(1) Hot working
Hot working refers to the plastic forming process of aluminum and aluminum alloy ingots above the recrystallization temperature. The plasticity of hot-processed ingots is higher and the deformation resistance is lower. It can produce products with larger deformation amount with equipment of smaller capacity. In order to ensure the organizational performance of the product, the heating temperature, deformation temperature, deformation speed, deformation degree, deformation end temperature and cooling speed after deformation of the workpiece should be strictly controlled. Common hot working methods of aluminum alloys include hot extrusion, hot rolling, hot forging, hot upsetting, liquid die forging, semi-solid forming, continuous casting and rolling, continuous casting and rolling, continuous casting and extrusion, etc.
(2) Improvement of cast structure by hot deformation
Aluminum alloys have high plasticity and low resistance at high temperature. In addition, the atomic diffusion process is intensified, accompanied by complete recrystallization, which is conducive to the improvement of the structure. Under the condition of dominant triaxial compressive stress state, hot deformation can most effectively change the cast structure of aluminum and aluminum alloys: given an appropriate amount of deformation, the cast structure can undergo the following favorable changes.
① Generally, thermal deformation is completed through repeated deformation in multiple passes. Since the hardening and softening processes occur simultaneously in each pass, the deformation breaks the coarse columnar grains, and the repeated deformation makes the material structure become more uniform and fine equiaxed grains. At the same time, some tiny cracks can be healed.
② Due to the effect of hydrostatic pressure in the stress state, the bubbles in the cast structure can be welded, the shrinkage cavities can be compacted, and the looseness can be compacted to become a denser structure.
③ Due to the enhanced thermal motion ability of high-temperature atoms, under the action of stress, with the help of the free diffusion and heterodiffusion of atoms, the ingot chemical composition is relatively reduced. Through thermal deformation, the ingot structure is changed into a deformed structure (or processed structure), and has a higher density, uniformly fine equiaxed grains and relatively uniform chemical composition, so the plasticity and strength indicators are significantly improved. Control of the particle size of hot-shaped products
(3) Control of grain size of hot-deformed products
The grain size of the product after hot deformation depends on the degree of deformation and the deformation temperature (mainly the final processing temperature). When processing aluminum and aluminum alloy materials within the temperature range of complete softening, in order to obtain uniform and fine grains, the deformation amount of each pass should be greater than the critical deformation degree. Usually, the deformation amount of each pass should be greater than 10%. For example, the critical deformation degree of 2024 alloy is 2%~8% when the deformation speed is high (such as impact deformation), and should be greater than 10% when the deformation speed is low (such as die forging or extrusion on a hydraulic press).
(4) Fiber structure during hot deformation
During the hot deformation process, the grains, impurities, second phases and various defects inside the metal will be elongated and thinned along the main deformation direction of maximum extension, and the strength in the direction of fiber formation is higher than the strength in other directions of the material (more obvious when there is an extrusion effect), and the material shows different degrees of anisotropy. In addition, deformation texture and recrystallization texture may also be generated simultaneously during hot deformation, which will also make the material directional and non-uniform.
(5) Recovery and recrystallization during thermal deformation
During thermal deformation, aluminum and aluminum alloy materials generally undergo dynamic recovery and recrystallization:
① Recovery of aluminum and aluminum alloys during thermal deformation.
The stacking fault energy of aluminum and its alloys during thermal deformation is large, and the self-diffusion energy is small. At high temperatures, dislocation slip and climb are relatively easy to carry out. Therefore, dynamic recovery is their only softening mechanism during thermal deformation. After high-temperature deformation, the aluminum alloy material is immediately observed, and a large number of recovery subgrains can be seen in the organization. Keeping the dynamic recovery organization has been successfully used to improve the strength of 6063 alloy building extrusion profiles.
② Recrystallization of aluminum during thermal deformation.
After the thermal deformation enters the steady state, comprehensive dynamic recrystallization occurs inside the aluminum material. As the deformation continues, recovery and recrystallization are repeated, and its organizational state does not change with the increase in deformation. However, the organization of aluminum softened by dynamic recrystallization is generally difficult to maintain because static recrystallization occurs quickly after the thermal deformation is completed and replaces the "processing structure". Therefore, the recrystallization during thermal deformation includes dynamic recrystallization that occurs simultaneously with deformation and static recrystallization that occurs during cooling after the deformation is completed between each pass. However, the main softening factor during thermal deformation is dynamic recrystallization. The research results show that the critical deformation degree of dynamic recrystallization is very large; dynamic recrystallization is easy to nucleate at grain boundaries and subgrain boundaries; since the critical deformation degree of dynamic recrystallization is much larger than that of static recrystallization, static recrystallization will occur immediately once the deformation stops; the higher the deformation temperature, the shorter the time required for dynamic recrystallization and static recrystallization.
