The effect of deformation degree on flow stress in copper tube rolling is that the flow stress increases with the increase of deformation amount at low deformation temperature, but the increase trend of flow stress decreases with the increase of deformation amount. When the flow stress increases to the maximum with the increase of deformation amount, the deformation amount continues to increase and the flow stress decreases, which indicates that dynamic recrystallization has taken place. At high temperature, the flow stress changes little with the increase of deformation, that is, the flow stress reaches the maximum when the deformation is small, and then increases slowly with the increase of deformation. With the change of deformation temperature, the influence of deformation amount on flow stress also changes. In the plastic deformation stage, before a certain amount of deformation, on the one hand, the increase of dislocation density leads to the hardening effect; on the other hand, the cross slip of screw dislocation and the climbing movement of edge dislocation related to diffusion make some dislocations disappear, the rearrangement of some dislocations results in a recovery softening mechanism, and the hardening effect is greater than the softening effect. Therefore, the flow stress increases gradually. When the strain increases gradually, the rate of dislocation disappearance also increases, therefore, the work hardening rate decreases gradually and the slope of flow stress-strain curve becomes smaller and smaller. The softening effect of dynamic recrystallization and the hardening effect of dislocation multiplication reach Dynamic equilibrium when the deformation amount reaches the critical deformation amount of dynamic recrystallization. As the deformation continues to increase, the flow stress remains basically unchanged.

The effect of temperature on the flow stress can be seen from the flow stress-strain curve. The higher the deformation temperature is, the lower the flow stress is. Copper is the material of face-centered cubic structure, and there are many slip systems. With the increase of deformation temperature, the thermal motion of atoms is intensified, the kinetic energy is increased, the binding force between atoms is weakened, the frequency of atom transition is increased, and the concentration of vacancy increases, the self-diffusion of atoms in metals, the climbing and grading of edge-type dislocations, the ability of screw-type dislocations to release vacancies are enhanced, the mutual destruction of dislocations is more obvious, the density of equilibrium dislocations is reduced, and the critical shear stress of deformation is reduced, it is possible that a new sliding system may be initiated, leading to a reduction in the flow stress during copper deformation. The essence of the plastic deformation of copper tube is the movement of dislocations, which will lead to the instability of the internal structure of the deformed metal and cause the work hardening at any stage of the deformation. However, with the increase of temperature, dynamic recovery and recrystallization may occur. The dynamic recovery and recrystallization can stabilize the internal structure, reduce the work hardening effect, and soften the deformation Jinli by the diffusion of point defects and dislocation migration, nucleation and growth of grains. With the increase of deformation temperature, the critical shear stress decreases and the slip system increases. The change of grain boundary properties is beneficial to intergranular deformation and elimination of intergranular damage. In addition, a new deformation mode may appear in the plastic deformation process to reduce the work hardening. When the temperature is low, it is mainly slip, twinning, kink band and deformation band. The mechanism of diffusion and intergranular sliding may occur when the temperature increases, which reduces the work hardening effect. 

Source: Web Collation

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