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19brendan81
16th Dec 2009, 02:55 PM
Here is a few photos from an aluminium cast I did a month or so back.

The first photo shows two pours cooling in the petrobond sand that I bought. This stuff is worth its weight in gold I reckon, it really makes casting a lot easier.

Second photos shows what happens when you spill molten aluminium on wood and concrete...i.e not much. I have heard people claim that molten aluminium explodes when it hits concrete. Well this photo shows that thats not always the case. In fact I have had a few spills over the last couple of years trying to cast aluminium, and nothing has resulted in anything remotely resembling an explosion.

Third photo is the casting straight from the sand.

Fourth photo is the finished product, I cast a rough cone to turn down to these threaded cones for use on my chopper frame jig.

Brendan

electrosteam
17th Dec 2009, 09:37 AM
Brendan,
The dangerous situation for exploding molten aluminium is when the melt covers a thin (not sure, but say 1 mm) of water in a concrete receptacle.

Apparently the water turns to superheated steam, and an exothermic reaction can occur, breakdown of the water into hydrogen and oxygen by catalytic action of the concrete, but not always.
The aluminium is oxidised by the oxygen and all the energy that was put into the material by the smelter comes out as the aluminium reverts to what it originally was in the bauxite ore.

I have seen video of experiments conducted in the US by Alcoa demonstrating the explosive forces that can be produced.
Apparently exothermic reactions do not occur in deeper water or on dry concrete.

The best advice would be to never spill molten aluminium on concrete.

Happy machining,
John.

19brendan81
17th Dec 2009, 10:50 AM
"The dangerous situation for exploding molten aluminium is when the melt covers a thin (not sure, but say 1 mm) of water in a concrete receptacle."

That would make sense, assuming we are talking about large quantity of the molten metal. Expecting an exposion from spilling half a cup of aluminium on a 30 year old slab of concrete, does not make sense.

I always thought you needed voltage to seperate the hydrogen and oxygen bonds in water, or super high temps (over 3000c). Do you know what the equation is for the reaction you mentioned above?

.RC.
17th Dec 2009, 05:27 PM
Great casting pics....Good to see someone doing the more unusual metal working... Much more satisfying then sitting in front of the television...

With regards to the explosion potential...I thought the problem was from any moisture in the concete being super heated and turning into steam..As steam occupies something like 30% more volume then water it blasts out...

One book I have says to do your casting work in a sandy area as that will absorb any spills..

Ropetangler
17th Dec 2009, 06:06 PM
Hi All,

Brendan,
The dangerous situation for exploding molten aluminium is when the melt covers a thin (not sure, but say 1 mm) of water in a concrete receptacle.

Apparently the water turns to superheated steam, and an exothermic reaction can occur, breakdown of the water into hydrogen and oxygen by catalytic action of the concrete, but not always.
The aluminium is oxidised by the oxygen and all the energy that was put into the material by the smelter comes out as the aluminium reverts to what it originally was in the bauxite ore.

I have seen video of experiments conducted in the US by Alcoa demonstrating the explosive forces that can be produced.
Apparently exothermic reactions do not occur in deeper water or on dry concrete.

I,m not sure that is strictly correct, but molten Al and even small quantities of water are a very bad combination.
In 1970 I worked briefly at Comalco in GeorgeTown, Tasmania, and recall the most thorough plant induction I would have until about 1997, when I was again employed on that site on a pollution control project.
We were shown a picture of a pile of debris along side a production furnace, which we were told was 10 tonnes of aluminium, the contents of that furnace. The operator who was collecting a sample of the furnace contents, by using a ladle about the size of your cupped hands, and with a 20 foot long handle, had failed to dry any moisture on the bowl of the ladle before plunging it below the crust of flux on top of the molten pool of Alumium. The resulting reaction was violent enough to empty the furnace contents on top of that poor operator! Needles to say, He never made that mistake again.
The consensus was that the ladle had been stored outside the potline building, because of its size, and perhaps there had been rain or even just some dew in the bowl. The correct procedure in that circumstance was to empty the bowl, and then poke the ladle into the furnace atmosphere, but to let it dry on top of the crust which floated on the molten pool of metal. the bowl was to be upside down while it dried, and only after it was thoroughly dry was it to be plunged below the crust and into the aluminium, to collect a sample. We were told that it would only take around a teaspoon of water to give a result like this. It seems that on that occasion he was distracted, and failed to carry out the procedure as he should have with tragic consequences.


I always thought you needed voltage to seperate the hydrogen and oxygen bonds in water, or super high temps (over 3000c).
Not so, did you ever have a science class where your teacher took a small sample of sodium metal from a jar where it had been kept under the surface of some light oil or kerro, and cut a tiny cube with sides of 1 or 2 mm, and then placed that tiny sample in water. That small bit of sodium would react with the water and buzz about like an angry bee, till it was consumed. If too large a sample was used there would be a small explosion as the resulting hydrogen was ignited by the molten sodium.
The reaction is 2Na + 2H2O –> 2NaOH + H2
There are other examples of where water can be broken down without using an electric current and at relatively low temperatures besides this one.

Nice looking casting Brendan, That is something I would like to try one day, but your method seems to be working well. Just make sure that you give any molten metal its due respect, as there is lots of potential for bad things to happen.
Rob

jatt
17th Dec 2009, 08:30 PM
The casting looks good :)


Not so, did you ever have a science class where your teacher took a small sample of sodium metal from a jar where it had been kept under the surface of some light oil or kerro, and cut a tiny cube with sides of 1 or 2 mm, and then placed that tiny sample in water. That small bit of sodium would react with the water and buzz about like an angry bee, till it was consumed.

Ah yes fun times. From memory column one elements in the Periodic Table: Lithium, Na, Potassium etc.. can all give similar results. They can be quite unstable once removed from under the protective coating.

John H
17th Dec 2009, 11:10 PM
Hi Brendan,

I have a bucket of aluminum offcuts I wouldn't mind getting melted into ingots/blocks. You doing any more casting soon?

Cheers
John

19brendan81
18th Dec 2009, 09:07 AM
Thanks guys. Casting is a lot more fun than the telly, thats for sure.

Im still striving to get a better surface finish on some of my castings, I think I'm packing my sand too tightly at the moment. Will keep playing.

John - yeah i'll be back in action during the new year. I need to get a new crucible though, my old bodgey one is shot. Going to lash out on something proper. Are your offcuts old extrusions?

Brendan

John H
18th Dec 2009, 03:38 PM
No worries Brendan. All my offcuts are plate aluminum. I also have a fair bit of chips from milling, but I read they may not be so good for re-melting.

19brendan81
21st Dec 2009, 08:58 AM
I melt my swarf too, but given that Aluminium oxidises instantly in contact with air, the oxide to pure metal ratio is much less than solid stock. So, you may get say 500g of molten aluminium out of 1kg of swarf, whereas with other stock and ingots etc you should be able to get that up to 900g of molten alloy from 1kg of stock/ingot etc.

I'll let you know when im firing up the furnace next.

John H
21st Dec 2009, 03:03 PM
Cool. No problems :)

clear out
22nd Dec 2009, 06:49 AM
TAFE shut down the foundry section at Ultimo after an explosion during pouring.
The apprentice before myself experienced one during his time there in the 60s he ended up with metal in his leg.Probably just too damp sand nothing fancy like Al.

When I was doing the TAFE foundry course at Illawarra they wouldnt put old tap fitting of which I had several bucketfulls in the melt as they couldn,t guarantee there was no moisture trapped inside them.

They had an induction furnace that would melt cast iron in 20 minutes but the cast size was limitted to what two blokes could lift in a ladle.

Dont know if they are still running this course but it was a beauty.
You could attend one day a fortnight or one night per week.
Did both over a few years, day one when self employed driving down from Sydney.
They had all sorts doing it male and female.
Sculptors, steam/rail freaks, car/bike dudes etc.
I was mainly doing bits for old woodworking gear I was restoring,mainly Wadkin.
The guy I did it with is down from Goulburn and has a bit of gear set up.

If you're boring out the cones why not use a core, less chance of probs with more uniform metal thickness besides less metal in the cast.

H.