Fracture toughness anisotropy of commercially pure titanium produced by laser powder bed fusion additive manufacturing

The effects of build orientation and hot isostatic pressing (HIP) were examined for CP-Ti manufactured from grade 2 commercially pure titanium powder using laser powder bed fusion (LPBF). Two orthogonal build orientations were combined with two HIP treatments, one above (950 °C) the β-transus temperature and one below (730 °C). Cracks tended to grow unstably and qualified Jc unstable fracture toughness values were measured for all groups except for the toughest X-Z oriented HIP 730 group where qualified Ju unstable fracture toughness values were obtained instead. For the as-built samples, the fracture toughness was relatively low due to the martensitic structure and was similar for the two build orientations. After HIP, the X-Z orientation demonstrated significantly higher fracture toughness compared to the Z-X orientation due to the influence of crystallographic texture. The X-Z oriented HIP 730 samples achieved the highest toughness value (KJu = 130.6 MPa√m) whereas the HIP 950 samples had lower toughness (KJc = 59.9 MPa√m) due to the larger grain size giving more cleavage fracture. Conversely, despite different HIP treatments, yield strengths, and grain sizes, all Z-X oriented samples had similar toughness values which was attributed to offsetting mechanisms of reduced yield strength and increased cleavage fracture with increasing grain size. The relatively higher fracture toughness of the X-Z oriented samples after HIP was attributed to a large number of grains aligned with their c-axes along the loading direction which activates symmetric pyramidal slip about the crack plane, promoting enhanced crack tip deformation and crack bifurcations.