IMPROVED MODEL TO PREDICT PROPERTIES OF ALUMINUM-ALLOY PRODUCTS AFTER CONTINUOUS COOLING

Previous models of quench sensitivity of age-hardening alloys have been extended to include loss of toughness as well as loss of yield strength upon postquench aging. Loss of toughness on slow quenching was modeled by the loss of solute to grain-boundary precipitates that promote intergranular fracture. The phenomena are modeled using differential equations, and the model includes temperature-dependent values of the minimum toughness and strength expected after extended isothermal hold times. Time-temperature-property (TTP) curves for the postaging yield strength and toughness were used to provide empirical kinetic and property data for fitting the proposed relationship. The model was tested against experimental data, both nominally isothermal and truly continuous cooling, for an Al-Cu-Li alloy plate. For nominally isothermally cooling, the model proved to be capable of accurately describing the loss of toughness and the loss of strength to a much larger loss in strength than previous models. The model also successfully predicted the loss of strength on continuous cooling but provided a conservative overestimate of the loss of toughness under the same continuous-cooling conditions. It is suggested that this bias arises from the lack of consideration of differences in the microstructure of the precipitates formed during isothermal treatments and those formed during continuous cooling.