Effect of Precrack Configuration and Lack-of-Fusion on the Elastic-Plastic Fracture Toughness of Additively Manufactured Ti-6Al-4V parts

Abstract A comparison between fatigue precracked and sharp-notched Charpy-type fracture toughness specimens is presented for characterizing the elastic-plastic fracture toughness of Ti-6Al-4V parts (produced by electron beam melting, a powder bed fusion method). The effects of processing and postprocessing conditions on crystallographic texture, grain morphology, and elastic-plastic fracture toughness of additively manufactured Ti-6Al-4V parts are currently under investigation at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. The specimens tested in this work were subjected to hot isostatic pressing (HIP) for 2 h at 900°C and 100 MPa in Argon environment (sub-β transus HIP), which is a commercial postprocessing step known to seal internal porosity in additively manufactured Ti-6Al-4V parts. Lack-of-fusion (LoF) pores were still visible after HIP treatment. These specific pores were exposed to the external surfaces and were thus immune to HIP treatment. In this work, the following variables and their effects on room temperature fracture toughness (measured by means of three-point-bending unloading compliance tests on Charpy-type specimens) were specifically addressed: notch configuration (fatigue precrack vs. sharp electrodischarge machining notch), specimens directly attached to the build plate (nonsupported) vs. connected to the build plate using standard thin wafer supports (supported), and LoF content. The results of this preliminary investigation will guide the choices for the remaining fracture toughness characterization of Ti-6Al-4V under various processing and postprocessing conditions.