Performance of GR5 titanium forgings under hydraulic press and forging hammer forging process
Preface
GR5 titanium alloy has good comprehensive properties and can be used in aerospace, petrochemical, marine engineering, transportation and other fields. With the rapid development of China's aerospace industry, the demand for GR5 titanium alloy forgings is also increasing. Therefore, Research on GR5 titanium alloy forgings is of great significance.
Forging is a special processing technology that can significantly improve the strength and hardness of metal materials. It has the advantages of high production efficiency, low production cost, and good product quality. With the development and progress of forging technology, forging technology is also constantly developing. and perfection.
In recent years, titanium alloy forgings have become more and more widely used in the aerospace field, and their performance requirements have become increasingly higher. How to prepare high-quality, high-performance GR5 titanium alloy forgings has become an urgent problem in the aerospace field.
Production process of titanium alloy forgings
Titanium alloy is a metal material with excellent comprehensive properties. Due to its low density, high specific strength, and good corrosion resistance, it is widely used in aerospace, marine engineering and other fields. At present, the manufacturing processes of titanium alloys mainly include forging and casting, of which forging is divided into cold forging and hot forging.
Cold forging uses a forging hammer to vertically strike titanium alloy forgings at room temperature, causing the metal to flow slowly and evenly along the length of the forging, so as to form a uniform microstructure inside the forging. The advantages of cold forging are simple equipment, wide forging temperature range, suitable for mass production and low production cost; the disadvantages are complex forging process, high deformation resistance and long forging time.
Hot forging refers to the formation of a large number of microstructure and structural defects, such as martensite, ferrite, etc., inside the forging through hot extrusion, isothermal forging, isothermal normalizing and other processes. The advantages of hot forgings are simple equipment and the ability to withstand large deformation forces and deformation temperatures; the disadvantages are long production cycles and low production efficiency.
During the forging process of GR5 titanium alloy, due to its own thermal expansion coefficient being different from traditional metal materials, it has a wide forging temperature range and good hot processing properties.
Therefore, GR5 titanium alloy forgings are generally produced by forging. In order to ensure the quality of GR5 titanium alloy forgings, they are generally heated to a process temperature range before being forged using a press.
For hot forging of titanium alloys, there are currently two main methods: one is to put the forged piece into the heating furnace and heat it to the process temperature before forging; the other is to hot forge the titanium alloy through hot die forging. Among them, hot die forging refers to a method in which titanium alloy that has been heated to the process temperature is placed into a mold and formed through the mold.
The advantages of hot die forging are small deformation resistance and uniform deformation, which can reduce internal defects of forgings and improve the quality and performance of forgings; the disadvantage is that it requires high dimensional accuracy and shape accuracy of forgings.
There are three main methods of hot die forging of titanium alloys: one is continuous forging, which uses machinery to continuously forge forgings, which is also the most widely used method at present; the other is semi-continuous forging, which means that the forgings rotate in the mold, but they cannot Continuous forging is performed; the third is free forging, that is, no rotation is performed during the forging process, but the forging can be bent.
Organizational Observation and Analysis
Observing the metallographic structure of GR5 titanium alloy forgings, it can be seen that the structure of the rough after forging is nearly circular columnar martensite + a small amount of retained austenite, while the structure of the forging is nearly circular columnar martensite + a small amount of retained austenite. There is an obvious difference between the two bodies, and the blank has a large lateral shrinkage after forging, resulting in a difference in the lateral size of the forging.
During the forging process, due to the high temperature of the hydraulic press, the high-temperature alloy is rapidly heated to above 1000°C, which results in excessive forging heating speed, insufficient quenching and surface oxidation.
At the same time, excessive forging temperature also leads to excessive deformation during the forging process of the hydraulic press, and oxidation and decarburization occur during subsequent heating. These two reasons lead to large differences in the lateral dimensions of forgings.
Analysis of the microstructure of the forgings shows that the cooling rate during the forging process is relatively large, while the cooling rate of the blank after hydraulic machine forging is relatively small. The excessive temperature rise during the forging process results in large differences in the transverse dimensions of the forgings. At the same time, during the forging process, the forging grains also grow to varying degrees.
Due to the rapid cooling rate of the blank after forging, the grain size of the forging is greatly different. The grain size of the forging during the forging process is significantly larger than the grain size of the blank after forging by hydraulic machine; the microstructure produced during the forging process is mainly nearly circular and columnar. Martensite + a small amount of retained austenite; during the forging process, the forging grains are coarse and unevenly distributed.
Due to the high forging temperature, the GR5 titanium alloy has high strength and hardness at room temperature, while the forging temperature is too low, resulting in insufficient plastic deformation ability, insufficient quenching and oxidation.
In addition, due to the high heating temperature of the hydraulic press, oxidation occurs during forging, and oxide scale appears on the surface of the final forged blank.
Tensile property experiment
The room temperature tensile properties of GR5 titanium alloy are affected by many factors, including alloy composition, grain size, strain rate, degree of deformation, etc.
First of all, the diameter of forgings is generally smaller than that of hot-rolled bars. The deformation temperature is lower during forging heating, and it is difficult to control the degree of deformation during the deformation process. Therefore, the tensile properties of forgings are greatly affected by the base material and the forging process. Under the same conditions, the tensile properties of GR5 titanium alloy forged by hydraulic press are better than those forged by forging hammer, but there is a key problem during forging hammer forging - hot compression deformation.
Because there is a certain degree of heating and cooling during the forging process of the hydraulic press, the deformation temperature and deformation rate can be better controlled during heat treatment, thus ensuring higher plasticity. Due to the lower heating temperature and faster cooling rate during forging hammer forging, the plasticity of forgings is not as good as that of hydraulic press forgings.
Under the same conditions, forging can significantly improve the tensile properties of GR5 titanium alloy more than hammer forging. For GR5 titanium alloy forgings of the same specifications, the tensile properties of hydraulic press forging are better than those of forging hammer forging; under the same conditions, forging hammer forging can significantly improve the tensile properties of GR5 titanium alloy than hydraulic press forging.
When the yield strength is the same, the post-forging tensile properties of GR5 titanium alloy forged by hydraulic press are better than those of GR5 titanium alloy forged by forging hammer. This is because hydraulic press forgings have small internal residual stress due to factors such as small deformation, low deformation temperature, and slow deformation rate; and the tensile properties of GR5 titanium alloy forged by forging hammer after forging are better than those after forging by hydraulic press.
This is because when forging with a forging hammer, large residual stress is generated during the forging and hammering processes, which causes large tensile stress inside the material, resulting in large plastic deformation of the material; while when forging with a hydraulic press, during the forging process The metal is in a free-flowing state and there is no residual stress inside the material, thus ensuring the degree of plastic deformation of the material.
Analysis of mechanical test performance results
The strength value and elongation of the GR5 titanium alloy sample after forging with a hydraulic press are greater than those after forging with a forging hammer. This is because a large number of impurity elements are produced inside the forging during the forging process, and the presence of these impurity elements causes the GR5 titanium alloy to undergo deformation. Severe recrystallization.
During the rolling process, due to the large rolling pressure, the stress concentration phenomenon is obvious. Some impurity elements are extruded into the interior of the GR5 titanium alloy and form a coarse Ti-rich phase on the grain boundaries, which causes a large amount of energy to be produced inside the forging. The dislocations and vacancies provide conditions for further deformation of GR5 titanium alloy.
The strength value of the GR5 titanium alloy sample after forging with a forging hammer is lower than that of the sample after forging with a hydraulic press. This is because a large number of dislocations and vacancies are formed inside the forging during the forging hammer forging process, and small particles are formed on the grain boundaries. The Ti-rich phase causes obvious recrystallization of GR5 titanium alloy.
It can be seen that the fracture characteristics of the GR5 titanium alloy sample after forging with a forging hammer are: mainly ductile fracture, supplemented by local brittle fracture.
This is due to the high forging pressure during the forging hammer forging process, which causes some impurity elements to form coarse Ti-rich phases on the grain boundaries. At the same time, a large number of dislocations and vacancies are generated during the forging hammer forging process, causing obvious changes in the GR5 titanium alloy. recrystallization phenomenon.
Due to the low forging temperature and fast forging speed during forging hammer forging, a large number of crack sources, pores and other defective elements are generated inside the material, resulting in obvious recrystallization of GR5 titanium alloy.
Application prospects and development direction
GR5 titanium alloy is widely used in aerospace, medical equipment, transportation and other fields due to its excellent comprehensive properties. Especially in the aerospace field, GR5 titanium alloy has become the main material.
GR5 titanium alloy has the advantages of low density, high specific strength and specific stiffness, high temperature resistance, and corrosion resistance. It is widely used in manufacturing key structural parts such as aircraft rotors, tails, fuselage reinforcement frames, and main and auxiliary fuel tanks.
Since GR5 titanium alloy has greater strength and plasticity at room temperature, solution treatment is required during forging to improve its properties. However, the structure of GR5 titanium alloy will be unevenly distributed during the forging process, and coarse α phase will be formed during the cooling process, resulting in a reduction in the mechanical properties of the alloy.
In order to improve the forging structure of GR5 titanium alloy, there are currently two commonly used methods: one is to perform solid solution treatment before forging, such as hydraulic press hot die forging; the other is to perform solid solution treatment during the forging process, such as forging hammer forging.
In actual production, due to the complex forging hammer forging process equipment, difficult operation and high production cost, hydraulic press hot die forging is currently a more commonly used process.
Compared with hydraulic press hot die forging, the surface quality of forging hammer forgings has been greatly improved, the grains are finer and more uniform, and large-diameter die forging dies can be used for production. However, the production efficiency of forging hammer forgings is low and the production cost is high. It has not completely replaced the hydraulic press hot die forging process.
In the future, with the development of forging hammer forging technology and the research on forging technology and equipment, it can be predicted that the production efficiency and quality of forging hammer forgings will be greatly improved.
In actual production, since a large amount of heat, force and impact load will be generated during the forging process, it is necessary to study the influence of isothermal forging on the structure, mechanical properties and mechanical life of forgings through thermal simulation experiments, and obtain appropriate process parameters to ensure that forgings quality.
Due to the high production efficiency and low equipment requirements of the forging hammer forging process, in the future we can try to use forging hammer forgings to produce key structural parts in aerospace and other fields, such as aircraft tails, main and auxiliary fuel tanks, fuselage reinforcement frames and other key structural parts. For some large forgings in the general civilian field, due to their large size, it is difficult to produce them using the forging hammer forging process.
Author's opinion
With the development of my country's aerospace industry, the quality requirements for titanium alloy forgings are becoming higher and higher, and the forging process is also one of the important factors affecting the quality of forgings.
However, with the development of hydraulic presses and forging hammers in our country, their applications are becoming more and more widespread. However, because the price of forging hammers is relatively high and the forging process is complex, most domestic aerospace companies prefer to choose hydraulic presses to complete products. Production, but with the increase in the number of hydraulic presses, its forging process and quality have also been greatly improved.
At the same time, because the forging parameters can be adjusted according to needs during the forging process, the forging process is becoming more and more flexible. However, at present, China's domestic hydraulic press technology and equipment cannot fully meet the requirements for forgings in the aerospace field. Therefore, the development of the aerospace field requires forging The demand for technology will also increase.
references
1. Yao Weidong: Research on the forging process and microstructure properties of GR5 titanium alloy forgings. "Journal of Beijing University of Aeronautics and Astronautics", 2017,18 (01): 1036-1037.
2. He Xiaolin: Research on the hot working behavior of titanium alloy forgings during the forging process. "China Forging", 2015 (11): 59-60.
3. Wei Guoli: The current situation and development trend of large-scale titanium alloy forgings production in my country. "Material Protection Technology", 2018 (05): 23-26.
