2005-07-30
Making Magnalium
Making Magnalium was one of the main reasons I started my Foundry work, I am surprised it took me this long to actually do it in modest quantities with my new furnaces.
Most people attempting this would use a camp oven, or similar cast iron pot with a close fitting lid. I decided to try it open-crucible, understanding that I'd probably burn quite a bit of the metal or otherwise have problems with Magnesium's insatiable lust for electron acceptors.
I had no suitable flux prepared, but learnt ages ago that if I did get into trouble with a Magnesium fire Sulfur would put it out for me. It is a very effective trick that has saved my butt several times now. The Magnesium Sulfide layer that forms is very stable, unlike the Oxide/Nitride layer that just foams and cracks away as it forms allowing more exposure of the metal to oxidisation rather than passivating like Aluminium does.
I set the propane burner to a slightly rich mix, providing a reducing atmosphere in an attempt to limit burning on the final alloy in the furnace. How effective this was in practice I can't really say. The crucible furnace was then warmed up, and the tin-can crucible burnt clean while 480 g of Aluminium ingots was weighed out to match the 540 g ingot of Magnesium. I was unsure of the exactly alloy of the Magnesium so extra was used assuming it would be at least several percent Aluminium. The target was 50:50 Magnalium.
The Aluminium went in first, and was melted, fluxed with a little Calcium Chloride and skimmed before the Magnesium ingot was dropped in. The Magnesium floats like a giant ice cube as it dissolves into the melt because of its lower density. Towards the end of the alloying process the furnace came alive with a growing cloud of white sparks, eventually the Magnalium melt caught and burnt slowly at the surface where the fluxing was insufficient.
On stirring and skimming the dross with trapped Magnalium burst into a shower of crackling sparks as soon as the spoon is removed from the furnace and the atmosphere had a chance to get at the metal. A little Sulfur was tossed onto the burning metal particles and they soon went out.
Once the alloying was completed a little Sulfur was thrown into the crucible to grow up a nice Sulfide layer. The Sulfur actually reacts more violently with the crucible than the melt, if you get too much on the walls of the can it will burn right through in a shower of sparks, use just a sprinkle, the Sulfur vapour will engulf the entire surface of the melt quite quickly and form a stable protective layer. The burner was then turned off and the tuyere blocked. (If the metal ignites some extra sulfur can be added) Finally the furnace is closed up with a fire brick to keep the oxygen level near the metal as low as possible during cooling.
Once cool the tin can was peeled off the Magnalium ingot and the metal wire-brushed to remove the bulk of the adhering rubbish.
It is hoped in the future better fluxing and a suitable mold will allow me to pour clean Magnalium ingots, my first attempt to do this in the pie trays was a disaster. The Magnalium kept burning slowly once poured and frothed up over the sides and onto the concrete where it reacted rather violently with the dampness causing steam explosions and lots of damage to the surface before I brought it under control with Sulfur. Most of the Magnalium was recovered despite this, but much oxide covered its complicated surface (a lot like broccoli). Separating this mixture mechanically seems infeasible, so this batch of Magnalium will probably be remelted once I have a suitable flux.
As in my previous work with Magnesium turnings, Carbide and Ammonia formation was noted when water was used to clean the bulk metal. A few "bubbles" of metal had pushed their way up through the grey-white crust on cooling, probably as a result of gas porosity. They were easily broken off during cleaning and were surprisingly clean on the surface, most of the Oxygen in the furnace was probably already scavenged by the time they erupted.
The bulk ingot was given a smart whack with a small cross-pein hammer and sheared into two pieces in an almost perfect latitudinal plane. Several "flakes" of razor sharp metal were also produced with the same hit. The centre of the ingot was quite porous towards the middle, as expected of this alloy, and the broken surface was amazingly smooth and shiny.
It is quite easy to nibble bits off the ingot at the edges with a hammer, and crush them into a powder in a mortar and pestle. The bulk Magnalium behaves a lot like Obsidian, it can be flaked into very sharp pieces that would make good knives if they were tougher, but are just too brittle for practical use. Pieces of 5 mm cross-section are easily snapped between the fingers or crushed to a granular powder with pliers. The crushing emits an odd squeaking sound, like a bit like Tin creak/cry.
A hit from a rusty hammer will fairly reliably generate a spray of sparks. Clean steel doesn't seem to do this, I am unsure if a Thermite reaction is occurring or if it is just incandescent particles of metal being ejected. Needless to say this observation has implications for pyrotechnic usage of the material!
In summary making Magnalium open-crucible is quite feasible, even without good fluxes or ideal equipment. Sulfur will get you out of trouble in most cases, but the short-wave light emitted by the burning metal is something you must be careful of if a major fire starts - ware suitable eye protection and start small.
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Attachments
| title | type | size |
|---|---|---|
| edited video of process | video/x-ms-wmv | 9.364 Mbytes |
| pile of Magnalium flakes | video/x-msvideo | 1.973 Mbytes |
| smaller and more compressed process video | video/x-ms-wmv | 1.206 Mbytes |
| striking sparks | video/x-msvideo | 1.296 Mbytes |