Effects of internal porosity and crystallographic texture on Charpy impact energy of Electron Beam Melting (EBM) titanium alloy (Ti-6Al-4V)*
Nikolas W. Hrabe, Ryan M. White, Enrico Lucon
The effects of internal porosity and crystallographic texture on miniaturized Charpy absorbed energy behavior of electron beam melting (EBM) titanium alloy (Ti-6Al-4V) are shown through investigation of material in two heat treatment conditions (As-Built and Hot Isostatically Pressed (HIPed)) and two specimen orientations (Vertical and Horizontal). Internal porosity was assessed by use of x-ray micro-computed tomography (CT) and fractography. Electron backscatter diffraction (EBSD) was used to characterize crystallographic texture. Charpy absorbed energy was measured over a range of temperatures (−196 C to 600 C). Results suggested internal porosity has a deleterious effect on Charpy absorbed energy. Internal porosity was observed in the As-Built condition (99.8 % dense) but not in the HIPed condition, and the HIPed material had a higher Charpy absorbed energy. An observed increase in α lath thickness due to HIPing also contributed to this trend in Charpy absorbed energy. Crystallographic texture and anisotropic grain morphology (i.e., prior-β grains elongated in the build direction) influenced the measured Charpy absorbed energy. It was observed that for similar texture, crack pathways that crossed more prior-β grain boundaries resulted in higher Charpy absorbed energy. However, differing textures were found to negate the prior-β grain boundary strengthening effect, emphasizing the influence of texture and variations in texture on Charpy absorbed energy. Mixed textures were measured for all heat treatment conditions and specimen orientations, ranging from predominantly ⟨001⟩_β to predominantly ⟨110⟩_β (both in the build direction), but none matched the most commonly reported texture for this material (i.e., exclusive ⟨001⟩_β-fiber in the build direction). Hypothesized processing parameters leading to these textures are discussed.