NSF-DFG: Laser Finishing of the Multi-Scale Surface Structure of Additive Manufactured Parts

该奖项支持基础科学研究，以发现激光重熔如何应用于增材制造的金属零件，以提高其最终表面质量并延长其使用寿命。这是由联邦机构在2015年和2016年发布的几个技术路线图推动的：（1）将金属部件（不仅仅是原型）的增材制造确定为确保美国制造业竞争力和国防的关键战略技术，以及（2）概述了改善增材制造零件表面光滑度的研究的迫切需要。增材制造零件的表面通常非常粗糙，外观类似于砂纸。这主要是由粘附在表面上的部分熔化的粉末引起的。这种部分粉末熔化也可导致显著的近表面孔隙率。粗糙的表面和近表面的孔隙可能会成为导致零件失效的裂纹源。此外，这些粗糙的表面可能会导致增材制造的零件无法满足设计规范。激光重熔使表面光滑，同时减少零件表面附近的孔隙数量。这意味着裂缝生长的来源更少，因此它应该更坚固、持续时间更长。从这个项目产生的知识将通过会议演示，期刊文章，开放式工程博览会，并通过整合到本科生和研究生课程中进行分发。该项目是威斯康星大学麦迪逊分校和德国不莱梅大学之间的国际合作项目。NSF只为威斯康星大学麦迪逊分校所做的工作提供资金;德国研究共同体（DFG）为不莱梅大学提供资金。 这种合作将使PI能够利用美国没有的研究设施来促进美国的创新，特别是在航空航天，汽车，医疗保健和国防行业。这代表了美国制造业的很大一部分，这项基础研究的结果可以为国内经济提供显著的实际利益。国际合作还提供了一个独特的美国研究生接触制造业研究在德国和美国的最高水平的复杂性。这项研究的目标是产生基础新知识，通过应用激光重熔来改善增材制造金属零件的表面光洁度和机械性能。激光重熔工艺为平滑和功能改进增材制造零件提供了重要但尚未探索的机会，并且它可以集成到现有的基于激光的增材制造设备中，以改善竣工表面光洁度。这项工作的科学贡献可归纳为三个部分：（1）提供对由粉末床激光增材制造产生的多尺度表面形貌的基本理解，以及这与工艺参数和原料的关系，（2）利用适当的模型，理解这些表面的激光重熔背后的物理现象，以实现平滑和孔隙率降低，以及（3）理解与完工部件相比平滑化和致密化对表面性能的影响（例如，疲劳寿命）。一个关键的科学理解将被调查如何粘附的水平部分熔化的粉末（即，与表面的物理接触和热接触）将影响其在给定的激光重熔参数下结合到表面中;以及由于未结合粉末的单独和聚集而引起的局部质量浓度如何重新分布（即，产生更光滑的表面）通过由激光参数变化引起的Marangoni流。该项目的一个重要目标是测量激光重熔区域中的局部波纹度，并在二次激光通过中调制激光加工参数（即，脉冲激光结构化），以引起抑制流动，从而克服在具有不可变激光处理参数的重熔中存在的潜在表面粗糙度最小化边界。

This award supports fundamental scientific research to discover how laser remelting can be applied to additively manufactured metal parts to improve their final surface quality and extend their useful service life. This was motivated by several technology roadmaps published by Federal Agencies in 2015 and 2016 that: (1) identified additive manufacturing of metal components (not just prototypes) as a key strategic technology for securing U.S. manufacturing competitiveness and national defense, and (2) outlined the crucial need for research on improving the surface smoothness of additively manufactured parts. The surfaces of additively manufactured parts are often very rough, with a similar appearance as a course sand paper. This is mainly caused by partially melted powder that adheres to the surface. This partial powder melting can also result in significant near-surface porosity. The rough surface and the near-surface pores can act as the source of cracks that can cause a part to fail. Furthermore, those rough surfaces can cause additively manufactured parts to fail to meet design specifications. Laser remelting smooths the surface while simultaneously reducing the number of pores near the part surface. This means that there are fewer sources from which cracks can grow, hence it should be stronger and last longer. The knowledge generated from this project will be distributed publically, through conference presentations, journal articles, open house engineering expositions, and through integration into undergraduate and graduate courses. The project is an international collaboration between the University of Wisconsin-Madison and the University of Bremen, Germany. NSF is only providing funding for the work done by the University of Wisconsin-Madison; funding for the University of Bremen is being provided by the Deutsche Forschungsgemeinschaft (DFG). This collaboration will be enable the PIs to leverage research facilities not available in the U.S. to the benefit of U.S. innovation, especially in the aerospace, automotive, health care and defense industries. This represents a very large portion of the American manufacturing sector, and the results of this fundamental research could provide significant tangible benefits to the domestic economy. The international collaboration also provides a unique exposure of an American graduate student to manufacturing research at the highest levels of sophistication in both Germany and the United States. The goal of this research is to generate fundamental new knowledge that will improve surface finish and mechanical properties of additively manufactured metallic parts through the application of laser remelting. The laser remelting process presents significant, yet unexplored opportunities for smoothing and functionally improving additively manufactured parts, and it can be integrated into existing laser-based additive manufacturing equipment to improve the as-built surface finish. The scientific contribution of this work can be summarized into three parts: (1) providing a fundamental understanding of the multi-scale surface topography created by powder-bed laser additive manufacturing and how this relates to the process parameters and feedstock, (2) understanding the physical phenomena behind laser remelting of these surfaces for smoothing and porosity reduction, with appropriate models, and (3) understanding the impact that smoothing and densification has on the performance of the surface as compared to the as-built part (e.g., fatigue life). A key scientific understanding will be investigated on how the level of adherence of a partially-melted powder (i.e., physical and thermal contact with the surface) will affect its incorporation into the surface for a given set of laser remelting parameters; and how localized concentrations of mass due to individual and agglomerations of unincorporated powder can be redistributed (i.e., create a smoother surface) through Marangoni flow that is induced through laser parameter variation. An important goal of this project is to to measure the localized waviness in a laser remelted area and modulate the laser processing parameters in a secondary laser pass (i.e., pulsed laser structuring) to induce suppressive flow to overcome potential surface roughness minimization boundaries present with remelting with non-variable laser processing parameters.