Towards highly fatigue resistant additively manufactured metals

Abstract

In the as-built condition, laser powder bed fusion (LPBF) Ni superalloy 718 and electron beam melted (EBM) titanium aluminide (TiAl) have distorted, non-equilibrium structures that have negative consequences for the mechanical properties. These issues can be mitigated through post build heat treatments, and opportunities also exist to create additive components that exceed the properties of conventional ones. Fatigue crack growth (FCG) performance of LPBF 718 was characterized after applying two common post-build heat treatments. The solution and duplex aging (S+DA) treatment retained many of the features of the as-built condition including large residual stresses, an elongated, textured structure, and solute segregation and lattice distortion within grains. Applying hot isostatic pressing (HIP) prior to the S+DA treatment resulted in complete relief of residual stress as well as significant but incomplete recrystallization and homogenization. At 20 °C, low load ratio FCG was accelerated in the S+DA condition due to residual stresses and lack of significant crack path roughness induced closure. At high load ratios, the intrinsic FCG resistance of the S+DA material was slightly reduced compared to the HIP+S+DA condition which was comparable to high quality wrought material. At 650 °C, the 30 Hz fatigue crack growth performance followed similar trends to the 20 °C performance. Under constant and 0.1 Hz loading, all materials exhibited intergranular oxidation crack growth. However, compared to wrought and HIP LPBF materials, the highly elongated S+DA LPBF grain structure nearly halted crack growth perpendicular to the build direction and gave much higher fatigue resistance for that orientation with cracks deflecting off of the mode I loading direction. EBM fabricated TiAl (Ti-47Al-2Cr-2Nb) has a fine, distorted, and non-equilibrium single phase structure in the as-built condition. Three novel HIP cycles were developed targeting microstructures that are expensive or impossible to achieve through conventional manufacturing. The first treatment achieved a fine, homogeneous, equiaxed dual phase structure with high yield stress and low failure strain. The other HIP cycles achieved a duplex structure with regions of lamellar colonies and equiaxed structures, lower yield stress, and higher failure strains. Different cooling rates were employed to control the lamellar spacing and were shown to increase the yield stress.