Development Trend of Precision Forging Process

Precision forging forming technology (net forming) refers to the forming technology that meets the requirements of parts after forging and forming, only a small amount of processing or no longer processing. Precision forging forming technology is an important part of advanced manufacturing technology, and it is also a widely used part manufacturing process in automobile, mining, energy, construction, aviation, aerospace, weapons and other industries. Precision forging forming technology not only saves materials, energy, reduces processing procedures and equipment, but also significantly improves productivity and product quality, reduces production costs, and thus improves the market competitiveness of products.

After more than 30 years of development, precision forging forming technology has been developed rapidly, and many research results have been obtained. In this paper, the development of precision forging forming technology in recent years is summarized from five aspects: process method, equipment, mold, forming process simulation and process optimization, and the future development direction of precision forging forming technology is prospected.

1. Precision forging process method
At present, there are many precision forging processes that have been used in production. According to the different forming temperature can be divided into hot precision forging, cold precision forging, warm precision forging, composite precision forging, isothermal precision forging and so on.

1.1 hot precision forging process (hot forging)
The precision forging process with the forging temperature above the recrystallization temperature is called hot precision forging. Hot forging materials have low deformation resistance, good plasticity, and are easy to form more complex workpieces, but due to strong oxidation, the surface quality and dimensional accuracy of the workpiece are low. Hot forging commonly used process method for closed die forging, due to the material is not allowed, mold design, manufacturing accuracy is not enough and other reasons, closed die forging after the mold stage deformation resistance is very large, causing greater damage to equipment and molds.

The common method to solve this problem is the principle of shunt pressure reduction, that is, the shunt pressure reduction cavity hole with reasonable shape and size is set at the place where the closed cavity is filled. When the cavity is completely filled with excess billet metal from the shunt cavity hole extrusion, so that both the billet volume and cavity volume can not be strictly equal to the contradiction, while reducing the internal pressure of the cavity, is conducive to improving the life of the mold.

As early as the 1950 s, due to the lack of sufficient gear processing machines, the Germans began to trial-produce straight bevel gears with closed-type hot die forging. The development and application of hot precision forging gear technology in China started in the early 1970 s and matured in the middle and late 1980 s.

In 1970, Shanghai Mechanization Technology Research Institute and Shanghai Automotive Gear Factory cooperated to carry out hot precision forging process forming tests on the planetary gears of American Grand Dodge T234 automobile differential, and invested in the establishment of precision forging workshop in 1973 for mass production [; In the 1980 s, Shandong University carried out research and development of bevel gear precision forging process and realized industrialization. Due to the significant economic benefits, hot precision forging technology has been widely used in recent years.

1.2 cold precision forging process
Cold precision forging is a precision forging process carried out at room temperature. Cold precision forging process has the following characteristics: the shape and size of the workpiece is easy to control, to avoid the error caused by high temperature. In the process of cold forging, the workpiece has poor plasticity and high deformation resistance, which requires high die and equipment, and it is difficult to form parts with complex structure. In order to overcome the problems of large deformation resistance and poor filling effect in the cold precision forging process, some new process methods have been developed, mainly including closed forging, floating concave die forging, prefabricated shunt forging, etc.

Blocking forging is a one-way or opposite extrusion metal forming in a closed die through 1 or 2 punches. The upper and lower dies used in the closed forging processing technology are all assemblies, which are respectively composed of an upper concave die, an upper punch, a lower concave die and a lower punch. In the forging process, the upper and lower concave dies are first closed to form a closed die cavity, and sufficient pressure is applied to it, and then the upper and lower punches are used to extrude the billet in the die cavity. In the forging process, the blank is in a strong three-way compressive stress state, the plasticity is good, and the parts with complex shapes can be formed at one time, the production efficiency is high, and the metal flow line is continuously distributed along the shape of the forging, and the mechanical properties of the forging are good.

The concave die in the floating concave die forging technology is not fixed and will float as the forging process proceeds. This design reduces the relative speed between the die and the metal deformation body, reduces the influence of friction on the contact surface, the filling performance of forgings is greatly improved, and the deformation resistance is greatly reduced.

The shunt forging method provides an overflow port at a certain position of the forging, so that the material always has room for free flow in the process of filling the cavity, thereby improving the filling of the cavity, reducing deformation resistance and processing load. In order to improve the filling situation of precision forging of spur gears, Zhang Qingping and others proposed a process method of prefabricated shunt hole-shunt forging. The process method pre-forging the upper mold and the lower mold with a boss, in the center of both ends of the blank forging out of the shunt area, in the final forging, the central shunt area plays the role of shunt material, improve the flow state of the material, thereby reducing the forming load, improve the blank filling.

In recent years, cold forging technology in the country to obtain a certain degree of development, Amazon Machinery Co., Ltd. the introduction of advanced forging machine and mold processing equipment, is the current cold forging equipment conditions of the forefront of enterprises. With an annual output of all kinds of precision cold and warm forgings for automobiles and motorcycles, the total output is 4000t. Some typical difficult cold forgings, such as car constant velocity universal joint jacket, star sleeve, gearbox drive shaft, etc., have been mass-produced in this factory.

1.3 warm precision forging process
Warm finish forging is a precision forging process performed at a suitable temperature below the recrystallization temperature. Warm forging precision forming technology not only breaks through the limitations of large deformation resistance, part shape can not be too complex, need to increase the intermediate heat treatment and surface treatment steps in cold forging, but also overcomes the problem of reducing surface quality and dimensional accuracy due to strong oxidation in hot forging. It has the advantages of cold forging and hot forging at the same time, and overcomes the shortcomings of both. However, the warm precision forging process has low forging temperature, narrow forging temperature range and strict requirements for its forging range, which requires high-precision special equipment, and has higher requirements for mold structure and mold materials.

1.4 composite precision forging process
With the increasing complexity of precision forging workpieces and the improvement of precision requirements, the simple cold, warm and hot forging process can no longer meet the requirements. The composite precision forging process combines cold, warm and hot forging processes to complete the forging of a workpiece, which can give full play to the advantages of cold, warm and hot forging and abandon the shortcomings of cold, warm and hot forging. Table 1 is a comparison of the technical performance of straight bevel gears produced by three different processes. It can be seen from the table that the mechanical properties, dimensional accuracy and surface roughness of the workpieces produced by the composite precision forging process are improved compared with the other two processes []. Therefore, the composite precision forging process is an important direction of the development of the current precision forging process.

1.5 isothermal precision forging process
Isothermal precision forging means that the blank is formed by die forging at a temperature that tends to be constant. In order to ensure constant temperature forming conditions, the mold must also be heated to the same temperature as the blank. Isothermal die forging is often used in the aerospace industry in the titanium alloy, aluminum alloy, magnesium alloy and other difficult deformation materials precision forming, in recent years also used in automotive and machinery industry precision forming of non-ferrous metals.

Isothermal forging is mainly used in metal materials with narrow forging temperature, especially titanium alloy which is very sensitive to deformation temperature. Isothermal forging parts generally have thin webs, high ribs and thin walls. The blank heat of such parts is quickly absorbed by the mold and the temperature drops rapidly. Using ordinary forging methods, not only the tonnage of the equipment needs to be greatly improved, but also it is easy to cause cracking of the mold.

Development Trend of 1.6 Precision Forging Process
With the development of manufacturing industry, the requirements for precision forging parts are getting higher and higher, and the research and development of precision forging process are also put forward higher requirements. At present, the main research directions of precision forging process are as follows.

1) Continuous process innovation. In order to meet the requirements of forming parts and reduce production costs, it is necessary to continuously develop new precision forging forming processes with high forming accuracy, long die life and high production efficiency.

2) Development of composite process. With the continuous improvement of the process requirements of forming parts, a single precision forging is difficult to meet the requirements, which requires the development of composite forming process, the forging process of different temperatures or different process methods to combine, learn from each other to complete the processing and manufacturing of a part. The precision forging process can also be combined with other precision forming processes such as precision casting and precision welding to improve the application range and processing capacity of the precision forming process.

3) Knowledge-based process design. With the continuous development of precision forging process, process design is becoming more and more complex. In order to improve the reliability and efficiency of process design, the development of knowledge-based expert system is an important research direction of precision forging process design in the future.