DISSOLUTION STUDIES OF Si METAL IN LIQUID Al UNDER DIFFERENT FORCED CONVECTION CONDITIONS Mehran Seyed Ahmadi 1 , Stavros A. Argyropoulos 1 , Markus Bussmann 2 , Don Doutre 3 1 Department of Materials Science and Engineering, University of Toronto 2 Department of Mechanical and Industrial Engineering, University of Toronto 3 Novelis Global Technology Center, Kingston, Ontario, Canada Keywords: Si-Al alloys, Dissolution, Forced convection Abstract The dissolution of solid cylinders of Si in a revolving tank of molten Al was studied, under both natural and forced convection conditions. The experiments were carried out at bath superheats ranging from 21ºC to 78ºC. The dissolution of Si increased with increasing bath superheat, and was accelerated by the forced convection. The increase of the mass transfer coefficient with increasing tank rotation rate clearly shows that the rate controlling factor in the dissolution of Si into Al is mass transfer across the concentration boundary layer, for the range of rotation rates tested. Introduction The assimilation rates of additions affect the production cost of alloys. The assimilation of a solid metal into a liquid metal can be distinguished as melting or dissolution: the former takes place through heat transfer, while the latter occurs when the solid comes in contact with liquid at a temperature below the melting point of the solid. This study focused specifically on the dissolution of solid Si into liquid Al. Al-Si alloys have attractive properties for the aerospace and automobile industries. However, the production process is very slow, because the time required for Si to dissolve into liquid Al is long. Industrially at Novelis, chunks of Si are added to molten Al, which is then stirred for up to 30 minutes. During that time, dross forms on the top of the melt that is skimmed off; the longer the alloy making process, the more dross forms. Therefore, we are seeking to accelerate the dissolution process to reduce material loss and energy consumption, by reducing the tap to tap time. Significant savings would be achieved by only a 10% decrease in the dissolution time [1]. When an addition (initially at room temperature) is introduced to a molten bath, the addition and the liquid quickly reach a thermal equilibrium, and then the solid begins to dissolve and its size gradually decreases. Dissolution of a solid addition in a metal bath takes place as two steps [2]: 1. Interface reaction at the solid-liquid interface, C Si →C Sat Atoms migrate from the solid phase into the melt. The concentration of the dissolving species in the liquid at the interface is given by the liquidus curve on the phase diagram, at the operating temperature, see Figure 1. Figure 1. Al-Si phase diagram showing the liquidus curve [3]. 2. Transport of dissolved species from the interface to the bulk liquid metal, C Sat →C b The dissolved species travels from the solid-liquid interface to the edge of the concentration boundary layer, and then from the edge of the boundary layer to the bulk liquid Al. The solute concentration profile next to a dissolving addition is shown in Figure 2. Figure 2. Solute concentration profile in the solid addition, at the solid/liquid interface, and in the liquid metal.