Binder jetting of aqueous polyvinyl alcohol for additive manufacturing of Inconel 718

Binder Jetting is an Additive Manufacturing technique which has been commercially available since 1996. It is characterised by the fabrication of three-dimensional components directly from a CAD model where the binder is selectively deposited on a powder bed repeatedly till the component is completely ‘printed’. Commercial Binder Jetting systems use binders which are proprietary, typically resins that are difficult to handle and store due to safety and environmental concerns. This increases the costs of the manufacturing processes involving Binder Jetting and decreases its appeal to the manufacturing industry.
Binder Jetting of Inconel 718 has not been studied thoroughly till date. This thesis explores the feasibility of using aqueous solution of polyvinyl alcohol as liquid binders, for the first time, for Binder Jetting of Inconel 718, a precipitation strengthened nickel super alloy. Liquid binder was made using deionised water and polyvinyl alcohol of different molecular weights. The metal powder was mixed with dry polyvinyl alcohol powder to make a modified feedstock as a novel approach. Printability was established by printing small cubical samples, and the process was then scaled up by printing tensile specimens. The freshly printed green parts were cured and sintered in different gas purged atmosphere to investigate the role of sintering environments on product quality, focusing on porosity and mechanical strength. Hot isostatic pressing was performed on some of the samples to study the effect of post-processing on the sintered parts.
It was inferred that the polyvinyl alcohol, having a molecular weight of 26,000 g/mol was the most suitable candidate as the combination of the liquid binder and modified feedstock, yielded the highest green strength, lowest porosity. Tensile strength and hardness of all the sintered samples were found to be low, but the specimens sintered in an argon environment improved their tensile strength by a factor of 4.26 after post-processing. Phase analysis of the parts indicated the absence of deleterious phases like σ, δ and Laves phases in the specimens.