The aluminium alloy profile strengthened by natural aging is quickly heated to a temperature of 210 ° C ~ 250 ° C, held for a short time (30 ~ 300s), and then quickly cooled to room temperature. Parked at room temperature, the normal natural aging can still be carried out.
This phenomenon is called regression phenomenon. If the above process is repeated, the regression phenomenon, the regression phenomenon of hard aluminium alloy profiles, can occur repeatedly.
The regression phenomenon is actually the GP region or metastable phase generated by the naturally aged aluminium alloy profile, which dissolves during rapid and short-term heating and becomes the original quenched state. Therefore, the performance of the aluminium alloy profile is also restored to the performance of the new quenched state, and the regression phenomenon has the following characteristics:
1. All aluminium alloy profile strengthened by natural aging have a regression phenomenon.
2. The regression process can be repeated many times. After each regression processing, its performance cannot be completely restored to the original state, there is always a little gap.
3. The higher the temperature of the regression treatment, the faster the regression process and the shorter the required heating time.
4. The corrosion resistance of the alloy after regression treatment has decreased.
Problems that should be paid attention to when using regression processing to restore plasticity:
1. The temperature of the regression treatment must be higher than the original aging temperature. The greater the difference between the two, the faster the regression and the more thorough the aluminium alloy profile. On the contrary, if the difference between the two is small, the regression is difficult to occur, or even does not occur.
2. The heating time of the regression treatment is generally very short, as long as the low temperature desolubilization phase is completely dissolved. If the time is too long, there will be a desolubilization phase corresponding to the temperature, which will increase the hardness again or over-age, and the regression effect will not be achieved.
3. During the regression process, only the GP area in the precipitation period (Al-Cu alloy also includes the θ" phase) is redissolved, and the products in the precipitation period are often difficult to dissolve.
Since a small amount of products in the desolubilization period are inevitably precipitated at grain boundaries during low-temperature aging, even in the most favorable case, the alloy cannot completely return to the new quenched state, and there are always a small amount of changes in properties that are irreversible. This will not only cause a certain loss of mechanical properties, but also easily cause intergranular corrosion of the alloy, so it is necessary to control the number of regression treatments.
The regression phenomenon has important practical use value in industry. The aluminium alloy profiles strengthened by natural aging can be regressed to soften them for processing. For example, many types and slightly environmental profiles, such as bar bending and forming, are processed after regression treatment. The rivets on the aircraft are riveted after regression treatment, and then they are naturally aged to increase their strength.