Powder-based additive manufacturing (AM) processes such as selective laser melting have been the subject of intense interest and engineering development in recent years. However the as-manufactured parts have rough surfaces that are generally not suitable for their intended use (e.g. medical implants) or as a part of bigger assembly (e.g. in the aerospace industry). As a result post-processing techniques (often semi-manual) are employed, adding significant cost to the AM process. Laser polishing provides the prospect of a fully-automated solution, that can controllably provide different surface textures on different areas of the AM part – a requirement e.g. for dental implants where a somewhat rough surface is required for the adhesive bonding surfaces, whilst a much smoother surface is required for the surfaces that will be in contact with soft tissue.
In laser polishing the focused beam is scanned over the surface in a specific pattern, locally melting a small volume of material. Due to surface tension effects the molten material flows and resolidifies providing a smooth surface. Compared to the currently-used electrochemical and mechanical polishing techniques, laser polishing offers easily-implementable automation, shorter processing times, zero waste (e.g. no abrasives/liquids involved), and more repeatable results. However, although the process is conceptually simple, there are the complicating factors of the variation of surface tension with temperature; the roughness of the initial surface; the interfaces between scanning lines; and the metallurgy of the re-solidified material – hence our research to define a suitably robust process as described in this paper.
We present laser polishing of AM components formed of two different alloys, cobalt chrome CoCr and titanium Ti6Al4V. We report our investigation of laser scanning speed, scanning patterns and the influence of initial surface quality on both flat and cylindrical parts.
We demonstrate surface roughness improvements of 98% for CoCr and 93% for Ti6Al4V.