A Casting Catastrophe

[Meldonmech]

Hi Guys,
             Decided to cast some ally ingots over the weekend, been waiting for some nice weather. I was using as usual mixed scrap from various sources. The metal was melted and the procedures carried which I have done successfully many times, and then poured into the ingot moulds. As the metal began to cool a dark purple crust started bubbling to the surface, and covered the whole of the ingots see pics 1&2. When the ingots cooled and examined the crust consisted of many small balls spreckled  with aluminium having approx. 5mm dia. The steel crucible was coated with the pitch like material up to about 5mm thick. I tried to chisel it off and found that beneath was a layer of aluminium that had soldered itself to the crucible. see pic 4. for detail of chippings. The only scrap that was painted white was a piece of alumimiun curtain rail, and paint usually burns off and forms slag. I am now wondering if this was powder coated with some form of plastic. Would welcome any comments and advice.  Any ideas on cleaning the crucible, the melt is contaminated. Thankful I wasn't casting into a complex mould.

                                           Cheers David

[Brass_Machine]

  I have no idea what that may be...

[vtsteam]

That's a pretty serious reaction!

I think it's a chemical reaction product from your description David, not just paint or powdercoat "dross".

I also have no idea what it is. I've never seen anything like it.

[tekfab]

Could be that your "scrap" contained some of that horrible die cast alloy thats commonly used nowadays. It has a lower melting point and it could be that you boiled it out of its happy state while the standardish aluminium stayed in its natural state so you end up with a sort of "alloy" dross on the surface and when its cooled . . . . well thats what you get.  Does that make sense ? 

Mike

[vtsteam]

I've cast with "diecast" or zinc alloys, pot metal, etc,  and never had anything like that black (or purple) substance come out of an ingot as it cooled.

Generally if the two get mixed accidentally you get the usual yellow white smoke from the zinc elevated above its burn temp by the aluminum, or if aluminum gets into pot metal -- well it doesn't improve it much, but aluminum in small amounts is usually a constituent anyway.

btw diecast metal is actually quite good for many purposes. Zamak is one high quality variety. The main problem with most cheap diecast is that it is cast into too thin webbed sections. Cast it into something substantial and it is very hard and tough stuff.

[vtsteam]

David, did you add "Lite salt" or a commercial flux by any chance?

In looking through the Internet I came across the following sentence relative to an aluminum waste dump:

"Secondary processing uses scrap aluminum and/or white dross to create aluminum by adding sodium and potassium chloride salts.  The byproduct of secondary processing is black dross.  White dross contains high aluminum content while black dross contains high levels of chloride, fluoride, and nitrate."

[Meldonmech]

Hi Guys,
              Thanks for your comments,
                                                        Mike I do use die castings in my melt together with 50% drawn aluminium, and have found that gives me a good casting that machines well, but your theory could be an explanation.
                        VT, I use table salt as a flux, and sodium carbonate to de gas the melt. There was a heavy crust of around
  25mm on the melt, which would account for the powder coating.
                                                         I am now wondering if any aluminium look alike small bits of scrap with a much lower melting point may have entered the melt, for example pewter.
                                                           The mystery continues.
                                                                                                    Cheers David