EAGER: Reduction of Melt Pool Balling in Metal Additive Manufacturing

这项探索性研究早期概念补助金（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。本研究的工作重点是一种特殊的增材制造工艺，其中激光束在一层金属粉末上跟踪零件形状。随着激光的移动，光束下的粉末融化并凝固。当痕迹完成后，涂抹新的粉末，重复痕迹。最终，一个坚实的部分就建成了。为了构建可用的部件，痕迹应该是平坦的，连续的层。这是由激光束的功率和传播速度决定的。在高功率和高速度下，熔化的材料凝固成一系列的液滴，而不是连续的一层。这就是所谓的球化，它导致无法使用的部件，不是完全固体。球化限制了可用部件的建造速度，因为在高功率和高速度下建造速度更快。令人惊讶的是，大多数用于增材制造的金属粉末成分都是基于减法制造过程中常用的成分（例如切割或铣削），并且没有针对增材应用进行优化。该团队将通过探索通过在粉末中添加合金元素来改变熔融金属的行为来控制球化的可行性，并通过展示一种新的、经济有效的方法来评估不同的合金成分来解决这个问题。EAGER奖项的两个研究组成部分都有潜力在增材制造和组件制造的粉末定制速度和成本效益方面取得重大进展。研究了选择性激光熔化增材制造中的起球缺陷及表面活性元素的影响。随着表面活性元素的加入，表面能的降低有望减少成球，因为它与高原-瑞利不稳定性和较差的润湿性有关。由于在激光焊接中，硫降低了表面张力，对熔池形状有很大影响，因此本研究将探讨硫合金含量对不锈钢合金熔池的影响。为了克服与制造小批量定制合金金属粉末用于研究目的相关的过高成本，该EAGER奖项将评估固体测试样品测试的适用性。固体测试样品将通过电弧熔化和机械加工工艺制造。最初，固体样品的散装材料组成将与商用金属添加剂粉末相同。激光再熔化材料的轨迹将在这些样品上在一定的光束功率和行进速度范围内进行，重点是功率和速度组合，期望过渡到球化。还将对样品进行不同粉末层厚度的测试，然后对不同硫含量的样品进行测试。将对熔池形状和显微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，用于评估使用固体样品的选择性激光熔化增材制造的合金成分，并展示了通过调整合金成分来减少与工艺相关的缺陷。该奖项反映了美国国家科学基金会的法定使命，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.