Development of an ultraprecision metal mirror on additively manufactured Ti-6Al-4V

Metal mirrors for precise optical applications are commonly fabricated by coating of a metal base substrate with a nickel-phosphorus alloy (NiP). The NiP layer is then processed by precision diamond turning and polishing to obtain a high-quality mirror surface. In this work, Ti-6Al-4V samples that were made by additive manufacturing, also called 3D printing, were used as a base for the development of metal mirrors. The additively manufactured samples were electroplated with a NiP coating and machined using single-point diamond turning (SPDT) to obtain a flat mirror with optical quality and low form error surface. The periodic structure of the SPDT toolmark was then removed by polishing postprocessing. Polishing optimization was first performed on NiP-coated aluminum test samples to find an optimal polishing setup. Based on this optimization, postprocessing of titanium samples was carried out by pitch polishing in combination with 1, 0.25, and 0.1µm diamond slurries. Using this polishing processing, a scratch-free surface was attained with surface microroughness below 0.5 nm.