Laser polishing for improving fatigue performance of additive manufactured Ti-6Al-4V parts

Additive manufacturing of metallic materials is rapidly growing due to the possibility of constructing customized products with complex geometries. The mechanical properties of additively manufactured parts often show inconsistent performance when compared against their wrought counterparts. The fatigue performance is often severely undermined by the presence of process-induced defects and in particular surface roughness, hence requiring a post-processing method to treat the surface of additively manufactured metallic parts. In this study, laser polishing is presented as a fast, efficient, and precise approach for post-processing of additively manufactured parts. The alpha-beta titanium alloy (Ti-6Al-4V) specimens were fabricated by laser beam powder bed fusion (LB-PBF) method and subsequently surface-treated by a continuous wave fiber laser in a controlled environment. A comprehensive study was performed to determine the optimum process parameters of laser polishing. Results indicated that the high cycle fatigue lives of the laser polished parts were somewhat longer than the as-built specimens due to reduced surface roughness. Inevitable residual stresses were introduced by laser polishing; however, it was successfully relaxed by a secondary stress relief process. Moreover, the laser polished and secondary stress relieved specimens had improved fatigue strengths at all life regimes.