Influence of defect morphology and position on the fatigue limit of cast Al alloy: 3D characterization by X-ray microtomography of natural and artificial defects

The objective of the paper is to evaluate the impact of the morphology and the position of a casting defect on the fatigue limit of cast Al-Si alloy. Natural defects such as shrinkages reveal relatively complex morphology so the question is to understand the scale controlling the fatigue limit: the local one associated to inter dendritic porosity or the macroscopic one associated to the global geometry of the defect? In order to answer, fatigue tests are conducted on samples containing a spherical artificial defect of 700 mu m. At the tip of the defect, three types of small defects aiming at representing inter-dendritic porosity are machined by EDM and FIB. Results show that there is no influence of a small defect at the tip of a big defect, meaning that the local morphology of the defect seems not to be the governing parameter. In addition, Finite Element simulations are conducted assuming that the global geometry of the defect could be described by an Equivalent Inertia Ellipsoid. Results show that this approximation of the defect gives good results for shrinkages. Finally, in order to understand the role of the position of the defect through the global volume of the sample, several samples have been analyzed through mu CT before fatigue tests. Results are analyzed using Finite Element simulations taking into account for local cyclic plasticity and show that the defect can be considered as internal when the size of the shortest distance from the defect to the surface is bigger than the size of the defect.