High-Cycle Fatigue and Fatigue Crack Growth Rate Behavior of Additively Manufactured Titanium Alloy and the Role of Internal Porosity and Crystallographic TextureWednesday (27.05.2020) 13:30 - 14:10
This talk discusses two related investigations on elelctron beam melted (EBM) titanium alloy (Ti-6Al-4V). In the first, the effects of internal pores and residual stress on ASTM E466 constant force high-cycle fatigue (R=0.1) properties of EBM Ti-6Al-4V material in as-built, stress-relieved, and hot isostatic pressed (HIPed) conditions were evaluated. Conventional techniques were used to measure the chemical composition and quantify microstructures, and neutron scattering was utilized to measure residual stresses. Post-processing did not alter chemical composition. Compared to the as-built condition, microstructure was unchanged for stress-relieved material and coarser for HIPed material. No significant residual stresses were measured for any of the three conditions. This indicates build platform and layer preheating lead to sufficient process temperatures to achieve full stress relief in-situ. The fatigue strengths at 10^7 cycles measured for the as-built and stress-relieved conditions were statistically similar and were measured to be 200 MPa to 250 MPa. A significantly higher fatigue strength at 10^7 cycles of 550 MPa to 600 MPa was measured for the HIPed condition. The increase in fatigue endurance limit was attributed to a reduction in internal porosity. In the second piece of work, the effects of internal pores and crystallographic texture on ASTM E647 fatigue crack growth rate (R = 0.1) of EBM Ti-6Al-4V were investigated by studying material in the as-built and HIPed conditions as well as in two orthogonal crack growth directions with respect to the build direction. Both internal porosity and crystallographic texture were found to affect the onset of unstable crack growth, but neither were found to affect Paris Law behavior. Implications on threshold behavior will also be discussed.