An aluminum alloy is a chemical composition where other elements are added to pure aluminum in order to enhance its properties, primarily to increase its strength.
These other elements include titanium, chromium, copper, manganese and nickel at levels that combined may make up as much as 15-25 percent of the alloy by weight.
Alloys are assigned a four-digit number, in which the first digit identifies a general class, or series, characterized by its main alloying elements.