Liquid metal flows in AC and DC magnetic fields and its application to casting processes

Electromagnetic fields provide an attractive tool for a contact-less control of flows in metallurgical and crystal growth processes. The presentation starts with a classification of magnetic field actions on melt flows and with a summary of the recently developed velocity measuring techniques for opaque metallic melts. Emphasis will be given to the Ultrasonic Doppler Velocimetry (UDV) as it provides full profiles of the mean velocity and is applicable even through channel walls.
Based on such experimental and numerical modelling capabilities, we first consider the design and application of various magnetic fields for the inflow control of the mould filling process of aluminum investment casting. This process consists basically of the flow in a U-bend showing a high pouring velocity at the beginning and decreasing velocity values during the course of the process. The high velocities during the starting phase are supposed to cause distinct problems like bubble or inclusion entrapment. We present results on the design and application of a DC magnetic field to control the pouring velocity. Numerical 3d transient calculations were performed to simulate the filling process and the effect of the magnetic field. In parallel, model experiments with a plexiglas model have been performed using the low melting eutectic GaInSn. UDV was applied to carry out detailed velocity measurements in the model. Those measurements served for the validation of the numerical calculations, thus allowing to scale up the simulations to the realistic aluminium casting process. Tests with molten aluminium have been performed at an industrial investment caster. The primary action of the magnetic field, i.e. the reduction of the velocity peaks at the beginning of the process, was clearly demonstrated. The amplitude of the DC field was tuned during the process as the braking action is only needed during the first part of the process. In this way, a clear reduction of the peak velocities is obtained without a significant prolongation of the overall filling time. Eventually, a remarkable diminishment of defects in the casting products was achieved.
A more active influence on the overall process can be obtained by application of a travelling magnetic field which brakes the flow at the beginning but allows to pump the melt at the end of the filling process. First numerical and experimental results will be presented showing the superimposed stirring action of such an electromagnetic pump besides its integral pumping action. For a homogeneous breaking or pumping a special design of the AC magnetic field is inevitable.
The application of electromagnetically driven flows during solidification improves the quality of casting ingots by promoting the formation of fine, equiaxed grains. However, the lack of detailed knowledge on the transient flow dynamics obstructs the optimisation of solidification processes by electromagnetic flow control and is consequently one of the reasons for the rather empirical application of melt agitation until now. Our results show that the forced convection influences significantly the concentration as well the temperature profile ahead of the solidification front. A flow effect can be supposed both on the presence of free nuclei in the melt and suitable conditions allowing them to grow as equiaxed crystals in competition with the columnar front.