This document describes a method for dispersing particles in liquid metals using simultaneous electromagnetic and mechanical ultrasound. Key points: - Electromagnetic fields are used to induce cavitation and acoustic streaming for particle dispersion, while mechanical ultrasound is injected via a probe for more localized mixing. - Experiments showed this combined approach helped disperse micron-sized silicon carbide, titanium nitride, and yttria oxide particles more uniformly in molten metals like tin, aluminum, iron, and copper. - Pulsed electromagnetic fields were also tested and shown to break oxide layers and induce strong stirring in molten tin, with potential for scaling up production of metal matrix composites.

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SIMULTANEOUSLY APPLIED ELECTROMAGNETIC
AND MECHANICAL ULTRASOUND
FOR PARTICLE DISPERSION IN LIQUID METALS
(Imants Kaldre)
(University of Latvia)
Imants.Kaldre@lu.lv

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Metal matrix composites
Metal matrix composites (MMC) are metallic alloys with evenly distributed particles or fibres
These materials have improved mechanical, thermal and radiation resistance properties.
If these materials could be produced in large quantities they could unlock new applications.
Production of such materials are complicated:
• Surface tension
• Poor wettability
• Particle agglomeration
• Oxidation
• Different densities
ODS steel (Oxide dispersion strengthened)
Fe-12%wt.Cr+0.3%Y2O3 exhibits improved thermal
creep resistance at high temperatures

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Existing production methods
• Mechanical stirring, inert gas stream
(For large particles only)
• Ultrasound transducer
(Slow, transducer erosion, low T only)
• Particle synthesis inside the material
(Difficult to control size)
• Powder metallurgy
(Low quality, impurities)
• Metal coated particles
(Low productivity, expensive)
• Laser sintering
(Low productivity, expensive)

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Particle dispersion by acoustic cavitation
• Cavitation occurs if fluid is subjected to rapid pressure change
• Cavitation is well known reason for channel wall decay
• Extremely high local parameters (temperature, pressure, velocity) are achieved
• Collapse of cavitation bubbles causes intense jets dispersing the particle
Cavitation bubble collapse

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Cavitation in molten metals
Comparison of various cavitation excitation methods in molten metals
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Induction melted cyllindrical sample is subjected to DC magnetic field
Contactless electromagnetically induced ultrasound
Pressure amplitude (thin skinlayer)
High fields necessary to exceed
cavitation threshold

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Variety of electromagnetic methods
• Permanent magnet stirring (initial stirring)
• Low frequency AC melt stirring
• Injection electric current+external fields
• AC+DC field (continuous pressure oscillations)
• DC+pulse field (larger amplitude, crucible scale flow)
• EM methods combined with other methods simultaneously
• Two stage process
a) stirring particles into the bulk of melt overcoming buoyancy and surface tension
forces
b) separation and uniform distribution of individual nanoparticles held together by
surface tension and Van der Waals forces
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Particle stirring under the surface
Permanent magnet stiring Powder metallurgy
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D=30 mm sample is prepared from metal powder and
nanoparticles by pressing it under 4000 bar
Particle mixing in liquid tin
(neutron radiography analysis)

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Particles (1-10 mm)
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Without stirring With stirring
Effective method to mix in particles (Funnel type flow)

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Contactless acoustic pressure induction
Sound in liquid metal is induced by combined AC and DC magnetic fields
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• Experimental setup (BDC=0.6 T, BAC=0.12 T,
f=9...18 kHz)
• Molten 30 mm liquid copper droplet

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Supermagnet experiment
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A) Electromagnetic inductor, b) 2 mm
thick water cooled copper screen; c)
Cross section
SiC dispersion in Al-Mg alloy, intense
cavitation signal, particles from the surface
cannot be mixed

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Contactless cavitation excitation
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Pressure distribution a) S=40, b) S=200
02
)(

ACACDC BBB
p
+
=
Al sound spectrum. BDC =4 T ,BAC =0.12 T,
thus pressure amplitude is 200kPa.
Cavitation onset starts at 40 kPa (10 kHz)
(Grants 2015. Journal of Applied Physics)
2
0RS =
Cavitation measurements by piezosensor

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Ultrasound injection into molten metals
• Silicon nitride probe
• Up to 1500 W
• Up to 770C
• Immersion depth 50 mm
• Diameter 22vmm

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Mechanical ultrasound injection into liquid metals
Experiment with electromagnetic and
mechanical ultrasound
SiC particle (d=2 μm) mixing in glycerol with ultrasound
probe. Different times: a) 0s, b)10s, c) 20s, d) 30s

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Liquid metal under DC+pulse field
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Liquid tin under DC(0.2 T)+pulse (0.6 T, 1ms)
• High peak pressure value
• Breaks oxide film
• Capacitor bank is discharged through inductor
• Peak magnetic field close to 2T
• Heating of the sample and circuit components is
reduced
• High electromagnetic impact can be realized

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Pulsed EM interaction
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( ) )sin(exp0 ttII  −=
β=R/2L
Dumped current oscillation
Experimental setup
Strong metal flows during pulsed field (13 kA, 1 ms)

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Experimental results
Samples solidified under combined AC/DC electromagnetic fields, XRF images:
a) Fe-TiN; b) Sn-SiC; c) Al-SiC; d) Cu-SiC
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Particle dispersion
Fe-12wt%Cr with 800 nm TiN particles dispersed with electromagnetically induced ultrasound: (a)
Scanning electron microscope micrograph, (b) TiN and Fe phase diagram.
Iron and Yttria are not compatible in high temperature

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Tin with 2 μm silicon carbide particles dispersed by electromagnetically induced
ultrasound: a) Scanning electron microscope image showing micron size particle
agglomerates; b) Individual SiC particles built in tin matrix

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Conclusions
• Proposed method showed promising results for some metal-particle
pairs in laboratory scale tests
• Different metals and particles behaves in a very different way
• Cavitation threshold for each metal is different and has to be defined
experimentally. Poor repeatability.
• Not all material pairs are chemically compatible in high temperatures
• Potentially this method could be used for Al/SiC and FeCr/TiN, Cu/SiC
composite production
• Pulse regime allows to mix in particles from free surface
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Thank You for attention !
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Acknowledgments: This work is supported by Postdoctoral research grant « Electromagnetic
methods for metal matric nano-composite production » No. 1.1.1.2/VIAA/2/18/264
Stirring of aluminium chips into a quickly rotating molten aluminium
vortex induced by rotating permanent magnets in contactless way