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Effect of Molding Device on Bending Formability of Profiles

Status of Bending Profiles

The curved profile has the characteristics of high strength, light weight, etc., and is increasingly widely used in aerospace, shipbuilding,Construction and other industries.

In particular, at present, all countries in the world are actively advancing the use of aluminum in the main parts of civilian motor vehicle bodies and bodywork. Considering cost, light weight, safety, etc., the space frame structure with aluminum alloy extruded profiles as the main body has been developed. future. Aluminum alloy profile space frame structure has the advantages of fewer components, short production cycle, low production cost, etc., and can be 40%~50% lighter than steel body, and has high safety, which is good for absorbing energy at the time of collision. To protect the safety of the driver and occupants to the greatest extent possible.

Aluminum alloy bumpers and frame bodies are all based on extruded profiles for bending. The traditional production of bending profiles is generally divided into two steps of extrusion and bending, and the production cost is mainly determined by bending.

Therefore, from the perspective of reducing the cost, in the premise of satisfying the performance requirements, the use of curved profiles is generally avoided in practical applications, and straight profiles are directly used instead.

However, for the structural design, due to the needs of aerodynamics, mechanics and aesthetics, the use of profiles in curved forms is unavoidable and increasing.

Traditional processing of curved profiles

Technical characteristics of extrusion processing of profiles

Extrusion of a profile refers to the fact that the metal blank flows under an extrusion force and passes through an extrusion die having the same shape as the cross section of the profile, increasing the length of the material and reducing the cross section of the material, thereby obtaining a hollow of a certain cross-sectional shape and size. Profiles.

According to the extrusion molding, the flow direction of the metal in the extrusion cylinder and the outlet of the extrusion die can be divided into forward extrusion, reverse extrusion and composite extrusion. According to the blank, it is heated before extrusion. Different temperatures, can be divided into hot extrusion, warm extrusion, cold extrusion, that is, room temperature extrusion.

Extrusion molding of metal is an advanced static and clean molding process. Except for cutting, almost no other follow-up machining is required. It has many advantages such as saving raw materials, lowering production energy consumption, shortening production cycle, improving component performance, etc. Therefore, profiles are widely used in design and use.

1, the basic principle of metal extrusion

The extrusion processing method can not only produce rod, tube, type, and line products with a simple cross-section shape, but also can produce profiles and tubes with extremely complex cross-section shapes.

Compared with other plastic processing methods, such as rolling, forging and other forming processing methods, extrusion processing features are:

(1) In the extrusion process, compared with rolling and forging, the metal is in a more intense and uniform three-dimensional compressive stress state in the deformation zone, and the plasticity of the processed metal itself is more fully exerted. In practical applications, in order to improve its organizational structure and improve its plasticity, it is also commonly used for extrusion of ingots. At present, extrusion is still the most superior method of directly producing products based on ingots.

(2) Extrusion processing methods can not only produce rod, tube, type, line and other products with simple cross-section shapes, but also produce profiles and tubes with complex cross-section shapes, which is difficult to achieve with other pressure processing methods, and even not Possible.

(3) The product produced by extrusion has high precision and good surface quality. With the improvement of the mold manufacturing level, it is now possible to produce ultra-thin, ultra-high-precision, high-quality surface profiles with a wall thickness of 0.6±0.01 mm and a surface roughness of R1.6-0.8 um through extrusion processing. Net shape greatly reduces and avoids subsequent processing steps, while also increasing the overall utilization of raw materials and the yield of workpieces.

(4) Extrusion process flexibility is high. Only by replacing the extruder and die can produce products with different shapes, sizes and materials on the same extruder. The replacement of extruders and molds is simple and less time-consuming. This processing method is most suitable for production plants that produce small batches, varieties, and specifications.

(5) Large deformation of the extrusion process can improve the mechanical properties of the metal. In particular, for some extruded aluminum alloys, after the corresponding heat treatment, the extruded products have higher longitudinal strength performance than those processed by other methods. The extrusion method can fully tap the performance potential of aluminum alloy materials, greatly improving the use value.

(6) The process is simple and the workers can master their operations and facilitate large-scale production. Compared to hot forming methods such as rolling and forging, extrusion can produce parts with a certain section and size at a time.

(7) Aluminum and aluminum alloys are good in plasticity, especially suitable for extrusion molding

(8) Easy to automate production. The extrusion production lines of straight aluminum profiles that are currently widely used in the construction industry have been fully automated and the number of operators has been reduced to less than three.

(9) It is easy to achieve closed production. When the material to be processed is radioactive, the extrusion line can be more easily closed than other rolling production lines and the unmanned production site can be realized.

Of course, there are some inadequacies in the extrusion process:

(1) The effective utilization of metal is not high. The main reaction is the waste at the initial stage of extrusion, and the residual pressure and squeezed end of the extrusion should be kept at the end of the extrusion, and the loss of the perforated head during the extrusion of the hollow pipe. For example, the remaining amount of pressure may generally account for about 15% of the weight of the ingot. Compared to the rolling process, the head loss and tail loss of the rolling stock are less than 3% of the weight of the ingot.

(2) In the normal extrusion process, the length of the ingot is limited, and the ratio of the length of the ingot to the diameter is generally controlled to 4 or less, which restricts the continuous production of the product and cannot effectively guarantee the structure and performance of the extrusion. Uniformity.

(3) The processing speed is low. Due to the large amount of deformation in the extrusion process, there is a huge friction between the metal and the mold, and the plastic deformation area is completely enclosed in the interior of the extrusion cylinder, causing the temperature of the metal in the deformation area to rise. It may reach the brittle temperature of certain metals or alloys, causing cracks or even cracks on the surface of extruded products. To avoid this potential problem, extrusion processing limits the speed of certain metal extrusions. In the rolling process, due to the small amount of deformation in each pass, the friction between the metal and the tool is relatively small, and the generated heat of deformation and frictional heat are not large. Therefore, the temperature of the metal in the plastic zone is difficult to approach the brittle zone temperature, so the forming speed is generally not limited in the actual rolling process.

(4) The law of continuous production is low, and a series of preparations are required before extrusion, such as the heating of metal billets and tools, etc. The preparation step takes a long time in one extrusion cycle, and the production efficiency is not high.

(5) High cost of work and tooling. The working stress of the extrusion process is very high and can exceed several times the metal deformation resistance. Especially on extruded gaskets, the average pressure is up to 400-800 MPa, and even up to 1000 MPa or more. Due to the huge friction and the high temperature and high pressure environment in the extrusion cylinder, the service life of the extruder and the die is much lower than that of other processing methods. Therefore, the expensive high-grade heat-resistant alloy must be used. Steel manufactures extrusion processors and molds, increasing production costs.

1.22 Development and Status Quo of Profile Extrusion Technology

The extrusion method appeared late in the field of metal plastic processing and is a new metal processing technology. According to historical records, around 1797, the British first invented an extrusion device for the production of lead pipes and then applied this principle to lead on the outside of the cable. Huge and continuous pressure has become an elusive problem, so you can only squeeze some relatively soft, low-melting metals, such as lead. This situation continued until 1894, the German A Dick designed and manufactured the first extruder can be used to squeeze brass. Since then, the extrusion process has developed little by little. If the extrusion equipment is used, the hydraulic press has a capacity of 125 MN. With the rapid development of aerospace, construction and automotive industries, the application of aluminum alloy profiles has become more widespread, accelerating the further development of the extrusion molding process, and some major breakthroughs in recent years.

(1) Extrusion product types and specifications are numerous. In the aspect of aluminum alloy hollow profiles, there are more than 10,000 kinds of varieties, and profiles can be processed to produce products with variable cross-section or variable wall thickness.

(2) High productivity of extrusion equipment. For example, a large hydraulic press with a capacity of 270 MN was designed and manufactured to fabricate structural materials such as integral wall panels for large transport aircraft and passenger aircraft. In 2001, the capacity of Southwest Aluminum Group was 8,000 tons of hydraulic presses. The dimensions of the produced profiles could reach a circumscribed circle of 500mm and a length of 26.5m.

(3) High degree of automation. The modern extrusion machine has completely freed itself from the stage of manually operating the dispenser and changed to advanced control methods such as long-distance centralized control, preset program control, and programmable logic control with a computer, greatly improving production efficiency. Significantly reduced staff and even unattended operation of the extrusion line.

(4) The continuous emergence of new extrusion technology. In the extrusion of aluminum alloys, isothermal extrusion technology can control the outflow rate and prevent periodic cracks from appearing on the surface of products. Cold extrusion technology and lubrication extrusion technology can increase the extrusion speed. Ingot extrusion and continuous extrusion of Con form and Casts can increase production efficiency and yield. After extruding the product out of the mold hole, direct quenching is performed using its own residual heat of extrusion, which can improve the production efficiency of the aluminum alloy profile. "Effective frictional squeezing" can effectively utilize the friction between the conventional in-press extrudate and the extruding barrel.

(5) There is a new breakthrough in extrusion theory.

Compared to the earliest extrusion methods that took place until the end of the 18th century, the study of extrusion theory was much less advanced.

At the beginning of the 20th century, HC Korna Koff first studied the flow and pressure of metals during extrusion.

Subsequently, Schweitz Gute studied the metal's flow law and the formation mechanism of squeeze tails when extruding brass. H Wenkel's clever use of plastic mud to study the flow of the extrusion.

Until the early 1930s, E Hibold first established a formula for calculating the extrusion force using the analytic method of rolling deformation work derived from C. Finkel.

Since the influence of uneven metal deformation and friction is not taken into account in this formula, the calculation result is far from the actual one.

Afterwards, Q. Sarkos and CH Gubkin successively used the flat section method to obtain their own formula for calculating the extrusion force. However, the flat section method still has the problem that the uneven metal deformation cannot be considered.

In the mid-1950s, EG Thomson et al. proposed the visual plasticity method, which combines the computational stress with the experimental measurement of metal flow.

The problem of plane strain or axisymmetric compression can be successfully solved by using the visual plasticity method.

In the 60-70s, Margara, Yamamoto and others successively used finite element thinking to solve the squeeze problem. This method can be used to calculate various parameters of the plastic deformation zone in the extrusion process. This method has been used to analyze the extrusion process. In the mid-1980s, R Hill passed careful mathematical processing. The slipline field theory was applied to solve the problem of plane strain extrusion.

Since then, researchers based on W Johnson have used the slip line field theory to successfully solve the problem of plane strain under various extrusion conditions.

13.1 defects and problems that may easily occur during profile bending

Some defects in the bending process directly affect the bending formability of the profile. Due to the profile, especially the profile with complicated cross-section shape, many defects are easily generated in the bending process, which greatly increases the difficulty of the bending forming of the profile. The existing bending process cannot effectively solve or avoid these problems and is a constraint on the profile bending process. Is an important factor. The main manifestations of these defects are:

Rebound, thinning and cracking, cross-sectional deformation, wrinkling, outer cracking and torsion, etc.

Three of them are the most influential factors affecting the accuracy of profile bending, which is difficult to avoid by existing processes.

(1) Cracking

Cold-formed forming is a process that experiences large rotations and finite strains. It has obvious complex geometric nonlinearity, physical nonlinearity, and non-linear characteristics of boundary conditions. The forming law is difficult to grasp. When the bending radius is too small, or in order to prevent springback and wrinkling of the inner wall and other defects, and to increase the additional tensile force in the tangential direction, there is a large tensile stress at the curved convex side of the profile, so that the convex side appears thinning phenomenon Until it breaks. As the profile section height increases, the cracking phenomenon becomes even more severe. During the bending process of the profile, the thinning and cracking of the outer wall is an indicator of the bending ability and bending limit of the profile.

(2) Cross-section distortion

Cross section distortion is the most prone to occur during the bending process of a profile. The deformation resistance of the hollow profile section is determined by the section shape and the material. In the course of bending deformation of hollow section, due to the thin wall thickness, if there is not enough support inside, it will easily cause deformation phenomenon; the section has a certain thickness, the deformation is not uniform due to the tension and pressure during forming, and defects such as wrinkling and collapse easily occur. The shape of the cross section of the original profile cannot be maintained. The generation of cross-section distortion is very difficult to control, and it also has a considerable impact on springback.

At present, there are two main ways to reduce or eliminate cross-sectional distortion:

1. It is a well-designed profile section shape,

2. In the production process, the cavity of the profile is supported (methods such as mandrel or filling objects can be used, such as fine sand used in factory production) to increase the stiffness of the profile section.

(3) rebound

Springback is a common problem in all bending processes and is the most difficult problem to solve. The main reason for the rebound is due to the presence of plastic deformation zones and elastic deformation zones, and a small amount of elastic deformation zones.

Therefore, when unloaded, the profile will naturally produce elastic recovery, and the curved shape obtained by the part will change, resulting in rebound phenomenon.

Profiles have complex cross-sectional shapes and there are problems such as cross-section deformation and cracking during the bending process. The combined effect of these problems will have a certain impact on springback. At the same time, the neutral layer in the process of bending deformation of the complex cross-section shape is difficult to determine, and can not be calculated using the traditional empirical formula, so the bending rebound of the profile is more complicated.

Especially in the actual production, the unloading process after the holding pressure molding is completely carried out by the operator on the basis of experience, there is no reliable, clear, quantitative operation specification, and the profile will produce an uncertain rebound after unloading.

The principle of reducing and preventing rebound can be basically divided into two types.

1. Compensate the amount of rebound by increasing the bending deformation to ensure the accuracy of the part shape after rebound. The main measure of this type of method is to modify the shape of the mold, reduce the bending curvature of the part or increase the bend angle so that the rebounded part meets the shape and size requirements.

2. The purpose of controlling the rebound is to control the stress on the workpiece. The more common measure is to apply additional tangential tensile stress on the workpiece to change the cross section of the workpiece.

The stress distribution makes the elastic recovery uniform and moderate, so as to achieve the purpose of minor rebound.

Springback is the most important factor in determining the accuracy of a part during bending. Because there are many factors that affect rebound, it isDevelopment of an extrusion-bending integrated molding device and its effect on the bending performance of aluminum alloy profiles

The combined results of interactions and impacts are predicted and permeate the entire bending deformation process, resulting in very high rebound problems.

Complex, difficult to predict, is a hot and difficult point in the study of profile bending and forming. Many scholars at home and abroad have conductedResearch and discussion.

In addition to the appearance defects described above, tensile stress remains in the profile on the convex side of the bent portion and remains on the concave side.

With compressive stress, these residual stresses will affect the normal use of the curved profile and may even be detrimental to subsequent processing.

And heat treatment. For example, in the heat treatment process, the residual stress is released under the influence of heat.

Causes bad or unpredictable changes in the shape and mechanical properties of the profile.

Therefore, for the bending deformation of the profile, we hope to avoid the above defects and deficiencies through a new process to reduce the adverse effects of the curved profile during use.

13.2 Traditional bending process

The traditional profile bending process is basically divided into two types from the forming principle: one is to rely on the shape of the die, such as drawing and bending, and the other is to rely on the relative movement between the work, the die and the workpiece, such as roll bending. Etc. The following will describe in detail the process and characteristics of the bending process for two types of profiles represented by tension and roll bending, and analyze their advantages and disadvantages.1.321 stretch bending

The pull-bend method is the most widely used profile bending process 2 in industrial applications and is usually divided into pre-stretching,

The three steps of bending and replenishing, in which the replenishing process is not necessary, it can be decided in the production according to the actual situation.

Whether or not there is a pull and bend process flow is shown in the figure.

Firstly, the two ends of the profile are fixed by means of a clamping device, and then a relatively large pretensioning force is applied to the profile. This force is slightly higher than the yield strength of the profile material; then the clamping device or the forming mold is started to move, when the profile comes into contact with When forming the mold, a forming moment will be generated in the middle part of the profile. With the moment acting area as the boundary, the profile is divided into two forming areas. As the clamping device or the forming mold continues to move, the forming moment generated by the clamping device increases. The larger the come, the greater the degree of deformation of the profile under the action of the forming moments. When the profile is deformed into the desired shape, the movement of the clamping device or forming die is stopped and held for a period of time.

After the bending of the profile is completed, the tensile force is not immediately removed and the profile is removed, but the tensile force is further increased.

The profile that has been bent and deformed is further stretched for final shaping.

The need for a pull-bend process allows a relatively large arc to be achieved in a single process, which is a process that requires multiple steps for other bending processes.

The springback process is relatively small. Throughout the entire bending process, there is a tensile force from beginning to end, so that the neutral layer moves from the center of the profile to the inside, and even the entire profile becomes a tensile stress zone, avoiding the different tensile and compressive stress of the inner and outer layers. The resulting bending moment can effectively reduce rebound. However, due to the presence of tensile force, the tendency of concaves in the outer layer of the profile increases, which can cause adverse effects on the bending quality of the profile.

Through the computer simulation, the forming limit and the forming precision were studied in two aspects. The influence of the pre-stretch amount on the bending deformation under different curvature radii was summed up, and the calculation principle of the pre-stretch amount was obtained.

The most important among the various process parameters of the pull-bend are: cross-sectional shape, radius of curvature, and pre-stretch amount. In these parameters, the cross-sectional shape and radius of curvature are determined by the design, and are parameters that cannot be changed in the pull-bend process. Therefore, choosing the appropriate pre-stretch amount determines the bending deformation quality. Studies have shown that the larger the radius of curvature of the profile, the smaller the amount of pre-stretch, the smaller the wall thickness reduction. For bending springback, the springback amount and the pre-stretch amount have a relationship in the range of the bending radius in the range of 500mm-10000mmn.

It can be seen from the figure that even though the bending radii are different, there is an area where the curves suddenly drop. After the pre-stretch amount passes through this area, increasing the pre-stretch amount no longer has a significant effect on the rebound angle.

This is because due to the increase of the pre-stretching amount, the strain-neutrality of the profile gradually shifts. When the pre-stretching amount is increased until the neutral layer is removed from the inner surface of the profile, the pre-stretching amount is increased. The influence of the impact angle is not determined by the factors of the forming limit and forming accuracy.

When the amount of stretching is small or insufficient, defects such as bottom wrinkling, poor mold sticking, and low forming accuracy may occur in the profile.

Therefore, the amount of stretch should at least ensure the bending of the profile, that is, there is no demoulding and wrinkling at the bottom, which is the most basic requirement of the stretching process.

Therefore, the minimum amount of stretching required to ensure that the profile does not exhibit defects such as demolding and bottom wrinkling in the course of pulling and bending but can complete bending deformation is positioned as the lower limit pre-stretch amount.

The pre-stretching amount in the profile stretching process should not be less than the lower limit stretching amount, and further increase the stretching amount according to the forming accuracy requirement.

The purpose of increasing the amount of stretching is to improve the bending accuracy of the curved profile and reduce the rebound, but may cause defects such as deformation of the cross-sectional shape and thinning of the wall thickness. When the amount of stretching is increased so that the neutral layer has been deflected to the underside of the profile, increasing the amount of stretching will no longer have a significant effect on springback. Instead, it will cause more severe cross-sectional deformation and wall thickness reduction. The phenomenon. Therefore, the amount of stretching that can just cause the neutral layer to move to the profile surface is defined as the upper limit pre-stretch amount.

In general, the prestretch amount should not exceed the upper limit prestretch amount.

The pre-stretching amount is a key parameter in the aluminum alloy profile bending process. When drawing the bending process, the pre-stretching amount must be higher than the lower limit stretching amount, and it should be as close as possible to the upper limit stretching amount, so as to satisfy a certain cross-sectional shape and wall thickness requirement, the highest possible value is obtained. Bending accuracy. It is also possible to use a strain-controlled pull-bend process to obtain a higher bending forming accuracy 1n, the principle of which is shown in the figure.

Both ends of the profile are fixed on the tool, and as the two parts of the tool rotate, they eventually reach a quantitative strain.