I needed to make some ZA-12 from pure zinc today, and don't recall if I ever mentioned how I've done that by melting the constituents together.
If not, here goes. I use a proportion of 88% zinc, 11% aluminum, and 1% copper. In my case the zinc was in ingots from a commercial foundry, the aluminum was soft aluminum round bar stock ends from a machining outfit making parts to be anodized, and the copper was some 3/8" soft copper tubing.
I put all ingredients together into a crucible today, and didn't try to melt the aluminum first -- which might have seemed more logical considering its higher melting point than zinc. And the copper has even a higher M.P. But, no, from past experience I guessed that the zinc would dissolve the aluminum, and the aluminum would in turn dissolve the copper, once it was in solution.
And this turned out to be exactly right. The zinc melted first, then attacked the aluminum, and finally the mix dissolved the copper. This was all done without having to go to an elevated temperature which would have fumed the zinc. The heat up was reasonably gradual in my Plaster of Paris lined propane foundry furnace, and I actually turned off the burner for a short period for a heat soak while the aluminum was dissolving and I pushed it under the surface a few times with a poker. It floats on zinc.
A faster and hotter furnace, might have been problematic, and it may have overheated the zinc before the aluminum even appeared to be dissolving. This brings up an interesting point. If I recall correctly I also once did a named aluminum copper alloy recipe as an experiment here, and successfully melted the copper into the aluminum, while Ironman reported that he'd had trouble melting copper into aluminum.
Since this earlier experiment and my zamak melt are both eutectic (dissolving) processes rather than true melting, maybe the reason for the problem was that his furnace was much faster to reach temperature -- Ironman, if you still read here, could that have been it?
With the furnace's admirable Kaowool insulation, reflective hot coating, and powerful burner, it's possible that the copper never had time to dissolve before the aluminum just had to be poured. As we all know from making lemonade and watching the sugar swirl around at the bottom of a pitcher, dissolving things can take some transitional time. Compared to melting from solid to liquid, which happens within a few degrees.
Anyway, that's how I made Zamak 12, today.
