Enhanced fracture toughness of additively manufactured Ti-6Al-4V ELI

Abstract

This study investigates the enhancement of fracture toughness in Ti-6Al-4V ELI, fabricated via laser powder bed fusion (LPBF), through a tailored cyclic heat treatment applied below the β-transus temperature to transform the martensitic microstructure into a bimodal configuration. Fracture toughness experiments were conducted using fatigue pre-cracked four-point bend specimens at room temperature, evaluating two orientations in additively manufactured (AM), heat-treated (HT) and wrought (WR) conditions. The findings reveal that stress-relieved AM samples demonstrated good ductility without compromising strength in uniaxial tension tests. However, they exhibited poor fracture toughness and pronounced anisotropy in crack initiation along directions parallel and perpendicular to the build orientation. This behavior is attributed to the \(\text{Widmanst}\ddot{\text{a}}\text{tten}\) microstructure and residual prior \(\upbeta \) grain boundaries. The cyclic heat treatment significantly enhanced fracture toughness in both orientations. This improvement is attributed to the larger colony size and higher initial strain hardening rate observed in the HT condition, achieving fracture toughness values comparable to wrought Ti-6Al-4V ELI. Fractographic analysis identified void-sheeting as the primary deformation mechanism governing crack propagation across all conditions. EBSD analysis further revealed that hard crystallographic orientations hindered crack initiation and propagation in HT samples. Additionally, ET1 twinning activity near the crack tip played a critical role in improving fracture toughness by blunting the crack tip and limiting its progression. This study offers valuable insights into the microstructural determinants of fracture toughness in additively manufactured Ti-6Al-4V ELI and underscores the potential of strategic heat treatments to achieve mechanical properties comparable to those of wrought materials.