The effect of HF-HNO3 chemical polishing on the surface roughness and fatigue life of laser powder bed fusion produced Ti6Al4V

Metal additive manufacturing is increasingly utilised for the production of customised components. Post-process operations are required to address the high surface roughness of as-built components which is detrimental for load-bearing applications. Parts produced by metal additive manufacturing often have complex- surfaces and geometrical features rendering mechanical finishing processes impractical. This study investigates the chemical polishing of laser powder bed fusion (LPBF) produced Ti6Al4V axial fatigue test specimens in HF-HNO3 solutions with different initial HF concentrations (1, 2, and 4 M) and a constant initial HNO3 concentration (3.17 M). Polishing efficacy was evaluated in terms of mass removal, surface roughness reduction, and resulting tension-tension fatigue strength. The 4 M HF solution resulted in both the highest mass removal and surface roughness reduction; however, when evaluated relative to resource consumption, the processing efficiency decreases more drastically than that of the 1 and 2 M HF solutions. Specimens polished in all three solutions had higher fatigue strengths and fewer surface crack initiation sites than the as-built condition. Specimens polished in 2 M solutions exhibited better fatigue performance than specimens polished in 1 M solutions while statistical significance was lacking between the fatigue strengths of specimens polished in 2 M and 4 M solutions.