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Ideas on a material controlled by a magnetic field in molten steel

  1. Jul 23, 2013 #1
    I was wondering if anyone could suggest a material that would stay in a solid state within molten steel/aluminum at least for several seconds. There are also two things: 1) the material should react to a magnetic field applied from the outside 2) material's density should be less than that of the melt. In the technique of making metallic foams, aluminum hollow spheres seem cool for that purpose but unfortunately they don't satisfy the requirement of being attracted/repulsed by a magnetic field.
    Thanks in advance to all who want to share their thoughts on this.
  2. jcsd
  3. Jul 23, 2013 #2
    Could you make it a bit clearer please..
  4. Jul 23, 2013 #3
    Thank you for your response. Could you please point out what exactly is vague?
  5. Jul 23, 2013 #4


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    just my initial thoughts.....

    firstly ... the molten steel/aluminium is likely to magnetically shield anything buried in it from the external magnetic field
    secondly ... the only thing that is going to significantly react to a magnetic field is nickel or iron and its likely to already be melting in your molten material

    lets see if anyone else has any thoughts :)

  6. Jul 23, 2013 #5
    Hey, Dave! Thanks!

    Could you please explain in detail why molten steel would magnetically shield anything that is inside?

    In production of metallic foams, hollow alumina microspheres are used because of the inherent resistance to high temperatures (molten steel) and high strength. In any other form, aluminum wouldn't withstand such high temperatures. I was wondering if there is a material that could be kinda like those aluminum spheres in terms of its ability to stay for some time in a solid state while submerged in molten steel, but also could be attracted by a magnetic field. Maybe some hollow iron spheres would do? Your ideas?

    My idea is to melt iron and then arrange some kind of spheres within it by a strong magnetic field. This idea of making metallic foams seems rather weird but who knows? Maybe it deserves to be given a go.

    Thanks again
  7. Jul 23, 2013 #6


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    I had the same first thought as Dave, that the molten iron would function as a magnetic shield. But then I remembered the Curie Temperature, above which permanent iron magnets lose their magnetic moments. So maybe the shielding effect would be less for the molten iron...

    So one idea would be to coat your iron rods with a very good insulator (ceramic? or Space Shuttle tile material?), and drop them into the molted iron. They should survive for a while, so that you can do your aligning or foam making or whatever...
  8. Jul 23, 2013 #7

    Vanadium 50

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    Cobalt has about the highest Curie temperature I know of. Molten steel is hot enough to demagnetize it, and it will sink in molten aluminum. So I don't think there is any such material.
  9. Jul 23, 2013 #8
    Thanks, berkeman!

    I was a bit confused about shielding and thought that maybe I didn't get something. But now you mentioned the Curie Temperature and made me sure that liquid steel wouldn't magnetically shield anything inside (or it would but only to an extent that we can neglect). I remembered how blacksmiths check if iron is ready with a small magnet. If it doesn't attract anymore then the temperature is just right for starting treatment. That's a good application for understanding the Curie Temperature.

    What are space shuttle tile materials exactly made of? (I guess, google is my friend in this question)

    Using ceramic coating is a good idea. In fact, it is already used as an alternative for aluminum spheres.

    Taking into account viscosity of the melt, how strong the field should be? What do you think?

    Thanks for sharing your thoughts!
  10. Jul 23, 2013 #9
    Thank you for your response, Vanadium 50!

    Cobalt does have the highest Curie Temperature (1400K). I know barely anything about cobalt. Isn't cobalt a bit expensive?

    Molten steel is hot enough to demagnetize it but maybe some ceramic coating would work to allow cobalt hollow objects (like spheres? I don't even know) to keep their temperature below the critical point for several seconds?

    Also, what do you mean it will sink in molten aluminum?
  11. Jul 23, 2013 #10


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    Are you trying to produce an open or a closed cell foam ?
    What do you want as the filling of the voids ?

    An iron melt will behave like aluminium to magnetic fields. The curie point will not prevent shielding because as a conductor it will take time for the magnetic field to penetrate. Use an estimate of about 5 metre per second for the magnetic penetration rate, (skin effect). Since hot iron is more resistive than cold, (proportional to absolute temp), it may penetrate faster.

    The rotating disk used in an energy meter is made from aluminium, you could induce the same motion in molten metal by providing the necessary phasing of external magnetic fields.

    If you implanted metal, ceramic or salt spheres with a gun, deep into the melt, you might be able to control their final positions, a bit like a 3D printer. Iron spheres would remain magnetic for a short time, so long as their kinetic energy on impact did not heat them above their curie point.

    It might be possible to implant rods using the same process. If the rod then melted and produced a line of liquid drops or gas bubbles you would get a fondue. With an open cell foam you could exchange the void filling later.

    Volcanic rock often has bubbles in it. If you introduce a gas under pressure that will dissolve in a high pressure melt then, when you release the pressure, you will get a foam like pumice stone.
    Hydrogen and nitrogen will effect the properties of the metal. Argon would make an interesting bubble in iron if it could be dispersed before decompression. The introduction of something that released oxygen would produce CO2 bubbles in a carbon steel melt.
  12. Jul 23, 2013 #11

    Vanadium 50

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    The density of cobalt is about 9, and aluminum is about 2.7. So a cobalt sphere will sink.

    As for whether you could use hollow spheres, or coated spheres, or something, I have no idea. You asked about materials. I know a little about materials. I know next to nothing about the process you want to use them for.

    Cobalt costs a bit more than tin.
  13. Jul 23, 2013 #12


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    This is a pretty interesting thread! :smile:

    I especially like the 3-D printer projectile injector angle, maybe in conjunction with coated pellets or some inert gas bubbles coming up through the iron melt. Is there a quick way to drop the temperature of the iron melt below the melting point while all of this is going on?

    EDIT -- BTW, what is the end use of an iron foam? High strength-to-weight structural applications?
  14. Jul 24, 2013 #13


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    It is very hard to get heat out of a molten metal. The phase transition will require a high thermal flux through what will be a good thermal insulator. Maybe if the voids were made by ballistic insertion of a solid that then boiled, and expanded, such as dry ice, the heat could be exchanged as the bubbles were formed and the foam solidified.

    Welds are weakened by internal gas (hydrogen) pressure so an open cell foam would quickly release any gas pressure and so strengthen the metallic glass that would result.
  15. Jul 24, 2013 #14
    If cobalt is used then the idea is that hollow cobalt spheres wouldn't be allowed just to sink in the melt due to applied magnetic field. (On the other hand, some sinking could be beneficial as in gravity-inflitration method of making metallic foams?) Thank you for your help, Vanadium 50!
  16. Jul 24, 2013 #15
    Hi, Baluncore. Thanks for your response!

    I would like to make a closed cell foam. Anything that has density significantly less than that of steel would work as a fillant of the voids. The price for that material should be reasonable.

    Sorry. I couldn't get what exactly you mean in the second paragraph. Could you please explain more about that? Isn't the Skin Effect closely connected to an AC current that produces a changing magnetic field? And we would like to use a constant magnetic field for taking those spheres into the melt (or maybe we can't use a constant field? Then why?) We probably shouldn't use inductive heating for creating the melt since eddy currents will be of the same trouble that you describe in the second paragraph. Then resistive heating?

    I didn't know that the disk is made of aluminum. Thanks for the info. Don't we need to get some current going through the plunged spheres in order to get the same motion as in an energy meter?

    Do you think it would be possible to use a gun for creating a relatively uniform distribution of voids without structural abnormalities (e.g. 2 voids in 10 cm^3 in one part vs. 10 voids in 10 cm^3 in another part of the resultant foam)? Wouldn't it also impose a restriction of the depth of the melt to which the projectiles would penetrate? (Or maybe we could adjust that by changing the force at which the projectile is "thrown" toward the melt? Then again, with a high force wouldn't we create a mess by destroying a pattern of the layers of voids below as we go up with new spheres?) Viscosity is something very important to consider in that technique borrowed from 3d printers (the melt is very viscous, so often agents that reduce it are added in making metallic foams).

    Mentioning gas bubbles you basically described several techniques of creating metallic foams (like gasar process). Their main disadvantage is the lack of uniform distribution. Research on metallic foams is going now in the direction of finding a cheap way of getting uniform distribution (surely without losing one of the main qualities: efficient energy absorbtion). Volcanic rocks do have bubbles in them but there isn't a strict pattern, so you can't rely on such a material.

    This is my bad, I should have said in the very beginning that distribution of the voids should follow some pattern without high deviations (otherwise the material becomes unreliable, right?). Syntactic Steel/Al Foams are leading in this field so far, and they are made with alumina hollow spheres.

    There are some cool applications. You can find them here http://www.ergaerospace.com/project-gallery.htm That includes Virgin Galactic's Spaceplanes and the Space Shuttle (R.I.P.) I'm new here and I hope it isn't against the rules of this forum to post a link.

    Thanks for your ideas, Baluncore!
    Last edited: Jul 24, 2013
  17. Jul 24, 2013 #16
    Berkeman, glad to know that it seems interesting! Thanks for your response.

    Using inert gas bubbles coming up and down through the iron melt isn't a new thing. It's been used for a long time. But the problem is that the voids are located in some chaotic pattern. I thought that maybe by applying a magnetic field there could be a way to control the injected spheres right in the melt at least for several seconds, thus creating some order. It is very hard to drop the temperature of the melt, so no luck with that.

    [This is a bit far from this thread but just in case you are interested in dropping temperatures very fast, check out the Skylon's SABRE (Synergistic Air-Breathing Rocket Engine). Helium is used to cool down the air from 1000°C to -150°C in 0.01 second. The engine is still under development.]
  18. Jul 24, 2013 #17
    Baluncore, shooting dry ice into the melt sounds pretty cool! Still, there is a problem of uniform distribution unless you somehow manage to shoot pieces of dry ice with different forces and do that all really fast as you build new layers by going upward from the very bottom to the surface of the melt.
  19. Jul 27, 2013 #18
    Oh, come on people. No ideas?
  20. Jul 27, 2013 #19


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    You want closed cell foam with regular packed voids. You could extrude that with a controlled void distribution. That has been done for over 50 years in aluminium, usually with hexagonal cells.

    You could lay down sheets of ceramic spheres bedded in a metal powder. Then sinter the product to fuse the metal powder. That could make a bone like structure.

    A foam on the other hand would have a distribution of void dimensions. To exchange the void contents and so prevent internal differential expansion stress it should be an open cell foam.

    You cannot form a density gradient free foam in a deep melt by using a gas because the pressure at the bottom will be significantly higher than the top. The foam will have to be made in thin sheets or drawn from the upper surface of a melt.

    The need for a magnetic field through a melt seems to be just a distraction.

    Life is a game, the aim of which is to discover the rules of the game. IMO it has been so hard to extract the detailed design constraints of your question that we are exhausted and have given up.
  21. Jul 29, 2013 #20
    Thanks, Baluncore. I should probably learn something from what you said.

    Baluncore, you now mentioned another type of materials. Those aren't foams and their main purpose isn't directed toward crash energy absorbtion which is the main application of syntactic metallic foams. Energy absorbtion goes first hence the material is an energy absorber (I hope I mentioned that above).

    Metal powder sintering isn't an option again since steel/aluminum foams made with metal powders absorb a lot less energy in comparison to foams made with techniques that deal with liquid metals. Some research is done on that in North Carolina. Energy absorbption per mass at densification for powder metallurgy carbon steel foams is 14.75 kJ/kg while it is 29.14 kJ/kg for medium carbon syntactic metal foams. Aluminum foams are already used in the FMTV. It's a question of health and subsequent financial expenditures in case a soldier gets some injury due to a contact with an IED. If there is a way to make a material that would have a better absorber zone in the stress-strain curve then it is amazing even if the method is bizarre (don't many new methods seem bizarre from the point of view of their preceding techniques?) Metal powders wouldn't help here (at least with current methods).

    Thin sheets wouldn't work either. The reason is the same.
    There would be some gradient as in any heating but hopefully it wouldn't be destructive.

    Life is a game, the aim of which is to discover the rules of the game. I agree with you on that. I expected someone on this forum to propose an idea of using ferrofluids that are put within alumina hollow spheres. Too bad that you and others have given up and that the challenge on this thread caused stagnation instead of bringing forward some bizarre but interesting ideas. I liked your idea with shooting pieces of dry ice.
  22. Jul 29, 2013 #21
    Search for Levitron Revolution device
  23. Aug 1, 2013 #22
    Thank you! I think it's bizarre enough. I put reading about Levitron Revolution devices on my schedule.
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