Effect of aging on quality index of an Al–Cu casting alloy

When the Al–Cu casting alloy A201 is aged, a plot of the tensile strength versus the elongation to fracture follows a circular pattern. This is in contrast with Al–Si–Mg alloys, which show a linear relationship between the two parameters when aged. The latter observation is the basis for the concept of quality index (Q) which is widely applied to Al–Si–Mg alloys but does not seem to be valid for alloy A201. As alloy A201 is aged up to peak aging, the yield strength increases while the ductility decreases. The quality index is high and remains nearly constant. When overaged, the yield strength decreases and the strain hardening rate at low strains is very high. At strains beyond 3–4% the strain hardening saturates. This limits the tensile strength and ductility, causing the quality index to fall. The circular pattern shown by the quality index results from the transition from the high Q value of the underaged condition to the low Q value of the overaged condition. An analytical model relating the quality index to material properties has been used to generate a quality index chart for alloy A201, incorporating the change in strain hardening characteristics with aging.