J-Integral Method for Determining the Fracture Toughness of Ti-6Al-4V Produced by Electron Beam Melting

Abstract

Electron beam melting (EBM) is a metal additive manufacturing (AM) a layer-by-layer process, reduces geometric constraints and enables fabrication of complex geometries to near-net-shape. Ti-6Al-4V is the most widely utilized alloy in metal AM due to its high specific strength, corrosion resistance, and biocompatibility. However, the repeated melting, solidification, and thermal cycling inherent to AM processes along with relatively low thermal conductivity and low specific heat capacity of titanium alloys, produce thermal gradients that result in unique directional microstructure and anisotropic material properties that must be considered during design and qualification. In aerospace sector, components are often designed using a damage-tolerant approach which assumes flaws exist in the material. As a result, the material property fracture toughness becomes an essential design criteria, defined by material’s resistance to crack extension in the presence of a flaw. In linear-elastic fracture mechanics the stress intensity factor K is used to characterize the stress fields at the crack tip and can be utilized for evaluating the fracture toughness, but it becomes insufficient in cases where may be significant crack-tip plasticity Under elastic-plastic behavior the fracture resistance is commonly characterized by J-R curve which represents material’s resistance to stable crack extension as function of stable crack growth. Limited research has been conducted on the elastic-plastic fracture resistance of Ti-6Al-4V produced by EBM. The present work addresses this gap in the additive manufacturing of Ti-6Al-4V literature by applying the ASTM E1820 unloading-compliance method to evaluate elastic-plastic fracture toughness of EBM-PBF Ti-6Al-4V investigate the influence of build orientation through comparison of horizontal (XY) and vertical (XZ) build orientation specimens on fracture toughness.