Influence of Casting Defects On Fatigue Strength of An Investment Cast TA6V Alloy

Cast components are well known for having complex geometries. These components can be manufactured by an investment casting process in one operation while keeping good static and fatigue properties but defects or microstructural inhomogeneities can reduce fatigue strength. This paper deals with the influence of casting defects on the fatigue behaviour. These defects can be internal defects like ceramic inclusions. Internal shrinkages or porosities are reduced thanks to Hot Isostatic Pressure (HIP). Casting defects can also be at the surface of components like pin holes or HIP-Sinks. It is necessary to study the influence of these defects on fatigue strength in order to assess defect size allowable in the component. In order to analyze casting defects harmfulness, fatigue tests are performed on artificial defects and natural defects. Artificial defects are spherical and introduced at the surface of fatigue test specimens. Fatigue tests are performed under a tension-tension loading (R=0.1). Thanks to their reproducibility, artificial defects allow understanding fatigue mechanisms from defects. It is important to understand the number of cycles required to initiate and to propagate the critical crack in order to assess the fatigue life using relevant modelling. In order to characterize fatigue lifetime, fracture surfaces are observed and length of the critical defect is monitored on each sample. This experimental study puts in relief the main parameters influencing fatigue strength of a cast Ti-6Al-4V alloy. All of these experimental results can be used to evaluate fatigue strength of a material containing defects thanks to a multiaxial fatigue criterion.