On the anisotropy in room-temperature mechanical properties of laser powder bed fusion processed Ti6Al4V-ELI alloy for aerospace applications

Extra low interstitial (ELI) grade of titanium alloy Ti6Al4V is used for the fabrication of components operating at cryogenic temperatures in satellite launch vehicles, given its excellent combination of strength and toughness up to 90 K. In the present study, test coupons required for strength and toughness evaluation were 3D printed in X, Y, Z, and 45°(along the direction of a body diagonal of an imaginary cubic build volume) orientations through laser powder bed fusion (LPBF) process and were vacuum stress relieved. Mechanical properties at room temperature met the minimum specified values against ASTM-F3001 standard in all orientations and were consistent within each orientation. However, the highest strength (~ 1080 MPa) and toughness were observed in the 45° orientation. The impact strength was in the range of 333–363 kJ/m2, and plane strain fracture toughness (KIc) values in X, Y, and 45° orientations were > 60 MPa√m. In the Z orientation, a marginally lower KIc value (58 MPa√m) was observed and is attributed to the higher probability of easier propagation of cracks in between the 3D printed layers. Microstructure in all the four orientations revealed a mixture of fine alpha (α) laths and acicular alpha prime (α′) in the transformed beta (β) matrix. Higher strength in the samples representative of Z and 45° orientations is attributed to higher dislocation density and higher grain boundary length. Higher fracture toughness observed in specimen representative of 45° orientation is attributed to the higher grain orientation spread (GOS) and lower volume fraction of pores, which results in a more tortuous crack propagation path. Fractography in all orientations showed typical ductile failure with the presence of dimples. Finally, based on the strength and toughness in different orientations, it can be concluded that LPBF processed Ti6Al4V-ELI components can be used in stress relieved condition for fracture critical aerospace applications at room temperature.