Effect of Altering the Sequence of Chempolishing and Electropolishing on Surface Properties of Additively Manufactured (AM) 316 Steel Components

Abstract

Abstract The surface roughness of as produced additively manufactured (AM) components is very high and may lead to component failure and undesirable coefficients of friction. In rough surfaces, small cracks form at regions of high surface roughness acting as a stress raiser or crack nucleation sites. Likewise, rough surfaces impact both static and kinetic friction that can impede desired motion and oppose desired mechanical forces. For using these components in many applications, it is necessary to reduce surface deviations drastically during postprocessing. For parts with complex geometries and enormous internal surface areas, this reduction presents a complex engineering problem. We have explored chempolishing (C) and electropolishing (E) to reduce the external and internal surface roughness of stainless-steel components in our previous studies. Chempolishing is an electroless etching process that can uniformly smoothen the accessible surfaces of complex AM components. Electropolishing can produce an extremely smooth surface to sub-micrometer level roughness. Our prior work showed that chempolishing and electropolishing produced very distinct surface microstructures. It is quite possible that in future surface finishing, chempolishing and electropolishing may be applied on the same AM component to reduce the surface roughness of complex AM components. The resulting microstructure after the sequential application of chempolishing and electropolishing may be quite different as compared to that of after chempolishing or electropolishing alone. Here, we report the application of altering the sequence of chempolishing and electropolishing to reduce the external and internal surface roughness of 316 steel components. It is unknown what will be the impact of manipulating the sequence of electropolishing and chemical polishing on surface roughness and microstructure of AM materials. This paper focuses on the post-process sequencing of chempolishing, followed by electropolishing (CE) and vice versa (EC). We found chempolishing followed by electropolishing reduced internal surface roughness by as much as 12 micrometers. Whereas the electropolishing followed by chempolishing reduced external surface roughness by an average of ∼15 micrometers. The structure and properties of the surface finished pieces were examined using: Scanning Electron Microscopy (SEM), Surface Profilometry, and Water Contact Angle Measurement. SEM provided direct insight that CE and EC process produced significantly different microstructures from each other and also from chempolished and electropolished processes. Water contact angle measurements performed on CE, and EC treated AM samples showed that surface energy was quite different. Hence, CE and EC are expected to perform quite differently under a corrosive environment and also yield various adhesion quality for the protective coatings. Confirmation of structural changes provided in this experiment shed light on the capabilities of postprocessing improvements we can make to materials performance.