Microtomographic study and finite element analysis of the porosity harmfulness in a cast aluminium alloy

An accurate methodology based on microtomography, image analysis and finite element analysis (FEA) has been developed to investigate the causes of fatigue fracture in the pressure-cast aluminium alloy AlSi9Cu3 (Fe). Five samples have been tested. In each case, porosity was the primary cause for failure. This is evidenced by the initiation and propagation of cracks from individual pores. Volumetric images of the samples internal porosity at the initial state have been used as input for finite element simulation of the mechanical response of the material to uniaxial strain. FEA coupled with morphological assessment of the pores allowed investigating the correlations between geometric parameters, stress concentration and crack initiation. The data were analysed according to two complementary approaches that focus either on the individual pores or on their local density. Results indicate that the volume affected by stress concentration at the vicinity of the pores is a valuable indicator of their propensity to initiate cracks. It is observed that the pores causing failure have sizes located at the high end-tail of the size distribution and, alternately, that the cracks occur in pore-rich regions.

► The fatigue behaviour of cast aluminium samples is studied.
► Microtomography is used in combination with finite element analysis.
► Two analysis of the criticality of pores with respect to crack initiation are compared.
► The volume of stress concentration is a valid indicator of a pore’s harmfulness.
► Cracks tend to initiate at the vicinity of pore-rich regions that concentrate stress.