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Aluminum quenching heat insulation is performed above the recrystallization temperature. According to the nucleation and growth theory of recrystallization, when the deformed metal is heated to a temperature higher than the recrystallization temperature, a recrystallization core will be formed on the deformed substrate and grown to complete recrystallization.

       However, in the process of aluminum quenching, the recrystallization is more complicated. Depending on the alloy and the extrusion processing conditions, recrystallization may occur at the time of quenching, or recrystallization may not occur. On the same workpiece, a part of the recrystallization occurs and grows into a large grain structure, and the other part does not occur at all. Crystallization still retains the original processed structure, forming a well-defined coarse-grained ring.

This difference in aluminum quenching structure is under what conditions, how to control or reduce this difference, discussed below. For convenience, briefly introduce the two nucleation mechanisms of recrystallization:

(1) Strain induces the grain boundary migration mechanism, namely the nucleation mechanism of grain boundary bowing. When the amount of deformation is small (less than about 40%), the deformation is not uniform, the dislocation density varies from one crystal grain to another, and the size of the cell structure on both sides of the grain boundary is also inconsistent. When heated, it may be at a high-angle grain boundary. The small side of the high-density side of the bow bow out; bow out of the area to become recrystallized nuclei growing up.

(2) subcrystalline workers nucleation mechanism. In the process of heating and heating, the temperature begins to be lower, and in the recovery stage, subgrains and subgrains merge, or the subgrain boundary migrates to grow. When the subgrains grow up, the same number dislocations originally belonging to each subgrain boundary are concentrated on the grown subgrain boundary, which increases the directional deviation and gradually becomes a high-angle grain boundary, resulting in a sharp increase in the migration rate of grain boundaries. The grown subgrains become the core of recrystallization and begin the recrystallization process.

The recrystallized nucleus is an undistorted new crystal region with low energy, and the matrix surrounding the nucleus is still in a state of high energy deformation. The difference in energy storage between the new crystal region and the original deformation region is the driving force of grain boundary migration. After the nucleus is formed, the grain boundary will be propelled to the surrounding deformed matrix under the driving force, so that the crystal nucleus grows. When the deformed matrix is completely replaced by new crystals without distortion, the recrystallization process is completed.

       The squeezing that this book talks about is the extrusion deformation in the hot state. The characteristics of hot extrusion deformation, first, a large degree of deformation, the general deformation rate of 80% -95%; Second, the ingot, work, mold are in a hot state, the temperature is higher, in the plastic deformation cone, due to heat of deformation If the temperature is higher, dynamic recovery may occur, resulting in the combination of subgrains and subgrains, or subgrain migration and growth into a recrystallization core, and then grow up to form recrystallized grains. In fact, under certain conditions, the extruded recrystallized grains can be observed when the 6×× series industrial aluminum profiles are rapidly extruded. After quenching and heating, in addition to the phase transformation in which the second phase dissolves in the solid solution, it also grows into a recrystallized crystal grain on the basis of the original recrystallized core; or the second recrystallized crystal grain grows up to become a large crystal. Granules, so the profile of the aluminum profile is not uniform recrystallized grain structure.

       However, with the profile that did not recrystallize during extrusion, the change in texture during quenching was different from the above. From the sampling inspection after cooling and quenching, it can be seen that no recrystallization occurs at the front end of the general extruded profile product, and the fibrous structure after hot extrusion is still maintained; as the sample is continuously transferred to the rear end of the aluminum profile product to a certain extent, It can be found that recrystallization has occurred in the periphery of the aluminum profile products, while the profile product center is still hot-worked fiber structure, and there is a clear demarcation between its processing structure and recrystallized structure, forming a coarse-grained ring. As the product moves backwards, the peripheral recrystallized portion increases, and the processed portion of the central portion decreases. That is, the coarse-grained ring develops toward the center, and its thickness increases. However, some alloys such as 6063 are extruded under certain conditions, and the front end of the aluminum profile may also form coarse-grained ring structures.

       Different alloy quenching organizations will be different. Some completely recrystallized, there is no coarse-grained ring; some products do not recrystallize at the front end, recrystallization occurs at the outer part of the rear end, forming a coarse-grained ring; some recrystallizes from the outer periphery of the front-end to form a coarse-grained ring. , After the aluminum products, the coarse crystal coarse depth increases. The coarse-grained rings will have adverse effects on product quality and must be controlled. Some may not even allow coarse-grained rings to exist. The following discussion focuses on the formation of coarse-grained rings.

        As described in the extrusion section, in the positive extrusion, the degree of deformation of the product is non-uniform in both lengthwise and cross-sectional directions, increasing from front to back along the length direction, and increasing from the inside to the outside in the cross-section. At the same time, in the extrusion process, deformation heat is generated in the deformation zone, which causes the metal temperature to rise and dynamic recovery occurs. The subgrain merges with the subgrain, and the same number dislocation aggregates to form a multilateral organization, but no undistorted crystalline core is formed. . The lower part of the deformation of aluminum products in the first part and the center of the product than the deformation of the larger part of the product after the first part and the outer part of the external friction due to the existence of the impact of the deformation of the metal after the deformation of the chip is different, the crystal lattice distortion The difference is that part of the deformation after deformation has a higher energy storage and the smaller part of the energy storage is low. Therefore, in the quenching and heating process, in the area with large deformation degree, due to large distortion, high dislocation density, high internal energy, subgrain merger or subgrain boundary migration occurs, and the growth of the subgrain belongs to each sub-region. The same number dislocations on the grain boundary are concentrated on the subgrain boundary, increasing the directional deviation and forming a high-angle grain boundary. The high-angle grain boundary bows out, the grain boundary migration occurs, and the recrystallization core is formed; the recrystallization core grows up. That is, recrystallization is completed. In regions where the degree of deformation is small, in addition to the dissolved phase transformation, no recrystallization core is formed and the original fibrous structure remains.

       Normal extrusion products generally have a smaller degree of deformation in the head and center portion, a more uniform deformation, a small distortion energy stored after deformation, and quenching does not occur recrystallization, so no coarse crystal rings appear. Therefore, the positive extrusion is reverse extrusion, and the friction between the extruded barrel wall and the ingot is eliminated when the extrusion is performed, so that the deformation degree and the uniformity on the section of the entire aluminum product are along the length direction, except for the final extrusion stage. In an extremely small part, due to insufficient metal replenishment, the degree of deformation at the periphery of the section or the center of the section increases, and the distortion can increase. After the quenched product, the length of the tail is very short, and a coarse-grained ring with a very shallow depth appears. The rest of the length is equivalent to positive. The degree of deformation and the uniformity of deformation at the start of the extrusion and the basic extrusion stage of the extrusion are maintained so that the fibrous structure during processing is maintained, and no coarse crystal rings are produced.

Changing the extrusion method can eliminate the influence of coarse-grained rings and greatly improve the microstructure and properties of aluminum profiles. The improvement of the forward extrusion process can also reduce the occurrence of coarse-grained rings to a certain extent and improve the organizational performance of the products.

       The non-uniformity of the deformation during the extrusion is due to the non-uniform friction. In forward extrusion, the ingot and the barrel wall are rubbed. The greater the friction, the slower the flow rate on the surface of the ingot. There is no external friction in the center of the ingot, only the internal friction, the internal friction is greater than the external friction. Much smaller, so the center flow rate is faster. Therefore, reducing the friction can reduce the flow rate difference. Similarly, the speed of the extrusion also affects the non-uniformity of the flow. The slower the extrusion speed, the slower the flow rate of the aluminum product, and the difference in flow rate at different parts of the same section. As the flow rate difference decreases, the relative ratio of flow rates may not change significantly, but the absolute difference peaks are significantly reduced. When the large peak of the absolute value decreases below the minimum value at which recrystallization nucleation occurs, recrystallization does not occur, and no coarse crystal ring occurs. Therefore, all measures that reduce the metal flow rate can improve the non-uniformity of metal flow and reduce the chance of coarse-grained rings. Increasing the temperature of the extruding tube and the temperature of the ingot can reduce the deformation resistance, that is, the frictional force is reduced, the non-uniformity of the metal flow is reduced, the extrusion speed is reduced, and the unevenness of the metal flow is also reduced. To increase the extrusion temperature, hard alloys and superhard alloys must be accompanied by a decrease in the extrusion speed. If the extrusion speed is not reduced, the temperature rise caused by the heat of deformation coupled with the increased extrusion barrel temperature and the temperature of the ingot may exceed the critical temperature of the deformation, resulting in cracking and scrapping of the aluminum profile product. Therefore, increasing the extrusion temperature and lowering the extrusion speed can reach or approach the conditions at the beginning of the basic extrusion stage, avoiding the occurrence of coarse grain rings or reducing the depth of the coarse grain ring. On the other hand, if an alloy such as 6061 as described above is reduced in extrusion temperature and the extrusion speed is maintained, it is possible to start from the front of the product and recrystallize on the outer periphery of the product to form a clear coarse-grained ring.