What is isostatic pressing? How to shape ceramics?

The stages of temperature and pressure rise, pressure maintaining, temperature and pressure reduction are called high-temperature and high-pressure cycle. According to the sequence of temperature rise and pressure rise, it can be divided into four different circulation modes.

Cycle 1: The pressure rise of the cold loading cycle is prior to the temperature rise, and both reach their respective peaks at the same time. This method is conducive to better control the geometric shape of the thin-walled metal sheath.
Cycle 2: The pressure is increased after the temperature reaches a certain value in the thermal loading cycle. This method is particularly important when using glass sheaths. Premature pressurization will break brittle glass.
Cycle 3: The post heat cycle is similar to the cold loading cycle in that the pressure rise is prior to the temperature rise. The difference is that the temperature rise starts after the pressure rise reaches the peak value and the pressure is maintained. This method promotes the recrystallization of powder particles through plastic deformation, thereby reducing the molding temperature.
Cycle 4: The most effective cycle is to increase temperature and pressure at the same time, so as to shorten the hot isostatic pressing time and obtain the highest efficiency
2.2 Working principle of hot isostatic pressing method
According to Pascal's principle, in a sealed container, the static pressure generated by the external force acting on the static liquid or gas will be transmitted evenly in all directions, and the pressure on its acting surface area is proportional to the surface area. Under the action of high temperature and high pressure, the sheath in the hot isostatic pressing furnace softens and shrinks, extruding the Laser Cutting Ceramics internal powder to make it move with itself.
The densification process of powder under the action of high temperature and high pressure is very different from that of ordinary pressureless sintering or normal temperature pressing.
1) Phase of particle approaching and rearrangement
Before heating and pressurization, there are a lot of pores between loose powder particles. At the same time, due to the irregular shape and uneven surface of powder particles, they are mostly in point contact, so the number of other particles (particle coordination number) in direct contact with a particle is very small. When an external force is applied to the powder, under the action of compressive stress, the following situations may occur in the powder: the randomly stacked powder will move or rotate to be close to each other; Some powders are squeezed into adjacent voids; Some large bridge holes will collapse. As a result of the above changes, the adjacent coordination number of the particles increases significantly, which greatly reduces the voids of the powder and rapidly increases the relative density.
(2) Plastic deformation stage
The densification in the first stage has greatly improved the density of the powder, the contact area between particles has increased sharply, and particles collide or wedge with each other. This is to make the powder continue to densify. The external pressure can be increased to increase the compressive stress on the particle contact surface, and the temperature can be increased to reduce the critical shear stress that is not conducive to the plastic flow of the powder. If the pressure and temperature are increased at the same time, it will be more effective to continue densification. When the compressive stress of the powder body exceeds its yield shear stress, the particles will produce plastic deformation in the form of slip.
(3) Diffusion creep stage
After a large amount of plastic flow of powder particles occurs, the relative density of the powder quickly approaches the theoretical density value. At this time, the powder particles are basically connected as a whole, and the residual pores are no longer connected, but dispersed in the powder matrix, like bubbles suspended in solid media. These pores began to exist in an irregular long and narrow shape, but under the action of surface tension, they became spherical. The volume fraction of residual pores will also decrease during spheroidization. The contact area between particles increases to such an extent that the effective compressive stress borne by the powder no longer exceeds its critical shear stress. At this time, the mechanism of plastic deformation caused by the sliding of a large number of atomic clusters will no longer play a major role. The densification process is mainly completed by the diffusion creep of a single atom or hole. Therefore, the densification process of the whole powder slows down and finally approaches to the maximum terminal density value.
It is worth noting that the above Ceramic Disc three stages are not completely separated. In the process of hot isostatic pressing, they often work at the same time to promote the densification of the powder, but when the powder is in different shrinkage stages, different densification processes play a leading role.
3. Hot isostatic pressing equipment
The hot isostatic pressing equipment usually includes five main components, namely, high pressure cylinder, hot isostatic pressing furnace, gas pressurization system, electrical and auxiliary systems.
High speed steel is a kind of high alloy steel with complex chemical composition. In the production of high-speed steel by traditional casting forging method, due to the large ingot size, the cooling is slow and carbide segregation is inevitable. This kind of segregation structure not only makes forging, rolling and other hot working difficult and damages various properties of products, but also limits the further increase of alloy content and hinders the development of high-speed steel.
With the advent of hot isostatic pressing technology, many high alloy high speed steels can be manufactured by powder metallurgy technology, thus overcoming the defects such as carbide segregation in melting and casting steel, and successfully introducing powder metallurgy technology into the production field of compact steel and alloy steel.
4.1.2 Hot isostatic pressing of cemented carbide
Compared with conventional sintered cemented carbide, HIP cemented carbide has the following advantages:
(1) Participatory voids are almost completely eliminated, and the density can be increased from the original 99.8% theoretical density to 99.999% theoretical density.
(2) When manufacturing large products or products with large height diameter ratio, the scrap rate is low, the surface defects are greatly reduced, and the highly polished surface can be obtained.
(3) Because the pore volume in the product is White Zirconia Ceramic significantly reduced, the fracture source is eliminated, and the performance and life of the product are greatly improved.
4.1.3 Consolidation of superalloy powder
Superalloy is a high-performance alloy that works under high temperature of 500~1200 ℃ and high dynamic load stress. It belongs to super heat resistant alloy, so it is called "super alloy". The preparation of powder superalloy by hot isostatic pressing is a great improvement in superalloy production. The research shows that the properties of hot isostatic pressing powder superalloy are comparable to those of cast forging alloy and have unique advantages.
4.1.4 Consolidation of titanium alloy powder
Titanium alloys are widely used in aerospace, aviation, navigation and chemical industry due to their high strength, high toughness, oxidation resistance and corrosion resistance. However, the high price of titanium products limits its application. The main reason for the high cost of titanium products is that the traditional manufacturing process is complex and the material loss of secondary processing is large. Generally, titanium smelting and processing processes include: sponge titanium pressed electrode; Two vacuum arc furnace smelting; Precision casting or forging, rolling and machining. Powder titanium alloy is considered to be the way to further improve the performance of titanium alloy and reduce its price. The hot isostatic pressing (HIP) process not only simplifies the melting process and cutting process, but also improves the properties of the products because the fine powder grains are conducive to the homogenization of the alloy structure.
4.1.5 Powder consolidation of ceramic materials
Ceramic materials include metal oxides, carbides, borides and nitrides. Such materials are characterized by high melting point, large elastic modulus, high hardness, low density, small thermal expansion, wear resistance and corrosion resistance. The conventional preparation methods of ceramic materials are powder pressing and sintering or hot pressing. Because of the high melting point and hardness of ceramic powder, it is difficult to form and sinter, so ceramic materials usually have large porosity and brittleness.
Hot isostatic pressing technology is used in the production of ceramic materials, which improves the forming and sintering conditions, significantly reduces the porosity of materials, thus improving the performance of materials, and provides an effective method for manufacturing special ceramics.
4.2 HIP Casting Treatment
Casting, especially precision casting, has the advantages of high alloying degree, simple process and equipment, low cost and easy to obtain complex shapes, so it is widely used. However, due to the large amount of shrinkage cavity, porosity and composition segregation in the castings, the properties of the castings are generally inferior to those of deformed alloy parts. The appearance of hot isostatic pressing technology has created conditions for eliminating porosity in castings.
The effectiveness and significance of heat and other depressurization treatment of castings can be summarized as follows:
(1) After HIP treatment, the scrap rate of castings in X-ray inspection and surface projection inspection can be reduced;
(2) Compared with untreated castings, HIP treated castings have fewer cracks after welding, thus reducing the cost of repair welding;
(3) The range of casting parameters can be increased and new casting alloy varieties can be expanded by hot isostatic pressing;
(4) HIP castings with improved fatigue strength and ductility can replace expensive forgings.
Hot isostatic pressing can not only densify the new castings, but also repair the castings in use, so that the properties of the castings reduced in use can be restored. Under the specified temperature and stress conditions, the casting has a certain calculated life value. After using for a period of time, micro defects will continue to occur, and the relative movement between grains will occur. Defects will appear at the grain boundary. These internal defects similar to common shrinkage cavity can be cured by hot isostatic pressing. The mechanical properties and fatigue properties of the engine parts in use can be restored to those of the new castings by such treatment.
4.3 HIP connection
HIP connection is the most original application of HIP technology. The equipment used for hot isostatic pressing connection is the same as that used for compacting powder. Although the application of hot isostatic pressing technology in connection is not as extensive as that in powder consolidation and casting treatment, it has the following advantages compared with general connection methods:
(1) The joining material has the characteristics of base metal, and there is no melting zone in the solder joint, thus eliminating the grain growth that is the degradation of solder joint performance;
(2) It can connect dissimilar metals that cannot normally be connected, and can prevent the formation of Kekendal vacancy under high hot isostatic pressure;
(3) It is not limited by the fixed mold, and can process and process any complicated shape parts;
(4) It can connect brittle materials or low ductility materials without fracture;
(5) The temperature limit is small;
(6) It can connect composite materials with little damage to fibers in composite materials.
4.4 Application in new fields
(1) The application of HIP in porous materials: due to the use of nitrogen as the medium, nitrogen compounds are generated at high temperatures, making HIP also have an effect on porous materials;
(2) HIP technology is combined with surface treatment such as nitriding to expand the function of HIP;
(3) HIP technology can be used to prepare high-purity materials by suspension melting process. Because the increase of the density of high-pressure gas can suspend Silicon Nitride Ceramic Rod the smelted material and realize crucible free smelting, thus greatly improving the smelting purity.
(4) The food industry adopts HIP technology: applying high pressure to food can achieve the purpose of sterilization and disinfection without destroying the nutrition and maintaining the original color and taste of food, which provides a new way for food processing.
5. Development and application at home and abroad
5.1 Overseas development and application
In 1965, the first hot isostatic pressing machine of Battelle Research Institute in the United States came out, marking the birth of hot isostatic pressing technology equipment. The hot isostatic pressing equipment is composed of high pressure vessel, heating furnace, compressor, vacuum pump, cooling system and computer control system. The high pressure vessel is the key device of the whole equipment.
At present, the advanced hot isostatic pressing machine is a frame structure with prestressed steel wire winding, and the connection between the end cover of the high pressure vessel and the cylinder body is threadless. Since both the cylinder and the frame are wound with steel wire prestress, the negative prestress obtained can be determined by calculation. Even when the device is in the maximum Alumina Zirconia working pressure state, its strong stress is borne by the prestressed wound steel wire, that is, the stress is eliminated centrally, and the bearing area is independent and safe. At the same time, the steel wire winding also plays the role of explosion-proof and barrier. Therefore, under the working conditions of high temperature and high pressure (2000 ℃ and 200 MPa), the hot isostatic pressing equipment with this structure does not need any special protective devices. Compared with the old hot isostatic pressing machine with threaded connection structure (between the end cover and the cylinder block), it not only has a compact structure, but also effectively ensures the safety of production.
The heating furnace is responsible for providing the heat necessary for heat isostatic pressing, which is usually a resistance type heating furnace. Different resistance materials can be used depending on the requirements of different temperature ranges: for example, when the maximum working temperature is 1450 ℃, molybdenum wire heating furnace can be used, and when the temperature is 2000 ℃, graphite heating furnace can be used. At present, in the advanced hot isostatic pressing equipment, the installation mode of the heating furnace is plug-in, the heating area is distributed at the bottom and side, which can realize rapid temperature rise and uniform heating, and control the temperature difference within 15 ℃ or even 10 ℃.
The compression system usually adopts non oil filled electric hydraulic compressor, and is equipped with overpressure protection, anti vibration device and automatic adjustment components, which can provide high pressure gas up to 200 MPa for hot isostatic pressure. The vacuum pump is of rotary impeller type, which is used for equipment evacuation and exhaust, and can remove water vapor, oxygen and other volatile impurities in the container.
The cooling system adopts internal and external circulation loop design. In order to protect the cooling system, the quality of cooling water is very important, deionized water is required, and the pipeline also needs to be treated with rust prevention. The external circulation carries out the heat of the internal circulation through the heat exchanger.
The computer control system can store all kinds of programs required by the hot isostatic pressing process in advance to realize the automatic control of basic process Al2o3 Ceramic parameters such as temperature, pressure, time, etc. The system is also equipped with a man-machine conversation PC monitoring subsystem, which is used to display online working status, fault monitoring alarm, etc., and can modify the program during the cycle.
The safety protection design for various aspects of HIP equipment can ensure its safe operation under high temperature and high pressure. For example, the high pressure valve and high pressure pipeline can withstand twice the maximum working pressure; In order to prevent overpressure, multi-stage pressure reducing valve is set in the high-pressure medium gas pipeline, and alarm device is equipped: when the pressure vessel is overpressure and overheated, the heater is overheated, the flow of cooling water is too small or the water pressure is too low, audible and visual alarm can be given, and the power of compressor and heater can be cut off at the same time; Reliable electrical and mechanical safety interlocking shall be adopted.
5.2 Domestic development and application
The General Institute of Iron and Steel Research has been studying the first hot isostatic press in China for more than 30 years since 1972. Through years of research and development, we have mastered the key technologies of the hot isostatic press -- high temperature and high pressure technology. The safety, reliability and stability of the equipment have reached the American ASEM standards. In 1986, we obtained the design qualification certificate of the hot isostatic press issued by the Ministry of Labor. The hot isostatic presses produced by the General Institute of Iron and Steel Research have been serialized. At present, the Institute can provide users with three series of hot isostatic presses of different models (from small hot isostatic presses for laboratory use to large hot isostatic presses for production). At the same time, the General Institute of Iron and Steel Research has produced dozens of hot isostatic pressing machines of various types for domestic and foreign users in more than 30 years of history. One large hot isostatic press was exported to Romania in 1991, and one large hot isostatic press and two large sintering hot isostatic presses were provided to Taiwan Chunbao Company in 1999. In addition, the RD690 large hot isostatic press (installed in the General Institute of Iron and Steel Research), which is currently the largest in China, won the first prize of metallurgical scientific and technological progress in 1987. Other hot isostatic presses are sold to many users in Shanghai, Tianjin, Beijing and Baotou. The main technical indicators of the hot isostatic press produced by Laser Cutting Ceramics the General Institute of Iron and Steel Research have reached the level of the existing hot isostatic press of ABB in Sweden. The General Institute of Iron and Steel Research had extensive exchanges and cooperation with international peers. In June 1999, it successfully held the International Conference on HIP '99 Hot Isostatic Pressing Technology on behalf of China in Beijing. The delegates at the conference came from 13 countries, including Sweden, the United States, Germany, the United Kingdom, France, Japan and Russia. International peers have highly evaluated the level of hot isostatic pressing machine produced by the General Institute of Iron and Steel Research.
In July 2005, Chuanxi Machinery Co., Ltd. adopted 13 key technologies, including domestic leading high temperature/high pressure rapid cooling, vacuum and ultra-high pressure isolation, and ultra-high pressure working cylinder. After more than three years of hard research and technical breakthrough, it successfully delivered one of the largest hot isostatic presses in China to Guizhou Anji Precision Casting Co., Ltd. of Guizhou Aviation Industry Group. The use of the hot isostatic pressing machine has filled the gap of domestic large-scale hot isostatic pressing technology, and created conditions for the development and production of high-tech material products such as high-temperature high-strength alloy, functional ceramics, composite materials, superhard materials, etc. in the fields of aviation, aerospace, nuclear industry, electronics, metallurgy, shipbuilding, etc.
In July 2008, the largest hot isostatic pressing machine in Asia manufactured by the General Institute of Iron and Steel Research( φ 1 250x2500 mm, 1 350 ℃, 150 MPa) has been successfully installed and commissioned and officially put into operation. At present, the equipment has been used to produce qualified high-quality powder turbine disk. The successful development of this equipment marks that Ceramic Disc the General Institute of Iron and Steel Research has reached the international advanced level in the field of research, development, manufacturing and use of hot isostatic pressing equipment.
6 Conclusion
With the continuous development of material science, HIP technology has occupied an increasing proportion in modern production technology. And more and more people use HIP technology to prepare new materials, such as metal matrix ceramics, carbon/carbon composites, hard alloys, tungsten molybdenum products, rare refractory metals and other products. HIP technology is also increasingly White Zirconia Ceramic penetrating into more technical fields, and with its unique advantages, it will play a broader role in the development space of new materials and new energy.