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, etcBrittle materialThere 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]
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 forcestressAnd strain (i.e. deformation).When the stress does not exceed theElastic limitThe resulting deformation isexternal forceAfter 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.stayforging and pressing, rolling, drawing, etcelastic 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 calledPlastic processing。[2]
mechanism
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Solid metal is composed ofgrainIntragranularatomThe regular spatial structure is arranged according to the body centered cubic, face centered cubic, or close hexagonal.For many reasonsatomic structureThere will be various defects.The linear variation of atomic arrangement is calleddislocation。Because of the existence of dislocation, the atoms in the crystal are easy to move alongDislocation lineMovement reduces the deformation resistance of the crystal.Through the transfer of dislocation motion, the arrangement of atoms slips andcrystal twin(Figure 1).slippageA 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 temperaturegrainTwo basic modes of internal plastic deformation.PolycrystalOfGrain boundaryIs adjacent grainatomic structureThe 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 metalWork hardening、internal stressandanisotropyAnd so on.
Work hardening
Principles of plastic deformation mechanics
Caused by plastic deformationdislocationProliferation,dislocation density Increase, dislocations in different directions will be delivered, and the movement of dislocations will be blocked, so that the metal will be producedWork 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 removedElastic recoveryIt will not be exactly the same, which will make the parts in the metal balance each otherinternal stress, i.eResidual stress。Residual stress reduces the dimensional stability of parts and increases the tendency of stress corrosion.
anisotropy
After cold plastic deformation of metal,grainInternal occurrenceslippageBand 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 directionatomAt 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,atomWhen the activation energy ofgrainThis phenomenon is called recrystallization.After recrystallization treatment, the grain distortion caused by cold deformation and the resultingWork hardening、Residual stressAnd 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.stayRecrystallization temperatureThe 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 causingWork hardeningIf it cools slowly, it will not appearinternal stress。[3]
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Cold deformed metal, when heated to slightly lower thanRecrystallization temperatureWhen, viaatomThe 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 calledstressAnnealing.[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 engineeringcompression ratioexpress.Such as original height of blank, height after upsetting(Figure 2), then the reduction △H=-, the compressibility of metal isforging and pressingThere 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 (seeForging 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 asFree forgingDuring upsetting, the surrounding of the billet protrudes outward, and the material is subject toTensile stressThe 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 calledBrittle material, such ascast iron。Brittle materials are generally not suitable for forging.
Deformation forcestayforging and pressingDuring the process, the inside of the blank is generally in three directionsStress state。Plastic deformationstressIt is not determined separately by the stress in one direction.Use 1, 2 and 3 to represent three mutually perpendicular directionsPrincipal stress(Figure 3)[4]Experiments show that if plastic deformation occurs to this elementElastic distortionIt can reach a certain value.Its mathematical expression is
Equation 3
WhereYIs 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 blankPrincipal stressThe 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 calledDeformation force。
Influence deformation forcePThe main factors of
Equation 2
WhereYIt 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 temperatureforging and pressingWhen the deformation resistance is lowWork hardeningThe post deformation resistance increases.AFor the forging force application directionCross sectional area。byStrain ratecoefficient。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 asFree forgingWhen the surface of the broken material side is not constrained,=1~2.5;Die forging andextrusionThe flow of metal is restricted by the mold chamber,=2.5~6。In addition, the surface roughness andlubricationThe condition also affects, and the forging die surface is smooth and well lubricatedSmaller;When the die surface is rough and without lubrication,Larger.