Knowledge

I. Explanation of Recrystallization Concept

1. Recrystallization refers to the crystallization process of solid metals and alloys without phase transformation. It is the process of softening metals or alloys that have undergone work hardening without phase transformation.

2. After cold deformation occurs in metal extrusion and then heating to a certain temperature, new undistorted grains will be regenerated in the deformed structure, and its properties will change significantly, returning to a completely softened state. This process is called recrystallization. That is, recrystallization is the process of nucleation and growth of various crystal nuclei.

3. The first process that occurs during recrystallization is the recovery process. At this stage, atoms undergo short-range limited diffusion and migration, and only point defects and dislocations move. Their migration and coalescence help eliminate the internal stress generated by cold deformation. However, the characteristics of cold deformation still exist, and only some substructures and physical properties change, with part of the energy stored during cold deformation being released.

4. Recovery annealing can also be called stress relief annealing.

5. After the recovery process occurs, if the metal is heated to a higher temperature, the diffusion and migration ability of atoms increase, and new grains will nucleate (as shown in Figure 3) and grow into soft equiaxed grains, with all internal stress completely released. This stage is called recrystallization.

6. After recrystallization is completed, the normal grains should be uniform and continuous. However, in some cases, the growth of grains is only the sudden and rapid coarsening of a few grains, resulting in an increasing size difference between grains (uneven grain size). This uneven growth of grains is like the occurrence of recrystallization again in the uniformly fine equiaxed grains after recrystallization, which is called secondary recrystallization. Materials with coarse grains formed by secondary recrystallization are generally not accepted by the market. Therefore, how to control secondary recrystallization is a key point for factory technicians.

7. The driving force for primary recrystallization is the reduction of stored energy due to deformation, while the driving force for secondary recrystallization is the reduction of interfacial energy.

 

II. Hot Working and Grain Structure of Aluminum Alloys

1. When the processing temperature of aluminum alloys is above 350°C, it is called hot working. Hot working is the simultaneous effect of the stored strain of processing and high-temperature processing annealing.

2. The processing temperature, processing amount (such as extrusion ratio, rolling ratio or forging ratio), processing speed, homogenization state, and alloy elements or alloy types all affect the recrystallization behavior.

3. The larger the processing amount, the lower the recrystallization temperature and the smaller the recrystallized grains. If the processing amount is too small, recrystallization may not be completed due to the high recrystallization temperature, resulting in a fibrous structure or a mixed structure of elongated recrystallized grains and fibers.

4. Mixed recrystallization and fibrous grain structures that are not completely recrystallized are commonly found in high-strength alloys, such as 7000 and 2000 series alloys.

5. If the processing temperature during hot working is very high, the amount of accumulated deformation energy or dislocations during processing becomes very small, and the recrystallization temperature of the fibrous structure material after high-temperature processing becomes higher. For high-strength alloys such as 2000 and 7000 series alloys, when the processing temperature exceeds 400°C, recrystallization during subsequent solution treatment becomes very difficult.

6. The recrystallization behavior of cast ingot hot working is similar to that of cold working + annealing heat treatment, but there are some differences. During the recrystallization process of hot working, the accumulated strain is already very small, so some high-alloy materials cannot be completely recrystallized after hot working.