Wavy ribbon formation during planar flow melt spinning process – a 3D CFD analysis

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

Planar flow melt spinning is a rapid solidification process used to produce amorphous ribbons for transformer core applications, etc. Molten metal is ejected via a nozzle onto a rotating cooling wheel for rapid solidification which bypasses crystallization. The study of surface feature (topography) is a measure of surface quality of the ribbon obtained. Experiments often result in formation of amorphous ribbons with different surface topographies such as wavy, dimple, herringbone, streak, etc which are imperfections, leading to non-uniform magnetic properties. An amorphous ribbon of polished surface topography is preferred for better performance of the magnetic core in the transformers. Hence to understand this phenomenon, a 3D numerical simulation of ribbon formation is performed to predict the ribbon surface. The computational domain consists of the space between the nozzle (from which the molten metal issues) and the rotating cooling wheel on which the molten metal falls, solidifies instantly and forms an amorphous strip. The Computational domain is extended on both sides of the nozzle to include the surrounding atmosphere. The solid cooling wheel is modeled as a curved wall boundary at constant temperature. A CFD technique called volume of fluid is used to simulate the two phase flow of melt in the air domain over the wheel surface. The conservation equations of Mass, Energy and Momentum are solved under transient conditions. Temperature dependent viscosity relation is used for the melt to employ the viscous changes in the ribbon flow. For a set of process conditions, wavy ribbon pattern is observed at lower melt ejection temperatures. By increasing the ejection temperature keeping other process conditions constant, polished ribbon is obtained. Upstream meniscus is observed to play an important role in the surface topography of the ribbon. Topographical changes in the ribbon are due to momentum transport mechanism during melt flow, which in turn depends on the surface tension and temperature dependent viscosity. Hence, a variation in ejection temperature leads to changes in surface topography of the ribbons. 3D model and simulations presented in the study are useful in predicting the surface topography of the ribbon for a set of process conditions selected for experimentation.