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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:

phase diagram for Al-Bi

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)?

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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.

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  • $\begingroup$ The melting point of silicon is 2500 F. But if I add silicon powder to the molten aluminum at 1200F, will it alloy? or will the silicon stay solid? $\endgroup$ – Ivan Viti Aug 6 '20 at 11:23
  • $\begingroup$ Also does it have to be pure silicon, or will SiO2 work too? $\endgroup$ – Ivan Viti Aug 6 '20 at 11:35
  • $\begingroup$ The ASM reference lists pure silicon or aluminum / silicon mater alloy to add Si, which sounds typical. Apparently sand ( Si O 2) won't work. The 12 Si alloy is A 132, also a 9 Si alloy D132. Also if melting temperature is your only criteria; 10 Mg melts at about 860 F. Also Zn alloys . As I suggested ,get a book. $\endgroup$ – blacksmith37 Aug 6 '20 at 15:19
  • $\begingroup$ are there any books you recommend? I assume there aren't any books titled "diy metallurgy for backyard enthusiasts". I doubt my forge can get past 1500F consistently. $\endgroup$ – Ivan Viti Aug 6 '20 at 17:25
  • $\begingroup$ American Society for Metals has many publications , there should be something to help. But I doubt anything for the "backyard". $\endgroup$ – blacksmith37 Aug 8 '20 at 15:38
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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.

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  • $\begingroup$ so if I add molten aluminum at 660 C to molten bismuth at 660 C, would it instantly solidify when mixing? or would it stay melted but separated? $\endgroup$ – Ivan Viti Aug 6 '20 at 11:26
  • $\begingroup$ @IvanViti I don't know what it would do, that's the point. $\endgroup$ – Tiger Guy Aug 19 '20 at 5:01

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