The role of microstructural evolution on the fatigue behavior of additively manufactured Ti-6Al-4V alloy

The fatigue behaviour of additively manufactured Ti-6Al-4V via Laser Powder Bed Fusion (L-PBF) was evaluated in three different conditions, as-built, heat-treated and hot isostatically pressed (HIP'ed). Fractography analysis interpreted together with the S-N curves indicates that fatigue failure in as-built and heat-treated conditions where <0.2% porosity was present, was mainly driven by early-stage crack growth. However, crack initiation was determined to be the main controlling factor for fatigue deformation of HIP'ed samples. Moreover, a strong correlation between the impact energy and fatigue limit was found. The findings were based on detailed microstructural and crystallographic characterization, as well as mechanical testing. The as-built and heat-treated conditions exhibited poor fatigue response in comparison to HIP'ed which is largely attributed to the lower levels of porosity identified. Even though similar levels of porosity are present in as-built and heat-treated samples, improvement in fatigue limit was determined in the heat-treated condition due to phase transformation and microstructural coarsening leading to reduction in micro-strain.