Magnetic Field Control of the Mould Filling Process of Aluminum Investment Casting S. Eckert1, V. Galindo1, G. Gerbeth1, W. Witke1, R. Gerke-Cantow2, H. Nicolai2, U. Steinrücken2 1Department Magnetohydrodynamics, Forschungszentrum Rossendorf P.O. Box 510119, D-01314 Dresden, Germany, http://www.fz-rossendorf.de/FWS/FWSH 2TITAL Ltd., P.O. Box 1363, D-59903 Bestwig, Germany Sino-German Workshop on Electromagnetic Processing of Materials Oct. 11-13, Shanghai, China

Aluminum investment casting Motivation Aluminum investment casting casting of complex shapes casting unit: U-bend with the filling channel (down sprue) and the mould being the legs connected by a horizontal channel the flow is exclusively driven by hydrostatic pressure Problem Too high fluid velocities in the early stage of the pouring process  entrapment of bubbles and impurities  worsening mechanical properties (first) Solution Damping of the turbulent flow with a D.C. magnetic field

Strategy Magnetic Field Control of the Mould Filling Process of Aluminum Investment Casting Numerical simulation - finite element code FIDAP Labor experiments with a “cold” liquid metal - Flow observation, velocity measurements - Validation of numerical results Extrapolation to industrial scales - Numerical simulation - Magnet system design Model experiments with GaInSn Experiments with Aluminum melt under industrial conditions

Numerical Simulation Finite element code FIDAP: solution of the Navier-stokes equation for the flow and, in the case of applied static magnetic fields, an additional „species“ equation for the electrical potential Φ 4 nodes (2d) or 8 node (3d) per element, bilinear interpolation Segregated algorithm for coupled equation system Volume of fluid (VOF) method for simulation of the filling process – void fraction is a new unknown scalar 2 turbulence models: Prandtl mixing length hypothesis and standard k-ε Sketch of the volume of fluid (VOF) method Interface: surface with a given constant value of the void fraction

Numerical Simulation (non-dimensional) Governing equations I momentum conservation: time dependent Navier–Stokes equation with Lorentz force density term mass conservation: where is the Reynolds number and is the electromagnetic interaction parameter. is the density, is the dynamic viscosity, is the electric conductivity, B0 is a characteristic magnetic field strength, L is a characteristic length and v0 is a characteristic velocity.

Numerical Simulation (non-dimensional) Governing equations II In the case of a present static magnetic field it is necessary to solve an additional equation for the electric potential electric charge conservation: taking into account the Ohm‘s and Kirchhoff‘s law, following equation for the electric potential holds: boundary conditions: isolating walls, non-slip

Material properties – characteristic numbers Numerical Simulation Material properties – characteristic numbers The system is defined through 2 independent non-dimensional characteristic parameters: Re and N Material properties Al GaInSn density ρ 2355 6360 Kg/m³ dynamic viscosity η 1.45 2.164 ×10-3 Pa s electric conductivity σ 3.73 3.2 ×106 -1m-1 taking L=0.03 m (channel height), v0=0.5 m/s and B=0.5 T we obtain: Characteristic parameters Al GaInSn Reynolds number Re 24362 44085 interaction parameter N 23.8 7.6

Numerical Simulation Application of a static magnetic field down sprue Electromagnetic force density acts as damping force mould model z y x pole shoes Sketch of the casting unit computational grid

3D-Numerical Simulation with the VOF Method Application of a static magnetic field Attenuation of the maximal velocity value at the beginning of the filling process t = 0.15 s B = 0 T B = 0.5 T Velocity vector plot on a cut plane in horizontal channel. Vmax=1.25 m/s corresponds to color red

3D-Numerical Simulation with the VOF Method Application of a static magnetic field t = 0.25 s B = 0 T B = 0.5 T Velocity vector plot on a cut plane in horizontal channel. Vmax=1.25 m/s corresponds to color red

3D-Numerical Simulation with the VOF Method Application of a static magnetic field t = 0.5 s B = 0 T B = 0.5 T Velocity vector plot on a cut plane in horizontal channel. Vmax=1.25 m/s corresponds to color red

3D-Numerical Simulation with the VOF Method Application of a static magnetic field t = 0.75 s B = 0 T B = 0.5 T Velocity vector plot on a cut plane in horizontal channel. Vmax=1.25 m/s corresponds to color red

3D-Numerical Simulation with the VOF Method Application of a static magnetic field t = 1 s B = 0 T B = 0.5 T Velocity vector plot on a cut plane in horizontal channel. Vmax=1.25 m/s corresponds to color red

Model experiments Perspex model model fluid: GaInSn, down sprue model fluid: GaInSn, liquid at room temperature D.C. magnetic field: up to 850 mT transparent walls mould model horizontal channel magnet pole shoe

Measuring techniques Visual Observation Ultrasound Doppler Velocimetry (UDV): see presentation about measuring techniques Inductive Flowmeter (IFM)

Measuring Techniques Inductive Flowmeter (IFM) Measurement of the perturbation of the magnetic field by the flow voltage is proportional to the flow rate high temporal resolution can be applied at high temperatures G: geometry factor

Experiments Velocity measurements obtained at a pouring experiment with a) InGaSn at room temperature b) AlSi alloy at about 700°C

Model experiments: UDV measurements

Model Experiments UDV measurements in the down sprue B = 0 B = 0.85 T

UDV velocity measurement in the model experiment Model Experiments UDV velocity measurement in the model experiment in the down sprue channel in the horizontal channel dangerous peak velocity in the early stage removed velocity fluctuations become smaller

Validation of the numerical simulation Experiments Validation of the numerical simulation Comparison of numerical and experimental results regarding the flow rate Q as a function of the magnetic field strength (related to the flow rate obtained at B = 0)

Al casting Experiments Casting units evaluation Visual inspection of the resulting metal surface UV-light visualization of surface defects B=0.25 T B=0.75 T in the first 5 seconds 18.11.2018 18.11.2018

Al casting Experiments Casting units evaluation - Statistics CNF 6-4 4-4 4-2 3-2 CWF 4-3 4-1 3-3 1-2 FNF 2-2 2-3 FWF 6-2 2-1 1-1 Clear tendency: the d.c. magnetic field always lead to an improved quality of the casting unit with reduced amount of entrapped oxides

Conclusions I Velocity and flow rate measurements are needed for better understanding of the flow phenomena and filling process of the investment casting of Al Low temperature metallic melt InGaSn has been used for model experiments; key advantage: at this temperature a sufficient number of different measuring techniques are available Validation of numerical codes using such liquid metal models provides a profound basis for an extrapolation of the numerics to the real scale problem and turned out to be essential for the reliable simulation of the real Al casting process Main problem in the pouring process: the occurrence of large velocity values at the beginning of the casting processes leads to an accumulated generation of vortices inside the pouring channel 

Conclusions II A high rate of turbulences in the flow is supposed to entail the transported impurities, oxides or gas bubbles from the walls and the free surface into the bulk of the casting patterns. As a result the mechanical properties are deteriorated The external d.c. magnetic field damps the high flow velocities at the beginning of the pouring process. A significant reduction of the peak velocities, leading to a generation of vortices inside the pouring channel, has been shown by model experiments and numerics, and has been demonstrated in the real Al casting process afterwards As a important input for the control system, a contact-less flow rate sensor has been developed and successfully applied The statistics of a multitude of cast units showed a clear tendency of reduced oxide entrapment due to the magnetic field influence

Perspective Next step: linear A.C. traveling field which brakes initially, and pumps at the end  constant flow rate during the whole process Scheme of the coil system to generate the traveling magnetic field Induced electromagnetic force density compute with a finite element Maxwell solver OPERA