What Is Plastic Deformation?

Plastic deformation is a deformation that cannot be recovered by itself. Engineering materials and components will undergo permanent deformation after being loaded beyond the elastic deformation range, that is, unrecoverable deformation or residual deformation will occur after the load is removed, which is plastic deformation. Not all engineering materials have the ability to plastically deform. Metals, plastics, etc. have different degrees of plastic deformation ability, so they can be called plastic materials. Brittle materials such as glass, ceramics and graphite have no plastic deformation ability. When designing engineering components, obvious plastic deformation is generally not allowed, otherwise the components will not be able to maintain their original shape or even break. ^[1]

The material deforms under the action of external force, and after the external force is removed, the elastic deformation part disappears, and the part that cannot be recovered and remains is the plastic deformation.

Solid metal is made up of a lot

The plastic deformation of a metal at room temperature has a great impact on the structure and properties of the metal, and work hardening, internal stress, and anisotropy often occur.

Plastic deformation work hardening

Dislocations multiply due to plastic deformation, dislocation density increases, and dislocations in different directions occur

Principles of Plastic Deformation Mechanics

During the delivery, the movement of dislocations is hindered, causing the work hardening of the metal. Work hardening can increase the hardness, strength, and resistance to deformation of the metal, and at the same time reduce plasticity, making subsequent cold deformation difficult.

Internal stress of plastic deformation

The distribution of plastic deformation in the metal body is not uniform, so after the external force is removed, the elastic recovery of each part will not be exactly the same, which will cause the internal stress, that is, residual stress, to be mutually balanced between the parts in the metal body. Residual stress reduces the dimensional stability of the part and increases the tendency to stress corrosion.

Plastic deformation anisotropy

After the metal undergoes plastic deformation in the cold state, slip bands or twin bands appear inside the grains. Each grain is also elongated and twisted along the deformation direction. When the amount of deformation is large (such as 70% or more) and it is along one direction, the orientation of the atom arrangement in the grains tends to be the same, and the inclusions inside the metal are also elongated along the deformation direction to form a fibrous structure. Anisotropy the metal. The strength, plasticity and toughness in the deformation direction are higher than those in the transverse direction. When the metal is deformed in the hot state, the orientation of the crystal grains will deviate from the deformation direction to different degrees due to recrystallization, but the direction of the fiber formed by the elongated inclusions will not change, and the metal will still be anisotropic.

Plastic deformation recrystallization and recovery

After the metal undergoes cold deformation, if it is heated to a certain temperature and held for a certain time, when the activation energy of the atoms is increased to a sufficient force, new nuclei will appear and grow into new grains. This phenomenon is called re-crystallize. After the recrystallization process, the grain distortion caused by cold deformation and the work hardening and residual stress caused by it will be completely eliminated.

Recrystallization temperature

The recrystallization temperature of the metal is usually the temperature at which recrystallization is completed after a one-hour incubation. The recrystallization temperature of various metals is approximately 40-50% of the melting point of the metal in terms of absolute temperature (K). The recrystallization temperature of mild steel is about 460 ° C. When the degree of deformation is small, the recrystallized grains are particularly coarse during the recrystallization process, especially when the temperature is too high. Therefore, if the grain size is to be fine, the metal material will have a large amount of deformation before the recrystallization treatment.

The recrystallization temperature is very important for the plastic working of metal materials. Plastic working and deformation performed above the recrystallization temperature are called hot working and hot deformation; plastic working and deformation performed below the recrystallization temperature are called cold working and cold deformation. During thermal deformation, the metal material continuously recrystallizes during the deformation process, which does not cause work hardening. If it is slowly cooled, internal stress will not occur. ^[3]

Reply

When the metal after cold deformation is heated to a temperature slightly lower than the recrystallization temperature, the defects of the crystal will be reduced by the diffusion of the atoms, the distortion energy of the crystal will be reduced, and the internal stress will be reduced; but no new grains will appear and the metal will remain Work hardening and anisotropy, this is the recovery of the metal. Such heat treatment is called stress relief annealing. ^[3]

Deformation and plasticity

The amount of plastic deformation is often expressed by different indicators depending on the deformation mode. Some use the change in cross-sectional area before and after the deformation of the blank, some use the change in length in one direction, and the size of the corner when twisting. The amount of upsetting and compression deformation is commonly expressed in engineering. Such as the original height of the blank

, After upsetting

(Figure 2), then the reduction amount H =

-

, Compression ratio

Formula 1

There is a limit to the amount of deformation that a metal can withstand during forging. The ability of a metal to withstand a large amount of deformation without breaking is called plasticity. The plasticity of a metal can be determined experimentally (see forging performance test). The quality of metal plasticity is related to factors such as chemical composition, internal structure, deformation temperature and speed, and deformation mode. Pure metals and metals with low alloying elements (such as aluminum, copper, low-carbon steel, etc.) have good plasticity, and high alloys and metals with many impurities have poor plasticity. Generally, metal has poor plasticity at low temperature and good plasticity at high temperature. The plasticity of the metal is also related to the deformation mode. For example, when free forging and upsetting, the periphery of the billet protrudes outward, the material is subject to tensile stress, and the metal has low plasticity and is easy to crack. During extrusion, the billet is pressed in three directions, and the plasticity of the metal is high. Metals that crack with minimal deformation are called brittle materials, such as cast iron. Brittle materials are generally not suitable for forging.

Deformation force During the forging process, the inside of the billet is generally in a three-way stress state. The stress at which plastic deformation begins is not determined solely by stress in one direction. Use 1, 2, and 3 to represent the principal stresses of three mutually perpendicular elements on any unit in the blank (Figure 3) ^[4] . Experiments show that if the element is to be plastically deformed, the three principal stresses will The elastic distortion energy should reach a certain value. Its mathematical expression is

Formula 3

Where Y is the deformation resistance of the metal, which is determined by a tensile test or a compression test. The above formula indicates the conditions that the principal stresses in the three directions should reach when plastic deformation begins at any point in the metal blank, which is called the yield criterion. During the forging process, plastic deformation occurs at various points on some faces in the blank. The external force applied at this time is called deformation force.

There are four main factors affecting the deformation force P , namely

Formula 2

In the formula, Y is the resistance of the metal to static deformation. It is related to chemical composition, temperature, and deformation process. Low deformation resistance of low carbon steel, high deformation resistance of high alloy steel; high deformation resistance at low temperature, low deformation resistance at high temperature;

Plastic deformation

The annealed metal at room temperature has low deformation resistance at the beginning of forging. After deformation and work hardening, the deformation resistance increases. A is the cross-sectional area in the forging direction of the forging.

Is the strain rate coefficient. When forging on a slow hydraulic press,

= 1 1.5; when forging on a forging hammer with a high strain rate,

3.

Is the excess work coefficient, which is related to the deformation mode, for example, the free material side surface is not restricted during free forging,

= 1 2.5; during die forging and extrusion, the flow of metal is restricted by the die cavity,

= 2.5 to 6. In addition, the surface roughness and lubrication of the die cavity also have an effect. When the surface of the forging die is smooth and has good lubrication

Small; when mold surface is rough and not lubricated,

Larger.