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home castings / metal alloys

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Dougie Swan30/11/2010 17:28:45
185 forum posts
40 photos
I like to make my own castings for freelance models, nothing too grand just a steam cylinder or two.
 
Recently I decided to try an amalgam of bronze and aluminium to reduce the weight of the castings.
 
For various reasons the castings were no use but from the runner I got a 22mm dia "rod" that I thought I could machine into bushes, bearings etc
WRONG. This stuff was as hard as glass and just as brittle, after blunting a couple of carbide and hss tools I gave it a light smack with a hammer and it shattered like glass, even dropping it on the floor caused it to shatter.
 
So my question is this, can I mix ali and bronze to make aluminium bronze, both metals were good quality and now are fit only for the scrap dealer, what did I do wrong
 
Any thoughts?
 
Thanks
Dougie
Peter Tucker30/11/2010 18:53:32
179 forum posts
Hi Dougie,
 
Not that I've ever done it but my understanding is aluminium bronze is made with fairly pure copper and alunimium, aproximatly 10% aluminium by weight.  By mixing the aluminium with bronze I think you may have lost both metals, sorry.
 
Good luck in future, and I'm very interested in how you get on.
 
Peter.
Speedy Builder530/11/2010 18:57:48
1815 forum posts
127 photos
I have often thought of melting down scrap water fittings to make bar stock etc. What sort of metal bar would this make,  and would it be suitable for steam fittings and boiler work ?
 
Can anyone offer good metalurgical advice here please?
wotsit30/11/2010 19:30:38
188 forum posts
1 photos
I don't know about melting down scrap water fittings (copper? brass?) - I have tried copper without conspicuous success: it is liquid at around 1000°c, which is not easy to achieve in a simple home built forge. Brass can be cast, but de-zincifies when liquid, so the final product is a bit unpredictable. Another point is that you cannot always be sure of the quality of the material you start with.
 
I have successfully done quite a bit of aluminium casting using the lost foam process, and made parts ranging from lathe wheels, to pulley wheels, and mountings for motors. Probably the biggest problem I have had was inclusions and small holes in the casting. Inclusions (carbon, dross etc), were probably my own fault for not skimming it off properly, but I have had cases of small (~1mm) holes in the metal. A finished casting looks fine, then on machining the holes are uncovered. Various causes are possible (I am told), ranging from a need to use a flux of some kind, to the melt temperature, or to leave the molten metal to stand to allow it to gas. I am too impecunious (tight?) to have tried flux yet. The other two suggestions seem inconsistant - sometimes the result is good, sometimes not.
 
Another thing I tried was dropping a small amount of copper into the melt. I was told this results in a harder metal (as in Dougies case), but I had no idea of quantities. I simply chopped off a couple of cms of copper water pipe, and dropped it in the molten ally (carefully!). I was a bit dubious that it would even melt, because ally is liquid at a lower temperature, but after pouring there was no lump of copper in the ally. 
The resulting alloy did seem a bit harder than ally, but was not brittle as Dougie describes - when turned in the lathe, it did not have the soft feel characteristic of ally - it turned OK, and is still in use (3 years on) as a pulley wheel on a circular saw.
 
There is reams of info in the Internet on aluminium casting - try the following as a start.
 
http://www.backyardmetalcasting.com/
http://members.optushome.com.au/terrybrown/welcome.html
 
I now prefer to cast parts wherever I can, rather than prefabricate them, because it is so much easier once you have the set up. Sorry I cannot add any 'hard' metallurgical advice, only my experience.

 
Howard Jones01/12/2010 09:20:54
70 forum posts
112 photos
the alloy description I've seen is 80% copper and 20% alooominum (as the yanks describe it) to produce aluminium bronze so the name is a misnomer.
Bronze is actually Copper and Tin in alloy. Just be careful replacing bronze with aluminium bronze, the subtly different properties have killed people.
 
mixing copper into the aluminium is non intuitive but is actually dead easy.
you get a melt of aluminium and take a piece of copper (tube or rod) and just stir away in the aluminium. the copper dissociates into the aluminium quite easily.
 
if you want light aluminium castings never mix a diecast piece in the melt. the diecasters use zinc which makes the aluminium flow nicely but makes the piece almost as dense as cast iron. ( I have the Tee shirt )
Dougie Swan01/12/2010 13:51:19
185 forum posts
40 photos
Thanks for the replies
 
I redid the castings in bronze and they turned out fine. In future I think I will stick to bronze and ali
 
The furnace I use is home made from an old gas tank and can melt a crucible of bronze in around 20 mins
I have had problems with air holes in ali castings but now use something called coverall flux which seems to have helped, I also read somewhere that a willow stick used to stir the melt helps, I tried it but didnt notice any difference
 
Regards
 Dougie
camper01/12/2010 15:00:32
11 forum posts
willow contains an acid salictic? related to asprin which allegedly removes free hydrogen from bronzes for ally i think you need a proper degass tablet bad news for the impecunious (my foundry experience was as an apprentice40 odd years ago and it all seemed a bit like sorcery at the time)
Terryd01/12/2010 15:25:48
1926 forum posts
179 photos
Hi Keith,
 
Impecunious means "penniless" or "without money" not quite 'tight' which I suppose should be 'miserly ' at least that's my wife's usage of the latter when talking of me.

Terry
Terryd01/12/2010 15:49:15
1926 forum posts
179 photos
Hi All,
 
Aluminium absorbs gases from the atmosphere if melted in air.  Coverall flux does just that protecting from the atmosphere as well as aiding the melt, I have read somewhere that common salt works as well, but that needs checking, I've never tried it.  We also used to use degassing tablets which were like large blue tablets and gave off an awful fumes, the newer ones aren't too bad, I'm not sure if my lungs were affected or not but we didn't do very much casting and had good extraction (eventually when H&E insisted).
 
It helps if you can exclude air from the melt, so at least use a cover on your crucible.  Another method of degassing used in foundries is to use a nitrogen lance and pump nitrogen gas to the bottom of the crucible during the last 10 minutes or so before pouring to flush out the Hydrogen, I think it is just a mechanical process. The nitrogen is inert in relation to the aluminium., not sure about the chemistry.
 
We used to get sample boxes of chemicals from Foseco of Tamworth but they don't give anything out these days.  Probably H&E.
 
Also Iron or steel should not come into contact with molten aluminium, it pollutes the melt, If you need to use steel tools such as stirrers and skimmers there is a ceramic coating which should be applied to protect the aluminium.  For that reason, steel crucibles should not be used.  Invest in a proper crucible which last a long time for our purposes.  John Winter is a good source of supplies apparently but I've never used the company but it is worth looking at the site.  Your local foundry might be willing to help with small quantities of chemicals to test, take along an example of your work and they are usually quite interested.  This is one area of industry which is still almost a cottage industry (at least the small local ones) and are often helpful.
 
Hope that is of some use, as someone said there is a huge amount of information on the internet.   I have yet to build my home furnace.
 
Terry
Terryd01/12/2010 16:09:38
1926 forum posts
179 photos
Hi Again All,
 
Sorry to be a bore, but I just found this section on the John Winter site,  They do a special section especially for model engineers and sell smaller package quantities of their products for folk like us.   I think that's rather thoughtful of them and if their prices aren't too bad they deserve supporting, usual disclaimers.
 
Terry
Nicholas Farr01/12/2010 19:42:51
avatar
1975 forum posts
936 photos
Hi, reading these posts takes me back to the company I used to work for, During the 80's they used to produce degassing tablets for Ali' casting. They were made from a powder mixture full of all sorts of nasties, which was hand fed into a small hopper on top of this secondhand tablet making machine. (I think I made the hopper and fitted it) The machine was a double punching affair producing tablets about 40mm diameter by about 20mm thick. About 25% of the tablets had to be ground down again and recycled because they would split or lumps would break out of them, didn't look good like that for the custmers. The machine was always going wrong and the nasties were a bit corrosive on the steel punches. It was muggins here who had to fix it, as the others said they didn't know anything about it, mind you, I was the one who was shown all the ins an' outs of it by the old boy who used to maintain it in its previous life. Realining the punchs if they came loose was the worst thing. Happy Days.
 
Regards Nick.
Ian S C02/12/2010 01:55:03
avatar
7447 forum posts
230 photos
I think you can use a steel crucible if first you line it with clay and bake it, it needs replacing every few melts, but can be made from a short length of pipe with a cap screwed on , or a disc welded on.
My attempts at casting aluminium have resembled Swiss Cheese.Ian S C
Howard Jones02/12/2010 06:38:14
70 forum posts
112 photos
To answer a few of the comments preceeding.
the blue degassing chemical is Aluminium Chloride. it is the same chemical used to impregnate the blue dot on those weather prediction wooden doohickeys.
when you plunge the pellet to the bottom of the mix it dissociates into the aluminium melt with the aluminium part just adding to the melt. the chlorine part gasses off through the melt combining with with the hydrogen to produce hydrogen chloride gas. if you get a wiff of this it is incredibly pungent, the gas combining with moisture to form hydrochloric acid in your nose. I suspect my own bad sense of smell to be due to scar tissue on my olfactory bits from use of degassing pellets.
 
Ian's comments about swiss cheese are certaily hydrogen embrittlement.
aluminium in the molten state can absorb huge amounts of hydrogen in solution which all comes out of solution as the aluminium cools from molten. it is the trapped bubbles of hydrogen that cause the swiss cheese effect in the aluminium.
you would think that if you placed water on molten aluminium  the temperature of the molten metal would just immediately evaporate the water. well it doesnt do that. the water dissociates into the molten aluminium just as salt does when dissolved into water. If you avoid melting aluminium on days of high humidity, if you make sure all your feed stock is scrupulously dry and clean, then you will avoid most of the hydrogen problems.
 
molten aluminium dropped on to moist anything will flash the moisture to steam and cause one hell of a grenade with molten blobs exploding out everywhere with lethal consequences. not something that you ever want to see. personally I like terry aspin's sand tray idea and have made about half a dozen for use while casting. they keep it safe.
 
Nicholas Farr02/12/2010 09:25:14
avatar
1975 forum posts
936 photos
Hi, I have been of the understanding that if you allowed water or damp ladles ect. to contact the moulten metal, it would have this explosive grenade effect. Is this the case?
 
Regards Nick.
Howard Jones03/12/2010 03:38:32
70 forum posts
112 photos
Nicholas it is dangerous stuff so only a complete idiot would ever try the experiment. from my experience with casting you will get the explosive effect if the moisture is trapped by themolten metal. if it isnt trapped it will dissolve into the melt with some being driven off as steam.
 
my experience with hydrogen came when I accidently used some old pistons, that had been out in the rain, in a melt. unknown to me the insides of some of the pistons was a wet pulpy mass. the resulting casting was like a plastic foam in appearance
 
btw the indicator that you will have a hydrogen embrittlement problem is the formation of lots of dross on the surface of the melt. if you get  an abnormal amount of dross then you will need to use the aluminium chloride blue pills to rescue the melt.
 
if you want to see good casting practise search youtube for a set of 14 videos by myfordboy. the guy uses a different furnace (mine was a waste oil burner)   but his technique is impeccable.
Nicholas Farr03/12/2010 08:53:46
avatar
1975 forum posts
936 photos
Hi Howard, yes I remember watching a short film about damp and moister in moulten metal when I first started college back in the late 60's. Can't remember all the details, but it involved a bloke lying in a hospital bed wih bandages over his face and eyes trying to fathom out what he had done wrong, although he'd gone through all his safety checks. Turned out that damp had formed on his ladle somhow, but as I say can't remember the details.
 
Regards Nick.
Terryd03/12/2010 09:18:12
1926 forum posts
179 photos
Hi Ian sc,
 
Yes you can use a steel crucible as long as it is lined and the aluminium can't contact the steel.  There is a ceramic coating available in small quantities from J Winter (link above) which can be applied fore this purpose.
 
Howard,
 
the idea of a sand tray is excellent as it prevents any spills from running away.  Concrete is likely to have absorbed moisture and cause the explosive effect you spoke of.  The tray should be large enough to hold the moulding flask and the crucible with plenty of space around, and of course the sand should be dry, we used to use spent greensand for this purpose.
 
Nick,
 
We always dried out all the tools which contact the melt, by leaving the working end resting on top of the furnace for a few minutes before pouring and while preparing the crucible for pouring.  I used casting in the school workshop for many years and had to be especially careful with a group of kids watching and trusted ones helping,  We never had an accident.
 
Regards
 
Terry
Nicholas Farr03/12/2010 09:25:44
avatar
1975 forum posts
936 photos
Hi Terry, in the short film I saw, it started of with a scene with his ladles ect. on top of his furnace for the very same reason. I just can't remember what happend that he overlooked that made the one he used damp.
 
Regards Nick.
 
P.S. looks like your losing a few years Terry.

Edited By Nicholas Farr on 03/12/2010 09:47:18

ady03/12/2010 11:26:08
612 forum posts
50 photos
if you want to see good casting practise search youtube for a set of 14 videos by myfordboy. the guy uses a different furnace (mine was a waste oil burner)   but his technique is impeccable.
 
Definitely worth repeating. Very informative.
Terryd03/12/2010 12:41:34
1926 forum posts
179 photos
Hi Nick,
 
I think I've developed the Benjamin Button syndrome (as in 'The Curious Case Of '). 
 
(Sorry if this garbage has ruined the thread, I really didn't mean to offend I hope another don't object too much and I get censored again.)
 
This site was mentioned above and is really worth a visit, it is a goldmine of ideas and information on backyard foundry work. He discusses founding brass and bronze as well as aluminium.
 
I would further endorse Myfordboys videos.  I came across them sometime ago when I was contemplating a home foundry and hope to get back to it when my workshop is rebuilt (if ever the builders come out of hibernation in the snow).
 
Terry

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