PRC Surface Finishing of Additively Manufactured Impellers

Abstract

Powder bed methods of additive manufacturing (AM) use either an electron beam (Electron Beam Melting – EBM) to sequentially melt powder, layer by layer, to build up a 3-dimensional object directly from a powder bed according to a computer aided design file. Complexity is free with AM processes, so parts such as the impeller are natural candidates for AM.With EBM. there are four steps to build each layer in a build. First, the beam is scanned to preheat the powder bed without melting, second, the contours (outer edge) of the area to be melted is traced and powder melted, third, the hatch (area within the contours) is melted and finally any supports that need to be built to support higher layers of the part being built are added. The side face of a part adjacent to powder after an EBM build.The major concerns for incorporation of AM into structural parts are (1) microstructural anisotropy/inhomogeneity, (2) porosity – open near the surface and closed internally and (3) surface finish that is strongly dependent on the orientation of the surface relative to the build direction. Improving the surface finish is critical for certain applications. While finish machining can be used where surfaces are accessible, one benefit of the complexity of part shape is the possibility of building parts with internal channels, inaccessible to machining.Therefore, this paper will discuss recent developments toward the demonstration of a process to electrochemically finish the internal passages of grade 5 titanium shrouded impellers produced by additive manufacturing. Specifically, we will discuss developments toward achieving optimal electrochemical processing conditions, surface finish, nominal tooling, and performance through structural spin test evaluations. Unlike conventional surface finishing processes that either use media or machine tools to physically modify the surface, electrochemistry can be used as a non-contact flexible approach, to finish hard to reach surfaces such as the internal channels of shrouded impellers that maybe inaccessible to conventional processes. This paper will focus on pulse/pulse reverse electrofinishing processes developed by Faraday Technology and the technique to scale this approach to functional parts of interest. Acknowledgements: The financial support of USAF Contract No. FA8814-15-C-0007 is acknowledged.