How to melt Aluminium in a Ceramic furnace..?? 3

[Amin37]

Hi everbody,
today I found this forum, and here I am with my first question...!!

In my company ( Ceramic MAterial Research ) we want to melt Aluminium ( either as powder or as piece ). I therefore have modified a 1700°C ceramic furance: Aluminium is placed in a small corundum cup, and nitrogen is blown on top of the aluminium surface at 20l/h to protect Aluminium from oxygen.

I Have tried twice: two hours at 750° and 800°C. Unfortunately, the powder did not melt.

Whats wrong?? Are two hours to short?? The same constellation worked out excellent with the melting of iron.

Thanks for a tip and regards,

Amin

 

[arunmrao]

Please check if you are having aluminium powder and not alumina. For the temperatures at which you have tried Aluminium pure or alloyed must melt. also there is no need to flush with Nitrogen during melting process. If necessary you can degass later.

 

[hacksaw]

use of aluminum powder is odd as opposed to solid wire or chunks, but am also puzzled about your use of N2. It is hardly considered an inert gas.

A cover on the crucible should be adequate.

 

[kenvlach]

1) You cannot get good results from aluminum powder without a flux; the aluminum does indeed melt, but the thin oxide shell on each particle prevents fusion into a single melt. Although this oxide shell may crack during heating (as the aluminum expands), the cracks will self-heal by ‘gettering’ any traces of O₂ in your N₂.

2) As hacksaw mentioned, N₂ is not inert w.r.t. liquid Al.
I have grown AlN-matrix composites by by high-temperature rxn. of liquid Al alloys with N₂. It was necessary to use (at least begin with) H₂-N₂ mixtures due to the inevitable traces of O₂.

3) Use bulk Al and either an Argon atmosphere or a borax flux or both to protect from oxidation. A crucible lid as suggested above is also a very good idea.
Note: the borax is basically a barrier; it will not allow melting of Al powder, for that you would need a Cl- or F-containing flux.

4) Commercially, Al is degassed with Cl₂; I doubt that you want to go this route.

 

[arunmrao]

If you must melt Aluminium powder can you consider compacting the powder into briquettes and then melting? This will increase the charge density and reduce the surface area for any further oxidation,thereby hastening the melting process.

 

[ornerynorsk]

Ditto for all of the above replies. Don't use powder, the surface tension caused by oxides will never allow your melt to puddle. If you want a "down and dirty" method just to try it, get some solid chunks of aluminum and get a box of 20 mule team borax from the grocery store for your flux.

 

[rd400guy]

I think the problem you're having is the use of nitrogen. Aluminum has a very high affinity for nitrogen, hence age hardening ability of aluminum alloys and aluminum additions in age hardening steels.
I also concur with everyone else's posts about using solids vs. powder as your melt stock. Aluminum oxide is a very stable refractory, so if there is any surface passivation (oxidation) on your powdered aluminum, it would definitely tend to resist melting.
As for the nitride, MatWeb lists aluminum nitride (if that's what you're forming) as having a melting point of 2227 C. So, even if you did get some of the aluminum to melt, the nitride would very readily form, and lo and behold, melting temp goes way up again. I hope this helps, and if I'm wrong, someone please correct.

 

[roperwerk]

Hi

From my experience it's very difficult to melt powder, also it depends on the qualtity. I recomend you first do not use nitrogen at the melting stage, the powder must be moved every now and then. If posible use small pieces of aluminium first to get the melt pool then add the powder. When you have produced the molten aluminium then blow pure nitrogen into the bottom of the melt through a stainless steel tube. then the dross can be scimmed off the top.
I hope this helps you