Method for Predicting Fatigue Crack Initiators in Porous Metals

In near-net-shape processing methods such as additive manufacturing and powder metallurgy, porosity is inherent. The predominant belief is that if pores are present in a metal preferential fatigue crack initiation will occur at these pores. However, recent work on Ti-6Al-4V produced by the Hydrogen Sintering Phase Transformation (HSPT) powder metallurgy method has shown contrary results. HSPT is a low cost titanium powder metallurgy method that utilizes hydrogen in the sintering atmosphere and in the form of titanium hydride powders. The use of hydrogen allows for complex phase transformations to occur which results in an ultra-fine grained lamellar microstructure with a relative density greater than 98%. Additionally, the as-sintered microstructure can be further processed without using thermo mechanical processing to produce a bimodal, equiaxed, or coarse lamellar microstructure. From fatigue testing of these microstructural conditions a phenomenon has been observed in which fatigue crack initiation occurred at microstructural features even when pores were present. Specifically, an inverse relationship between the microstructure size and propensity for pore related crack initiation was observed. Using common material properties a constitutive model has been developed in which the minimum pore size required to act as the preferential fatigue crack initiator is correctly predicted based on the microstructure type & size and pore location & morphology.