Binder jetting additive manufacturing of copper foam structures

In binder jetting additive manufacturing (BJAM), the part geometry is generated via a binding agent during printing and structural integrity is imparted during sintering at a later stage. This separation between shape generation and thermal processing allows the sintering process to be uniquely controlled and the final microstructural characteristics to be tailored. The separation between the printing and consolidation steps offers a unique opportunity to print responsive materials that are later "activated" by temperature and/or environment. This may allow a new paradigm in mull-scale, multifunctional materials. This concept is preliminarily demonstrated using a foaming copper feedstock, such that the copper is printed, sintered and then foamed via intraparticle expansion in separate steps. The integration of foaming feedstock in BJAM could allow for creation of ultra-lightweight structures that offer hierarchical porosity, graded density, and/or tailored absorption properties. This work investigates processing protocol for copper foam structures to achieve the highest porosity. The copper feedstock was prepared by distributing copper oxides through the copper matrix via mechanical milling, and that powder was then printed into a green geometry through BJAM. The printed green parts were then heat treated using different thermal cycles to investigate the porosity evolution relative to various heating conditions. The heat treated parts were then examined for their resulting properties including porosity, microstructural evolution, and volumetric shrinkage. Parts that were initially sintered in air and then annealed in a hydrogen atmosphere led to higher porosity compared to those sintered in hydrogen alone. It was also found that the annealing of parts at 600 degrees C for 2 h resulted in the highest final porosity (59%) and the lowest volumetric shrinkage of 5%. Anisotropy in linear shrinkage in X, Y, and Z direction was also observed in the heat treated parts with the largest linear shrinkage occurring in the Z direction.