Lack of fusion-induced cracking effect on tensile and fatigue behaviours of laser powder-bed fusion-processed Ti-6Al-4V implant

Abstract

In the laser powder-bed fusion (L-PBF) of Ti-6Al-4 V (Ti64) alloy, cracking defects often caused by lack-of-fusion (LoF) defects cannot be avoided. These defects strongly impact the structural stability of Ti64 implants. In this work, the influence of LoF defects on the tensile and fatigue behaviours of Ti64 implants at different energy densities is evaluated. Three primary approaches were used to characterise the LoF defects in the 10 mm gauge volume of near-net-shaped produced hourglass samples: Image-J software, scanning electron microscopy (SEM), and X-ray computed tomography (µ-CT). A low-cycle fatigue (LCF) test at a stress ratio of −1 and an extensive fractography study of the fractured Ti64 samples were used to investigate the LoF porosity-induced cracking and fatigue effect. The tensile properties of Ti64 samples processed by the L-PBF are generally more affected by microstructural changes resulting from variations in energy density than by LoF defects. A fine microstructure enhances tensile strength, while a coarser microstructure favours ductility. This is evident in samples LPBF-AD and LPBF-BD1, which demonstrate finer and coarser microstructures but show the lowest (8.31 %) and highest ductility (17.58 %), respectively. The best and worst fatigue performance samples were LPBF-AD (600 MPa) and LPBF-BD2 (200 MPa), respectively, and LoF defects significantly impactedthis finding. Moreover, it has been demonstrated that cracks originate and propagate faster from the surfacethan the internal sites. As a result, the former demonstratesa negative effect on the fatigue life of L-PBF-processed Ti64 samples under cyclic loading. The sub-surface morphologies and pore formation with different energy densities are rationalised by high-fidelity thermal-fluid dynamics simulations.