Plastic deformation

Unrecoverable deformation

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Plastic deformation is a kind of 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, irrecoverable deformation, or residual deformation, will occur after the load is removed, which is called plastic deformation. Not all engineering materials have the ability of plastic deformation. Metal, plastic and so on have different degrees of plastic deformation ability, so they can be called plastic materials. Glass, ceramics, graphite, etc Brittle material There is no plastic deformation capacity. In general, no obvious plastic deformation is allowed in the design of engineering components, otherwise the components will not maintain their original shape or even break.^[1]

Chinese name: Plastic deformation
Foreign name: Plastic Deformation
Definition: Material deforms under external force

Nature: physical phenomenon
Discipline classification: Metallography
application area: Plastic processing

catalog

▪ Deformation and plasticity

brief introduction

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

Plastic deformation

Material is produced under the action of external force stress And strain (i.e. deformation). When the stress does not exceed the Elastic limit The resulting deformation is external force After removal, the material will be completely eliminated, and the material will return to its original state, which is a reversible elastic deformation. When the stress exceeds the elastic limit of the material, the resulting deformation cannot be fully restored after the external force is removed, while the residual deformation cannot restore the material to its original shape. This residual deformation is irreversible plastic deformation. stay forging and pressing , rolling, drawing, etc elastic deformation It is much smaller than plastic deformation and is usually ignored. This kind of processing method that uses plastic deformation to form materials is collectively called Plastic processing 。^[2]

mechanism

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Solid metal is composed of grain Intragranular atom The regular spatial structure is arranged according to the body centered cubic, face centered cubic, or close hexagonal. For many reasons atomic structure There will be various defects. The linear variation of atomic arrangement is called dislocation 。 Because of the existence of dislocation, the atoms in the crystal are easy to move along Dislocation line Movement reduces the deformation resistance of the crystal. Through the transfer of dislocation motion, the arrangement of atoms slips and crystal twin (Figure 1). slippage A part of the grains slide along the plane and direction where the atoms are arranged most closely. The sliding of many atomic planes forms slip bands, which together become visible deformation. Twins refer to the relative movement of a part of grains in a certain direction relative to a certain crystal plane, which is called twin plane. The distance of atoms moving is proportional to the distance of twin planes. The arrangement direction of atoms between two twin planes changes, forming twin bands. When the slip and twin are at low temperature grain Two basic modes of internal plastic deformation. Polycrystal Of Grain boundary Is adjacent grain atomic structure The transition zone of. The finer the grains are, the larger the grain boundary area in unit volume is, which is conducive to the movement and rotation of intergranular. Some metals can have an elongation of up to 300~3000% without fracture through grain boundary deformation under specific conditions of fine grain structure.

influence

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The plastic deformation of metal at room temperature has a great impact on the structure and properties of metal Work hardening 、 internal stress and anisotropy And so on.

Work hardening

Principles of plastic deformation mechanics

Caused by plastic deformation dislocation Proliferation, dislocation density Increase, dislocations in different directions will be delivered, and the movement of dislocations will be blocked, so that the metal will be produced Work hardening 。 Work hardening can improve the hardness, strength and deformation resistance of metal, and reduce the plasticity, making it difficult to deform in cold state later.

internal stress

The distribution of plastic deformation in the metal body is uneven, so after the external force is removed Elastic recovery It will not be exactly the same, which will make the parts in the metal balance each other internal stress , i.e Residual stress 。 Residual stress reduces the dimensional stability of parts and increases the tendency of stress corrosion.

anisotropy

After cold plastic deformation of metal, grain Internal occurrence slippage Band or twin band. The grains also elongate and twist in the direction of deformation. When the deformation is large (such as 70% or more) and along one direction atom At the same time, the inclusions in the metal are elongated along the deformation direction to form fibrous structure, which makes the metal anisotropic. The strength, plasticity and toughness along the deformation direction are higher than those in the transverse direction. When the metal is deformed in the hot state, due to recrystallization, the grain orientation will deviate from the deformation direction to varying degrees, but the direction of the fiber formed by the inclusion elongation remains unchanged, and the metal still has anisotropy.

Recrystallization and recovery

Cold deformed metal, if heated to a certain temperature and kept for a certain time, atom When the activation energy of grain This phenomenon is called recrystallization. After recrystallization treatment, the grain distortion caused by cold deformation and the resulting Work hardening 、 Residual stress And so on will be completely eliminated.

Recrystallization temperature

The recrystallization temperature of metal is usually the temperature at which recrystallization is completed after one hour of heat preservation. The recrystallization temperature of various metals is about 40-50% of the melting point of the metal according to the absolute temperature (K). The recrystallization temperature of low carbon steel is about 460 ℃. When the deformation degree is small, the recrystallized grains are especially coarse during the recrystallization process, especially when the temperature is high. Therefore, if the grain is small, the metal material will have a large deformation before recrystallization treatment.

Recrystallization temperature is very important for plastic processing of metal materials. stay Recrystallization temperature The above plastic processing and deformation are called hot processing and hot deformation; Plastic processing and deformation below recrystallization temperature are called cold processing and cold deformation. During thermal deformation, the metal material continuously recrystallizes during deformation, without causing Work hardening If it cools slowly, it will not appear internal stress 。^[3]

Cold deformed metal, when heated to slightly lower than Recrystallization temperature When, via atom The diffusion of C will reduce the crystal defects, reduce the distortion energy of the crystal, and thus reduce the internal stress; However, no new grains appear, and the metal still retains work hardening and anisotropy, which is the recovery of metal. Such heat treatment is called stress Annealing.^[3]

Deformation and plasticity

Equation 1

The amount of plastic deformation is often expressed by different indexes according to different deformation modes. Some are expressed by the change of sectional area before and after blank deformation, some are expressed by the change of length in a certain direction, and others are expressed by the size of rotation angle during torsion. The deformation of upsetting and compression is commonly used in engineering compression ratio express. Such as original height of blank

, height after upsetting

(Figure 2), then the reduction △ H =

－

, the compressibility of metal is forging and pressing There is a certain limit for the deformation that can be withstood in the process. The ability of metal to withstand large deformation without fracture is called plasticity. The plasticity of metals can be measured experimentally (see Forging performance test )。 Metal plasticity is related to chemical composition, internal structure, deformation temperature and speed, deformation mode and other factors. Pure metals and metals with low alloy elements (such as aluminum, red copper, low carbon steel, etc.) have good plasticity, while high alloys and metals with more impurities have poor plasticity. General metals have poor plasticity at low temperatures and good plasticity at high temperatures. The plasticity of metal is also related to the deformation mode, such as Free forging During upsetting, the surrounding of the billet protrudes outward, and the material is subject to Tensile stress The metal has low plasticity and is easy to crack. When extruding, the billet is pressed in three directions, and the metal has high plasticity. The metal that cracks under very small deformation is called Brittle material , such as cast iron 。 Brittle materials are generally not suitable for forging.

Deformation force stay forging and pressing During the process, the inside of the blank is generally in three directions Stress state 。 Plastic deformation stress It is not determined separately by the stress in one direction. Use 1, 2 and 3 to represent three mutually perpendicular directions Principal stress (Figure 3)^[4] Experiments show that if plastic deformation occurs to this element Elastic distortion It can reach a certain value. Its mathematical expression is

Equation 3

Where Y Is the deformation resistance of metal, which is measured by tensile test or compressive test. The above formula indicates three directions when plastic deformation starts at any point in the metal blank Principal stress The condition that should be reached is called yield criterion. In the process of forging, plastic deformation will occur at all points on some surfaces of the blank, and the external force applied is called Deformation force 。

Influence deformation force P The main factors of

Equation 2

Where Y It refers to the static deformation resistance of metal, which is related to chemical composition, temperature, deformation process, etc. Low carbon steel has low deformation resistance, while high alloy steel has high deformation resistance; High deformation resistance at low temperature and low deformation resistance at high temperature;

Plastic deformation

Annealed metal at room temperature forging and pressing When the deformation resistance is low Work hardening The post deformation resistance increases. A For the forging force application direction Cross sectional area 。

by Strain rate coefficient 。 When forging on a slow hydraulic press,

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

Juan 3.

Is the coefficient of redundant work, which is related to the deformation mode, such as Free forging When the surface of the broken material side is not constrained,

=1～2.5； Die forging and extrusion The flow of metal is restricted by the mold chamber,

=2.5～6。 In addition, the surface roughness and lubrication The condition also affects, and the forging die surface is smooth and well lubricated

Smaller; When the die surface is rough and without lubrication,

Larger.

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