What alloy am I making (Al-Bi)

Question

I have been melting aluminum in a DIY furnace for a few months in order to make lost wax casts. I was thinking that since bismuth has a much lower melting point than aluminum, that I could add a small amount of bismuth to lower the melting point (9 parts Al, 1 part Bi). Qualitatively this seems to be working. But I am confused.
Looking up a phase diagram for Al-Bi, I find:

Which if I understand it correctly seems to mean that the melting point actually went up. Am I reading it correctly? If both metals separately have a lower melting point than the alloy at that ratio (and I am not reaching the new melting point), then what am I making?
Besides lowering the melting point, my hope was also to lower the viscosity so that the mix fills in my mold better. Will adding bismuth help accomplish this goal? Should I be adding tin instead? Is there a tin-bis-al ratio that would yield minimum viscosity at temperatures below 1500 °F (ideally mostly made out of Al)?

blacksmith37 · Accepted Answer

You are not making anything ,usable. Bi does not raise the melt point until over 3.4 % Bi, that is lost in this diagram. If your aim is to make aluminum castings , look in a book and do it the way the rest of the world does, add Si. At about 12% Si the melting point is down to about 998 F, about as low as you will get . And,suprise, this is a common composition for aluminum alloy castings.

Correct answer by blacksmith37 on December 26, 2020

Tiger Guy · Answer

Combining metals does not result in uniform melting points according to the melting points of the components.  A classic example is tin & lead, the melting point of which varies according to the ratios, but I believe is always less than either of the component metals.
I supposed we'd need a chemist to explain why this is, but I submit that you should just not expect that melting points should be so straightforward.

What alloy am I making (Al-Bi)

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