Experimental and numerical study about the solidification of a PbSn alloy exposed to a rotating magnetic field

An important factor to influence the microstructure of a metallic alloy is the convection in the melt during the solidification process. The application of time varying magnetic fields can be considered as an effective tool to organise a well-defined flow structure in the liquid phase affecting the nucleation and growth process. The aim of our work is to improve the understanding of the basic mechanisms allowing us to find a strategy to refine the microstructure of castings by an optimal combination of magnetic field intensity, field frequency and cooling rate.
Directional solidification experiments were carried out with a Pb85wt%Sn alloy contained inside a cylindrical crucible with a diameter of 50 mm. A melt height of 60 mm was chosen. The container bottom is positioned on a water cooled copper chill allowing a directed solidification by a vertical heat flux. A rotating magnetic field was generated by an inductor system with 6 coils. A maximum field amplitude up to 25 mT can be applied. The frequency of the field can be varied between 10 and 400 Hz.
Local temperatures were determined during the solidification process using a set of thermocouples. Cooling curves measured at different locations inside the sample reveal the significant influence of the electromagnetic convection on the local heat transfer. Profiles of the melt velocity were obtained applying the ultrasonic Doppler method..
The Pb-85wt%Sn alloy shows a microstructure with primary tin-crystallites and eutectic. Specimens solidified without a rotating magnetic field showed a columnar dendritic microstructure which is orientated in heat flux direction. The tin crystallites and the eutectic are homogeneously distributed over the whole sample. If the alloy solidifies in a rotating magnetic field the microstructure changes. The shape of the dendrites changes from columnar to equiaxed in direction from the bottom to the top of the specimen and in the same direction the volume content of the eutectic increases.
Numerical calculations were performed using a continuum two-phase model for the directional solidification of a binary alloy. The model includes mass, momentum, energy and species mass conservation equations written in compressible form in order to be able to model shrinkage flow. The Lorentz force term has been included into the momentum conservation equation in order to take into account the effect of the applied magnetic field. The temperature-solute coupling has been described by conduction dominated solidification rule. The two-phase mushy zone has been treated by means of a porous medium approach.
The system of equations has been solved by means of a Finite Volume method. SIMPLE algorithm was used. The results are compared with numerical solutions for diffusion dominated solidification of PbSn and with the data obtained from the experiment.
It was demonstrated by the numerical simulations that a rotating liquid phase forms a mushy zone front showing a convex shape with the maximum on the axis of rotation. This phenomena is confirmed by the experiment.