Pore evolution in a direct chill cast Al-6 wt.% Mg alloy during hot rolling

A series of laboratory-scale rolling experiments was performed at 400 °C to simulate low reduction per pass (3–5.5%) industrial hot rolling of a homogenised direct chill cast Al–6% Mg alloy. The evolution of porosity was examined at intermediate passes (4, 8, 12 and 16) during rolling of a 20 mm plate with a reduction of 0.6 mm per pass using two-dimensional metallography and three-dimensional X-ray microtomography (XMT). Metallographic analysis showed that while pore closure took place near the surface, in the central region of the plate classic inter-pore coarsening appeared to have occurred. Contrary to this hypothesis, XMT analysis indicated that in fact intra-pore Ostwald ripening occurred during rolling resulting in the originally tortuous pores spheroidising. Finite element modelling was used to simulate the through-thickness stress and strain distribution during rolling. This showed that during the initial low reduction ratio passes the central region of the plate experienced a tensile, rather than a compressive, hydrostatic stress, explaining the initial pore ripening. It was found that the hydrostatic stress in the central region was directly proportional to the ratio of the contact length between the roll and the plate (L) and the initial height of the plate (H). It is concluded that in order to prevent pore coarsening an L/H ratio greater than 0.3 is required.