I have seen many articles and books that most of the Aluminium alloys are having 4%cu (Al-4%Cu-0.5%Mg) and explained it as good for structural applications. But most of the guys not explained why only 4% cu is used.
Because solubility of copper in aluminum is limited in solid state: it approaches 5.5% in binary alloy but is even reduced by the addition of other alloying elements. As a consequence, a chemical threshold is in place.
I think, you are referring to 2024 Al alloy which is age hardenable. If you look at the Al-Cu binary phase diagram you will note that the terminal solid solubility of Cu in Al is 5.65 wt% or 2.53 at% at 548.2 deg C. The solid solubility decreases as the temperature decreases. The super saturated copper will precipitate out as CuAl2 (theta phase) and improve the mechanical properties.
The question is why only 4% not 5.65%? As answered by Vittorio Alfieri, we need other alloying additions such as Si, Mg, Zn etc to increase the hardenability, castability, and formability etc. These solid solution strengtheners are going to decrease the solubility limit of the copper. 4wt% Cu gives optimal mechanical and fabrication properties. More than 4 wt% Cu may result in a large vol% of theta phase that may be incoherent with the matrix and lower the corrosion resistance. Lower than 4wt% Cu may not be sufficient to precipitate harden the alloy (otherwise you have to increase the Mg and Zn content).
Krishnan Raja Thank you very much sir for giving clear idea about my questions.
Al thought eh solubility limit of Cu in Al is 5.6 wt% at 548 C, one will encounter other problem of incipient melting during treatment of duralumin. This will cause degradation of mechanical properties.