The small-scale limits of electron beam melt additive manufactured Ti-6Al-4V octet-truss lattices

Abstract

The emergence of powder-based additive manufacturing (AM) processes, such as electron beam melting (EBM), enables the one step manufacture of microarchitected metamaterials from topology optimized models. However, many applications are optimized by low relative density lattices with slender trusses whose diameter approaches small multiples of largest powder particles, potentially resulting in surface roughness. The thermal history experienced by alloy powders also modifies the alloy microstructure, and thus mechanical behavior, posing a significant challenge to metallic metamaterial designs and fabrication. We therefore build and characterize the multiscale structure and mechanical properties of EBM manufactured Ti-6Al-4V octet truss lattices with strut diameters approaching the particle diameter-imposed fabrication limit. We measure the dependence of their relative density, elastic modulus, and compressive strength on the fabrication process-controlled truss topology and microstructure, and compare them to identical smooth surface structures made from an annealed, wrought version of the same alloy built using a snap-fit assembly method. Micro-x-ray tomography confirmed that the lattice strut surfaces were covered with partially melted powder particles, resulting in about 29% of the lattice mass that inefficiently supported the applied loads. The use of a powder bed held at a temperature of 600-700 °C also resulted in a lamellar α/β phase microstructure with an elastic modulus, yield strength, and a ductility that were less than the equiaxed α/β microstructure of snap-fit assembled structures. However, the higher tangent modulus of the lamellar AM processed alloy resulted in significant strengthening of EBM lattices that failed by inelastic buckling during compression. The ability to increase the alloy tangent modulus during an EBM build process therefore provides a promising approach for increasing lattice compressive strength and therefore compensates for surface roughness induced losses.