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

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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.