After quenching the natural aging of the aluminum alloy (such as aluminum - copper) reheated to 200 ~ 250 Â°C, and then quickly cooled to room temperature, the alloy strength decreased, re-softened, the performance returned to just quenched state; such as at room temperature, As with new quenched alloys, normal natural aging can still be performed. This phenomenon is called regression. The explanation for the regression phenomenon is that when the alloy is naturally aged at room temperature, the size of the GÂ·P zone is smaller. When heated to a higher temperature, these small GÂ·P zones are no longer stable and re-dissolve in the solid solution. As soon as it cools to room temperature, the alloy is restored to its new quenched state and can still be naturalized again. In theory, the regression process is not limited by the number of treatments, but in practice, it is difficult to completely re-dissolve the precipitated phases during the regression treatment, resulting in local precipitation during the later aging process, and gradually reducing the time-strengthening effect. At the same time, in the course of repeated heating, the crystal grains of the solid solution have an increasing tendency, which is unfavorable for the performance. Therefore, the regression treatment is only used for repairing the rivet alloy for aircraft, and this phenomenon can be utilized to perform riveting at any time, but it has no use value for other aluminum alloys.