A study of parameter and post-processing effects on surface quality improvement of Binder Jet 3D-printed Invar36 alloy parts

Binder Jetting Additive Manufacturing technology, besides its high-productivity manufacturing potential, it also offers a high accuracy fabrication route for less common metal alloys or special purpose applications. In the present work, Binder Jet 3D-printed Invar36 alloy parts’ surface quality improvement is studied. Invar36 is a Fe–Ni alloy which presents near zero coefficient of thermal expansion below its curie temperature (279 °C) and it is widely used for high precision instrumentation in space environment. D90 < 22 µm grade Invar36 powder was used and printing process parameters were optimized following the Taguchi DoE methodology, to reduce sintered parts’ surface roughness. Additional sandblasting and electropolishing operations were performed and part surface roughness was reduced from 5 µm Ra to 1.5 µm Ra. Near surface closed porosity emerged and enlarged during the electropolishing process, leaving a non-homogeneous surface appearance. Consequently, a Hot Isostatic Pressing (HIP) thermal treatment was applied to reduce part overall and near-surface porosity, reaching a relative density of 99.8%. After the sandblasting and electropolishing of HIP-ed parts, surface roughness was further reduced to 1 µm Ra maintaining and homogeneous and clean surface. The results of the study showed that Binder Jet 3D-printed Invar36 parts with low surface roughness can be obtained, getting this process and material closer to future space optics developments.