Laser polishing of additively manufactured CoCr and TI6AL4V parts and subsequent stress relieving

Abstract

In recent years additive manufacturing (AM) has been a subject of increased interest and development as it offers many advantages over traditional subtractive machining techniques. As-built surface quality however is rarely suited to the desired application and in a majority of cases (e.g. medical implants), requires post processing.Two main finishing methods are currently used if a high level of surface finish is required: electrochemical polishing and mechanical (abrasive) polishing. The former is not selective, and the latter is hard to implement for the free-form surfaces typical of AM parts, and so instead a manual process is typically used. Laser polishing is an industrially applicable alternative as it is selective, capable of polishing free- form surfaces and can be easily automated.By scanning the laser beam across the sample surface, a localised, small volume of molten material is created. Driven by its surface tension, this molten material flows locally and resolidifies providing a smoother surface. Compared to electrochemical and mechanical polishing, laser polishing offers shorter processing times, more repeatable results, can be easily automated and is more environmentally friendly (no liquids or abrasives used). A variety of surface finish characteristics can be produced on specified areas of the target.In this paper we focus on laser based polishing of both flat and curved cobalt chrome (CoCr) and Ti6Al4V additively manufactured parts. A range of different approaches are tested including different scanning patterns, scanning speeds and spot overlaps. We demonstrate an improvement in surface roughness of >90% in all cases. We also present analysis of residual stresses following the polishing process and demonstrate that such stresses can be alleviated by subsequent heat treatment.