Unravelling anisotropic deformation behaviour of Ti-6Al-4V ELI fabricated by powder bed fusion additive manufacturing

Abstract

The uniaxial compression tests with respect to the different loading directions (i.e., parallel, and perpendicular to the building direction) were performed to investigate the anisotropic deformation mechanism of Ti–6Al–4 V ELI (extra low interstitial) alloy fabricated by laser powder bed fusion (L-PBF). The tests showed anisotropic mechanical properties; the sample loaded perpendicular to the building direction (TDx) has a higher fracture strain and lower yield strength, but a superior hardening effect than those of the sample loaded along the building direction (BD). Electron backscatter diffraction (EBSD) and digital image correlation (DIC) analyses were conducted used at regions of interest (ROI) in as-built and deformed samples. The kernel average misorientation (KAM) analysis revealed that a trend of dislocation evolution is strikingly different according to the loading directions; uniform KAM distribution is observed in the BD sample, whilst localization at prior β grain boundaries is found in the TDx sample. In addition, it was identified via a microscopic strain distribution map that the unique layered microstructure (i.e., layer band) and prior β grain boundary are more sensitive to compressive loading than the surrounding matrix. The layer band and prior β grain boundary were formed with their directivity during the additive manufacturing process; thus, their morphology is different depending on the loading direction. As a result, the layer band and prior β grain boundary play a key role in anisotropic behaviour, more precisely, the anisotropic deformation behaviour originates from the morphology of the layer band and β grain boundary, which is changed with respect to the loading direction.