Chen Xuewen, Chen Jun, Zhao Zhen, Wu Gongming, Ruan Xueyu, National Die CAD Engineering Research Center of Shanghai Jiaotong University, Shanghai 200030 Cold forging technology is a precision plastic forming process with incomparable advantages of cutting, which is widely used in the manufacturing of key parts of various medium and mechanical products. This paper comprehensively discusses the development status and trend of cold forging technology from the aspects of shape, material, process innovation, productivity, numerical simulation technology, digital/intelligent design technology application, and optimization technology of cold forging parts. Key words: cold forging, process/die design, numerical simulation, knowledge-based process design, design optimization State of The Art and Advance of Cold Forging Technology CHEN Xue wen, CHEN Jun, ZHAO Zhen, WU Gong ming, RUAN Xue yu National Die and Mould CAD Engineering Research Center, Shanghai Jiao Tong University, Shanghai 200030 ABSTRACT: Cold forging is a kind of precision forming technology, and has the unique advantages compared to the traditional machining technology. In this paper, the start of art and advance of the cold forging technology is systematically illustrated in the aspects of part shape, part material, process innovation, production efficiency, application of numerical simulation & knowledge-based design, and design optimization technology. Keywords: cold forging, process/die design Aerospace and machine tool industries have a wide range of applications. At present, the rapid development of the automobile industry, motorcycle industry and machine tool industry has provided the driving force for the development of the traditional technology of cold forging. For example, the total output of motorcycles in China in 1999 was more than 11.26 million [1]. According to the preliminary estimate in 2000, the total demand for automobiles in China will reach 3.3 million by 2005, including 1.30-1.4 million cars, and the demand for forgings in the automobile industry alone is more than 500,000 to 600,000 tons [2]. Although the cold forging technology started not too late in China, there is a big gap between the development speed and that of developed countries. Up to now, the weight of cold forgings on cars produced in China is less than 20Kg, which is half of that of developed countries. The development potential is great. Strengthening the development and popularization of cold forging technology is an urgent task in China.
1. The shape of cold forging is becoming more and more complex. The shape of cold forging parts is becoming more and more complex, from the original step shaft, screw/nut and conduit, to the parts with complex shape. As shown in Figure 1, there are spline shafts and spline sleeves of motorcycles of different sizes. The typical process of spline shafts is: forward extruding the rod part - upsetting the middle head part - extruding the spline; The main process of spline sleeve is: backward extruding cup shaped part, punching the bottom to make annular part, and forward extruding shaft sleeve. As shown in Figure 2, automobile output shaft and input shaft, as well as other cold forged products. As shown in Figure 3, there are various bevel gears, spiral bevel gears and other disk parts [1] for automobiles/motorcycles made by swing rolling technology in China. As shown in Figure 4, there are cold forged parts produced by a Japanese company. As shown in Figure 4, the scroll supercharger has been included in the national "10th Five Year" key project [3]. At present, the cold extrusion technology of cylindrical gears has also been successfully used in production. In addition to ferrous metals, the cold extrusion applications of copper alloys, magnesium alloys and aluminum alloys are more and more extensive.
2 Continuous technological innovation Cold precision forging is a (near) net shape forming process. The parts formed by this method have high strength, high precision and good surface quality. At present, the total amount of cold forgings used in an ordinary car abroad is 40~45Kg, of which the total amount of gear shaped parts is more than 10Kg. The weight of a single piece of gear formed by cold forging can reach more than 1Kg, and the gear profile accuracy can reach level 7. Continuous process innovation has promoted the development of cold extrusion technology. Since the 1980s, domestic and foreign precision forging experts have begun to apply the split flow forging theory to the cold forging of spur gears and spiral gears. The main principle of diffluent forging is to establish a material diffluent cavity or channel in the forming part of the blank or die. During the forging process, when the material fills the mold cavity, part of the material flows to the splitter cavity or channel. With the application of diffluent forging technology, the less and no cutting machining of high precision gears has rapidly reached the scale of industrialization. As proposed in [1], for the extrusion parts with a length diameter ratio of 5, such as the piston pin, the cold extrusion can be formed once through axial separation by adopting the method of axial residual block, and the stability of the punch is good; For the flat spur gear forming, the cold extrusion forming of the product can also be realized by using the radial residual block. Closed die forging is a kind of near net shape precision forging without flash, which is formed by extruding metal in one direction or opposite direction with one or two punches in a closed die. If the precision parts of some cars, such as planetary and half shaft gears, star sleeves, cross shafts, etc. are machined by cutting methods, not only the material utilization rate is very low (less than 40% on average), but also the labor consumption is high and the production cost is very high. In foreign countries, closed die forging technology is used to produce these net shaped forgings, which saves most of the cutting and greatly reduces the cost. The development of cold forging technology is mainly to develop products with high added value and reduce production costs. At the same time, it is also constantly infiltrating into or replacing cutting, powder metallurgy, casting, hot forging, sheet metal forming process and other fields, and can also be combined with these processes to form a composite process. For example, the hot forging cold forging composite plastic forming technology proposed in literature [4]. The hot forging cold forging composite plastic forming technology is a new precision metal forming process that combines hot forging with cold forging. It takes full advantage of the respective advantages of hot forging and cold forging: the metal has good plasticity in hot state and low flow stress, so the main deformation process is completed by hot forging; The precision of cold forging is high, so the important dimensions of parts are finally formed by cold forging process. The hot forging cold forging composite plastic forming technology appeared in the 1980s, and has been more and more widely used since the 1990s. Parts manufactured with this technology have achieved good results in improving accuracy and reducing cost. Figure 1 Motorcycle Spline Shaft and Spline Sleeve Figure 2 Automobile Output Shaft/Input Shaft and Other Cold Forging Products Figure 3 Products Produced by Swing Rolling Technology Figure 4 Cold Forging Products Produced by a Japanese Company
3 When the numerical simulation technology is used to check the rationality of the process and die design. When the cold forging process is used to form parts, because the metal has plastic deformation in the cold state, its deformation resistance is very large, so the deformation of the parts is relatively difficult. During the deformation process, it is easy to have defects such as insufficient metal and cracks in the forged parts. At the same time, excessive deformation resistance will seriously reduce the service life of the die. For a long time, the process design and die design of metal plastic forming in China have always used the traditional method of relying on experience and experiment. This design method is difficult to meet the requirements of net shape manufacturing process. With the rapid development of computer technology and the development of plastic finite element theory in the 1970s, many difficult problems in plastic forming process can be solved by finite element method. In the field of cold forging process, through modeling and determination of appropriate boundary conditions, the finite element numerical simulation technology can intuitively obtain the stress, strain, die stress, die failure and possible defects of forgings during the metal flow process. The acquisition of these important information has important guiding significance for reasonable die structure, die material selection, heat treatment and final determination of forming process plan. The current effective numerical simulation software is based on the rigid plastic finite element method. These software include DEFORM, QFORM, FORCE, MSC/SUPERFORM, etc. The finite element numerical simulation technology can be used to verify the rationality of process and mold design. For example, literature [5] proposed a new process for forming straight cylindrical gears from hollow billets: pre forging split flow zone - split flow final forging. Numerical simulation research was carried out with the three-dimensional finite element numerical simulation software DEFORM-3DTM, and the forging load stroke curve and the stress, strain, velocity distribution in the whole forming process were obtained, and compared with the results of traditional closed upsetting extrusion process simulation. The analysis shows that the traditional closed upsetting extrusion forming of spur gear has large forming load, which is not conducive to the filling of tooth profile. By adopting the new technology of pre forging split flow zone - split flow final forging, the forming load can be greatly reduced, and the filling property of materials can be significantly improved, so that the gear with full tooth shape corner can be obtained. Document [6] used the three-dimensional large deformation elastic-plastic finite element method to conduct numerical simulation of the cold precision forging process of gears, and carried out numerical simulation analysis of the deformation flow of the two-step forming mode with closed die forging as the pre forging and closed die forging, hole diversion and constraint diversion as the final forging. The results of numerical analysis and process experiments show that it is very effective to reduce the working load and improve the corner filling ability by adopting the diffluence, especially the restriction hole diffluence, in the final forging.
4 Application of digital intelligent design system23
4.1 Application of CAD/CAM technology in cold forging forming process/die design In order to meet the requirements of modern production for short cycle, high quality and low cost of forging die development, it is an inevitable trend in the field of forging die design to introduce advanced design theory and methods and computer technology, especially CAD technology, into the traditional forging die design process. With the advantages of high automation, high precision and high efficiency, mold CAD technology is continuously promoting the reform of traditional mold design and manufacturing methods. Since the 1970s, many foreign organizations have begun to carry out extensive research on forging die CAD/CAM. T. Altan of Bethel Columbus Laboratory in Ohio, the United States, and others first developed the axisymmetric forging die CAD system. The expert systems of the former Soviet Union, such as Jiejielin, studied the principles of die forging design automation and optimization, put forward a full set of theories from the aspects of methodology and algorithm model, and developed an automatic design system for rotary forging. In China, this aspect started relatively late. Shanghai Jiaotong University, Tsinghua University, Huazhong University of Technology, Harbin University of Technology and Nanchang University developed design systems for different types of forgings based on certain platforms, but most of them are limited to two-dimensional systems. So far, although many researchers have successively developed forging die design systems suitable for different parts, realizing automation and intelligence in several aspects of process and die design, there is no mature forging die CAD/CAM system to market, and many engineering designers can only use general mechanical CAD/CAM software to carry out detailed structural design.
4.2 Knowledge based design technology and its application in cold forging forming process/die design Cold forging forming process and die design are knowledge and experience intensive processes. The experience and knowledge accumulated by die designers in their long-term work have a very important impact on die design. The traditional CAD technology is separated from the knowledge in the design field, lacks the support for the knowledge in the design process, and is unable to meet the requirements of "high quality, short cycle, low cost" in mold development. Therefore, it is necessary to upgrade the traditional forging die CAD technology to the knowledge-based design level. The way to realize this leap is to introduce artificial intelligence (AI) technology and knowledge-based engineering (KBE) technology into the field of cold forging forming process/die design, and develop a knowledge-based design support system by combining it with traditional CAX technology. Knowledge Based Engineering (KBE) mainly refers to the application of knowledge to solve various engineering problems. It is the application of artificial intelligence technology in engineering. As the cold forging forming process/die design is a complex process involving the inheritance, integration, innovation and management of knowledge, after the introduction of knowledge-based design method, the experience and knowledge sharing of cross domain experts can make the design more creative and predictable, and the consistency of domain expert knowledge in the entire product design life cycle ensures the success rate of product development, greatly saving the development time, The design quality is improved. So that the whole design pattern changes from empirical design to scientific design. The Columbus Batel Laboratory of the United States developed a knowledge-based design system for preform geometric dimensions. Because the shape of the preform is a spatial geometry, its geometric shape must be operated, so the reasoning process cannot be described simply in general language. For the geometric information of parts, the frame method is used to express, and different grooves are used in the frame to define the basic components of parts and the topological relationship between them. Design rules are expressed by production rules and reasoned by OPS tools. J. C. Choi and C Kim [7] developed a knowledge-based integrated process design system for cold forging and hot forging, and established process design rules for cold forging and hot forging respectively. The application of knowledge-based design method in the design of cold forging process and die will completely change the traditional plastic forming state of relying on personal experience of designers, repeatedly modifying in the design process and low design efficiency. It uses artificial intelligence, pattern recognition, machine learning and other technologies to extract appropriate knowledge from the system knowledge base during the design process to guide the cold forging process and die design. At present, this technology is developing further. In recent years, knowledge-based design method has become a hot topic in the research of intelligent forging process/die design technology.
With the introduction of optimization technology, low cost, high quality and high efficiency are the goals pursued by the manufacturing industry. In forging industry, in order to improve design efficiency, reduce manufacturing costs and improve product quality, it is necessary to optimize various process parameters that affect forging quality during forging process. Because forging deformation is a very complex problem, it is difficult to achieve the desired effect by using traditional design methods for the optimization of its process parameters. With the continuous development and improvement of computer technology and plastic finite element theory and technology, numerical simulation methods represented by finite element method have been widely used in the solution and analysis of various metal forming problems. Therefore, the application of optimization design method based on finite element analysis in cold forging process/die design is not only possible, but also an inevitable trend. From the perspective of practical application, the most representative optimization methods based on finite element analysis include the optimization method based on sensitivity analysis and the fitting optimization method based on objective function value. The optimization design method based on sensitivity analysis belongs to gradient optimization design method. In the specific implementation of this method, first determine the objective function and design variables, then find out the relationship between them, derive the sensitivity derivative formula of the objective function to the design variables, solve the sensitivity information according to the existing value of the design variables, and then use the optimization algorithm to determine the optimal search direction of the design variables to obtain better design variables, and then solve the sensitivity information, and so on, Until the optimization iteration converges [8]. The fitting optimization method based on the objective function value comes from the extrapolation method. This method approximates the functional relationship between the objective function and the design variable with a simple interpolation function, and approximates the extreme point of the real function by solving the extreme point of this approximate function. When the method is used to optimize the forging process parameters, the objective function value is realized through the finite element program. At present, some general finite element analysis software, such as (DEFORM, Marc), have been widely used in the numerical simulation of the forging process, which can easily calculate the stress, strain and other information. Therefore, the fitting optimization method based on the objective function value can separate the finite element program from the optimization algorithm, which is suitable for different forging processes, and is more convenient for the parameter optimization of the forging process [9]. Although many achievements have been made in the research and application of forging process optimization technology based on finite element analysis, it is still in its infancy. From the construction of objective function, the selection of optimization design variables to the specific application of optimization methods, we can see that there are still some problems in this field. (1) The research and application of optimization methods based on sensitivity analysis have made many achievements. Because this method belongs to gradient optimization method, its convergence speed is relatively fast. However, there are still some shortcomings in its practical application: in the process of optimization design, it is necessary to solve the sensitivity information (i.e. derivative) of the objective function relative to the optimization design variables, and it is difficult to derive the sensitivity information for the complex metal plastic forming process. This method requires that the program code for solving sensitivity information be embedded in the numerical analysis program code, and the finite element analysis code and optimization algorithm code need to be written, so the programming workload is large. At the same time, the optimization program of this method has poor universality. (2) The advantage of the fitting optimization method based on the objective function value is that the optimization algorithm is separated from the finite element program, which can take full advantage of the powerful commercial finite element analysis software. This method has strong universality. Its main disadvantage is that the convergence speed is relatively slow. At the same time, in the fitting problem, when more design variables are obtained, there will be many complex problems, such as the fitness problem, which makes the fitting process fail. (3) Forging forming is a complex process. The ideal forging should not only have an accurate shape that meets the design requirements, but also have uniform deformation, reasonably distributed deformation force and ideal quality (without macro and micro defects). All the above aspects are the goals pursued by forging production, so it is necessary to carry out multi-objective optimization for forging process optimization design [10]. At present, there are few researches in this field.
Conclusion The forming precision of cold forging is higher than that of warm forging and hot forging, and it has its unique advantages in the field of precision forming. This paper comprehensively discusses the development status of cold forging technology from the aspects of shape, material, process innovation, productivity, numerical simulation technology, digital/intelligent design technology application, and optimization technology of cold forging parts, analyzes the existing problems of cold forging technology, and points out the future development direction. Cold precision forging is a (near) net shape forming process, which has a very broad application prospect. Reference 1. RUAN Xueyu, PENG, Yinghong&Wu Gongming et al Development of plastic processing technology - more refined, more economical and cleaner. Proceedings of the 8th National Annual Plastic Processing Academic Conference, November 2-6, 2002, Beijing: 1-4. 3. Wei Zhe, Li Yuan, Cao Fei, etc Numerical Simulation Research on Precision Flow Control Forming Process of Aluminum Alloy Air conditioning Scroll Supercharger, Proceedings of the 8th National Annual Conference on Plastic Processing, November 2-6, 2002, Beijing: 106-109. 4. Jiang Peng Hot forging cold forging composite plastic forming technology, metal forming process. 2000 (1): 27-29. 5. Xia Shisheng, Wang Guangchun, Zhao Guoqun, etc Numerical simulation of new cold precision forging process for spur gear Hot working process. 2003. (2): 22-23. 6. Kou Shuqing, Fu Peifu, Yang Shenhua, etc The International Journal of Advanced Manufacturing Technology, Issue: Volume 16, Number 10/August 21, 2000, pp 720 - 727 8. Gao, Zhenyan, ” Microstructure optimization in design of forging processes”, International Journal of Machine Tools and Manufacture Volume: 40 Issue: 5, Jan, 2000, P 691-711 9. Luo Renping. A new method for optimizing the shape design of the preform die -- micro genetic algorithm. China Mechanical Engineering. 2001, 02: 202-204 10. Zhao Xinhai et al. Research on multi-objective optimization design of forging preform. Journal of Mechanical Engineering. 2002, 04: 62-65