Forging

Application of precision forgings in the automotive field

The variety of parts (including complexity, weight, and precision) and the number of parts manufactured by the "cold and warm forging" near-net-shape process on an automobile reflect the level of advanced automobile manufacturing technology. At present, the number of cold and warm forged parts produced by domestic technology and equipment for a car in my country is only one third of foreign countries. For cold forged parts with complex shapes and high precision, they are either purchased directly from foreign automobile groups, or imported from Germany, Japan's extremely expensive cold and warm forging presses and large precision molds are produced. Generally speaking, the R&D capability and manufacturing technology level of my country's automobile industry are not enough, and there is a certain gap between the overall automobile quality and foreign countries. Especially in the aspect of precision forging, the level of precision forging technology of auto parts is far behind that of advanced countries. Precision forging technology has become one of the core technologies that automobile companies must master to maintain their competitive advantage.

The advantages of precision forging in the auto parts industry chain

Due to the high strength and toughness of the parts manufactured by the forging process, it is suitable for mass production, so nearly 60% of the parts on the automobile are forged. In recent years, with the rapid growth of automobile production, the integration of parts and components is also being planned simultaneously when the OEMs are increasing and integrating the production capacity of complete vehicles. In order to reduce the size of the CVT gearbox and meet the requirements of light weight, Japan JATCO combined the gear and the shaft into a pulley and achieved near-net shape. After the modification, the structure is more compact and the environment is more friendly. More and more OEMs are pursuing lightweight and fuel-saving rates, and replacing cutting processing technology with precision forging technology has become the main direction of new product development, all of which provide opportunities for the development of precision forging.

From the current practice, precision forging can reduce the manufacturing cost of the entire supply chain, and the general saving rate is more than 15%. After the precision forging process is adopted, although the forging cost is increased, the machining cost is reduced, thereby greatly reducing the total cost. For example, combining the overall forging of gears to replace the manufacturing process of separate processing of synchronous gears and bodies can save about 25% of production costs, which represents the trend of future automotive transmission gear processing.

Another important feature of precision forging is that the manufacturing cost gradually decreases as the production volume increases, which mainly reflects the advantages of the die forming process, high efficiency and high consistency. Although the fixed cost in production is high, the production efficiency is also high. For mass-produced parts, the production capacity can be greatly improved with only a small investment in capacity improvement.

In order to enhance the competitive advantage of auto parts, traditional automobile forging enterprises need to invest in precision forging research and investment, so it is imperative to develop precision forging.

The main process form of precision forging development

Precision forging refers to a process that can produce near-net shape forgings with high surface quality and dimensional accuracy. It is difficult to obtain near-net shape forgings effectively by the traditional hot forging process. In order to solve this contradiction, on the basis of the original hot forging, the forging process is improved by using such as plasticity theory, material science and tribology, and the precision forging technology is developed. At present, there are many classifications of precision forging technology, which can generally be distinguished according to forging temperature and forming method.

1.Forging temperature distinction

From the temperature range, it is divided into cold forging, warm forging and warm forging (hot forging)-cold forging compound forging.

(1) The cold forging process refers to the forming of forgings after pickling, phosphating and saponification of steel at room temperature. The precision of its forgings can reach IT8-11 grades.

(2) The warm forging process in the industry refers to the forging process carried out in the range of higher than room temperature and lower than the final forging temperature of hot forging (1000℃～1250℃). Level 14.

(3) Warm forging (hot forging)-cold forging combined forming process, that is, the metal is made into a blank basically close to the final shape of the product by the warm forging method. After cleaning and lubricating treatment, the blank is finished by cold forging. Or shaping, the precision of the finishing part of the final product reaches the level of cold forging.

2. Forming method distinction

In the process of developing the traditional closed die forging process, on the basis of flashless die forging (i.e. closed die forging), closed forging and split forging have been added. simple mold structure. Compared with the traditional forging method, there are the following special processes.

(1) Extrusion.

Under the action of three-way uneven compressive stress, the blank is extruded from the cavity of the die to reduce the cross-sectional area, increase the length, and become a forging. The processing method is called extrusion. Mainly used for cold extrusion of various splines, warm extrusion of various valve bodies, hot extrusion of half shaft casings, etc.

(2) Block forging.

Blocked forging is a die forging of near-net shape precision forgings without flash by extruding metal one-way or oppositely through one or two punches in a closed die. Block forging can realize flash-free forging, high material utilization rate, small equipment tonnage and high dimensional accuracy.

(3) Split forging technology.

The main principle of shunt forging is to create a shunt cavity or shunt channel of material in the forming part of the blank or die. During the forging process, while the material fills the cavity, part of the material flows to the shunt cavity or shunt channel. The application of shunt forging technology has made the less and no cutting processing of high-precision gears quickly reach an industrial scale, which is mainly used in the cold forging of spur gears and helical gears.

Introduction of precision forging process for typical parts

At present, the precision forgings on automobiles are mainly concentrated in the gearbox (Fig. 1) and the rear axle assembly (Fig. 2). In addition, the transmission assembly in the car parts and the half shaft on the rear axle drive car are typical automobile precision forgings. For our traditional automobile forging enterprises, the following main types of parts have adopted the precision forging process, some of which have entered mass production, and some are still in the research and development stage.

Figure 1 Gearbox composition

Figure 2 Anatomy of the posterior bridge

Spur bevel gears

The precision forging technology of bevel gears was applied earlier in production. Its development has developed from the initial two fires and two forgings to one fire and two forgings. Larger diameter gears with complex shapes can also be formed by hot forging with minimal steps. At present, the bevel gear of the automobile differential has basically eliminated the cutting technology and turned to the precision forging process.

With the improvement of product accuracy requirements, the dimensional accuracy of hot precision forging gears can no longer meet the product accuracy requirements. For example, the planetary axle gears on cars have begun to use cold forging processes such as block forging and cold oscillating forging technology, and the product accuracy is IT7 level. However, the applicability of cold forging is low. At present, only gears with a diameter below φ90mm can be produced in China.

Combining the advantages of hot forging and cold forging, the development trend is the hot-cold or warm-cold composite forging process, or the straight bevel gear with higher precision can be directly formed by warm precision forging. Figure 3 and Figure 4 show the warm forging + cold finishing process, and its precision can reach IT7.5 level.

Figure 3 Warm forging block forming

Fig. 4 Cold finishing

Gearbox combination teeth

With the improvement of domestic automobile energy saving, emission reduction and light weight requirements, the gear structure of the combined teeth (Fig. 5) in the transmission is developing towards an integration, and the combination of hot forging forming and cold forging finishing is used to produce the synchronous gear of automobile transmission. An integral forging integrated with the body. For the combined tooth profile, hot precision forging or warm forging is used, and then the precision forging is cold-finished to ensure the tooth profile accuracy. During cold finishing, the tooth shape, end chamfer and cutting machining positioning surface are formed, and then the combined tooth chamfer angle is processed by the radial diameter reduction method. After comprehensively controlling each process link of precision forging, the difficulty of the production process has been degraded, and the stability of the process system can be achieved.

Figure 5 Gearbox combined with tooth precision forging

In China, Jiangsu Pacific uses a hot die forging press to carry out closed hot die forging and cold forging to form combined tooth fine forging gear blanks, realizing mass production of warm and cold composite forming with combined teeth as a typical part. Compared with the traditional machined-formed coupling gear, the precision forged coupling gear has softer tooth profile, impact resistance, fatigue resistance, low production cost and relatively stable quality, and has been widely used in the automotive industry.

Spur Gear

The spur gear is the main part of the transmission, transfer case and drive axle. Due to the lack of draft angle of spur gear precision forging, it is difficult to eject the die and to form the tooth tip, so the degree of popularization of this process is relatively low. The process methods of precision forging mainly include hot precision forging, cold precision forging, cold extrusion, hot forging-cold pushing, warm forging-cold extrusion, etc. Figure 6 is a manual transmission reverse gear part developed by considering the split flow principle and adopting the gear split forging technology. Since the tooth surface of the reverse gear has a certain taper requirement, a taper cold forging should be performed on the tooth surface after the tooth is formed.

Figure 6 Split forging process

The use of positive extrusion to form sheet gears and pinions is a more domestic process. A typical car starter spur gear is a cold extrusion process. First use the cold extrusion process to produce the involute tooth spline shaft (Fig. 7), and then use the method of wire cutting the spline shaft to produce the small modulus spur gear, the precision can reach IT7.5 level. Its application has developed from peripheral components such as automobile starter motors and window lifters to key assemblies such as automobile steering gears and gearboxes.

Figure 7 Spline shaft cold extrusion process

The spur gear adopts the combination of warm forging and open cold extrusion, which can effectively reduce the forming force and improve the life of the die. The forming process scheme combining warm forging and cold extrusion has become an important development direction of precision forging.

Spiral bevel gear

Due to the large size of the spiral bevel gear, the deep tooth groove, and the influence of the helix angle, its precision forging is much more difficult than that of the straight bevel gear. Precision forged helical bevel gears are manufactured in developed countries with high precision and have begun to be widely used in the automotive industry.

At present, the domestic spiral bevel gear adopts the closed die forging process without flash. The billet is heated with little and no oxidation. First, the billet is made by the expansion rolling process (upsetting, punching, and hole reaming). After the billet is finished, pre-forging and final forging are performed, and a small amount of allowance is left on the tooth surface for fine milling.

Near net shape technology. The process is hot forging + warm finishing or hot forging + cold finishing. In the sub-hot forging range (950 ° C ~ 1050 ° C), the metal is plastically deformed in a closed mold and quickly flows to fill the cavity to obtain a qualified tooth shape. Precision forging gear blanks; perform fine shaping and precision pressing within the range of (800℃～650℃) warm forging to improve the tooth shape accuracy of precision forging gear blanks.

The development method and strategy of precision forging

Compared with ordinary forging, the development of precision forging is accompanied by high investment and high risk. I think that the development of precision forging technology by traditional forging enterprises is more based on the existing resources of the enterprise. Whether it is through introduction or self-development, it must combine the existing foundation and advantages of the factory to develop precision forging technology with its own characteristics. The development route and focus need to be considered in terms of personnel, equipment, management and development mode.

Literacy preparation for technicians

The design of precision forgings is closer to product design. The requirements for some features are the final requirements of the product. It is necessary to have a deeper understanding of product design and use. Special product designers are required in the factory to develop and produce products. China maintains close contact with the assembly and has a greater say in the product, because it is no longer a blank design in the traditional sense for the product R&D department.

This requires technicians to master more product design knowledge, such as gear transmission principle, tooth profile design method and detection method. New process developers need to have an in-depth understanding of equipment parameters and characteristics, mold material characteristics, raw material characteristics, lubrication and other knowledge, which are the keys to the success or failure of the process.

Selection of equipment for precision forging products

Precision forging can use a variety of technology, generally choose precision forming on general-purpose presses and special-purpose presses. Generally speaking, traditional auto parts forging enterprises are more familiar with general-purpose presses, so the risk of developing with general-purpose presses will be smaller. However, the existing equipment should be restored or improved as much as possible to meet the precision requirements of precision forgings. In order to realize the precision forging process, innovations are made in the structure of the gripper and the die. For example, a hydraulic die frame can be designed on the hot die forging press to realize complex double-motion forming to realize precision forging. At present, the closed forging die is used to mass-produce the flashless gear blank of the car gearbox, and the dimensional accuracy is ±0.5mm; the closed precision forging of the φ140mm side gear is realized by the hydraulic die set on the 12.5MN; The semi-finish forging of the φ400mm helical bevel teeth (tooth surface allowance is 1.0mm) is realized by the hollow shunt method of the spring blocking die. Using ordinary hydraulic press, cold finishing process can be realized.

Of course, to be a real leader in technology, the use of special presses, especially fully automatic special presses, is the ultimate goal. For example, using HATEBUR to produce flashless gear blanks, the production efficiency can reach 4,000 to 6,000 pieces per hour. Due to the fast cycle time and less oxidation, the minimum machining allowance can reach 0.5mm. When using the warm forging process, due to the more stringent temperature requirements, the use of a multi-station warm forging press can truly meet the requirements.

In general, the adjustment of equipment use, stability and production cost are the prerequisite factors for selecting a process. The advanced nature of technology should always be subject to specific adaptability and pertinence. The choice of forging equipment and tooling is mainly based on the actual situation and economic strength of the unit, in order to give full play to the economic efficiency of its equipment, so as to implement scientific and advanced operation methods.

Management Improvement

The level of precision forging technology reflects the management level of the forging enterprise to a certain extent. Therefore, to develop precision forging, the existing management level must be improved. Compared with traditional forging production, precision forging management is stricter, and there is no room for manoeuvre after production unlike traditional blanks. In traditional forging, in order to reduce the risk, it is possible to increase the machining allowance in the design and add compensation in the production, which cannot be realized in the precision forging. For example, in flashless forging, if the blank and die are poorly controlled, longitudinal burrs will eventually be formed, which will greatly reduce the technological level. As for the process parameters involved in the whole process of warm forging-cold finishing production line, including warm forging accuracy and cold forging accuracy, ensuring the stability of each link is the key to the success of the comprehensive forming technology of warm forging and cold finishing. Based on this, it is possible to start with relatively simple precision forging due to the phased nature of management improvement.

Innovation in development mode

We should start from the basics, first realize flash-free forging on the hot die forging press, and gradually accumulate experience. In the development of the extrusion process, the input and output shafts of the gearbox with market competitiveness can be developed first (Figure 8). Through the development process, the cold forging experience has been accumulated, and the spur gears can be further extruded by this method. In short, the development mode should be combined with factory practice, and gradually realize the complete turn of precision forging.

Figure 8 Cold extrusion output shaft

In addition, there are more interdisciplinary subjects involved in precision forging, and manufacturing technology, equipment technology, and process technology can no longer be realized by just relying on one company. It is necessary to strengthen the connection with university research institutes, and carry out basic research on precision forging with scientific research institutes as the main body and precision forging application research with enterprises as the main body. By participating in special projects, such as "automatic multi-station warm forging production line for gears", systematically develop complete sets of equipment, supporting processes and mold technology, and realize technology with independent intellectual property rights.

In a word, auto parts forging enterprises need to grasp the development trend of precision forging process, choose the precision forging process suitable for their own development, and get the first opportunity to build the core competitiveness of enterprise development.

Concluding Remarks

In order to enhance the competitiveness of our automobile manufacturing industry, make forging a source of high-tech parts, and at the same time enable auto parts companies to win the market, we must develop precision forging. For various precision forging processes and products, forging enterprises should follow the development trend of products and the actual situation of the factory, take a road suitable for their own development, develop precision forging equipment and processes suitable for the actual factory, and contribute to my country's precision forging technology. due contribution.