Two-phase induction melting with tailored flow control

Induction heating is widely used in metals processing. In many cases, additional wishes exist for the flow structure in the melt and the heat and mass transport processes connected with it. The induction heating itself causes a flow in the melt which in a cylindrical configuration typically consists of a double vortex driving the flow on the melt surface radially outwards.
We present the solution of a two-phase stirrer which allows a flexible control of the melt motion in addition to the induction heater. In order to demonstrate this approach, we consider the case of a float-zone arrangement working with an RF induction heater. The two-phase stirrer basically consists in an additional coil superimposed to the primary induction coil. The additional coil is not connected to any power supply, but it is part of a secondary circuit with adjustable capacitor and resistance. The current in the secondary coil is solely induced by the primary coil. In that way, an electromagnetic pump is created. Its intensity and resulting flow direction can easily be adjusted to the process needs. The flexible system parameter are the location and distance of the secondary coil with respect to the primary one, and the capacitor and resistance of the secondary circuit.
We present numerical and experimental results for two applications: the float-zone crystal growth of Ni-based single crystals, and the solidification of the magnetic material NdFeB under varying flow conditions at the solidification front. In the first case, the main interest is directed to a change of the phase boundary geometry in order to obtain a single crystal at all. In the second case, the interest is focused on the microstructure of the solidified material resulting from strongly differing flow conditions in the molten phase. Compared to the usual induction heater, the two-phase stirrer allows to provide a much stronger flow in both directions, i.e. radially inwards or outwards at the solidification front, or an almost stagnant melt. The theoretical part contains the solutions of the electromagnetic problem and the heat & fluid flow equations with a free form of the solid-liquid interface. Experimental results will be given for the resulting phase boundary in single crystals of Ni97Si3, and the convection influenced microstructure of NdFeB alloys.