Increasing the productivity of laser powder bed fusion: Influence of the hull-bulk strategy on part quality, microstructure and mechanical performance of Ti-6Al-4V

To increase the productivity of Laser Powder Bed Fusion (LPBF), a hull-bulk strategy can be implemented. This approach consists in using a high layer thickness in the core of the part, hence reducing the build time, and a low layer thickness in the skin, to maintain a high accuracy and good surface finish. The present study investigated to what extent this strategy affected the surface roughness, relative density, microstructure and mechanical properties of Ti-6Al-4 V parts. Ti-6Al-4 V specimens were built using two distinct sets of process parameters, one optimized for a 90 mu m-layer thickness in the bulk and the other for a 30 mu m-layer thickness in the hull. In addition to surface roughness and relative density measurements, a thorough microstructure analysis was done using both optical microscopy and SEM. Additionally, EBSD measurements and numerical reconstruction of the parent beta grains were performed to evaluate the mesostructure and texture evolution from hull to bulk. Microhardness measurements and tensile tests were done to assess the effect of the hull-bulk strategy on the mechanical properties. This analysis was completed on both as-built and stress-relieved specimens. The present study demonstrated the possibility of using the hull-bulk strategy to build high-quality Ti-6Al-4 V parts, without impacting their tensile properties, hence increasing the productivity of the process by a geometry-dependent factor, typically ranging between 25 % and 100 %.