The evolution of porosity during homogenization heat treatment of direct chill (DC) cast Al–Mg alloys was studied. Homogenization heat treatment was performed at 530 °C for various holding times (0, 1, 10 and 100 h). The evolution of porosity was quantified using two-dimensional metallography and three-dimensional X-ray microtomography (XMT) techniques. The metallographic data suggested that the mean pore size, maximum Feret length and percentage porosity all increased during homogenization, which might be explained by classical inter-pore Ostwald ripening. However, the pore number density also increased, which is not expected when inter-pore coarsening is the controlling mechanism. XMT was performed to elucidate this apparent contradiction. XMT data revealed that the tortuousity of the pore networks formed in DC casting was very complex and that there was no increase in maximum pore length during homogenization. Instead, intra-pore Ostwald ripening of the tortuous pore networks was the key mechanism driving the evolution of pore morphology, with coarsening of both the asperities and interconnects being driven by their high local curvatures. A one-dimensional simulation of vacancy diffusion was developed and corroborated this conclusion.
- X-ray tomography
- DC casting