Prospective profit by using modulated magnetic fields during unidirectional solidification of metal alloys

AC magnetic fields are used in industrial practice for melt stirring. The requirements are manifold for miscellaneous metallurgical operations or casting technologies, mainly the magnetic field application should provide an efficient mixing of the melt in order to achieve homogeneous distributions of solute and/or temperature. Applications of different magnetic fields (rotating, traveling, pulsating and combinations thereof) are connected with the occurrence of a more or less symptomatic flow pattern. Various forms of electromagnetic stirring have been studied in our laboratory. Here we consider exclusively the use of a rotating magnetic field (RMF), which has already become widespread in industrial practice. For instance, the rotary stirring during solidification has been proved to be a striking method in order to achieve a purposeful alteration of the microstructure of casting ingots, such as a distinct grain refining or the promotion of a transition from a columnar to an equiaxed dendritic growth (CET). However, the imposition of an RMF on a metal column also causes problems like the occurrence of typical segregation pattern or a deflection of the upper free surface leading to surface defects or the entrainment of gas. The RMF application provides a permanent radial inward flow along the solidification front. Such flow is responsible for the transport of solute to the axis of the ingot resulting in typical freckle segregation pattern in form of vertical channels filled with alloy of eutectic composition. In this paper we present a new innovative method of electromagnetic stirring using a modulated RMF which offers a considerable potential for a well-aimed modification of casting properties
Solidification experiments as well as numerical simulations were carried out considering the directional solidification of Pb Sn alloys from a water-cooled copper chill. A modulated rotating magnetic field (RMF) was applied for melt agitation. Thermocouples were used to measure the temperature field during solidification. The velocity field in the liquid phase was determined by means of Ultrasound Doppler velocimetry (UDV). Our numerical model is based on the classical mixture formulation. To calculate the viscosity in the mushy region the model proposed by Roplekar and Dantzig (2001) was implemented into the code. The comparison between numerical simulations and solidification experiments delivered a good agreement. Our results demonstrate the modulation magnetic field enables an effective control of the flow field and the structure of the solidified samples. Modifications of the grain structure and macrosegregation effects are discussed with respect to the details of the flow field.