making cast alnico

[Mystic]

can I melt and recast an alnico magnet?

[David King]

http://www.wipo.int/pctdb/en/wo.jsp?...2&DISPLAY=DESC

Not sure they can be remelted after the fact but either way you'd need to get them really hot...

[Mystic]

I thought maybe I could melt a few (off the shelf) mags and then pour the melted metal into my own size and shape mold. Your link says cast alnico is brittle. I'm not going to machine it so that shouldn't matter. HAs anyone here done this?

[David King]

Mystic, what that article makes clear to me is that the process is complicated, you need to grind up your current magnets into a very fine powder and melt them in an induction furnce and spin cast them while maintaining them at a specific temperature (Curie point) for several hours under a strong magnetic field. Then they are tempered at a lower temp for several more hours.
(the article goes on to describe pulverizing those chunks and sintering them in a separate process that produces a sintered magnet with unique qualities.

I say if you have an oxy-cetalene torch and a fume extraction hood you should have at it and see if you can actually melt one down. A solid graphite crucible and graphite molds would also be helpful though I'm sure casting sand can be used.

[Steve Conner]

They probably need to be melted under some kind of protective atmosphere like argon.

I built an induction heater that can get pickup-magnet-sized lumps of steel to melting point, but it just burns and oxidizes as soon as it starts to melt, and makes an icky looking slag.

The exhaust from an oxy-acetylene torch is pretty much all carbon dioxide and water vapour, so it will tend to protect the work by itself. It might even work better than an induction furnace in this case.

[Joe Gwinn]

They probably need to be melted under some kind of protective atmosphere like argon.

Exactly. Small bottles of argon are easily obtained from welding supply houses.

There is probably a more or less standard gas mix used in production of alnico, and a little web work should come up with a few recipes.

The aluminum is probably the problem here - it will get the oxygen, and turn to aluminum oxide grit. The spin casting may be required to force grit and dirt to the ends of the ingot, which ends can then be cut off and discarded.

Spin casting was first used industrially in the casting of cannon barrels. The centrifugal force cased all the slag and dirt to rise to the center of the cannon. This dirty center was subsequently bored out to accommodate the cannonball and powder. The resulting cannons were far better than static cast, as the metal was far cleaner and uniform, so fewer cannons burst when fired, a very good thing. At least for the gunnery crew.

Boring cannon barrels also led to the development of the modern theory of heat and energy. http://en.wikipedia.org/wiki/An_Expe...ed_by_Friction

I built an induction heater that can get pickup-magnet-sized lumps of steel to melting point, but it just burns and oxidizes as soon as it starts to melt, and makes an icky looking slag.

Definitely need an oxygen-free atmosphere. For magnetic materials, hydrogen is one standard. For this, one uses ammonia, which cracks to hydrogen and nitrogen in the heat of the furnace. Use enough to consume all the oxygen, and then some.

The exhaust from an oxy-acetylene torch is pretty much all carbon dioxide and water vapour, so it will tend to protect the work by itself. It might even work better than an induction furnace in this case.

If you make the flame fuel-rich (slightly smoky) and thus rich in hydrogen and carbon monoxide, it might just work.

But I think that argon plus a little ammonia (to scavenge stray oxygen) is a better bet.

[salvarsan]

Patents 2578407, 4604147

-drh

[David Schwab]

Do Alnico magnets need to be cast in an oriented magnetic field?