Curvature of micropores in Al–Cu alloys: An X-ray tomography study

Micropores formed in Al–Cu alloys cast under controlled conditions have been analyzed using high-resolution X-ray tomography. The influence of inoculation conditions, copper content, cooling rate and initial hydrogen content on the morphology of pores has been investigated. Based on the three-dimensional reconstructed shape of the pores, the distribution of curvature was estimated. It is shown that the mean curvature of pores in either non-inoculated or inoculated Al–4.5 wt.%Cu alloys can be as large as 0.35 μm−1 near the end of solidification and can be fairly well approximated by a set of interconnected cylinders growing in between the primary phase dendrites. The so-called “pinching” effect, i.e. the restriction of the pore curvature by the solid network, is a function of the volume fraction of the primary phase and of the secondary dendrite arm spacing. If the fraction of porosity is highly dependent on the initial hydrogen content, the curvature itself is only weakly influenced by this parameter. Based on these results, it is concluded that curvature plays a major role in porosity models and that the analytical pinching model developed by Couturier et al. offers a fairly good and simple approximation of this contribution.