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

Abstract

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-4V parts. Ti-6Al-4V specimens were built using two distinct sets of process parameters, one optimized for a 90 µm-layer thickness in the bulk and the other for a 30 µm-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 β 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-4V parts, without impacting their tensile properties, hence increasing the productivity of the process by a geometrydependent factor, typically ranging between 25% and 100%.

Dimensional Errors Due to Overhanging Features in Laser Powder Bed Fusion Parts Made of Ti-6Al-4V

“…Alternatively, simply the scanning pattern that is employed within the bulk of the part can be confidently chosen for the down-facing area. The layer thickness plays a significant role in the productivity of the process [35] and should therefore be chosen depending on the required build times and quality. The quadratic regression equation can be used to make this decision as this paper also shows the effect of the layer thickness on the dimensional error of the L-PBF part.…”

A comprehensive investigation of abrasive barrel finishing on hardness and manufacturability of laser-based powder bed fusion hollow components

“…It is well known that increasing the production rate in LB-PBF negatively affects the surface quality and mechanical properties. de Formanoir et al [14] showed that by increasing the layer thickness from 30 to 90 µm, the roughness on the vertical and horizontal surface increases from 4.32 to 15.24 µm and 6.22 to 10.13 µm respectively, which can increase the probability of failure under mechanical loading. Khorasani et al [15] proposed a model to predict the average areal surface roughness (Sa) and analysed the effect of laser power, scan speed, hatch space and scan pattern angle on the quality of the surface of LB-PBF of Ti-6Al-4 parts.…”

Development of an empirical process model for adjusted porosity in laser-based powder bed fusion of Ti-6Al-4V

“…Besides the aim to generate FGM, a related technique, the hull-bulkstrategy, is also applied to increase the productivity of the process. For this purpose, the core is typically processed with a larger layer thickness than the hull resulting in higher porosity in the core [49,50]. The mentioned studies focussed on the realization of porous structures or FGM by implementation of lattice structures or LOF defects.…”