Application of a DC magnetic field to the filling process of aluminium investment casting

We present results on the design and application of a DC magnetic field in order to control the filling process of the investment casting of aluminium alloys. The configuration basically consists in a U-bend, and the main request is to reduce the high flow velocities during the starting phase of the filling process. They are considered as the main source of problems like bubble or inclusion entrapment.
At first, the process was simulated using a plexiglas model and the eutectic melt InGaSn which is liquid at room-temperature. Local velocity measurements have been performed using the ultrasonic Doppler velocimetry, whereas flow rates have been determined independently using the contact-less transmitter technique. Measurements and video visualizations clearly show effects like flow rate oscillations or gas bubble entrapment. Depending on geometric and process parameters, the time-scale to get rid of these entrapments may become longer than the process itself. In order to decrease the maximum values of the velocity at the beginning of the process, an external steady magnetic field has been applied. The measurements show that it is capable of reducing the velocity peaks significantly.
Numerical simulations have been performed for the filling process with DC magnetic field control. The free surface problem which occurs in the riser of the casting process is taken into account by a Volume of Fluids Method. The simulations have been compared to the model experiments, and afterwards scaled up to the realistic aluminium casting process. For the influence of the DC magnetic field, a good agreement is obtained between numerics and measurements.
Real tests have been performed with molten aluminium at an industrial investment caster. They clearly show the primary action of the magnetic field, i.e. to reduce the velocity peaks at the beginning of the process. In a second set of experiments the amplitude of the DC field has been adapted in such a way that at the beginning of the process the full breaking action takes place, whereas the field strength has been reduced during the course of the filling process. In this regime, a clear reduction of the peak velocities is obtained with an only slight increase of the overall filling period.
For the flow rate measurement in a pipe, a contact-less solution based on a pair of alternating magnetic field transmitter and receiver has been developed. Test results from a laboratory model and the industrial Al casting process will be presented.