Working with low-melt alloys

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Working with low-melt alloys

Postby Doug Coulter » Sat Jun 28, 2014 6:57 pm

There are a number of these out there, usually named Cerro-something (and sometimes the same alloy has more than one Cerro-X designation). Cerro-safe, sold to cast rifle chambers seems for example to be about the same as Cerro-bend.

These alloys have numerous uses. They are most often some mix of bismuth, lead, tin, cadmium and perhaps one or two other elements. By adjusting the bismuth compared to the others, you get interesting variations in properties. Some shrink on hardening - that's really cool for holding something delicate as a temporary fixture for machine work. The stuff can then be melted right off your tiny part, and it recast if required to do some other side of it.

We (EG, BillF) were lucky to obtain quite a large amount of what was called at the time, Cerro-safe. Some new meaning of the word safe here - yes, it melts at less than 100c, so you only get burned really badly if you spill it on yourself. But ours has cadmium in it, which is pretty doggone unsafe if it gets in your body. Well, actually, according to the Rare Metals Handbook, no one has died from Cd poisoning, because it makes you so sick so fast, you toss your cookies, and the previous couple meals instantly.

When the medical and X ray tech business was even less sophisticated than now, they used this stuff (which was our source of it) to limit X ray exposure by putting the cancer patient into a lead coffin, which had a square hole in it. A medical tech would cast a plate of this stuff to fill that hole, with a cutout of the size of your cancer, then they simply more or less flood the place with X rays. Not real slick, so it was kind of gross to recover all these plates - names and dates on them, and you know all those folks are long-dead. But at current prices of stuff like this, I'll remelt it and use it. MidwayUSA is listing 1/2 lb for $19.99. Here's some good words and reviews of their version. That particular alloy has an interesting use, casting internals of guns to measure later. For about half an hour after hardening, it shrinks. Another half an hour and it's back to full size and you can put a micrometer on it and be exact.

These alloys do have a wear factor. Various of the components have different oxidation and slagging rates with use - this might be why we have the batch we have, you can tell some of it is not the same as other of it. Remelting and remixing helps a little there, but each time I do 20 or so lbs (did I mention we got a super good deal here? We got many 100's of lbs.) I get a rather astonishing amount of crud I skim off on each melt - and that's melting it underwater. I do that because A: I can, and B: in theory, I have at least a crude temperature gauge that way, since I'm using the otherwise dirt-simplest gear there is - an old turkey cooker burner and an old aluminum pot, outdoors of course.

My own use of this huge amount is going to be adding shielding for the fusor. We've recently had a breakthrough in Q and output I estimate (because it was so large most of my data aq was made to fail by it) at around 2800. I know that even without the "secret sauce" I'll be describing later on another thread that we effortlessly exceed 10 million neutrons/second (and a heck of a lot more of incidental gammas) just idling at low current and with the same old power supply. OK, I was willing to tolerate some exposure, but this is way too over the top. So, I'm looking at stopping what are potentially useful power-producing amounts of fusion byproducts, while trying to keep flexibility so I can still get to it and tweak, and not have to put the mess into a cave and run remotely - the Mark 1 eyeballs have just been too useful (along with the other 5 or so senses ;) ) to give that up easily.

So, here we go with how I'm working with this stuff. Right now, while I tihnk about how I'm going to apply it, I just need to remelt and remix the basic stock (for uniformity and any usefullness whatever) and make basic building blocks for shielding. They have to be thick, as any Cd that eats a slow neutron (they are always with you when you're making fast ones) makes one heck of a capture gamma itself - and gammas aren't my favorite food. In fact, at present, if you believe in Sieverts (well, at least they are trying to get it right), they're still the main risk here.

Here's the basic rig I'm using for melting.
MeltStart.JPG
The basic rig.

Ranch grade all the way, baby. Fire set on about the lowest it will go, and a bit of water in there to help thermal contact with the raw chunks and warn me if things are getting too hot. I've had pure Cd catch fire...you really don't want to go there.
MeltBegin.JPG
Just getting going here.


One hassle with the low melting point stuff is just that - it's low melting and takes quite a long time to get hard. This is exacerbated by the fact it has a weird phase-change heat as various things get hard and new eutectic compositions are formed. Short story, it takes "forever" to get hard again, outdoors in 80f weather. And, since it gets hard so slow, the crystals of this and that intermediate compound (none are really the pure elements as far as I can tell, but different eutectics or intermetallics) are huge. Which makes the stuff weak. Here's one I broke after only waiting a couple hours for it to cool off before dumping it out of the mold. Not everything I do works on the first try...
BrokenXtal.JPG
I broke one. I'll need some smaller pieces anyway, I hope. Lucky to get 2-3 a day of these since they take so long to do.


I've had sub-batches of this stuff permanently expand when hardening. I strongly suggest John Strong's hints in Procedures for Experimental Physics re casting - leave some draft. And use a mold release. Here I'm using one from Midway designed for bullet casting, and a really fancy stainless steel mold pan, I think made originally for drywall mud. It has plenty of draft and the stuff just falls out.
Mold.JPG
Like I said, ranch grade - anybody can do this with the best gear and unlimited time/money.
A good engineer can do for a nickel what any damn fool can do for a dollar, after all.
All that other junk is used to level the pan each casting so things come out almost uniform.


Well, we have to melt it before pouring it. This takes awhile, and one should NOT turn up the fire to make it quicker. This stuff just takes time, and you don't want to overheat it.
Slag.JPG
This stuff will happily sit under boiling water above its melting point and not melt for quite some time. It makes water look like a lousy phase change heat storage media.


See that big pile of slag I skimmed to the side? Those pieces were cleaner than that - some of that was oxidation while doing the melt and taking all that time. Another reason for using the water to reduce the air exposure (water doesn't hold much oxygen when it's boiling). That slag is a deadly poison (Cadmium oxide and so on), FWIW - dispose of it properly.

So, now (not quite done yet, have half a week more to do here) I have a bunch of 4.5" wide, 19" long by around 3/4" thick blocks of this. I plan to use the rounded edges so they can overlap a bit, and mount them with some sheet lead I also have to keep leaks down and hold it all together. We'll see if this will take tapped holes strong enough to hold its weight. I know the lead won't - they'll pull right out, so the bolt heads with washers will go on the lead side of things (outer).

It's too bad I can't cast this right onto the fusor, but hey - that gets hot and it would be inconvienient to have all your shielding slump to the floor at the wrong time. I suppose you could make a case for using this as liquid cooling if you wanted to go to the trouble to have heaters for the pump and lines too...but that's an "any damn fool" project, not for this engineer.

I'll add more to this thread when the project is further along - I'll have more to say then anyway. In the meanwhile, I am trying to catch at least a back of envelope gamma spectrum of things as they are (and very quickly, it's a lotta rads to be in a room with) so we can see how things change, where the bigger leaks and sources are and so forth. Maybe I'll even show the use of some of this as a machinist tool - it's really cool for that too if you get the type (or change this alloy) in a way that it shrinks on hardening - or expands, depending on the holding task at hand (adjust lead/bismuth ratio for that). This stuff I've got is near-neutral, but a little variable due to long use and oxidation before it came to me.

And oh, here's a couple of the resulting "raw bricks" I plan to customize a bit before use. This stuff saws easy as pie FWIW.
Bricks.JPG
Bricks of gamma/neutron shielding before customizing and mounting on the fusor.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Working with low-melt alloys

Postby solar_dave » Sun Jun 29, 2014 9:50 am

I remember using some similar stuff back in the '80s to hold thin 304 stainless parts to a rotary surface grinder. The parts were water cooled while grinding, but one guy thought he was pretty smart and turned the coolant pump off, claimed it gave a better finish. I had to LMAO when the parts got hot, and the part lifted and exploded the 12 in diameter grinding wheel throwing the part about 30 feet. The safety guys heard about it and then called the EPA bunch came in and made us get rid of the stuff because of the metals content, casting with no vent hood ....

We then switched to Cocall(sp) which is just fancy plaster of paris to hold those parts, the cuts were reduced to about 20% of what you take using the metals and the setup time was about tripled but what the heck it paid by the hour.

Those parts were for a mask used in ion beam coating machine. Don't ask me what or why, I can't say.
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Re: Working with low-melt alloys

Postby Doug Coulter » Sun Jun 29, 2014 1:10 pm

Yeah, you ALWAYS have to pay attention when large forces and lots of heat are around (not to mention poisonous stuff). Mostly this kind of thing was used for dinky parts and milling, not big stuff and grinding, for just those reasons. Things that would pull apart under the machining forces could be cut while embedded in this, for example, without hurting them.

Not all the low-melt alloys have Cd, though. In some of them, the main "nasty" is merely lead. The safety procedure for that being "wash your hands, dummy", more or less.
I'm not even sure if this batch of stuff I got has Cd or not (I do have Cd and other interesting things bought at Rotometals, though, which is a good source of this kind of thing in general).

I found this further documentation on the wayback machine.
http://web.archive.org/web/200503202159 ... usible.htm

So my "Cerro-safe" might actually border on being safe. I'll have to devise or find some chemical test. For this, I'd really rather prefer no Cd, not so much because it's poisonous, I'm not going to eat the stuff, but because of the capture gammas when hit by neutrons, which are in the "fierce" energy ranges.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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