Most commercial die cast alloys contain less than 5% copper; up to this level, a solid solution of copper in aluminum is formed on solidification but on further cooling, the CuAl2 compound is precipitated from solution.
Generally copper is added to aluminum alloys to improve strength and hardness: between 2.8 to 5% is necessary to give good response to heat treatment. Lower copper contents have some hardening effect but the alloys are not responive to heat treatment. Corrosion resistance is reduced by copper, and for optimum resistance the copper content should be less than .05%. As copper content increases, there is a gradual increase in corrosion attack.
Street, A.C. (1977) The Die Casting Book. Red Hill, England. Portcullis Press, Ltd