STUDY BY MICROTOMOGRAPHY OF THE IMPAIRMENT BY POROSITY OF THE FATIGUE RESISTANCE OF AL-Si_9Cu_3 CASTINGS

摘要

The fatigue resistance of casting alloys is influenced by several factors specific to the material used and to the loading conditions. Among the former, the porosity, in competition with the dendrite spacing and the possible presence of precipitates, plays a non-negligible role. The objective of this study is to quantify the impairment by pores of the life of AI-Si_9Cu_3 castings shaped by pressure diecasting. The experimental approach is based on the nondestructive characterization by X-ray microtomography of fatigue specimens that are then tested in cyclic tension. The microtomographie data are processed so as to segment the initial specimen into two constituents, the matrix and the pores. The geometry and distribution of the pores are analyzed individually and statistically. The segmented volume image is also used as input for a finite-element analysis. This analysis, based on an elastic calculation, identifies zones of stress concentration. A refined pore meshing technique has been developed on this occasion. The spatial distribution of the Von Mises equivalent stress, which takes the form of a scalar field, is exported as a volumetric image that can be manipulated using the same tools used for the tomographic images. To each pore are thus associated a Von Mises stress and a zone of influence, which are then linked to the geometrical and morphological parameters of the pore and/or of its vicinity. This experimental and numerical approach reveals a strong correlation between the stress developed in me vicinity of the pores and their volume, and, to a lesser extent their minimum distance from the surface of the specimen. The stress concentration also serves to predict the zone of crack initiation.