Making a metal casting in aluminium

03 March 2019

Interview with 

Rob Thompson, University of Cambridge

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We love hearing from our listeners and every now and then we get some unusual requests from people, who want to know if science can solve their problem. Georgia Mills has been working on an interesting request from Emma…

Emma - I came to The Naked Scientists with a bonkers one. My poor, old, dad passed away in May last year and that was very hard but I wanted my boys to have something to remember him by. So I fished through some of his coat pockets and sure enough there was French chalk. He loved French chalk. Some people call it engineer's chalk. He was an engineer; it writes on pretty much anything and I just thought what I'd love to do is take the chalk and somehow make a present for my boys.

I put out the message to all the scientists I know, to say to them, “can I wrap the chalk in metal?” All of them came back saying, “well, this is the first time we've ever been asked do something like this”. And then Chris Smith from the Naked Scientists got back and went "Aha! I might have a cunning plan..."

Georgia - A cunning plan indeed which involved getting the help of a friendly neighbourhood

Rob - I'm Rob Thompson. I am a teaching fellow in the matierials science department and I still manage to carry out a bit of research into mechanical properties of alloys and ceramics.

Georgia - Well, it wasn't possible to wrap the chalk. What they could do was make a metal copy through something called metal casting which apparently involves lots and lots of sand.

Rob - We're going to use this sand and we're gonna pack that into a mould. We pack that in nice and tight and then we can pull the pattern out leaving behind the shape that we want to make. We'll cut some holes through the sand, pour the molten metal in, and then we'll have some holes to let the air out so that we end up filling the mould completely.

And then we let it cool down break all the sand off and we're left with the cast iron.

Georgia - Why sand?

Rob - Sand is pretty convenient because it's quite fine so we can pack it nice and tight so we get good surface detail but also it's mostly silicon dioxide so it's pretty stable and it doesn't really react with anything. That means that when we pour molten aluminium, at say 800 degrees, it doesn't really do anything, it just sort of sits there and takes it. Also, sand because it's a particulate, you don't get thermal shock. The expansion of the local sand particles, the sand grains, happens into the space around them so it doesn't fall apart when you hit it with something hot or as if you just had a solid ceramic mould then thermal shock would be quite likely to just break it apart.

Georgia - I know we're going to have to come back to put the metal in, why can't we pour in now?

Rob - We've bound all of this together with water so that it sticks together much like a sandcastle. We want that to evaporate and just glue all of the sand together but also water, when it gets hot, forms steam and that will make bubbles in our casting. At best it's going to give us a rubbish casting but it could potentially build up pressure and blow the casting apart, neither of which is great.

Georgia - Fine. So fast forward to two weeks later and we're back in the workshop with Rob blow torching any excess water off the mould.

Now it's time for the bit I was looking forward to, melting the aluminium. Although images of an enormous fireplace were way off, all of it took place in a little crucible wrapped in a coil.

Rob - This is an induction furnace and we've got coils of wire around the crucible. We’re gonna pass an alternating current through those. That alternating current is going to induce a current inside the aluminium in the crucible and the currents there will start heating it up just by resistance of the aluminium. We keep our wires, that we're driving all of this with, cool by passing water around them and then in that way we just pour electrical energy in and heat up the metal.

Georgia - How long does it usually take to melt aluminium?

Rob - This is a eutectic aluminium where we've mixed some silicon in. That lowers the melting point, I think it's around six hundred, six hundred and fifty...... Whereas say steel would be more like fifteen hundred, sixteen hundred degrees, so it’s a lot hotter. It's a bit quicker than that for steel.

Georgia - The aluminium started melting very quickly, while Rob added various substances like flux which makes it flow better, and a de-gasser which, well, de-gases it. Then very, very carefully, he pours aluminium that's so hot it's orange into the mould and the rest gets dumped unceremoniously into a bucket of water.

Then we chip away the sand to see our final product...

Wow! It came out and you can see the imprint, it looks amazing. One now metal French chalk preserved for time immemorial. And this technique is useful for a whole host of things.

Rob - It's often used because it's quite a quick and easy method to make something. So if you're doing a few one off bits where you just need a bit of metal, about this sort of shape, maybe you need to just finish it off afterwards with a bit of machining or something. It's quite a quick method to prototype things or make things quickly and easily. It's also used because when you're casting something like steel, there's relatively few options for what you can cast it into.

With low melting point metals, we use something called die casting where you make a reusable mould and you make 10 million pencil sharpeners out of your little magnesium. That's nice and cheap and you can make millions of them. But for something like a car engine block made of steel, you can't cast steel into steel, it would melt the steel. Then, you run out of options for what you're going to make it out of. It's not going to suffer from this thermal shock or react with the metal in some way because it's so hot. Car engine blocks are a good example of something in industry where we cast with sand and then you normally machine it out afterwards just to get the final details exactly right.

Georgia - And what about Emma, happy with the final result?

Emma - It was brilliant. We ended up with a perfect cast of my dad's French chalk and I gave it to my boys and they were absolutely thrilled. It was lovely.

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