A study on the effects of surface finishing methods on the internal surface roughness of 316L PBF-LB tubes

This paper addresses the effects of different post-processing techniques on the internal surface roughness of 316 L stainless steel tubes, obtained via the Laser Powder Bed Fusion (PBF-LB) technique. The tubes have a cylindrical or triangular section and were manufactured horizontally to accentuate the as-built surface defects on the internal overhang surfaces. Mechanical, electrochemical and mechano-electrochemical-based post-processing techniques were applied to the tubes, according to industrial standards. The internal surfaces were scanned using a 3D profilometer and the Ra and Sa parameters were assessed. The results show that, despite the simple geometry of the tubes, obtaining low surface roughness is a difficult task, mainly due to accessibility issues. Only the micromachining process based on selective roughness processing achieves a very low surface roughness, below 2 μm. Mechanical and electrochemical processes allow for reducing surface roughness by almost half, and sometimes removed larger surface defects of the overhang surface, but they are generally limited techniques when facing internal geometries. A vibratory mass finishing technique also allows for reducing surface roughness by around 40%. Finally, one of the applied mechano-electrochemical finishing processes was not compatible with the tested geometries, due to the length/diameter ratio of the tube.

This study contributes to a better understanding of the key factors that must be considered when comparing post-processing methods for additive manufacturing. It emphasizes that surface accessibility is a critical practical parameter, as many processes fail precisely because of this limitation. It also shows that traditional roughness parameters alone are not sufficient to characterize additively manufactured surfaces, since singular point- or line defects cannot be properly captured by such metrics.