The effect of reheating layers in Metal Additive Manufacturing on the external surface finish of a printed part

Abstract

Additive Manufacturing (AM) is expanding out of the Rapid Prototyping space to an end user product manufacturing technology. The commercial interest in industry, has created research opportunities to study how quality assurance can be provided for AM end user part. There are several concerns lacking solutions at this stage, one of which is the surface finish quality on a metal AM part. The unmodified printed part extracted from an AM metal printer, often has a rough finish on the exterior surface. When the support material is removed from the part, a rough texture from the prongs bonding the support layers and part layers often leaves an even rougher texture in associated areas. For this reason, AM parts require further maintenance, often media polishing, to achieve the parts required surface finish. In cases where structural and mechanical quality is required, the surface finish can have a detrimental impact. While there are several beneficial methods employed in Subtractive, Formative and Joining manufacturing processes to improve metal surface finish, the method of interest in this study is laser reheating. More specifically, the scope of this paper studies effect laser reheating has on the surface finish of AM prints. A review of similar processes for other AM metals is studied to determine testing parameters of interest. The experimental work performed focuses on testing specimens printed in a Direct Metal Printer (DMP), applied to Stainless Steel 17-4 PH powdered material. The aim of this experiment is to determine, through stronger bonding and further melting, whether a smoother and more polished surface can be achieved. The results of the experiment performed showed high laser power and scan speed result in a better polish with each repetition, but increased reheating on just one layer caused an inward collapse. Several quality inspection techniques are compared to determine which proves the most fruitful in a studying the surface finish of the samples. Testing methods utilized, include visual inspection techniques (human eye and Scanning Electron Microscopy (SEM)), mechanical tests (compression and micro-hardness testing) and NDE roughness profiling techniques (dye-penetrant, AFM and Surface Profilometer testing).