Effect of aging on duality index of an Al-Cu casting alloy

When the Al-Cu casting alloy A201 is aged, a plot of the tensile strength v ersus the elongation to fracture follows a circular pattern. This is in con trast with Al-Si-Mg alloys, which show a linens relationship between the tw o 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 n ot 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 in dex is high and remains nearly constant. When overaged, the yield strength decreases and the strain hardening rate at low strains is very high. At str ains beyond 3-4% the strain hardening saturates. This limits the tensile st rength and ductility, causing the quality index to fall. The circular patte rn 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 conditi on. An analytical model relating the duality index to material properties h as been used to generate a quality: index chart for alloy A201, incorporati ng the change in strain hardening characteristics with aging.