Process study of stamping and forming of large welded elbow
The elbow is the main connecting part in the pressure pipe fittings. The research of low energy consumption and high quality elbow forming process has become a hot spot in the research and development of enterprises in recent years. At present, the stamping process of plate butt welding elbow still has the characteristics of high energy consumption and poor forming quality. However, with the rapid development of manufacturing technology and process and the upgrading of product competition, improving the forming process, improving the forming quality, and improving the energy utilization rate is urgent.
In this paper, the elbow’s finite element model is established using the finite element software Dynaform. The shape and size of the elbow blank are optimized, and the thickness distribution and formability of the elbow are analyzed. The elbow hot pressing process is changed to cold pressing to reduce energy consumption. The finite element software ABAQUS was used to establish the cold stamping and springback model of the elbow, and the position of the sheet was optimized. The effects of the element type, finite element algorithm, and unloading path on the springback simulation were analyzed.
In order to improve the forming quality of the elbow, the stamping-welding elbow was thermally shaped, and the whole diameter thermal shaping model of the elbow was established. The necking ratio of the elbow was optimized, and the optimization parameters of the whole diameter thermal shaping were given. The forming results of the thermal shaping elbow were measured and analyzed, and the parameters such as the circumference, diameter reduction rate, and ovality of the elbow were tested. All the parameters of the elbow have reached the standard index of the pipe fitting, which proves the feasibility of the elbow thermal shaping. Combined with the parameters of plate blanking, cold stamping butt welding, and whole diameter thermal shaping simulation optimization results, and based on the existing mold parameters of the factory, the mold shape was improved, and the elbow cold stamping-shaping combined mold was designed, which reduced the mold cost and improved the mold utilization rate.
With the power, chemical, petroleum, and other industries on the pipe fitting equipment requirements continue to improve, the future needs of industry on the pipeline will inevitably move in the direction of high-tech. Along with the continuous improvement of industrial production and scientific and technological development in the chemical industry, petroleum, and electric power, the need for industrial piping will be very large; pipe fittings manufactured parts will also be widely used in various fields. Plastic processing and molding of pipe fittings occupies a very important position in the production of modern industry. Pipe fittings processing and molding uses plastic deformation, and processing production of pipe fittings requires parts of the processing techniques. To realize the needs of the contemporary use of pipe fittings, over the years, a fairly well-developed processing technology has been developed through the accumulation of a large amount of experience and a constant stream of innovations. The main goal of modeling, simulation, and optimization of the machining system is the organization and good operation of the machining system.
Along with constructing the West-East Gas Transmission Project and exploiting offshore oil, China’s demand for the number and variety of pipe fittings is expanding. Whether it is tabularized production or with fittings for oil, heat, and gas transmission, as long as there are fittings, connecting fittings joints are essential. Pressure pipes are these parts that connect pressure fittings. Whether in the important parts of the piping system to change or some of the equipment interface most of the pressure pipe fittings installed, their role is mostly to change the size of the pipe diameter or the direction of the pipe fittings system, used to realize the pipe fittings to connect, local reinforcement or pipe fittings branching. Piping system stress concentration location most of the pipe fittings application, is a variety of media delivery facilities are very important. This requires high-quality pipe fittings, which means that the curved pipe fittings forming process and the processing level will be very important, related to the domestic pipe fittings that can replace the imported products and meet international standards.
1.2 Overview of pressure fittings and safety
In life, production may cause poisoning or explosions and other major hazards of some facilities, such as flammable, explosive, toxic media steel pipes, natural gas, oil and gas pipes, steam pipes, etc., and pressure fittings. Pressure fittings can be defined as fittings in which the pressure of the medium inside the pipe is greater than or equal to 0.1MPa. Most substances transported by pressure pipes are dangerous and must carry a certain pressure, but they are indispensable facilities in our life and production.
Production of indispensable facilities. It is the complexity of the application conditions of the pressure pipe fittings and a wide range, from time to time, will be a threat to people’s lives and property; the danger is greater, therefore, in our country, the pressure pipeline as a special facility must be subjected to safety inspections.
1.2.1 Classification and application of pressure fittings
The connection method can be divided into four pressure fittings: flanges, socket fittings, welded fittings, and threaded fittings. Most of the pipe fittings material and pipe are the same: equal tee, reducer tee, elbow, flange, cross, and other fittings. Most elbows change the direction of the piping system; the flange is used for pipe to pipe connection; three pipes connection requires a tee; the same four-way is to complete the four pipes intersection connection; reducer is for the connection of different pipe diameter steel pipe.
1.2.2 The development of pressure pipe fittings at home and abroad
In foreign countries, many developed countries, such as Italy, the United States, Slovakia, and Japan, have done much research on X80 pipeline steel, and there are many examples of successful applications. As shown in Table 1-1, the current users of X80 are mainly Transco in the UK, Mannesmann in Slovakia, and so on.
Table.1-1 Global X80 pipeline projects have been completed
|Serial number||Years||Country||PROJECT||Length/km||Steel pipe factory||Diameter/mm||Wall thickness/mm|
|2||1986||Slovakia||Fourth Gas Pipeline||1.5||Mannesmann||1422||15.6|
|3||1990||Canada||Nova Express East||26||NKK||1067||10.6|
|4||1992||United States||Ruhr Gas/Schluech Larter||115.5||European Steel Pipe||1219|
|5||1993||Ruhr Gas/Wame Wetter||144|
|7||1995||East Alberta System||55|
|8||1997||Central Alberta System||91|
|9||1997||East Alberta System||27|
|10||2001||Britain||Cambridge M.G||47||European Steel Pipe||1219|
Compared with foreign countries, the research and application of pipeline steel started late in China, but the research and application cycle for X60 and X70 steel is very short. The research on X80 and X100 pipeline steel has started, especially for applying X80 has also formed a certain scale.
1.3 Elbow Molding Research Overview
Elbows in modern industry are widely used in oil, gas, chemical, electrical, and textile. Its forming methods mainly include stamping – butt welding, hot push, cold push, simmering, extrusion and pressing, etc.; forming methods are diverse, but each has its characteristics.
1.3.1 Stamping-butt-welding forming
At present, stamping – butt-welding forming in the hot stamping forming method, his forming process is as follows: raw material inspection → cutting material (with a cutting machine) → hot pressing blanks (need to be heated first, and then put into the hydraulic press stamping) → billet processing (excision of redundant machining allowances, and conducive to the piece) → piece spot welding → welding → ray flaw detection → shaping (necking of the elbow) → coil head → heat treatment → pipe end beveling → surface Processing → finished product inspection → finished product into storage.
The most important forming process is the material, hot press forming, and butt welding, as shown in Figure 1-1. The first is to select the appropriate plate thickness of steel cutting (see Figure 1-1a)); then put on the press to press molding (see Figure 1-1b)); and finally, butt-welded into an elbow, this time the inner arc of the elbow and the outer arc of the elbow will be left with a weld (see Figure 1-1c)).
Figure.1-1 Elbow Forming Process Flow Chart
Stamping – the advantages of the butt-welding elbow are a simple mold structure, easy to use and flexible, low mold costs, does not require special hydraulic equipment (you can make full use of the first hydraulic press), smaller investment costs, can be processed in a wide range, and can be processed wall thickness of the thicker pipe fittings.
1.3.2 Hot push into shape
The hot push elbow processing process generally uses devices with medium-frequency induction heating equipment, bullhorn-shaped mandrels, and unique horizontal elbow push-bending machines. The machine pushes the billet in the mold forward movement and is heated by the induction coil simultaneously in the bullhorn mandrel expansion bending forming. Hot push elbow forming characteristics are the application of metal deformation volume before and after the principle of constant to determine the diameter of the billet, the general application of the pipe diameter to be smaller than the elbow diameter, through the bullhorn mandrel to control the billet forming process, bending compressed billet metal flow at the inner arc, to compensate for the thinning due to the expansion of the other parts of the elbow, and ultimately get the elbow of the wall thickness of the various parts of the more uniform.
The hot push forming process elbow has a uniform wall thickness and smooth appearance. It is suitable for mass production, mostly alloy steel or carbon steel elbow processing, but also some types of stainless steel elbow processing.
Processing heating methods are reflective furnace heating, flaming heating, and high-frequency or medium-frequency induction heating, the application of which method of heating depends on the requirements of the molded parts and energy conditions. Chemical, oil, natural gas, and other equipment fittings have been widely used in processing localized medium-frequency induction heating elbow.
1.3.3 Other forming
The cold push forming process applies the tunnel forming principle of processing and forming methods, processing equipment shown in Figure 1-2.
Figure.1-2 Forming device
The main pressure F causes the upper mold 1 to move toward the lower mold 2 and generates a clamping force after contact, and the horizontal cylinder pusher 3 pushes the tube blank 4 from the mold guide section into the cavity and forms it under the support of the mandrel 5. For easy release from the mold, the surface of tube blank 4 is previously coated with vegetable oil and animal oil mixed with molybdenum disulfide or other lubricants. Cold pushed the elbow processed using the method of good quality, less wall thinning, production efficiency is also high, 2.5 – 6 inch butt-welding elbow forming speed is only 20 – 60s, can realize the whole automated production, the average unit cost is low, so it is suitable for mass production. But the cold push elbow is a one-time large investment, requiring unique equipment, and requires uniform wall thickness, elasticity, and plasticity of the pipe is better.
The press-bending molding process is the earliest process applied to the mass production of elbows. This processing method is to put the billet into the mold with a press directly into shape and should be placed in the billet core and end mold, using mold support and constraints on the role of the billet forming. Now, most of the specifications of the elbow forming almost do not use this processing method, replaced by some other processing technology. However, some special models of elbows and press-bending molding processes are still used. Press-bend forming elbow characteristics are shown in Table 1-2.
Table.1-2 Characteristics of press-forming elbow
|Scope of application||Economic benefits||Forming effect||Using tube billets||Wall thickness distribution||Equipment|
|Single piece, small batch||Low cost||The appearance quality is not as good as push bending||Too thick, too thin||Severe thinning||Press|
Stamping forming is divided into two methods, cold stamping, and hot stamping, generally according to the material properties and equipment capacity to choose the processing method. The cold extrusion elbow processing method uses the unique elbow forming machine, the pipe fittings into the outer mold, the upper and lower mold after the mold, the push rod under the thrust, the pipe along the outer mold, and the inner mold of the reserved gap forward to complete the processing process.
Using internal and external mold extrusion methods, forming elbow shape is more beautiful, the wall thickness is also very uniform, and the size deviation is also very small, so the processing of stainless steel pipe fittings, especially the processing of thin-walled fittings when using this process more. However, it is relatively high to apply the inner and outer mold precision requirements; for the elbow wall, thickness deviation requirements are also very strict.
In addition to several common processing technologies, elbow processing allows the tube extrusion to the outer mold and then through the tube within the ball rounding process. But this method is more troublesome; the operation is also relatively troublesome, and the elbow quality could be better than other processes, so this process is rarely used to form. Through the above processing methods, each processing and molding process has its advantages and disadvantages, so their scope of application, processing quality, and molding efficiency are also different; all in the production of elbow fittings have an extremely important role in the process. In each company, the process must be combined with the existing actual situation and their needs to achieve the best production efficiency.
1.4 Research and development of stamping – butt welding molding method
Relative to foreign research in China for pipe fittings forming, research work started late. Still, with the development of the pipeline transportation industry, for the pipe fittings processing and forming process, research has been more and more, but also more and more in-depth. Among them, research reports have yet to see much for the press welded elbow.
As early as the Soviet period, the Lenin Machine Building Plant in Novokramatolsk developed and applied the process of producing smoke pipes and elbows from thin sheet metal rolled material and designed the manufacturing molds. According to this process, pipe elbows were produced by the soviet national standard ГOCT 19904-74 using thin sheet metal rolled material of CT3 steel with a thickness of 1 mm. However, the thickness of the elbow produced by this process and its specifications are small, which is not fully applicable to the current large elbows.
Our Xingdan machine shop studied the use of plate pressing small radius elbow; the method is first pressed into a semi-circular ring with a plate, after shaping trimming two processes, the two semi-circular rings butt-welded into a ring pipe and then sawed into different angles of the elbow as needed. But the process only applies to thin-walled small radius elbow production, and forming mold, shaping mold, and trimming touch three sets of molds.
Liaoning Panjin Oil Construction Company Metal Structure Factory studied the bending radius equal to or less than the nominal diameter of 180 ° stamping elbow manufacturing technology; the process is designed specifically for producing oilfield special heating furnace fire tube elbow. However, the process only applies to 180 ° elbow; the use of the process to produce the elbow specifications is small. The process used in the elbow unfolding method for the plane unfolding is not an accurate and complex practice.
Jiangsu Province, the fourth construction engineering company, and the Beijing Yanshan Petrochemical Company Machinery Factory have adopted hot stamping and welding technology to produce different elbow specifications. The fourth construction engineering company in Jiangsu Province, with an annual output of about 10,000, the specifications include: low-pressure elbow DH (pipe outside diameter, the same below) 89 × 4 – 426 × 10; medium-pressure elbow DH273 × 8 – 426 × 12; seamless steel pipe DH89 – 159; the center of the radius of curvature R = 1.5 DH. Beijing Yanshan Petroleum and Chemical Corporation Machinery Plant produced several caliber Φ800, wall thickness S = 14mm, large-diameter 90 ° elbow. = 14mm large caliber 90° elbow. Xi’an, Shaanxi China National Petroleum Corporation Pipe Research Institute for the domestic application of a pipeline in the X 70 trunk pipe diameter 1016mm, wall thickness of 26.2mm, the use of hot pressing – butt welding molding method, the development of a diameter of 1016mm, wall thickness of 32mm, radius of curvature of 1.5D 90 ° X 70 elbow. The method is to first select the appropriate thickness of steel plate for cutting, taking into account the wall thickness of the elbow and the unevenness of each part of the pressure, selecting the wall thickness of 32mm steel plate as the raw material for processing elbows.
The cut steel plate was placed in the heating furnace, heated to the appropriate temperature, and then pressed in the press to form; finally, the butt weld, the inner arc of the elbow, and the outer arc of a weld at each. However, these enterprises and units use hot press molding technology, which consumes a lot of energy.
Molding technology, energy consumption is greater, which is currently a more widely used method. Although stamping-butt welding technology has been used in the production of different specifications of the pipe, there are still the following problems:
- (1) Theoretical research has yet to mature; the bending pipe-forming mechanism is imperfect.
- (2) Currently, most of the processing of larger pipe fittings uses a hot stamping and forming process; energy consumption is greater, and research and development are very rare for the cold stamping and forming process.
- (3) For the elbow unfolding material technology, the current use of experience and drawings unfolding a combination of ways under the sheet area is large on the waste of raw materials is serious.
- (4) Forming and shaping using two sets of molds, the mold utilization rate is low.
1.5 The research significance of the subject and the main research content
Existing stamping and welding molding process has the phenomenon of inaccurate material waste of raw materials. And the existing process uses hot stamping – butt welding forming, the whole forming process needs to be heated several times, and the energy consumption is relatively large. Therefore, the study of modern computer technology to complete the precise feeding cold stamping welding forming process, improve the technology level, save raw materials, and reduce energy consumption are very necessary.
With the development of computer technology and the further improvement of finite element analysis capability, numerical simulation technology has become one of the main tools for solving many engineering problems. For many engineering problems that cannot be solved in practice and practice need to waste a lot of workforce and metal material resources to solve the engineering problems, using numerical simulation technology to solve these engineering problems has become an inevitable trend. Finite element numerical simulation technology has been widely used in metal pipe forming.
In this paper, we take the 90° X80 elbow with a diameter of 700mm, wall thickness of 20mm, and radius of curvature of 1.5D, and the 90° X80 elbow with a diameter of 1200mm, wall thickness of 20mm, and radius of curvature of 1.5D as the object (from now on referred to as “Φ700 elbow” and “Φ1200 elbow”), and carry out the numerical simulation to solve the engineering problems of metal pipe forming. (from now on referred to as “Φ700 elbow” and “Φ1200 elbow”), to carry out the design of cold press shaping combination molds and improve the cold press-butt welding process research and its application. The main research contents are as follows:
- (1) Plate precise discharging technology. Utilizing the BSE (plate size calculation) function with DYNAFORM software, unfolding the workpiece to get a reasonable blank size is convenient.
- (2) Plate cold press molding process. Use ABAQUS software to simulate the sheet stamping and rebound processes to get the tile shape.
- (3) Elbow straightening and straightening technology. Use the stamped tile’s shape to establish the elbow model after butt welding in UG, and then import the model into ABAQUS software to simulate the straightening and straightening process.
- (4) According to the results of computer simulation optimization, the stamping shaping combination mold is designed.
Chapter.2 Elbow Unfolding Accurate Undercutting Study
This chapter is mainly based on the basic principles of the finite element method, the characteristics of large deformation of metal plate and pipe shaping, and the characteristics of the DYNAFORM finite element software to determine the type of finite element cell and the type of mesh and other parameters of the selection principle, the establishment of the elbow undercutting back-calculation finite element model to solve the elbow stamping process in the required blank shape, and to obtain a more accurate size of the blank.
2.2 One-step numerical simulation technology for plate forming
The shape of the blank has a greater impact on the results of plate stamping and forming; when the initial shape of the blank is unsuitable, the stamped parts are prone to wrinkles or cannot even be formed at all, while a reasonable blank shape can save raw materials. For this reason, people have put forward such as empirical graphical method, slip line method, and geometric mapping method to solve the problem of predicting the initial shape of the sheet. Still, such solutions have yet to be widely solve the general problem. Computer inverse simulation methods have been proposed with the development of numerical simulation technology and computers. Domestic Northwestern Polytechnical University, Shandong University, and other colleges and universities have applied this method to solve many practical problems. Reverse simulation is based on plastic forming theory, from the metal forming mechanism to solve the blank shape method, to be able to get the initial blank shape and size required for forming parts from the given size and shape of the part, along the direction opposite to the molding process to simulate deformation. The prediction of the amount of DYNAFORM downstream is mainly used in the BSE (Sheet Size Element Calculation) module, which uses a finite element One-step forming inverse algorithm; the process is shown in Figure 2-1. The calculation speed is very fast and can accurately predict the initial shape of the sheet.
Fig.2-1 Process of BSE module
The finite element software for sheet forming based on incremental theory can simulate the whole process of forming and generally has a high computational accuracy. Still, the method has certain defects, as shown in Table 2-1. Therefore, there is an urgent need for simple, fast, and technicians in the finite element knowledge is not demanding, according to the product or process number of mold information, in the design and manufacture of the pre-stage prediction of the deformation of the numerical simulation technology.
Table.2-1 Disadvantages of numerical simulation based on incremental theory
|Computing time||Long time|
|Development cycle||Long time|
|Requirements for technical personnel||Mastering knowledge in this area|
At present, in the design and manufacture of the pre-stage prediction of the deformation of the part, more and more use of one-step fast simulation technology and make it a research and application of hot spots. A one-step method based on the finite element full amount of deformation theory to solve the solution speed, so it is often also known as the fast method; in addition, the one-step method uses the part shape and size information to back-calculate the size of the blank shape and size and be able to predict the deformation. It is also known as the inverse method. Therefore, this method has the advantages of simple operation, fast calculation speed, can be used for any shape of the workpiece, and can predict the formability of the stamped part through the inverse of the stamping and forming process. The idea of the one-step method is to deduce the size and shape of the initial blank through nonlinear finite element analysis based on certain boundary conditions and inversely deduce the positions of the grid nodes in the blank from the distribution of the grid nodes in part, to deduce the size and shape of the initial blank. The calculation process of the one-step method involves the principle of virtual work to establish the equilibrium equations, the treatment of boundary conditions, and the selection of yield criterion is a complete finite element analysis system, which is equivalent to a loading step of the incremental method, but there is no contact search judgment. In addition, the assumption in the one-step method that the part is formed in one step will greatly affect the convergence of the nonlinear equations. If the mesh quality is poor, a pathological stiffness matrix will be formed, which will also affect the convergence of the equations.
2.3 Introduction to DYNAFORM software and modeling of unfolding undercutting
American LSTC company and ETA company developed Dynaform software specially used in the slab stamping and forming simulation simulation of special software. Still, ETA/FEMB before and after the processor and LS-DYNA solver with the perfect fit is now one of the most popular mold designs and slab stamping and forming of the CAE tools. Dynaform is much more functional; Dynaform is a very powerful pre-processing system with many functions. It has strong graphic input capabilities, can be combined with various CAD software, has many cell types, and has very advanced contact and boundary condition processing capabilities, especially with hundreds of material models. Its world-famous LS-DYNA solver uses explicit and implicit computational methods to simulate the process of sheet stamping and forming and has very good problem-solving capabilities. At the same time, Dynaform software has a lot of very useful post-processing capabilities; eta-post is a special ETA company designed to apply Dynaform’s post-processing software, which allows the user to get the calculation results very intuitive.
Dynaform, as a plate stamping and forming simulation software, can very accurately predict the deformation process of the slab cracks, wrinkles, thinning rebound, etc., used to judge the quality of the forming of the plate and then for the blank stamping and forming process and tooling design to provide an effective reference. Currently, Dynaform is used in many universities and research institutions and by major aircraft and automobile companies worldwide.
When using Dynaform software to simulate the plate, blank stamping and forming generally have three main parts: the establishment of the simulation model, solving, and calculating the results of the analysis it is the whole process is shown in Figure 2-2.
Figure.2-2 DYNAFORM software analysis flowchart
In this paper, the use of Dynaform software to expand the elbow under the finite element analysis process is as follows: first read in the CAD software UG established in the standard elbow geometric model and finite element mesh; and then define the blank attributes, such as the thickness of the plate and the choice of material model; and finally submitted to the computation and the results of the post-processing, evaluation of molding results.
2.3.1 Geometric Modeling
Dynaform finite element software supports various CAD systems; you can use 3D drawing software UG to establish a 3D model, export the model to IGS format, as shown in Figure 2-3, and then import it into the Dynaform software before the processor.
Processor in the Dynaform software.
Figure.2-3 Export IGS format from UG
In this study, the UG was established in the diameter of 711 mm, wall thickness of 20 mm, radius of curvature of 1.5D 90 ° X80 elbow and diameter of 1219 mm, wall thickness of 20 mm, radius of curvature of 1.5D 90 ° X80 elbow model. Because Dynaform imports are sheet structures, this paper to elbow outer wall size is the standard to establish the model. To bevel the next step in the elbow processing process, the edge of the pipe wall modeling leaves 10mm of processing allowance.
2.3.2 Grid division
For the forming simulation calculation, the reasonable choice of cell type is an important process; whether to choose a reasonable cell type on the calculation accuracy and simulation results will produce a very large gap. Therefore, we must follow the different press-forming processes using finite element calculation simulation to select a reasonable cell type. When dealing with the bending problem, the thickness of the slab is very small compared to the size in other directions. Suppose we use three-dimensional solid units, to get more accurate results. In that case, we have to set at least 6-8 three-dimensional solid units in the direction of the thickness of the plate to calculate the bending effect. Still, the result is that the total number of units is very large, which reduces the efficiency of numerical simulation greatly. Therefore, two-dimensional shell cells are mostly applied in numerical simulations for calculating plate blank forming.
This paper uses two-dimensional shell cells to divide the mesh, as Figure 2-4 shows the mesh division results of Φ700 large-radius elbow, with a mesh size of 5mm and a total of 25538 meshes. At the same time, Φ1200 large radius elbow mesh is the same as this.
Figure.2-4 Elbow mesh delineation
2.3.3 Material properties
The metal material selected in this paper is X80 pipeline steel. An important trend in developing national pipeline engineering is high-pressure transportation, large diameter, high steel-grade pipe fittings. The international X80 pipeline steel has been a large development and successful application, and production technology has matured. In our country, the second line of the West-East Natural Gas Pipeline Project X80 pipeline steel pipe is also used, and in the second line of the West of the project, construction of the new product has been fully applied. High-strength pipeline steel X80 uses the U.S. classification model; line steel pipe X80’s minimum yield value is 80,000 psi, with a minimum yield strength of 552 MPa. API standard in 1973 added the X70 pipeline steel, and in 1985 added the X80 pipeline steel, and now has been developed in the X100 steel, the carbon equivalent of 0.02 – 0.05%, the relative carbon equivalent has been reduced to 0.35%, the carbon equivalent has also been reduced to 0.05%, the carbon equivalent of 0.05%, the carbon equivalent of 0.05%, the carbon equivalent of the carbon equivalent has been reduced to 0.35%. Equivalent has also been reduced to less than 0.35, thus realizing the contemporary meaning of multiple micro-content pipeline steel. X80 pipeline steel to low C – Mn – Nb system is the main, while adding Cr, Mo, Ni, Cu a variety of micro-alloying elements to strengthen the matrix, especially in the thick specifications of the X80 pipeline steel, must be added to a certain degree of hardenability elements such as Mo, Cr, etc., to ensure the uniformity of the surface and the heart of the organization. Consistent. The main chemical composition of the following table 2-2.
Table.2-2 Thick specification X80 pipeline steel composition (mass fraction, %)
X80 material mechanical properties are as follows: yield limit σs = 552 MPa, strength limit σb = 802 MPa, Poisson’s ratio μ = 0.3, modulus of elasticity E = 210 × 103 MPa, uniform elongation under unidirectional tensile δ = 21%, the material density ρ = 7.83 × 10-9 T/mm3, the equivalent force-strain relationship in plastic deformation is 0.20 σ = 0.20 σ relationship is 0.20σ = 1107.89ε0.20, where the hardening coefficient is 1107.89 MPa and the hardening index is 0.2. The stress-strain curves at different temperatures are shown in Figure 2-5.
Figure.2-5 Stress-strain relationship
2.4 Analysis of elbow press formability
The one-step method is a complete finite element analysis system in terms of the solution process, which can not only inverse the blank size and shape but also predict the formability of the stamped part. Open the Dynaform post-processing module, and you can get the FLD diagram, as shown in Figure 2-6, and the wall thickness distribution diagram, as shown in Figure 2-7, respectively. It can be concluded from the two diagrams:
- There is no danger point in the forming process, and there is no danger of rupture, serious thinning, or wrinkling; the inner arc of the pipe wall is slightly thinned, and the outer arc and the bottom of the pipe are slightly thickened; from the inner arc of the pipe wall to the outer arc of the pipe wall along the symmetric surface shows a trend of thinning to thickening and then to thinning and finally thickening; the overall wall thickness distribution of the elbow is relatively uniform. Therefore, the plate cold press molding elbow process is feasible.
Figure.2-6 Φ700 large-radius elbow FLD Figure
Figure.2-7 Φ700 large radius elbow forming wall thickness distribution
2.5 Analysis of blank back-calculation results
Elbow model after DYNAFORM plate back-calculation to get the results shown in Figure 2-8. The figure can be seen at the elbow after the expansion of the X-Y surface to form a closed curve; that is, the computer inversion of the plate shape should be obtained after the outer contour curve. DYNAFORM back calculation of the contour curve obtained after the IGS format export, and then in the UG will be imported; you can establish the plate model and get the blank size.
Figure.2-8 Elbow expansion
2.5.1 Analysis of Φ700 elbow blank back-calculation results
As shown in Fig. 2-9a), the shape and size of Φ700 large-radius elbow are optimized after back-calculation by computer, in which the area of the sheet is 1946735.18 mm2. As shown in Fig. 2-9b), the shape and size of the blank are obtained using the empirical undercutting technique in the factory, which is the best way to obtain the shape and size of the blank.
As shown in Fig.2-9b), the shape and dimensions of the optimized blank are obtained using an empirical factory undercutting technique in which the slab area is 2189175.40 mm2.
Figure.2-9 Φ700 large radius elbow different blank shape and size comparison
A comparison of the above figure shows that if the use of rectangular discharging material, the computer back-calculated the resulting blank, then the factory under the blank to save about 11% of the metal material, reducing the waste of metal materials. And in the bending port by computer expansion.
The result is a bending curve from the factory under the material obtained for a straight line. After stamping, the curve elbow port is relatively flat and in a plane, while the straight elbow port is not flat and not in a plane (this part will be demonstrated later in the Chapter 3 simulation).
2.5.2 Φ1200 elbow blank back-calculation results analysis
The same can be obtained after optimizing Φ1200 large radius elbow blank shape and size, as shown in Figure 2-10a.
Figure.2-10 Φ1200 large radius elbow different blank shape and size comparison
Its area is 5571927.13 mm2. If the factory experience undercutting technique is used, the resulting blank shape and size are shown in Figure 2-10b), in which the plate area is 6143354.76 mm2. It can be obtained that the optimized blanks using rectangular release undercutting are about 10% less than the factory undercutting blanks, improving the material’s utilization rate.
2.6 Summary of the Chapter
This chapter mainly uses computers to carry out elbow unfolding and undercutting techniques. Which briefly introduced the finite element method, plate forming one-step numerical simulation technology, and Dynaform software, and how to establish the elbow to expand the finite element model, including geometric modeling, mesh division, and determining material properties. The feasibility of the cold press forming elbow process is demonstrated, and the advantages of computerized back-calculation of blanks are reflected by comparing blank back-calculation results with the shape and size of the blanks discharged from the factory. Using Dynaform software simulation technology, the blank shape and size can be obtained relatively accurately, providing a basis for the elbow stamping blank.
Chapter.3 Cold Stamping of Elbow Sheets
This chapter mainly uses the characteristics of plastic deformation of sheet metal, the basic principles of finite element calculation method, and the functional characteristics of ABAQUS simulation software to determine the simulation model mesh type, boundary conditions, loading paths, and other parameters of the principle of selection, the establishment of sheet metal cold stamping finite element model. As shown in Figure 3-1, for the elbow tile after sheet stamping, to obtain a better shape of the tile, need to optimize the location of the sheet placement; this chapter also analyzes the elbow tile stress, strain distribution characteristics, spring back after stamping, and at the same time the shape of the spring back analysis.
Figure.3-1 Elbow tile
3.2 Cold stamping simulation modeling
Most people agree that ABAQUS is a very powerful and accurate simulation software that cannot only analyze and calculate complex mechanical structure models but is also especially suitable for analyzing complex and huge problems and simulation of a highly nonlinear simulation. ABAQUS provides users with many practical functions, especially since the operation is simple. For complex and integrated problems, users can easily simulate them using a combination of different functions.
The Part module of ABAQUS/CAE allows the creation of a wide range of part types, including 2D, 3D, and axisymmetric deformed or rigid parts. At the same time, the Part module also provides customers with a very large number of geometric model creation, modification, and processing functions; he also supports the use of other CAD software to build the model as long as the choice of the appropriate model format will be able to make the model imported into ABAQUS/CAE. In this paper, we use the specialized modeling software UG to establish the three-dimensional model of the component, export the model according to the IGS format, and finally import it into ABAQUS/CAE to establish the simulation model. Its numerical simulation process is shown in Figure 3-2.
Figure.3-2 ABAQUS numerical simulation roadmap
3.2.1 Geometric Model Creation
The elbow stamping process is placing the under-slab blank on the lower mold of the hydraulic press and pressing it into elbow tiles using a special mold. The stamping model is shown in Figure 3-3, where the shape and size of the sheet are obtained by backcalculating with Dynaform software. Because the upper and lower molds in ABAQUS software are set to discrete rigid bodies, the model is established as a sheet, only the mold work surface model.
Figure.3-3 Cold Stamping Assembly Model
3.2.2 Selection of cell type
We should use 3D hexahedral cells (brick type) when solving 3D problems because they can give the best simulation results with the lowest computational cost. ABAQUS/Explicit has neither regular second-order cells nor full integrals. The sheet material simulated in this paper is a solid body, so we choose the C3D8R in the ABAQUS/Explicit solid cell library (To suppress the expansion of the “hourglass” mode, it is necessary to set the hourglass control parameters to Enhanced, Relax, etc., and at least one of the parameters in the thickness direction should be set to “Hourglass.” To suppress the expansion of the “hourglass” mode, you need to choose to set the hourglass control parameters into Enhanced, Relax, etc., and in the thickness direction should be divided into at least four cells; this paper simulates the thickness of slabs in the direction of the set of eight cells, and the upper and lower molds are set to the discrete rigid body, choose the discrete rigid body unit R3D4 (four-node three-dimensional bilinear rigid quadrilateral unit).
3.2.3 Boundary condition processing
The plate blank stamping forming principle is complex is more complex three-dimensional space large deformation forming, but also about the Z-X symmetric structure, to facilitate the later data processing, respectively, established about the Z-X symmetric simulation model and the
Full simulation model in two ways.
- (1) Determination of friction conditions. The relative movement between the blank and the mold in the process of plate stamping and the size of the friction force in the deformation process greatly influence the product’s forming quality, and the treatment of friction greatly influences the realism of the simulation results. Friction is a complex physical phenomenon associated with the hardness of the contact surface, humidity, normal stress, and relative sliding speed. Some assumptions and simplifications are often made when performing finite element simulations. The following friction models are often used: penalty function friction, Coulomb friction, kinetic friction, Lagrange friction, and other models. In this paper, the Cullen friction model is selected, and the Cullen friction condition is satisfied between the blank and the mold, the friction coefficient is set to 0.1, and the normal direction is hard contact.
- (2) Loading mode. In the stamping process, the lower mold is fixed, and the upper mold moving displacement is the loading mode; when the two molds are only one plate thickness away from each other, the movement is stopped, in the displacement is loaded linearly.
3.2.4 Solver module selection
ABAQUS/Standard is a general-purpose analysis module capable of solving linear and nonlinear problems, which can solve dynamic problems, static problems, electrical problems, thermal problems, etc.
ABAQUS/Explicit is a finite element analysis module that uses display dynamics algorithms. It is suitable for simulating brief, transient dynamic events such as impacts and explosions. In addition, it is also very effective in dealing with highly nonlinear problems with changing contact conditions similar to metal plastic molding.
Considering the computational speed, this paper selects the ABAQUS/Explicit solver for the cold stamping simulation process. CONSIDERING THE COMPUTATIONAL ACCURACY, the ABAQUS/Standard solver is selected for the rebound simulation process.
3.3 Evaluation of sheet metal stamping model
3.3.1 Energy Analysis
For a model is not the formation of the correct static response assessment, the most universally significant way is to check the establishment of the model of the various energy conditions, according to the model of the law of change of the various energies and the ratio of these energies between the law to determine whether the model is reasonable. The following equation is the energy balance equation in ABAQUS/Explicit:
- EI-Internal energy (including elastic and plastic strain energy) (j).
- EV-energy absorbed by viscous dissipation (j).
- EKE-Kinetic energy (j).
- EFD-Energy absorbed by frictional dissipation (j).
- EW work done by external forces (j).
- Etotal-Total energy in the system (j).
If the numerical simulation model developed is a quasi-static numerical simulation model, the work done by the external force in the system should be almost equal to the value of its energy inside the system. Unless the system contains discrete dampers, viscoelastic materials, or material damping is used in the developed numerical simulation model, then the viscous dissipated energy is mostly very small. Since the value of the deformation velocity of the sheet material in the numerical simulation model established in this paper is not large, its inertia force is so small that it can be neglected in the quasi-static response process. With the above two conditions, we know that the sheet’s kinetic energy during the system’s quasi-static response is also very small. As a general rule, in most numerical simulations, the kinetic energy of the deformed material will not exceed a very small percentage of its internal energy (typically 5-10%).
The variation of kinetic energy, pseudo-strain energy, and the internal energy of the sheet with time during the simulation is shown in FIGS.3-4. As shown in the figure, the result can be concluded that with the change of time, its internal energy increases continuously, while its pseudo-strain energy is within a small range of basically no change, so relative to its internal energy and its pseudo-strain energy is negligible, so it can be concluded that the simulation model does not appear more serious “hourglass” phenomenon, and also can prove that the simulation model is not a good choice for the simulation. Therefore, it can be concluded that the simulation model does not have a serious “hourglass” phenomenon. It can also be proved that the network accuracy applied in the simulation model is sufficiently fine; there is no longer a need for further mesh division, and there is no need to quote the hourglass stiffness. It can also be concluded that the kinetic energy of the sheet is negligible compared to its internal energy, and the ratio of its internal energy is also very small, completely below the ratio of 5%, which is also within a reasonable range. As shown in Figure 3-5 kinetic energy history results, the kinetic energy response is related to the forming process of the sheet. Still, there is little turbulence with the change of time, so the kinetic energy of the sheet is reasonable and appropriate. Therefore, through the above analysis, it can be concluded that the whole deformation process of the cold stamping simulation model of the sheet material established in this paper can be regarded as a quasi-static response process, and the loading method applied in the simulation model is also acceptable.
Figure.3-4 Energy History
Figure.3-5 Kinetic energy history
3.3.2 Wall thickness distribution law
Figure 3-6 shows the results of wall thickness measurement on the symmetry plane of cold stamped Φ700 elbow tile. Figure 3-6a) shows the left end of the elbow tile at the inner arc welded bevel of the pipe wall, and the right end is at the outer arc welded bevel of the pipe wall, while Figure 3-6b) shows the wall thickness distribution along the left end to the
The right end of the wall thickness distribution. The conclusion from Figure 3-6: the outer arc welding bevel and elbow tile bottom slightly thickened and slightly thinner wall arc welding bevel; from the outer arc welding bevel to the pipe wall arc welding bevel to the formation of the inner arc welding bevel from the thickening to the thinning and then to the thickening of the last to the thinning of the trend; the entire wall thickness of the elbow distribution is relatively uniform, there is no large thickened or thinned area. It can be seen that this is consistent with the results of the stamping feasibility analysis of the elbow obtained in Chapter 2 using the software Dynaform. Therefore, the correctness of the cold stamping model is verified, and the model can more accurately simulate the elbow cold stamping forming process.
Figure.3-6 Φ700 elbow symmetry surface thickness measurement
3.4 Optimization of plate blank placement position
Although the computer-optimized shape of the elbow slab blank was obtained in this paper’s previous chapter, the blank’s placement in the mold has yet to be determined. Elbow slab blanks in different positions inevitably cause different shapes after stamping and forming. If the blank placement is not appropriate, it will not be able to get a satisfactory or qualified elbow shape, so optimizing the blank placement will become very important.
3.4.1 Determination of optimization scheme
First, the coordinates of the blank in the X-Y plane after optimization by Dynaform software are kept unchanged as the initial position on the die for stamping simulation; if a satisfactory stamping shape cannot be obtained, the blank placement position needs to be changed. Because the blank and the upper and lower molds are on the Y-Z surface of the symmetrical structure, the blank after stamping the biggest defect is the resulting elbow tile front, and the back wall is not flush enough. There is a high before and after the low or low before and after the high phenomenon, so the blank in the X-axis of the position remains unchanged; you only need to adjust the position of the Y-axis of the stamping simulation. Suppose you still cannot get satisfactory results. In that case, you need to continue to adjust the position of the blank in the Y-axis and keep simulating until you get more satisfactory results.
3.4.2 Optimized simulation results
Figure 3-7 shows the optimized forming process of Φ700 elbow tile. As can be seen in the figure, when the blank is put into the initial position after stamping, the elbow tile is low in the front and high in the back of the shape and does not meet the requirements of the tile; at this time by the shape of the elbow tile can be seen, the blank needs to be moved several distances to the negative direction of the Y-axis when stamping so here is tentatively set to the blank to the negative direction of the Y-axis to move a distance of 10mm, however, stamping to get the front and back of the high shape of the elbow tile; the use of the principle of the dichotomy method, it is Adopting the principle of dichotomy, it is changed to move a distance of 5 mm from the initial position to the negative direction of Y-axis, and then the elbow tile with the shape of front low and back high is obtained. Continuing to adopt the dichotomy method and changing the distance from the initial position to the negative direction of the Y-axis to move a distance of 7.5 mm, the elbow tile with the front and back wall edges almost flush is obtained, and the optimal placement of elbow tile blanks is obtained as a result. Using the same principle and process, Φ1200 elbow tile optimization forming process: when the blank is placed in the initial position after stamping the elbow tile was low in front and high in the back of the shape, and does not meet the requirements of the tile; at this time by the shape of the elbow tile can be seen, the blank stamping needs to be moved a number of distances to the negative direction of the Y-axis, so here is a tentative blank to the Y-axis to the negative direction of the distance of 10 mm, however, the stamping is still The elbow tile with the shape of low front and high back is obtained; at this time, the blank is moved to the negative direction of Y-axis by another 10 mm, and the elbow tile with the shape of high front and low back is obtained after stamping; later, the optimization area is gradually narrowed down by using the dichotomy method, and the optimal placement position of Φ1200 elbow tile blank is finally obtained: it is moved by 12 mm to the negative direction of Y-axis from the initial position.
Figure.3-7 Stamping and forming results at different positions
3.5 Sheet rebound modeling
Because the process used in this paper to save energy and reduce energy consumption, in the stamping-butt welding elbow stamping process to establish the simulation model is the sheet cold stamping molding model, but after cold stamping molding sheet rebound is slightly larger than the hot stamping molding after the sheet rebound, so the simulation of the cold stamping rebound process of the sheet will also be essential.
In the simulation modeling process, since more factors affect the rebound simulation accuracy, it is necessary to study the influence of various factors on the rebound to establish a rebound simulation model with high computational accuracy.
3.5.1 Influence of cell type and integration scheme
For the rebound simulation, the simulation of the bending stress field is very important for the forming process. To improve the bending stress simulation accuracy, increasing the number of Gaussian integrals in the material thickness direction is an effective method, and the more Gaussian integrals, the more accurately the bending stresses can be simulated. FValente and LiXP pointed out that when the number of Gaussian integrals is greater than 9, the improvement of the simulation accuracy is small. Still, it significantly increases the CPU computation time. Considering the simulation efficiency, 7 Gauss integration points are taken along the plate thickness direction.
3.5.2 Influence of Finite Element Algorithm
There are generally two kinds of static implicit and dynamic explicit algorithms when simulating and calculating the finite elements of the rebound process. Different from the molding simulation process, there are many difficulties in applying the dynamic explicit algorithm in the rebound simulation. Because the system unloading process on the part of the boundary conditions of the constraints is very low (close to the free boundary), you cannot use the speed of the external force applied or mold movement to determine the end of the calculation time, can only be applied to gradually consume the damping effect of the system deformation process of elastic strain energy, which makes the whole simulation of the computation time is very long and the calculation of the error accumulation of a large number of easy to cause the computational failure.
Although the static implicit algorithm could be more efficient and has better convergence when solving large molding problems, it is very efficient when solving the rebound problem, and often, good results can be obtained after several iterations or even one step. Using a static implicit finite element algorithm to simulate the rebound is a sacrifice of computational efficiency to ensure the accuracy of the calculation measures.
This paper uses the static implicit algorithm to simulate the spring back after stamping.
3.5.3 Influence of unloading path on rebound simulation
There are generally two calculation methods for choosing the path to simulate the deformation of rebound simulation: the modal method and the modeless method. According to the theory of springback simulation of the molded method, at the end of the forming stage, the unloading and springback process of the part is a very complex nonlinear deformation process, which must accurately simulate the unloading action of the mold, which is very demanding for the contact friction model and leads to computational inefficiency due to the involvement of fine incremental steps and contact friction nonlinear iterative process. According to the theory of moldless springback, although the unloading springback of a part is a very complex deformation, the springback problem mainly belongs to the linear elasticity problem, so the final springback simulation results are not much affected by the deformation path. Therefore, the final result of elastic recuperation can be calculated by reversing the loading of the equivalent nodal force. This method is more efficient because it does not involve contact calculation.
In this paper, considering the effect and efficiency of simulation calculation, this paper adopts the moldless method in the rebound simulation.
3.6 Analysis of plate cold stamping springback forming results
3.6.1 Φ700 elbow tile end face results analysis
After the cold stamping springback has been formed elbow tile, this time from the ABAQUS output tile end face of each node of the three-dimensional coordinate values, the value will be converted to Excel table form to save to facilitate numerical processing. The Z-axis coordinates are set to 0, then the projected coordinates of each node of the end face of the shingle in the X-Y plane are obtained. Then, the projected coordinates of the X-Y surface are fitted to the curve of the discrete points, and the results shown in Figure 3-8 are obtained. From the figure, it can be seen that all the projected points are almost on the same straight line, and the equation of the fitted straight line is:
y = -0.9945 x -41.456 (3-1)
Where: the fitting calculation R2 variance is 0.9999, which can be regarded as all points are on the same straight line. Therefore, it can also be concluded that the computer back-calculated blanks obtained after cold stamping rebound elbow tile, its end face mouth in the same plane, that is, the port is flat and straight.
Figure.3-8 Tile end face projection fitting curve
Using the same principle and process, it is also verified that the Φ1200 elbow tile end face ports obtained after cold pressing of the blanks back-calculated using Dynaform are in the same plane, i.e., the ports are straight. As mentioned earlier, the actual production of the factory under the sheet ends is straight; after stamping and welding to form, the elbow port is not flat and cannot be connected to the straight pipe; you need to go through a disk head process will be milled flat port. Relative to the factory material, the computer back-calculated sheet ends for the curve, and after stamping, the port formation is straight. It can be concluded that the back-calculated blank shape is more reasonable.
3.6.2 Effect of Cold Stamping Springback on Forming
As shown in Figure 3-9 for the intercepted elbow tile symmetry before and after the springback position comparison, to see more clearly, the figure the deformation effect will be enlarged twice the results, and the actual springback deformation for the figure half. The figure shows that the rebound simulation will be fixed at the left end; the rebound effect will be fully concentrated on the right. After measurement, the left end of the rebound distance of 13.2 mm; although the actual elbow tile after demolding will be both ends of the rebound, the total amount of rebound should be the same. Because of the use of cold press molding, the total amount of rebound after molding the elbow tile has more than half the thickness of the slab, which makes the rebound change the ellipticity of the elbow tile, so the calculation of the elbow tile rebound simulation cannot be ignored.
Figure.3-9 Elbow tile symmetry surface rebound diagram
3.7 Summary of this chapter
This chapter briefly introduces the algorithm of finite element simulation software ABAQUS and how to establish the plate cold stamping finite element model, including the material model, friction model, contact model, geometric model, unit type, and solver selection. Plate material selection of X80 steel, in the mold and blank of each contact surface tangential to the use of hard contact, normal to the use of Cullen friction contact, the friction coefficient of 0.1. blank using a three-dimensional solid model, choose three-dimensional reduced integral first-order (linear) interpolation of hexahedral solid unit C3D8R; mold using discrete rigid body model, and choose discrete rigid body unit R3D4. Choosing an ABAQUS/Explicit solver greatly reduces the time required to simulate the material model; the simulator is a good choice. An explicit solver is used to reduce the calculation time and improve the calculation efficiency greatly.
This chapter uses computer simulation experimental research to develop the elbow cold stamping sheet placement scheme to optimize the best placement of the sheet. It also establishes a plate rebound model that analyzes the effects of cell type, finite element algorithm, and unloading path on the rebound simulation calculation. It also analyzes rebound effects on elbow cold stamping forming.
Chapter.4 Thermal Shaping of Elbows
After the cold stamping of the plate after the spring back to form a tile, but in the tile after two butt welding elbow pipe diameter, the standard requirements of the nominal diameter is larger than the elbow oval degree does not meet the standard, so the elbow also needs to necking plastic. At present, the factory is only necking the tube mouth; it is obvious that the other parts of the elbow still need to meet the standard. In this chapter, we will study a whole diameter thermal shaping method so that the entire elbow pipe diameter reaches the standard pipe diameter within the error range. Due to the larger pipe specifications, taking into account the larger diameter straightening force and the impact of the rebound after the straightening, this paper adopts the whole diameter thermal shaping, which reduces the shaping force and can make full use of the existing hydraulic equipment, but also reduces the impact of the rebound so that after shaping the shape of the elbow does not have a large change.
4.2 Finite element thermal coupling basic equations
In the elbow thermal deformation process, the effect of temperature on the molding is very obvious; the temperature of the elbow, along with the elbow shape change and changes significantly, and in the case of different temperatures, the mechanical properties of the elbow material are not the same, showing the deformation effect is not the same. In the metal plastic forming process, the temperature of the mold is generally lower than the blank temperature; mold and blank contact surface will produce a heat conduction phenomenon so that the local temperature of the blank falls rapidly, and the blank and it’s surrounding air thermal radiation and thermal convection phenomenon so that the heat of the blank to cause serious loss of the blank, the blank’s local temperature changes rapidly; blank in the process of plastic deformation due to deformation of the work generated by the part of the transformation into internal energy, there is The work generated by the friction between the mold and the blank is also partially converted into internal energy, which makes the temperature of the blank has a certain increase. In short, in the material thermal deformation process, the blank temperature field change is very complex, so the blank deformation results have a great influence. Therefore, the study of material plastic deformation problems must be combined with the metal deformation flow and thermal effects for thermal coupling analysis.
4.2.1 Thermal coupling
In the process of plastic deformation of materials, along with the forming process of the blank, a part of the plastic deformation work and friction work into internal energy to heat conduction to the mold and the blank in the way, at the same time, the blank continues to heat convection in the form of heat to the surrounding environment, the mold and the blank and the way to heat conduction between the heat transfer of heat, the whole process is a source of heat is extremely unstable heat conduction process can be applied to the following The basic equation to express the heat convection and heat conduction characteristics.
(kTj)j + f = (ρCpT) (4-1)
In the equation:
- k-Thermal conductivity (W/(m.°C)-1);
- f-proportion of heat generation; and
- T-temperature (°C).
- ρ-material density (kg/m3).
- CP-Specific heat capacity (J(kg.°C)-1).
Where (kTj)j represents the proportion of heat transfer and (ρCpT) represents the proportion of internal heat generation.
The proportion of heat generation within the deformed body due to plastic deformation can be expressed by the following equation:
r = aσε (4-2)
In the formula: a-percentage of heat energy converted from strain energy, generally 0.9 – 0.95.
4.2.2 Heat of deformation and change of temperature field
In calculating the temperature field, we can regard the plastic deformation work of the material as a body heat source. The friction work is equivalent to the exchange of heat flow between the contact surface of the blank and the mold so that we can express the body heat source as well as the friction work Q into the following formula:
q = μus (4-4)
In the equation:
- γ-plastic deformation work into heat efficiency (generally take γ = 0.9 – 0.95).
- μ-mold and blank parts of the friction (N).
- us-mold and blank parts of the relative speed of movement (mm.s-1).
Usually, to deal with the convenience and simplicity, it can be assumed that the billet and mold absorbed 1/2 of the total frictional work. The temperature field on the plastic deformation of the material can usually be assumed that the flow stress is the equivalent strain rate, the equivalent strain, and the temperature of the function.
4.3 Elbow straightening thermoplastic modeling
4.3.1 Elbow weld treatment
Whether using the original technology of the factory under the plate material or computer back calculation of the plate material, the plate material, after cold stamping springback elbow tiles, needs to go through the trimming and beveling process. Sheet after stamping and molding to get the elbow tile, but at this time, the tile wall edges are not straight, so the two tiles will not be able to align or aligned after the pipe diameter is too large, so the need to trim. The welding process is used for the elbow tile connection, so beveling is indispensable.
Because of the complexity of the welding decision, the computer simulation of welding is also quite complex. Still, the width of the weld area relative to the diameter of the elbow is very small. From the general weld to weld, the material properties of the weld area are significantly better than the other regions of the material properties of the elbow. Therefore, the weld will be dealt with in most of the rigid body simulations. In this paper, the elbow diameter straightening process, the weld area relative to the pipe diameter is too small, with almost no effect on the straightening, so in the establishment of the elbow model without a weld.
4.3.2 Geometric model
Because the ABAQUS software simulation elbow tile trimming beveling and welding process is very complex, and establishing the simulation model is very difficult, it is also difficult to ensure the reliability of the simulation results and these two processes.
On the back of the elbow thermal shaping process is very small, so these processes in the three-dimensional modeling software UG simplify the processing.
First of all, the ABAQUS software will be obtained in the press elbow tile profile curve on the coordinates of each node output, the use of these node coordinates in the software UG to re-construct the elbow tile model, and then use the software UG in the “trimming body” function instead of trimming process will be the elbow tile wall edge trimming, and then use the “Mirror Body” function. “Mirror body” function will flatten the elbow tile along the corrected surface mirror; Finally, the two elbow tile “sum” to replace the welding process, and ultimately obtained as shown in Figure 4-1 after the butt welding elbow geometric model, due to the cold stamping to leave machining allowances, elbow Due to the machining allowance left in the cold stamping, the elbow has a straight edge section and the pipe diameter is larger than the standard elbow, the need for subsequent reduction and shaping process to eliminate the straight edge section and the pipe diameter compressed to the standard diameter.
Figure.4-1 Butt-welding elbow model
As shown in Figure 4-2, the model of the upper and lower molds uses a discrete rigid body, so they only need to make contact with the elbow with the working surface. To compress the elbow diameter to the standard pipe diameter, the upper and lower molds’ working surface will form the outer surface of a standard elbow.
Figure.4-2 Upper and lower mold models
4.3.3 Material Thermal Properties
The elbow material is still X80, and the material’s room temperature properties, as described in Chapter.2 remains unchanged, but in the straightening is the first elbow heated to high temperatures and straightening, this time for the metal hot forming process. The computer simulation of thermoforming thermal coupling problems needs to know the material’s thermal properties, the specific heat capacity of X80 steel, and the internal thermal conductivity with the temperature change of the law curve shown in Figure 4-3.
Figure.4-3 Material properties
4.3.4 Setting of Other Conditions
In this paper, the simulation model adopts the dynamic display thermal coupling analysis step; the elbow applies three-dimensional four-node linear thermal coupling unit C3D4T, and the upper and lower molds are selected discrete rigid body unit R3D4, that is, four-node three-dimensional bilinear rigid quadrilateral unit; there are four unit cells divided in the thickness direction of the elbow; the elbow and the mold to meet the coulometric friction conditions, so the coulometric friction model is selected, the friction coefficient setting The friction coefficient is set to 0.12, and the normal direction is set to contact hard;
The lower mold is fixed, the displacement of the upper mold is loaded, and the upper and lower molds are combined; ABAQUS/Explicit solver is finally selected.
4.4 Elbow necking ratio optimization
The elbow straightening and straightening process reduces the entire pipe fitting. As shown in Figure 4-4, if the elbow obtained after cold stamping butt welding reserved straight edge section is too large, the shaping process of shrinkage ratio is large, with the increasing deformation force reserved for the straight wall section first into the yield state and ultimately the formation of instability, the material in the mold at the folding of the instability or wrinkled defects, cannot meet the process requirements.
Figure.4-4 Elbow instability folded and wrinkled
Suppose you want to avoid the elbow straightening and straightening of folding instability or wrinkle defects. In that case, it is necessary to use simulation to optimize the elbow shaping and straightening of the limit of the shrinkage, and to avoid defects; it is necessary to give a reasonable amount of necking and shaping. To this end, this paper on the different specifications of the two elbows, respectively, uses different necking amounts of the whole diameter of the straightening process to optimize the simulation experimental research. The optimization experiment aims to provide a safe and reliable elbow straightening and straightening technology necking ratio or necking amount; the results of the simulation optimization experiment are shown in Table 4-1. As can be seen from the table, for Φ700 elbow, when the straightening and straightening shrinkage is greater than 58.46 mm (or shrinkage ratio is greater than 2.7%), there will be wrinkles or even the danger of unstable folding. Therefore, the simulation results show that, to reduce the scrap rate in this paper, it is recommended that the necking amount should not be greater than 50 mm (or the reduction ratio is not greater than 2.2%). Similarly, the Φ1200 elbow diameter straightening reduction amount should not exceed 50 mm (or the necking ratio is not greater than 1.3%).
Table.4-1 Optimization of necking parameters
|Pipe specifications||Pipe circumference (mm)||Standard pipe circumference (mm)||Reduced diameter (mm)||Reduction ratio||Forming effect|
|Φ 700||2176.31||2107.94||68.37||0.031||Unstable folding|
|Φ 1200||3912.84||3829.6||83.24||0.021||Unstable folding|
4.5 Elbow shape inspection
For a molded elbow to be qualified, the industry has made the standard requirements for its shape and size, so the elbow, after molding, should be its shape and size inspection. For example, the pipe diameter of the elbow should be within the error range of the nominal pipe diameter; elbow angle error should be within a certain range; standard elbow fittings for the round cross-section, so the cross-section oval degree of certain requirements; elbow cross-section thickness error also has requirements. All of the above is to determine whether the elbow, after forming the main parameters of the qualification, so to do the main analysis.
4.5.1 Elbow pipe circumference and necking ratio
Figure 4-5 is the Φ700 elbow after cold press butt welding, and after reducing the size of the final elbow shape and its equivalent strain, the figure can be seen in the elbow weld area. The symmetry surface of the intersection of the equivalent force is the largest; this is also the most likely to be the first place to appear or defects. To avoid instability folding or surface wrinkles, the initial circumference of the stamped butt weld elbow is 2146.63 mm.
Figure.4-5 Φ700 elbow shrinkage shaping results
After measurement, after reduction and shaping of the elbow end face circumference of 2109.06mm, the shaping reduction ratio is 1.75%; for the middle symmetry face circumference of 2106.65mm, the shaping reduction ratio is 1.86%. Standard Φ700 elbow tube circumference of 2107.94mm, tube end, and symmetry surface circumference relative error value are less than 1‰, completely within the circumference error.
4.5.2 Elbow end face angle
Φ700 cold stamping – butt weld elbow reduction shaping simulation, from ABAQUS output elbow end face of each node of the three-dimensional coordinate values, and then import the values into the Excel table to facilitate numerical processing. First of all, the Z-axis coordinate value of each node is set to 0, then the coordinate value of the projection point of each node of the elbow end face in the X-Y plane is obtained. Then, the X-Y surface of the projected point coordinates of the value of the discrete points of the curve fitting, the results shown in Figure 4-6, from the figure can be seen in the X-Y surface of all the projected points almost in the same straight line. The equation of this fitted straight line is:
y = -x-42.896 (4-5)
Where the calculated variance of the fit R2 value is 0.9994, it can be considered that all the projected points within the X-Y plane are on this fitted straight line. Therefore, it can also be concluded that Φ700 elbow after reduction and shaping of the elbow end face profile obtained in the same plane, that is, the port of the elbow is straight, according to the slope of the equation and the symmetry of the elbow can be calculated that the angle between the two port planes of the elbow is 90 °, which is fully in line with the application standards of the elbow.
Figure.4-6 Elbow end projection fitting curve
Here, we can use the same principle and calculation process to derive the following results: Φ1200 elbow after reduction and straightening of the elbow port; elbow angle between the two port planes of 90 °, also reached the elbow end face shape.
Also, meet the elbow end face shape standard.
4.5.3 Elbow end face and symmetry surface ellipticity
Reduction and straightening of Φ700 elbow end face and symmetry of the node coordinates of the surface input to the software Imageware for elliptic curve fitting, the results shown in Figure 4-7.
Figure.4-7 Fitting results
As can be seen from the figure, after shaping and straightening Φ700 elbow end face long half-axis length is 355.9780 mm, the short half-axis length is 355.1898 mm, the elbow symmetry surface long half-axis length is 355.1841 mm, the short half-axis length is 355.1439 mm.
Pipe fittings relative nominal diameter calculation formula:
D = (ab)/2 (4-6)
In the formula.
- a – ellipse long axis length (mm).
- b – ellipse short axis length (mm).
Pipe fitting ellipticity calculation formula:
λ = 2(a-b)/(a+b) (4-7)
Formula: λ – pipe fitting ellipticity.
Calculated from the above formula (4-6) and formula (4-7) can be obtained: after shaping and straightening Φ700 elbow end face relative to the nominal diameter is 711.17 mm, ellipticity of 2 ‰; elbow symmetry relative to the nominal diameter of 710.33 mm, elbow symmetry surface ellipticity of 0.1 ‰.
4.5.4 Elbow end face and symmetry surface thickness distribution
As shown in Figures 4-8 and 4-9, the elbow wall weld as the starting point along the counterclockwise direction in the elbow end face of an average of 44 selected nodes and was measured at each node of the elbow thickness. Measured results show that the maximum thickness of the elbow end face is 20.50 mm, the minimum thickness is 19.82 mm, the average thickness is 20.14 mm, and the average thickness variance is 0.042.
Figure.4-8 Elbow end face
Figure.4-9 Distribution of wall thickness at end face
As shown in Figures 4-10 and 4-11, the same elbow wall weld as the starting point along the counterclockwise direction in the symmetry of the elbow symmetry surface of an average of 44 nodes was measured at each node thickness. After measuring the elbow symmetry surface, the maximum thickness is 21.07 mm, the minimum thickness is 19.14 mm, the average thickness is 20.18 mm, and the average thickness variance is 0.042.
By the elbow end face and symmetrical surface thickness measurement results, although the average wall thickness of the elbow after necking and straightening is slightly larger than the required elbow wall thickness of 20 mm, the relative error of the wall thickness is less than 1%, within the error, fully able to meet the needs of the project.
Figure.4-10 Elbow symmetry surface
Figure.4-11 Symmetrical surface wall thickness distribution
4.6 Summary of the chapter
This chapter establishes the elbow diameter thermal shaping thermal coupling finite element model, including the thermal coupling model, elbow weld treatment, geometry model, material thermal properties, and other parameter settings. The elbow necking ratio was optimized experimentally, and the whole diameter thermoforming optimization parameters were given. The forming results of the thermo-shaping elbow were measured and analyzed to check the elbow shape.
Chapter.5 Mold Design
Mold design and manufacturing process levels are related to automotive, aerospace, agricultural machinery, instrumentation, military and daily light industrial products and other fields of product quality, reliability, life, and cost, directly affecting the rapid development of these fields, so the development of mold design and manufacturing technology has attracted high attention. Specifically, to reduce the total cost of mold design and manufacturing shorten the total cycle of mold design and manufacturing, with engineering and practical level of optimization technology and accurate and rapid computer simulation technology as a means to systematically carry out the development and application of a complete set of technologies, including process, equipment, design, materials, management, the formation of process, equipment, design, materials, management and development of the system to improve the advanced design, advanced materials, advanced Manufacturing comprehensive efficiency and overall level of plate forming manufacturing. Therefore, it is necessary to understand the structure of the mold study and improve the various technical indicators of the mold for mold design and development of stamping technology.
In this paper, the study of the subject of the mold is stamping – butt-welding elbow forming the main process equipment, forming elbow surface quality, dimensional accuracy, production efficiency, and economic benefits with the mold structure and its rationality of the design of the relationship is very large. The main working part of the mold (convex die, concave die) of the structural shape and size requirements not only on the deformation process of metal blanks and metal flow has an important impact, but also an important factor affecting the quality of elbow forming. Under modern technology, a reasonable structure shape and size of the working part of the mold should be designed according to the deformation simulation results of each process.
5.2 Mold design
The sheet material discharging simulation, cold stamping simulation, and straightening and straightening simulation results to get the sheet shape and size, sheet placement position, and proved that the process and mold geometry model used in this paper can produce qualified elbows. Provides a reference basis for the structural design of the mold.
5.2.1 Factory currently uses mold
As shown in Figure 5-1, the factory is currently used to produce the elbow stamping die. Due to the large size of the mold, the mold is formed by casting to save material and use a hollow structure. Because of the large area under the plate, stamping and butt welding result in a larger elbow size, so the need for shaping. Figure 5-2 shows that the factory used for port shaping mold. Although the mold can be able to shape the size of the port, the rest of the elbow size cannot be improved, so the shaping method of the elbow, in addition to the port of the other parts of the size of the actual, did not reach the elbow within the error requirements.
Figure.5-1 Factory existing stamping die
Figure.5-2 Elbow port shaping molds
We can see that the current production in the factory stamping shaping each requires a set of special molds, and shaping the process of the elbow port only orthopedic, which will inevitably be difficult to ensure that the size of the entire elbow is qualified. In the use of existing equipment based on this paper to improve the utilization rate of the mold and ensure that the size of the elbow qualified, designed a stamping – shaping a combination of molds.
5.2.2 Mold Structure Design
Although the actual use of two sets of molds in the factory production practice reduces the mold’s utilization rate significantly, the mold’s structure and size can still be used to design a new stamping-shaping combination of molds to provide a favorable reference basis. Therefore, this paper is guided by computer simulation technology, and based on the existing molds in the factory, we will design a stamping-shaping combination mold.
From the overview in the previous sections of this paper, it can be concluded that the working contact surface dimensions of the stamping die do not need to be changed, and the factory die dimensions can still be used to design a new stamping die. The working surfaces of the lower stamping die and the lower shaper die is slightly different in shape and dimensions. However, they still overlap to a large extent, and it is possible to consider the lower shaper die as part of the lower stamping die. Therefore, it is only necessary to make the stamping lower die into a block structure, and it can be used as a shaping lower die after taking away some extra pads when shaping, as shown in Figure 5-3 for the lower die structure of the stamping-shaping combination die. Among them, block 1 and block 2 with the lower mold using M20 hexagon socket countersunk head screws to connect and fix; block 2 is installed with three “positioning shims” on the slab position for positioning, “positioning shims” and block 2 with M10 screws fixed connection. This does not use positioning pins and refers to the “positioning shims” mainly to facilitate the replacement of different sizes of positioning shims and change the slab’s positioning so that the slab size requirements are more flexible and variable.
The shapes and sizes of the working surfaces of the stamping die, and the shaping die are different, so they must be designed separately. As can be seen from the stamping simulation, the hot stamping upper die used in the plant can still be used in cold stamping and yield good elbow shingles. Drawing on the structure of the hot stamping upper die used in the factory shown in Figure 5-1, the cold stamping upper die shown in Figure 5-4 was obtained:
- To save material, the body of the upper die is hollow and has reinforcement to ensure strength;
- To facilitate the fixed to the hydraulic press and meet the requirements of the hydraulic press stroke, in the mold above the connection of a welded structure called “bench”;
- The “bench” is welded to the mold.
From the plastic simulation, results can be obtained, shaping the upper mold and the lower mold working surface shape and size is identical, so you can shape the upper mold and shape the lower mold designed for the structure and size of the structure is similar to the structure, only shaping the upper mold does not need to open, and to reduce the weight of the material to save, the upper mold non-working surface dimensions of a smaller.
Figure.5-3 Lower mold and blank assembly diagram
Figure.5-4 Stamping upper die
5.3 Summary of this chapter
This chapter mainly introduces the design of a stamping and shaping combination mold different from the original mold, changing the original stamping mold and shaping mold into a block-type combination mold. This set of molds in the stamping die directly using the original stamping die structure does not change make full use of existing resources; the stamping die takes away the block that is shaping the mold, improves mold utilization; the original elbow port shaping die into a whole diameter shaping die, improve the quality of elbow forming.
This paper simulated the use of finite element software Dynaform and ABAQUS on the plate butt-welding elbow material optimization, plate cold press forming, and elbow diameter shaping process. According to the factory’s existing process as well as the simulation and optimization results of the design of stamping and shaping combination of molds, this paper has obtained specific conclusions are shown below:
- (1) The elbow unfolding finite element model is established, and the accurate undercutting of plate butt-welding elbow is obtained through the one-step numerical simulation technique of plate forming.
- (2) A numerical simulation model of elbow cold stamping and rebound was established to optimize the best placement position of the slab, and the cold stamping forming technology of plate butt-welding elbow slab was obtained.
- (3) Through the numerical simulation study of the straightening thermal shaping process, the shrinkage rate of the shaping elbow is optimized, and the technical method of straightening and shaping of plate butt-welding elbow is proposed.
- (4) The plate butt-welding elbow stamping and shaping combination mold technology was established Using the parameters obtained from the simulation optimization and based on the original mold parameters.
Combined with the research work of this thesis, to better guide the actual production, the authors believe that the following further research work needs to be done:
- (1) It is possible to optimize the simulation of more specifications of the elbow’s shaping parameters to find out certain laws.
- (2) Apply the parameters of the simulation study to guide the actual production of experimental research to test the optimization of the simulation parameters further.
Author: Wang Huaipan