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

Abstract

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).