Stress and strain during extrusion deformation of

2022-09-21
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Forging technology: stress and strain during extrusion deformation

in actual production, it is found that deep shrinkage holes appear at the end of aluminum parts being extruded. Shrinkage holes are one of the quality problems of extruded products. Starting from improving product quality, we should analyze these facts and find out ways to prevent and eliminate them. Therefore, we should study the metal flow during cold extrusion

there are many methods to study metal deformation and stress. At present, the applied methods include coordinate method, simulation experiment method, hardness experiment method, photoplasticity method and dense grid moire method. These methods are described below

1. Coordinate method. Coordinate congruence is a widely used method. As shown in figure L-5, the blank is divided into two half cylinders, one of which is engraved with coordinates (Figure b), the grid size is 33 mm2, and the other has a smooth and flat surface (Figure C). In order to separate the splicing surface. Apply lubricant on the two semicircular mating surfaces. For some materials that are sensitive to surface scoring, it is best to polish the surface of the sample, coat it with photosensitive film, and then coat it with correct coordinate negatives. After photosensitization and washing, fine coordinates can be obtained. Then put two cylinders (Fig. l-6a) into the concave die for extrusion. From the half piece of metal engraved with coordinates after extrusion, we can see the areas with large deformation, areas with difficult deformation and flow, and the state of uneven deformation

figure the coordinates carved on the blank

have a comfortable and durable touch at the same time

(a) the blank bonded together (1, 2) (b) the coordinate half blank is carved on the cutting plane (1)

(c) the half blank with smooth cutting plane

2 Simulation test method: it is used to conduct simulation tests to study the flow of metal. This plastic body is made of yellow wax, vaseline and chalk in the ratio of 9:18:73. The samples are made in layers of different colors. After extrusion, cut the specimen along the symmetry plane, and observe the deformation from the position change of the delamination. The figure shows the plastic sample after the test. Of course, because the physical properties of the metal are different from those of the plastic body, the extrusion result of the plastic body cannot be exactly the same as that of the metal during extrusion, so this method can only be used as a simulation test

figure plastic body extrusion flow

3 Hardness test method: during cold deformation, the hardness of the metal increases with the increase of the degree of deformation, so as long as we know the change of the hardness of the deformation part, we can generally understand the deformation value and the uneven distribution of deformation. The figure shows the hardness changes of various parts of steel parts after cold extrusion by different methods

figure study on deformation distribution of steel parts by hardness test

left reverse extrusion medium forward extrusion solid part right forward extrusion hollow part

4 Photoplastic method: at present, the main method to study the stress distribution in plastic deformation is optical analysis. It uses polarized light (or polarized light) to pass through isotropic deformed samples to produce various colored lines, so as to observe the distribution of stress in the samples. The photoelastic method is basically the same as the photoelastic method of experimental stress, but the materials used in photoelasticity, in addition to meeting the requirements of general Photoelastic Measurement, should also be suitable for plastic deformation tests, which requires good transparency, optical properties, isotropy in mechanical properties and easy machining. At the same time, the optical properties and mechanical properties should be stable when the temperature changes little, It can have large plastic deformation with a production cycle of about 4 days. Commonly used materials include celluloid, gum, oleoresin, etc

5. Dense grid moire method: the dense grid moire method is to use the prepared high-precision dense grid plate and reprint it on the film by photographing Cowell and other methods to make the dense grid film. Because this film not only needs to use a lot of electronic expertise, but also sometimes needs to have rich on-site experience to paste it on the sample as the test grid, and GFW is printed on the glass as the reference grid. The moire test can also be carried out on metal materials (moire is engraved on the materials by chemical corrosion or light), or models made of other materials can be used for testing, and then the stress and strain on the real object can be calculated according to the similarity theory

II. Strain during extrusion

the punch acts on the metal material to press the face blank, and the concave die prevents the metal blank from flowing around and also gives the metal blank a pressure. Under the action of the punch, the blank only flows out to the die orifice (figure a) (forward extrusion), or flows along the gap between the punch and the die (Figure B, reverse extrusion). In addition, due to the relative movement between the metal flow and the die, there is friction to prevent the metal flow. In this case, the metal will flow from the die orifice or along the gap between the punch and the die, and the punch potential must exert greater pressure to overcome friction and metal resistance

during extrusion, the stress state and stress state at any point in the deformation zone can be represented by principal stress diagram and principal strain diagram. It can be seen from the above analysis that the basic stress state in the extrusion deformation zone is tri compressive stress, but the order of stress and strain is different in different regions of the deformation zone. Next, we will study in different areas

for the forward extrusion of solid parts and the reverse extrusion of cup-shaped parts, assuming that the friction is small and the height diameter ratio of the blank is small, the deformation can be divided into two distinct regions according to the deformation mechanics, as shown in the figure

when the solid part is being extruded (Fig. a), area 1 is the directly compressed area. Its deformation is similar to that of the ring blank upsetting in the closed die cavity. It is subjected to three-dimensional unequal compressive stress, and the strain is two-dimensional compressive strain and tensile strain. It has a very wide speed regulation range and the moving distance of the beam, but the deformation of zone 2 is similar to that of the meridian circular shaft, which is still subject to three-dimensional compressive stress, with tensile strain in the axial direction and compressive strain in the tangential and radial directions

when the cup-shaped part is backward extruded (Fig. a), area 1 is the directly compressed area, which is similar to the upsetting of a circular blank between two flat plates. It is subject to three-dimensional compressive stress, and the strain is two-dimensional tensile strain and one-dimensional compressive strain. Region 3 is an indirectly compressed region, which is subject to three-dimensional compressive stress, and the strain is two-dimensional compressive strain and tensile strain

figure stress and strain of metal during extrusion deformation

such stress and strain state is conducive to improving the mechanical properties of metal and reducing the damage caused by internal defects of materials. In addition, low plasticity and low strength metals can be used to replace high strength metals

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