Briefly describe the media and cooling methods for aluminum alloy annealing and cooling

The annealing and cooling speed of aluminum alloy must be sufficient to avoid the precipitation of coarse excess phases. On the other hand, due to the low strength of aluminum alloys, despite good thermal conductivity and small internal stress generated by quenching, distortion and cracking are still inevitable. Controlling the quenching transfer time and selecting appropriate quenching media are important measures to ensure the quality of aluminum alloy annealing.

(1)Quenching transfer time. Quenching transfer time refers to the time it takes for the workpiece to be transferred from the heating furnace to the quenching tank. If the transfer time is too long, the supersaturated solid solution will decompose during the transfer process, causing the strength of the alloy to significantly decrease after aging and the corrosion resistance to deteriorate.

Generally, when the thickness of aluminum alloy is less than 4mm, the quenching transfer time shall not exceed 30s. When the number of batches of workpieces to be quenched at the same time increases, the transfer time can be increased. It can be increased to 20-30s for hard aluminum and forged aluminum alloys, and to 20-30s for super-hard aluminum. Can be increased to 25s.

(2)Quenching cooling medium and cooling method

1) Water temperature adjustment and quenching. The most commonly used quenching medium for aluminum alloys is water. The quenching cooling characteristics of water are related to the water temperature, and the cooling rate decreases as the water temperature increases. Water at normal temperature has a large cooling capacity, and the maximum cooling speed can reach more than 750 degrees. If the cooling is too fast, large internal stress will be generated in the workpiece, resulting in quenching distortion or cracks. The quenching cooling rate close to the ideal can be obtained by adjusting the water temperature. Generally, a quenching device that adjusts the water temperature can be used to achieve this. The water temperature during production should generally be maintained within the range of 10 to 30 degrees. For large workpieces with complex shapes, in order to prevent distortion and cracking, the water temperature can be raised to 30-50°C. Under special circumstances, the water temperature can be increased to 80°C.

2) Liquid-gas atomization medium. Liquid-gas atomized medium quenching is a type of spray quenching. General liquid jet cooling quenching has a larger cooling speed and cooling intensity than immersion cooling quenching. Liquid-gas atomized medium cooling quenching, especially light mist spray quenching, makes the cooling speed of the workpiece relatively gentle and can be adjusted within a wide range. It is suitable for aluminum alloy workpieces with different thicknesses and different requirements for cooling speed.

3) Graded quenching. In order to reduce distortion during quenching of forgings and die forgings, a graded quenching process can be used after solid solution heating, in which they are first held in a medium with a higher temperature for a short period of time and then cooled in room temperature water.

4) Quenching of polymer aqueous solution. The cooling rate of the polymer aqueous solution is between room temperature water and boiling water. Water temperature adjustment is used for quenching. The temperature is difficult to control and consumes a lot of energy. Therefore, the use of polymer aqueous solution quenching is a promising new quenching process.

There are nearly 20 kinds of polymer aqueous solutions as quenching media. Among them, aqueous solutions of polyvinyl alcohol, polyether, and polyethylene oxide have begun to be used in production. The polymer aqueous solution has inverse solubility. When the hot workpiece is quenched into it, the liquid temperature around the workpiece rises sharply, the polymer precipitates from the water, the medium becomes turbid, the viscosity rises, and a continuous and uniform film is formed on the surface of the workpiece. As the workpiece cools, the film gradually dissolves, which accelerates the cooling of the workpiece at low temperatures. In this way, the workpiece has a relatively uniform cooling rate at both high and low temperatures, thereby reducing distortion and cracking of the workpiece due to uneven cooling. For workpieces with different thicknesses and complex structures, due to this self-adjusting cooling process, all parts of the workpiece can be cooled nearly uniformly to prevent distortion.

During heat treatment, cooling of the workpiece is essential. This article briefly introduces the quenching cooling medium and cooling method. If you have any questions, you can call us for consultation.