The time has come – Casting Day. My last post took me to step (9) of the Wikipedia page process for lost-wax casting. Now its ‘No 10’: Pouring. Big Day!
Loading up the Crucible
I loaded up my steel crucible with:
- Aluminium (!)
- A bit of chlorine tablet
- Some charcoal
I have been stock-piling aluminium scraps. In addition to quite a few bits of powder-coated aluminium extrusion (from work samples), I also had a few bits-&-bobs of cast aluminium and aluminium plate. I used these, rather than the extrusions since (1) I’ve read that extruding alloys (6000) are not great candidates for casting, and (2) The powder-coating is tough to remove and if cooked off in foundry will be a pain in the arse.
I got the volume of the dog’s head from the 3D model, plus some for the funnel and sprues. I needed around 750gm of aluminium. I had a lump of zinc too – an anode for a boat hull – so I chucked this in to make up the weight – I think I ended up with about 900gm. I figured the zinc could not do any harm, zinc-aluminium alloys are often used for casting…it says here…(I knew that. Ha).
!!!!Spoiler alert!!! 900gms was not enough. I would say, doubling what you think you need would be a safe bet.
So, I read somewhere that hydrogen in the aluminium melt can be a problem: it causes metallurgical problems (hydrogen embrittlement) and can affect surface quality. The pros use various degassers and fluxes to counter this – flushing the melt with nitrogen seems popular. I saw somewhere that chlorine was an option, so I bought some solid chlorine tablets (for swimming pools – PoolStar HandiTabs). They are made of Trichloroisocyanuric acid (or TCCA).
To start I popped in a little bit, about the size of a sugar cube, at the bottom of the crucible to start. I read that it’ll disassociate at about the melting point of aluminium and mop things up etc as it goes. All seemed quiet on the Western front as the I hit 700C so at that point it seemed to have ‘worked’, in the ‘no visible ill-effects’ sort of way. I then decided to add a little more aluminium and, for some reason, a massive slab of chlorine for good measure.
The garden looked like no-mans land in the Somme. It was no longer all good on the Western front. A thick green gas crept over the ground. And to think I nearly did this in the garage!?!? There was also a kid’s party next door at the time which went kind of quiet. They are all alive though – since they asked to borrow some milk the next morning.
Oxidation & Dross: Carbon
Another issue is the formation of dross, and the subsequent melt loss. 1kg of aluminium ‘in’, does not mean 1kg ‘out’. Quite an interesting article here. Even in an industrial process, the writer reckons you might have a melt loss of 17-18% for chips/ turnings.
As that article says,
Melt loss is defined as how much metal is lost during the melting process due to oxidation of the aluminum. Oxidation of aluminum and the creation of dross can occur throughout the melting process and handling of the molten aluminum.
As dross generation is inevitable when melting aluminum, the only controllable factors are the amount of dross generated
It’s inevitable because you’re aluminium charge material – even if stripped of paint, oil and dirt – is going to have a layer of aluminium oxide on it. To I suppose the greater the surface area (i.e small pieces), the more dross there is going to be – all other things being equal.
The article goes on the say:
Dross is basically a mixture of oxide films and aluminium. Aluminium-oxide is wetting; it absorbs molten aluminum like a sponge, creating fine aluminum droplets encapsulated by aluminum-oxide. Some of the primary factors causing dross generation and melt loss include: disturbance of the clean aluminum surface; turbulence; oxide surrounding the scrap; dirt, oil, or other contaminants on the scrap;
I also read somewhere that dross for heavy metals, like steel, floats on the molten metal making it easier to fish out – not so in aluminium where it can sit pretty much anywhere depending on composition. I THINK (from here) that my chlorine tab is kind of helping with dross as a drossing flux. Not sure.
I read here that carbon can be used to retard oxidation at the melt surface (and making the foundry reasonably airtight and not agitating the melt surface too much seem to be good ideas as well). The carbon (charcoal) presumably intercepts any oxygen knocking around, turning into carbon monoxide/dioxide. I came up with a little cradle to sit on top of the crucible so the carbon was not in the melt.
Unfortunately, at around 500C, this started billowing out black smoke. I feel I should have had faith and that this would reside (like on a BBQ) but I bailed on the carbon and pulled out the cradle.
Not sure the cradle really would have worked anyway from what I have read- the carbon needs to be at melt surface, so floating on the metal. I will try it again though.
I lifted out the crucible and started to pour….and missed the funnel on my mould pouring molten aluminium everywhere except where it was supposed to go.
I lost quite of bit of molten metal.
Combining my pissing away good molten metal and melt loss from dross and guess what? I did have enough stupid metal to fill the f-ing mould. I probably 2/3 filled it, which is a fat lot of good. Half a dog’s head.
Pretty upsetting. Have to make the wax copy and plaster mould again now. Results from this hashed attempt in next post.