Sheet forming properties
The ability of a sheet to adapt to a variety of press forming operations is referred to as the stamp forming performance of the sheet. Specifically, it refers to whether or not a high-quality stamped part can be efficiently produced from a blank using a simple process method. Press forming performance is a comprehensive concept, and it involves many factors, including two main aspects: on the one hand, the forming limit, and the desire to reduce the forming process as much as possible; on the other hand, it is necessary to ensure that the stamping product quality meets the design requirements. Discussed separately below.First, the forming limit
In press forming, the maximum deformation limit of a material is called the forming limit. For different forming processes, the forming limit should be expressed using different ultimate deformation coefficients. For example, the minimum relative bending radius of the bending process, the limiting drawing coefficient of the drawing process, and the like. These limit deformation coefficients can be found in various stamping manuals and can also be obtained experimentally. What is the basis for determining the ultimate deformation factor? It depends on what factors affect the normal process of forming. The external force during stamping can directly act on the deformed area of ​​the blank (such as bulging), or through the non-deformed area, including deformed areas (such as deep drawing) and areas to be deformed (such as necking, flare, etc.). Force passed to the deformed area. Therefore, factors that affect the normal forming process may occur in the deformed area or in the non-deformed area. To sum up, there are roughly the following situations:1. Problems that belong to the deformed area
Stretch-type deformation is generally due to the tensile stress is too large, the material is too thin, the local instability and the resulting fracture, such as bulging, turning hole, flaring and bending outside the area of ​​cracking. Compression deformation is generally because the compressive stress is too large, exceeds the critical stress of the plate, so that the plate loses stability and causes wrinkling, such as shrinkage, wrinkles without crimping, etc.2. Problems that belong to non-deformed areas
The carrying capacity of the force transmission area is not enough: when the non-deformation area is used as the force transmission area, the deformation process cannot be continued because the deformation force exceeds the bearing capacity of the force transmission area. Also divided into two situations: 1) Cracking or excessive thinning; for example, drawing is the force transmission zone using the deformed area as the pulling force. If the deformation force exceeds the tensile capacity of the deformed area, cracks or local severe thinning will occur in the area. The workpiece is scrapped. 2) Loss of stability or plasticity: For example, the process of flaring and necking is to use the area to be deformed as the force transmission area of ​​the pressure. If the deformation force exceeds the bearing capacity of the tube blank, the area to be deformed will be buckling due to instability. Or plastic deformation occurs. The non-transmission zone is destroyed under the action of internal stress: when the non-deformation zone is not the force transmission zone, due to the non-uniformity of the metal flow in the deformation process, excessive internal stress may also be generated and destroyed. According to the different parts of the problem, it can be divided into: 1) When the deformed area is cracked or wrinkled: For example, in the subsequent drawing process of the box-shaped member, the velocity of the metal to be deformed into the deformed area is inconsistent, the inflow rate is fast in the straight edge part, and the metal inflow speed in the corner part is slow. Under the mutual influence of the two parts of the metal, the straight edge part is easily cracked, and the corner part is easily buckling and crumpling along the height direction. 2) Cracked or wrinkled in deformed zone: If the thin-walled part is back extruded, if the velocity of the metal flowing from the deformed zone to the deformed zone is not uniform, the quicker part is likely to wrinkle due to the additional compressive stress. The slow part is susceptible to cracking by additional tensile stress. In summary, regardless of whether it is an elongation type or compression type deformation, whether the problem occurs in the deformed area or the non-deformed area, the instabilizing forms are nothing more than two types: the necking fracture occurs at the tensioned part, and the pressured part is compressed. wrinkle. In order to improve the stamping forming limit, from the material point of view, it is necessary to increase the plasticity index of the plate and enhance the tensile and compressive resistance.Second, the forming quality
Stamping parts not only require the required shape, but must also ensure product quality. The quality specifications of the stamped parts are mainly the thickness thinning rate, dimensional accuracy, surface quality and the physical and mechanical properties of the formed material. The volume of metal does not change during plastic deformation. Therefore, in the elongation type deformation, the plate thickness should be thinned, it will directly affect the strength of the stamping parts, so the stamping parts that require strength are often required to limit the maximum thinning rate. The main factors that affect stamping part size and shape accuracy are rebound and distortion. Since the plastic deformation is accompanied by the elastic deformation, there is a rebound phenomenon after unloading, resulting in a decrease in the dimensional and shape accuracy. The surface quality of the stamped parts mainly refers to the scratches caused during the forming process. The cause of scratches is due to the irrational or uneven die clearance and the rough surface of the die, often due to the material sticking to the die. For example, stainless steel deep drawing is very easy to have this problem. Sheet metal forming performance testFirst, the sheet metal stamping forming test method
Sheet metal stamping performance test methods are generally divided into three types: mechanical testing, metallurgical testing (collectively referred to as indirect testing) and process testing (direct testing). The commonly used mechanical tests include simple tensile test and biaxial tensile test to determine the mechanical properties of the sheet metal. Metallurgical tests are used to determine the hardness, surface roughness, chemical composition, crystal orientation, and grain size of the metallic materials. Etc.; process test, also known as simulation test, it is to simulate the actual production of a certain stamping process method to measure the corresponding process parameters. For example, Swift's deep-drawing test measures the limit draw ratio LDR; T ZP tests measure the T value of the comparative drawing force; Erichsen test measures the limit bulging depth Er value; KWI reaming test measures the limit reaming rate λ, etc. . The following is a brief introduction to the simple tensile test of a sheet.
(b) Plate tensile test
The tensile test of a plate is also called a uniaxial tensile test or a simple tensile test. With the tensile test method, many test values ​​can be obtained for evaluating the stamping properties of the sheet, so the application is very common.Because of the different purposes of the test, the tensile test method for the evaluation of sheet metal stamping performance and the obtained test values ​​are different from the tensile tests for evaluating the material strength properties. A brief introduction is as follows:
Figure 1 tensile test sample
Test equipment: tensile testing machine (mechanical or hydraulic).
During the test, the tensile force P and the tensile stroke (elongation of the sample) ΔL were measured using a measuring device, and an s-d curve was made based on these values. (figure 2). The following mechanical properties can be obtained from the test:
Figure 2 stretch curve
1) Yield limit ss or s0.2; 2) strength limit sb; 3) Yield ratio ss/sb; 4) uniform elongation du; 5) The total elongation d; 6) Elastic modulus E; 7) Hardening index n; 8) Thickness index g Relationship between sheet metal mechanical properties and stamping forming performance The mechanical properties of the sheet are closely related to the stamping properties of the sheet. In general, the higher the strength index of the sheet, the greater the force required to produce the same amount of deformation; the higher the plasticity index, the greater the amount of ultimate deformation that can be sustained during forming; the higher the rigidity index, the higher the resistance during forming. The greater the ability to lose wrinkles. The mechanical performance indicators that have a great impact on the sheet metal forming performance are the following: 1) The yield limit ss The yield limit ss is small, the material is easy to yield, the deformation resistance is small, the deformation force required to produce the same deformation is small, and the yield limit is small. When the compression deformation, the material with small yield limit is not easy to appear due to easy deformation Wrinkle, the rebound is small rebound deformation. 2) The yield-to-strength ratio ss/sb is smaller than that of the ss/sb, which means that the σs value is small and the σb value is large, that is, plastic deformation is easy to occur and cracks are not easily generated, that is, there is a large plastic deformation interval from yielding to cracking. . Especially for the deep-drawing deformation in compression type deformation, it has a great influence. When the deformation resistance is small and the strength is high, the material in the deformation area is easy to deform and is not easy to wrinkle. The material in the force transmission area has higher strength and is not easy to pull. Cracking is conducive to improving the deformation of deep drawing deformation. 3) Elongation In the tensile test, the elongation at which the sample is broken is called the total elongation or the elongation d. The elongation at which the sample began to locally concentrate (when necking) was called the uniform elongation du. Du indicates the ability of the sheet to produce a uniform or stable plastic deformation. It directly determines the stamping forming properties of the sheet during elongational deformation. It is verified from experiments that the degree of hole opening deformation and uniform elongation of most materials. The rate is proportional to. It can be concluded that elongation or uniform elongation is the most important parameter affecting the performance of the hole-turning or hole-expanding forming. 4) Hardening Index n The uniaxial tensile hardening curve can be written as s=Ken, where the index n is the hardening index, which indicates the degree of hardening in the plastic deformation. When n is large, it indicates that the material in the deformation is hardened and the real stress increases. When the sheet material is stretched, the entire deformation process is not uniform. First, uniform deformation occurs, then concentrated deformation occurs, a necking is formed, and finally the sheet is broken. During the drawing process, on the one hand, the size of the material section is continuously reduced to reduce the bearing capacity, and on the other hand, the deformation resistance is improved due to work hardening, and the bearing capacity of the material is also increased. In the initial stage of deformation, the effect of hardening is predominant, so the carrying capacity of a material somewhere is strengthened in the deformation. Deformation always obeys the law of least resistance, which is the principle of "first deformation in weak regions". The deformation is always carried out at the weakest surface, so that the deformation region is continuously transferred. Therefore, the deformation is not concentrated on a certain section, but it is uniformly deformed on a macroscopic scale and the bearing capacity is continuously improved. However, according to the characteristics of the material, the hardening of the sheet is gradually weakened as the degree of deformation increases. When the deformation reaches a certain moment, the effect of hardening and reduction of the section on the bearing capacity is exactly the same. In this case, the weakest section bears the load. The ability is no longer improved, so the deformation began to focus on this local area, can not be transferred out, developed into a narrow neck, until pulled off. It can be seen that when the value of n is large, the material is hardened by work hardening, and the hardening enhances the strength of the material, thereby increasing the range of uniform deformation. For elongation type deformation such as bulging, the material with large n value makes the deformation uniform, thinning and reducing, uniform thickness distribution, good surface quality, increasing the ultimate deformation degree, the parts are not easy to crack 5) Thickness heterogeneity index g Due to the fibrous structure and other factors that occur during sheet rolling, the plasticity of the sheet material may vary due to different directions. This phenomenon is called plastic anisotropy. Thickness anisotropy coefficient refers to the ratio of width strain and thickness strain of a uniaxially stretched specimen, ie: g=eb/et (1) In the formula, eb, et—the strain in the width direction and thickness direction. The thickness anisotropy index indicates the deformation ability of the sheet material in the thickness direction. The larger the g value, the less likely the sheet material is to be deformed in the thickness direction, that is, the thinning or thickening is less likely to occur, and the g value is deepened in the compression type deformation. The effect is large. When the g value increases, the sheet material is easy to deform in the width direction, the possibility of wrinkling may be reduced, and the thickness of the sheet material is not easy to be thinned, so that cracking is not easy to occur, so the value of g is large. It helps to increase the degree of deep drawing deformation. 6) Plate Planar Anisotropy Index?g The sheet material has different anisotropy index in different directions, resulting in anisotropy in the plane of the sheet. Use ?g to indicate: ?g=(g0+g90+2g45)/2 (2) In the formula, g0, g90, and g45—longitudinal sample, transverse sample, and 45° sample thickness anisotropy index with the rolling direction. The larger ?g, the more anisotropic in the plane of the plate, the uneven lug phenomenon at the end of the part during drawing, which is caused by the anisotropy of the material, which wastes material and adds a trimming process .Stamping materials and their mechanical properties
The most commonly used materials for stamping are metal sheets, and sometimes non-metal sheets. The metal sheets are divided into two types, ferrous metals and non-ferrous metals. According to the nature of black metal sheet can be divided into: 1) Ordinary carbon steel plates such as Q195 and Q235. 2) High-quality carbon structural steel plate The chemical composition and mechanical properties of these steel plates are guaranteed. Among them, carbon steel is more commonly used for low-carbon steels, and the commonly used grades are: 08, 08F, 10, 20, etc. The stamping performance and welding performance are good, and it is used to produce stampings with little force. 3) low-alloy structural steel plate commonly used such as Q345 (16Mn), Q295 (09Mn2). Used to make important stampings with strength requirements. 4) Electrical silicon steel plates such as DT1, DT2. 5) stainless steel plate Such as 1Crl8Ni9Ti, 1Cr13, etc., for the production of corrosion-resistant rust-proof parts. Commonly used non-ferrous metals are copper and copper alloys (such as brass), grades are T1, T2, H62, H68, etc., their plasticity, electrical conductivity and thermal conductivity are very good. There are aluminum and aluminum alloys, commonly used grades are L2, L3, LF21, LY12, etc., have good plasticity, deformation resistance is small and light.The following table lists the mechanical properties of some commonly used metal sheets.
