributions of this work are validation of a cost-effective test method to evaluate alloy composition for selective laser melting additive manufacturing using solid samples, and a demonstration of process-related defect reduction through adjustments to alloy composition.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.