Influence of the solidification microstructure and porosity on the fatigue strength of Al-Si-Mg casting alloys

The fatigue behavior of Al-Si-Mg casting alloys is affected by the solidification microstructure and especially by defects such as gas pores, shrinkage pores and oxide films. This paper reports the microstructural characterization of a die cast engine head made of the A356 (G-AlSi7Mg0.3) alloy and the long-life fatigue strength determination using specimens extracted from the cast. The microstructural characterization was achieved by optical microscopy and digital image analysis to identify the different classes of solidification defects and to evaluate the correlation between local SDAS and the size and shape of the defects. Fatigue testing was performed under rotating bending and the stress amplitude vs cycles to failure dependence showed a large scatter. The reason for this is related to local stress concentration induced by the pores, which was analyzed by the finite element method, developing models of a material volume containing solidification pores characterized by 2D and 3D techniques (i.e. optical microscopy with image analysis and X-ray tomography, respectively).