Modelling of the Phase Distribution and Residual Stress in a Composite Mill Roll
Post-Doctoral Research - Montanuniversität Leoben
Current
Supervised by Dr. Andreas Ludwig. Production of quality sheet metal in rolling mills dictates the need for high strength rolls with excellent wear resistance. Composite rolling mill rolls satisfy these demands with a high hardness, crack resistant outer shell and a nodular iron core material. Production of a composite roll requires a successive casting and treatment process which includes centrifugal casting of the shell material, subsequent conventional ingot casting of the core material into the shell-mold assembly, and final heat treatment. During casting of the core a thin layer of the shell material re-melts and mixes with the core material producing a layer of intermediate composition. A critical problem with composite rolls is this intermediate bonding layer between the shell and core, which is often the source of spalling and cracking due to poor cohesion between the layers. Additionally, the contamination of the core material with undesired alloying elements from the shell (e.g. Cr.) layer can be a problem. During casting of the core alloying elements can migrate from the shell-core interface into the core center during the partial re-melt of the high-Cr-content shell. These factors clearly have a significant influence on the microstructure of the shell, the intermediate bonding zone, and the core. Achieving a balance of re-melting to obtain optimal bonding at the shell-core interface without undesired contamination of the core material is a central objective of this investigation.
The initial stage of the project is focused on the mold filling and solidification of the core, including the melting and re-solidification of the shell and the shell-core interface, the interaction at the interface, and the solute transport during solidification of the core. For this an Euler-Euler multiphase model will be used which incorporates the mass, momentum and species transport phenomena, as well as the interactions among the liquid melt and solid phases during solidification. Secondary stages of the project will investigate solid state phase transitions and accompanying stress-strain evolutions with a separate FEM model.
My research group at Montanuniversität Leoben. Information in German and English.
Industry partner: Eisenwerk Sulzau-Werfen
Gas Fluxing of Aluminum: Mathematical Modeling and Experimental Investigations
Ph.D. Research - University of California at Berkeley
June 2001 – June 2006
Supervised by Dr. James W. Evans.Chlorine fluxing is an essential purification step in aluminum refining in which impurities such as Ca, Na, Li, and Mg are removed by bubbling a mixture of chlorine and argon gas through molten aluminum. The gas is injected into the fluxing vessel through a rotating shaft and impeller which simultaneously agitates the melt, while breaking up and dispersing gas bubbles through the liquid phase. The efficiency of impurity removal and control of toxic chlorine and chloride emissions are dependent upon the extent of gas dispersion or mixing, residence time of the bubbles, and surface area of the bubbles. Clearly the gas injection and distribution within the liquid metal cannot be directly observed and such operations are often poorly controlled and not well understood. Problems arise when the injection gas, i.e. chlorine, is not completely consumed by reaction with impurities and the excess is reported as emissions of chlorides such as toxic HCl. The intention is to improve the technology to eliminate this waste (saving on the energy entailed in the chlorine production and reducing pollution) by better dispersion of the injected gas throughout the metal.
Previous experimental investigations using a capacitance probe, capable of immersion in liquid aluminum for several hours, have been carried out to detect bubbles in an industrial fluxing unit at the Alcoa Technical Center. Bubble frequency data have shown the bubbles to be fairly well dispersed in the areas of the fluxing unit, decreasing in observed bubble frequency with increasing distance from the impeller (source of gas injection). To gain further insight and add to our experimental findings, two computational models have been developed to simulate the complex two-phase fluid dynamics of a rotary gas injection system. The results of these two modeling approaches are presented and analyzed and compared to experimental bubble measurements gathered using the capacitance probe. Bubble size distributions and residence times from the discrete phase model were incorporated in an external demagging reaction model to predict chlorine utilization efficiency. This simplified model included several assumptions regarding the kinetics and reaction path, however the model showed reasonable agreement to prior experimental magnesium removal data and provides valuable information related to the interplay of reaction progress in a fluxing unit and the fluid dynamics, in terms of bubble size, trajectory and resulting bubble residence time.
Extended Summary of my Research at Berkeley