Why does aluminum coil curl?

In the process of making aluminum coil, in order to help change its shape, we need to heat it at high temperature. It will be softer at high temperature, and its shape is easier to undergo some changes. Adjust the shape. After that, the temperature can be lowered to maintain a certain hardness. However, in some cases, after the high-temperature cooling is completed, the aluminum coil will have some curling problems. What is the reason?

The bending phenomenon occurs during cooling. The reason is: under general air-cooling conditions, the heat transfer coefficient of each part of the aluminum coil in contact with the air is the same, but because the wall thickness or shape is different, the heat dissipation rate of each part is uneven, resulting in The heat dissipation speed of the thick-walled part or hollow tube is slower than that of the thin-walled part, which will eventually cause it to bend to the hollow part or the part with thicker wall when cooling.

The reason and process of bending are as follows:

1. The temperature of the thin-walled part drops quickly, and the contraction force occurs first, and the temperature of the thick-walled part or the hollow tube part drops slowly, and there is almost no contraction force;

2. The cross-sectional area of ​​the thin-walled part is small, and the shrinking force generated is small, or it is eliminated by the traction force of the tractor;

3. When leaving the tractor, the temperature continues to drop;

4. The thick-walled part or the hollow tube has a large cross-sectional area, and gradually produces a larger shrinking force as the temperature drops, and the temperature of the thin-walled part has dropped significantly, and no shrinking force is generated or the shrinking force is small;

5. The shrinkage force on the cross section of the aluminum coil is uneven, and it is bent toward the thick wall part or the hollow tube part in the extrusion direction.