Modeling of Fluid Flow and Heat Transfer in Twin-Roll Casting of Aluminum Alloys

Abstract

A two-dimensional finite element based mathematical model of coupled turbulent fluid flow, heat transfer, and solidification in horizontal twin-roll thin strip casting was developed. Basic formulations for simulating the coupled thermal and flow fields are described in this paper. A k-e turbulence model was used to calculate the turbulent viscosity in the melt pool. A variable viscosity model was used to model the mushy region. Inlet velocity, strip/roll heat transfer coefficient, alloy composition, and melt superheat were the main process variables considered. The effect of the above process variables on the sump depth, mean strip exit temperature, roll surface temperature, and temperature gradients inside the roll, was analyzed. Twin-roll thin strip casting of aluminum alloys, is an ef- ficient way of obtaining hot-rolled strips directly from the molten metal. In this process, the molten metal is fed from a refractory feed tip into the gap between the steel rolls. The material undergoes a considerable amount of hot rolling be- fore it leaves the rolls. The heat is extracted from the melt by the water-cooled rolls. The process involves solidification of liquid metal and rolling of solidified metal before it leaves the kissing point, which is the point of least roll separation, of the two rollers. Solidification of molten metal starts at the point of first metal-roll contact and is over before the kissing point. The process has many advantages over the conventional casting technique. The primary advantages are of reduced number of steps of operation and very high cooling rates. A very high cooling rate provides microcrystalline structure with improved mechanical properties. But the process also suffers from many types of defects in the cast strip, which is pronounced at higher casting speeds and inlet temperatures. Increasing the casting speed leads to an increase in the mean exit strip temperature and thus rolling force decreases. 1) The major defects encountered are the centerline segregation, heat line formation, sticking, and surface cracks due to the induced thermo-mechanical stresses. Extent or the severity of these defects is very sensitive to the process conditions. Proper op- timization of these process conditions can lead to minimiza- tion of these defects. Hence, a detailed analysis of fluid flow and heat transfer is necessary to find the effect of these vari- ous process parameters on the extent of these defects. Various mathematical models 2-7) have been suggested in the literature to predict the fluid flow, heat transfer, and so- lidification in twin-roll casters. Most of the papers deal with fluid flow modeling in vertical twin-roll casters, but only a few papers 6, 7) deal with the fluid flow modeling in horizon- tal twin-roll casters. Miyazawa and Szekely 2) were the first