Fatigue Crack Growth Characterization of Additive Manufactured Electron Beam Melted Ti6Al4V

Abstract

The rapid advancement of metal additive manufacturing (AM) has led to a stronginterest in producing AM components for load-bearing and safety-critical applications. Compared to subtractive manufacturing, benefits of AM include printing at near-net shape and allowing more complex geometry. With this design flexibility, parts can be built with reduced weight, and subsequently, reduced cost. Additionally, with a powder bed fusion (PBF) process, powder reuse is possible, further lowering material costs. This dissertation investigates a titanium alloy (Ti6Al4V) manufactured by a PBF AM process called electron beam melting (EBM). EBM is one of the most popular PBF processes used on the market due to its relatively high build speeds, its ability to produce as-built (AB) Ti6Al4V with negligible residual stresses, and its near-vacuum environment that reduces oxidation effects and contamination. Similarly, Ti6Al4V is a widely used alloy, known as the ”workhorse” of the aerospace industry due to its high strength-to-weight ratio and corrosion-resistance. Despite the many benefits of EBM Ti6Al4V and the research done on its high cycle fatigue (HCF) and fracture toughness (FT) properties, there is still limited understanding of its damage tolerance (DT), specifically fatigue crack growth (FCG) when a defect/crack is present in the part. Results show that resistance to FCG (ΔKth) is influenced by microstructural features, specifically larger α features (α-laths, grain boundary α, and α phase volume fraction), where larger values contribute to improved ΔKth. There is a positive trend between both thickness and build height on ΔKth, as well as a subsequent positive effect with the α grains. Similarly, after HIP+machining, α microstructural features increase, improving ΔKth. Although there is not a strong relationship between microstructural feature size and the orientation of the specimen, there is a significant difference in ΔKth depending on orientation. This is due to the orientation of the prior β grains and GB α parallel to the build direction, leading to differences in crack growth resistance from microstructural boundaries so that ΔKth,vert > ΔKth,flat > ΔKth,horiz. ΔKth of EBM Ti6Al4V considering different build factors is predicted using analytical models based on linear-elastic fracture mechanics (LEFM). The Hartman-Schijve equation is used to model the FCG curve and predict ΔKth for long crack growth and the Kitagawa-Takahashi diagram is used to predict ΔKth in the presence of small defects/short cracks < 1000μm. Numerical simulation is also performed using finite element analysis (FEA) to validate the FCG behavior of EBM Ti6Al4V. This research contributes to a deeper knowledge of DT of EBM Ti6Al4V that supports integration of this technology as a reliable manufacturing process for application in safety-critical components in the aerospace industry and beyond.