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Control of liquid metal by AC magnetic fields : examples of free surfaces and solidification

Control of liquid metal by AC magnetic fields : examples of free surfaces and solidification. Y. Fautrelle EPM lab./CNRS/Grenoble Polytechnic Institute. Outline. introduction action on free surfaces action on solidification conclusions. Context.  Free surface is a key-factor for

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Control of liquid metal by AC magnetic fields : examples of free surfaces and solidification

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  1. Control of liquid metal by AC magnetic fields : examples of free surfaces and solidification Y. Fautrelle EPM lab./CNRS/Grenoble Polytechnic Institute

  2. Outline • introduction • action on free surfaces • action on solidification • conclusions

  3. Context • Free surface is a key-factor for - pollution, inclusion entrapment - mass transfers • Many defects occurs during solidification due to fluid flows, both in the liquid and mushy zone : segregations, structures Both topics are strongly influenced by AC magnetic fields

  4. 1 : Action on free surfaces : static deformations

  5. The electromagnetic forces are responsible for two kinds of effects : static free surface deformation : dome effect, levitation free surface agitation : surface stirring, emulsion …

  6. coil liquid metal drop  60 mm substrate Scheme of the apparatus Example of non-symmetric static free surface

  7. Static deformations of a flat gallium drop The free surface may take complex static shapes R = 3cm, f = 14 kHz B = 0 - 40mT

  8. Example of static deformations(ACHF) Axisymmetric shaping may not be always possible!

  9. 1 : Action on free surfaces : agitation

  10. Free surface motions (ACLF) • Low frequency magnetic fields generate various types of surface waves • Forced (symmetric) waves Unstable (non-symmetric) waves symmetry breaking digitation emulsion

  11. gallium circular drop (ACLF=1.5 Hz)simple transition axisymmetric forced waves  azimuthal unstable waves

  12. gallium elongated drop (ACLF)simple transition snake-type

  13. gallium elongated drop (ACLF + DC)the symmetry breaking is suppressed BAC = 1 - 15% BDC BDC= 2.2 T BAC = 0.3 T

  14. Emulsion of a gallium drop (ACLF = 6 Hz)droplet formation

  15. Increase of the area / perimeter A being almost constant, increase of the surface area occurs through an increase of the drop perimeter p thus let us consider the non-dimensional perimeter NB : for a circle p+ = 2p = 3.54 A

  16. Evolution of the non-dimensional perimeter versus the coil current drop perimeter log (p+) 2/3 theoretical minimum emulsion threshold coil current log (I)

  17. molten salt +Zr liquid Al-Cu main frequency f1 = 14 kHz modulation frequencyf2 = 1- 10 Hz Enhancement of mass transfer through liquid-liquid interfaces two-frequency system : bulk + surface stirring

  18. Fluoride salt dark layer containing Zirconium Sans modulation Avec modulation Al-Cu Al-Cu Without surface agitation with surface agitation after 40 mn after 10 mn The surface stirring promotes the transfer of Zirconium from the salt to the liquid metal

  19. 2 : Action on solidification : segregation control by moderate AC fields

  20. Modèle FHP Stirring in the mushy zone Hypotheses : Darcy approximation in the mushy zone Columnar solidification Two-phase statistical model + envelope model Laminar flow regime Some effects of AC fields on solidification can be understood by numerical modelling

  21. Case of rotating magnetic fields z gravity liquid zone 1 cm rotary stirrer  10 mm 2 cm mushy zone heat extraction Alloy : Pb-Sn10%wt Cooling rate :1K/min

  22. Résultats Cm Results in the pure-natural convection mixing concentration maps [ Cm min = 4,7 % ; Cm max = 19,3 %] Time : 1350 seconds channels horizontal cross-section at h = 5 mm.

  23. Contexte Sarrazin – Hellawell experiment 88 (Pb-Sn) Dark channels Freckles

  24. Cartes Cm Effect of a moderate rotating e.m.s. h = 20 mm. h = 15 mm. h = 10 mm. h = 5 mm. h = 0 mm. Appearance of a central segregation Cm min = 5,13% ; Cm max = 25% Central channel

  25. Pompage d’Eckman Interpretation : stirring in the mushy zone - - + + + + Rotation of the liquid Low pressure High pressure Flow in the mushy zone The solute is drained from the wall toward the centre The mushy zone is « washed » by the fluid flow

  26. Effect of moderate travelling fields on the segregations during solidification Two kinds of electromagnetic forces : force of constant amplitude F0 force with a sinusoidal amplitude F0 sin(2pt/p) e.m. stirrers F0 Extracted heat flux 10  5 mm 2D-ingot

  27. Brassage Effect of steady electromagnetic forces (b) Pb-10wt%Sn, F0 = 1000 N.m-3 Liquid zone Segregated channels Mushy zone Heat flux Natural convection electromagnetic stirring Evolution of the averaged solute concentration (Medina et al. 2004)

  28. Experimental evidence : Zaidat et al. (2004) 1mm TMF effect B = 0,07 T B = 0 B = 0,35 T Al-Ni3.5wt.% Travelling magnetic field Cylindrical rod R =5mm B = 30 mT Central channel segregate

  29. Freckle suppression by modulated electromagnetic forces Al-7wt%Si, 10  5 mm 2D-ingot, GT = 1000 K/m, Cooling rate = 24 K/min constant e.m. force modulated force (period = 10 s) averaged solute concentration

  30. Time evolution of the solidification of a Al-Si 7%wt ingot under modulated e.m. stirring Initial fluid motion liquid fraction

  31. Conclusions • Free surfaces • AC magnetic fields may be destabilizing even at high frequencies • It is possible to create various functions : stirring, emulsion • 2. Segregations during solidification • The liquid pattern has a significant influence on the segregation • stirring in the mushy zone is able to control (partly) the segregations

  32. A l interpretation by energy balance Magnetic energy : with vol = h a2, A p l Surface energy : thus : Emulsion occurs when : l < lc

  33. Stability diagram of a mercury drop unstableregion f4 f5 f6 f7 Inductor current (A) stable region Frequency (Hz)

  34. gallium elongated drop (ACLF = 2Hz)simple transition saussage type

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