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，以发现如何将激光重熔应用于增材制造的金属部件，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年发布的几项技术路线图所推动的：(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；(2)概述了研究提高增材制造零件表面光滑度的关键需求。增材制造零件的表面通常非常粗糙，具有与过程砂纸相似的外观。这主要是由于部分融化的粉末附着在表面造成的。这种部分粉末熔化也会导致显著的近表面孔隙。粗糙的表面和近表面的孔隙可能成为裂纹的来源，从而导致零件失效。此外，这些粗糙的表面可能导致增材制造的零件不能满足设计规范。激光重熔使表面光滑，同时减少了零件表面附近的气孔数量。这意味着可以产生裂缝的来源更少，因此它应该更坚固，持续时间更长。从这个项目中产生的知识将通过会议演讲、期刊文章、开放的工程展览以及通过整合到本科和研究生课程中公开分发。该项目是威斯康星大学麦迪逊分校和德国不来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的研究提供资金；不莱梅大学的经费是由德国研究协会（DFG）提供的。这种合作将使pi能够利用美国无法获得的研究设施来促进美国的创新，特别是在航空航天，汽车，医疗保健和国防工业方面。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重大的有形利益。国际合作还为美国研究生提供了一个独特的机会，让他们了解德国和美国最高水平的制造业研究。本研究的目标是通过激光重熔的应用，产生新的基础知识，以改善增材制造金属零件的表面光洁度和机械性能。激光重熔工艺为增材制造零件的平滑和功能改进提供了重要的、尚未开发的机会，并且可以集成到现有的基于激光的增材制造设备中，以提高成品表面光洁度。这项工作的科学贡献可以概括为三个部分：(1)提供对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)通过适当的模型，了解这些表面的激光重熔背后的物理现象，以平滑和减少孔隙率；(3)了解与成品相比，平滑和致密化对表面性能的影响（例如疲劳寿命）。对于给定的一组激光重熔参数，将研究部分熔化粉末的粘附程度（即与表面的物理和热接触）如何影响其融入表面的关键科学理解；以及如何通过激光参数变化诱导的马兰戈尼流重新分布（即创造更光滑的表面）由未结合粉末的个体和凝聚引起的局部质量浓度。该项目的一个重要目标是测量激光重熔区域的局部波幅，并在二次激光通道（即脉冲激光结构）中调制激光加工参数，以诱导抑制流，以克服激光加工参数不变的重熔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.