Thermoplastic deformation mechanism of metal forgings

2019-03-14 11:54:59 4

The metal deformation mechanism of metal forgings mainly includes: intragranular slip and twinning, grain boundary slip and diffusive creep. Among them, intragranular slip is the most important and common, and the twins occur mostly at high temperature and high speed deformation. For hexagonal metal, this mechanism also plays an important role. The grain boundary slip and diffusion enthalpy are only changed at high temperature. Play a role. With the change of thermal deformation conditions (such as deformation temperature, strain rate, three-dimensional compressive stress state, etc.), the mechanism and role of these mechanisms in the plastic deformation of forgings also change.

(1) intragranular slip

Under normal conditions, the main mechanism of thermal deformation of forgings is intragranular slip. This is due to the increase of the atomic spacing at high temperatures, the increase of thermal vibration and diffusion speed of the job, the slippage, climbing, cross-slip of the dislocations and the anchoring of the dislocation nodes are easier than the low temperature, and the slip system is more The flexibility of the slip is improved, the coordination of deformation between the crystal grains is improved, and the hindrance of the grain boundary to the dislocation motion is weakened.

(2) In the grain boundary sliding thermoplastic deformation, since the grain boundary strength is lower than that in the crystal, the grain boundary sliding is easy to proceed, and the damage caused by the grain boundary sliding is eliminated in time due to the increase of heat diffusion. Therefore, the deformation of the grain boundary sliding is larger than that of the cold deformation. The effect of the three-direction compressive stress will repair the crack generated by the high-temperature grain boundary slip in time by the plastic cohesive welding effect, resulting in large intergranular deformation. Nevertheless, under conventional thermal deformation conditions, the grain boundary slip is small relative to the amount of intragranular slip deformation. The grain boundary sliding mechanism plays a major role only under the superplastic deformation of fine grains, and the grain boundary sliding is carried out under the condition of diffusion creep.

(3) Diffusion creep diffusion creep is caused by the directional movement of vacancies under the action of the stress field. Under the action of the stress field, the vacancy concentration of the grain boundary of the tensile stress is higher than that of other parts. Due to the difference in the chemical potential energy of the vacancies in each part, the directional movement of the vacancies is caused, that is, the vacancies are released from the grain boundaries perpendicular to the tensile stress, and are absorbed by the grain boundaries parallel to the tensile stress. According to the different diffusion pathways, it can be divided into intragranular diffusion and grain boundary diffusion. Intragranular diffusion causes elongation deformation of crystal grains in the direction of tensile stress, or shortening deformation in the direction of compression; and diffusion of grain boundaries causes "rotation" of crystal grains.

The diffusive creep of forgings will continue to occur with the continuation of time even under low stress induction, but the speed is very slow. The higher the temperature, the finer the grains and the lower the strain rate, the greater the effect of diffusion bats. This is because the higher the forging temperature, the greater the kinetic energy and diffusion capacity of the atom; the finer the grain, the more the grain boundary and the atomic diffusion path are shorter; and the lower the strain rate, the more abundant The time to spread. In the plastic deformation below the recovery temperature, the effect of this deformation mechanism is not obvious, only necessary at low strain rates, and plastic deformation at high temperatures of forgings, especially in superplastic deformation and isothermal In forging, this diffusive creep plays a very important role.