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According to this phase diagram, the melting point of 30% copper aluminum bronze is around 548°C.

Lets say I create molten aluminum at 680°C, can I add small pellets of copper and swirl them around to create a molten alloy? or will the alloying process not start until Copper's melting temp has been reached at around 1080°C?

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(Short answer below) A phase diagram like the one you posted is to be intended true for a solution(either solid or liquid) of the two components(i.e. Al and Cu). Adding Cu to molten Al does not properly mean to have a solution(the two components must be dispersed in thw whole bath in a uniform way). Also a phase diagram is drawn as a "succession of equilibrium state", thermodynamic gergon for "waiting a long long time between any temperature or concentration variation".

This means that if you put Cu pellet in a molten Al bath the following happens: slowly the outer shell of copper will dissolve into Al (as tey are soluble as the diagram shows at the 30% wt%), at a certain point all the Cu will be melt in the bath. How long will it take? It is difficult to say(it can be computed though), depends on diffusion coefficient, temperature, stirring, ...

What is done for a practical application: Al and Cu are melt at about 1100°C (melting point of Cu) and then cooled down slowly below 582°C. When I say slowly I mean really slowly in order for the proper grain structure of the eutectic to be reached and to be uniform. I am not sure for the specific case but rather often this process has to be carried out in a controlled temperature environment for the cooling to be slow enough, this means atmospheric air cooling is often non viable.

TL;DR

From a thermodynamic point of view adding the copper pellets will cause them to melt even at the lower temperature but from an engineering/practical point of view it will take too long and material properties might not be the desidered ones.

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I suggest using established alloys; there is no alloy in the range of 30 % Cu. Aluminum bronze is 12 % Al maximum and it is hard and brittle. Normally an aluminum bronze is no more than 9 % Al. Aluminum alloys with Cu contain no more than a couple % Cu. Either way it dissolves easily , at temperatures modestly above the aluminum melting point. Think of dissolving salt in water , you don't need to melt the salt to dissolve it.. Stirring will be necessary to make it uniform,( argon bubbling ?). You are not describing aluminum bronze which requires Fe and Mn , and is significantly improved with Ni.

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  • $\begingroup$ If you plan an ornamental statue ,composition may not be important . But if you want structural parts , the ratios of Cu, Fe, and Mn are critical , just getting them into specification ranges is not sufficient. . $\endgroup$ – blacksmith37 Aug 27 at 19:03

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