Flexural properties of Ti-6Al-4V sandwich structures with novel lattice core geometries produced by electron beam powder bed fusion
Eric Bol, M. Ramulu
Abstract
• Flexural properties of additive sandwich structures are not well known. • TPMS-type core geometries tend to resist core buckling than strut-types. • Failure behavior of a core geometry was unaffected by the thickness grading. • Stress minimization thickness grading was superior to compliance minimization. • Gyroid core achieved the highest flexure yield stress regardless of grading method. Within aerospace, metallic sandwich structures are utilized where high specific flexural strength and temperature resistance is required. However, traditional joining methods (e.g. brazing) for attaching the face sheets to a core structure limit the geometric complexity, and are plagued by void defects. Therefore, this work presents the ability of electron beam powder bed fusion (EB-PBF) technology to additively manufacture a monolithic Ti-6Al-4V sandwich structure with novel topology optimized cores. Four lattice core geometries were selected, two strut types: Octet and Kelvin, and two triply periodic minimal surface (TPMS) types: Gyroid and Primitive. Two methods for functionally grading the core thickness were applied to each based on a 4-point bend simulation, and a uniform core thickness provided a baseline. One grading method used a compliance (density-based) minimization strategy, and the other a von Mises stress minimization strategy. The flexural properties were determined by ASTM C393 4-point bend experiments, which revealed that grading of the core thicknesses delayed flexure yielding, and the TPMS-type core geometries demonstrated a higher resistance to core buckling. The stress minimization strategy provided superior performance in maximum load and modulus, and the Gyroid core geometry achieved the highest maximum load and flexure yield stress.