Interrupted aging and secondary precipitation in aluminium alloys

It is shown that, if elevated temperature aging of aluminium alloys is interrupted with a dwell period at a low temperature (65 °C), age hardening continues due to so called secondary precipitation. If elevated temperature aging is then resumed, significant improvements can be obtained in mechanical properties compared with those available using a conventional T6 temper. Average increases in 0.2% proof stress and tensile strength of 10%, combined with improved fracture toughness, have been achieved in a wide range of alloys. These effects arise primarily because interrupted aging promotes the formation of more finely dispersed precipitates in the final microstructures. The concept of interrupted aging is described in some detail with respect to the model system, Al-4Cu, and examples are then given of the effects of the treatment on the microstructures and mechanical properties of several wrought and cast aluminium alloys.