Effect of Notch Geometry 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 effect of processing and post-processing conditions on crystallographic texture, grain morphology, and elastic- plastic fracture toughness of additively manufactured Ti-6Al-4V parts are currently under investigation at NIST in Boulder, Colorado. Industrially relevant processing variables include the use of support structures (supported and non-supported) and variations in scan lengths (20 – 90 mm). The specimens tested were also subjected to Hot Isostatic Pressing (HIP), which is a commercial post-processing step known to seal internal porosity in additively manufactured Ti- 6Al-4V parts. The sub-beta transus HIP cycle (900 °C, 200 MPa) was applied to seal the internal gas porosity. This cycle is not expected to drastically change the crystallographic texture. 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) are specifically addressed: notch configuration (fatigue precrack vs. sharp EDM notch), specimens directly attached to the build plate (non-supported) vs. connected to the build plate using standard thin wafer supports (supported), as well as the presence of macroscopically visible lack of fusion pores on the fracture surface of some of the specimens tested. The results of this preliminary investigation will guide the choices for the remaining fracture toughness characterization of Ti-6Al-4V under various processing and post-processing conditions.