Development of hot isostatic pressing technology and its application in precision casting
Hot isostatic pressing, as one of the modern molding techniques for materials, is a branch of isostatic pressing technology. At present, hot isostatic pressing technology has been widely used in aviation, aerospace, energy, transportation, electrical, electronic, chemical and metallurgical industries, for the production of high-quality products and the preparation of new materials. Hot Isostatic Pressing (Hot Isostatic Pressing, referred to as HIP) process is a nitrogen, argon and other inert gases for the pressure transfer medium, the product is placed in a closed container, the temperature of 900 ~ 2000 ℃ and 100 ~ 200MPa pressure under the joint action of the products to the products of the same pressure to the products for the pressing and sintering process technology. Under the joint action of high temperature and high pressure, the processed parts are pressurized in all directions in a balanced manner. Therefore, the processed products have high density, good uniformity and excellent performance. At the same time, the technology has a short production cycle, fewer processes, low energy consumption, small material loss and other characteristics.
1 hot isostatic pressing equipment and working principle
1.1Characteristics of hot isostatic pressure equipment Hot isostatic pressure equipment consists of high-pressure vessel, heating furnace, compressor, vacuum pump, gas storage tank, cooling system and computer control system, of which the high-pressure vessel is the key device for the whole equipment.
high-pressure vessel of the current advanced hot isostatic pressure equipment is composed of unthreaded, the bottom of the closed steel wire winding prestressed cylinder and steel wire winding and prestressing frame. Steel wire is rectangular cross-section, cold-rolled spring steel strip, cylinder by forging and heat treatment, the frame consists of two cross-frame and two columns, metal shell packaging, the application of prestressing, its structural features are: (1) cylinder in the tangential direction of balanced compression, which can prevent axial rupture; (2) the frame of the compression of balanced, which can prevent the tangential rupture; reliable and safe, the bearing area is independent; (3) the pressure vessel can be calculated accurately at various points of stress (3) the stress at each point of the pressure vessel can be calculated accurately; stress concentration is eliminated; (4) the cylinder and frame are not subjected to any tensile loads; the steel wire winding plays the role of explosion-proof and barrier. 1.1.2 Heating Furnace High-quality Heating Furnace is an indispensable key component of the advanced hot isostatic pressure equipment. At present, the advanced installation of the heating furnace is inserted, there are two heating zones, can be designed for three gears of the maximum operating temperature of 1200 ℃ (for Fe-Cr-Al furnace), 1450 ℃ (for Mo furnace), 2000 ℃ (for graphite furnace) and different atmospheres of the multi-heating furnace, due to the inserted, the user can be conveniently replaced according to the sintering temperature, the atmosphere requirements of the heating furnace, each kind of Each kind of heating furnace can realize rapid heating and cooling, and the temperature difference inside the furnace is less than ±15℃.
1.1.1Graphite heating furnace is made of mixed materials of carbon essence and graphite mixed with fiber reinforcement, its strength, reliability and safety are better than the conventional pure graphite, with high resistivity, extremely suitable for vacuum and low voltage work.1.1.3Compressor and Vacuum Pump Thermal Isostatic Pressure Equipment is usually used in the non-oil-injected electro-hydraulic compressor, which eliminates the need for compressed air device and is configured with over-pressure protection, vibration-proof device, and automatic adjusting parts. The vacuum pump adopts a rotary impeller, which is used for vacuum suction in product sintering, and at the same time pumps out oxygen, moisture (water) and other impurities in the vessel.1.1.4 Cooling deviceCooling water passes through the regenerative cooling circuit, and the pipeline exchanges heat between the cooling water and the shell of the pressure vessel with de-ionized water and antirust agent to ensure the quality of the cooling water and to protect the cooling system.1.1.5 Computerized control systemIt is powered by an IBM-compatible PC, floppy disk drive, color SYGA monitor, keyboard, IBM-compatible printer output, data acquisition, control software and desktop components supported by manual control. During the hot isostatic pressing process, it can realize the program control of temperature, pressure and vacuum and display all the working status. Between program control and manual control, the programmable controller provides safe and reliable interlocking.
1.2 Thermal isostatic pressing process flow thermal isostatic pressing process has three kinds, namely, first pressure and then temperature, applicable to the manufacture of metal cladding workpiece; first temperature and then pressure, applicable to the glass cladding manufacturing in line with the material; at the same time, temperature and pressure, applicable to low-pressure molding, loading a large amount of heat preservation time for a long period of time, the manufacture of the workpiece.
1.3 Thermal isostatic pressure working principle according to Pascal’s principle, in a sealed container, acting on the static liquid or gas generated by the external force of the static pressure, will be uniformly transmitted in all directions, in its role in the surface area of the pressure is proportional to the surface area. Under the action of high temperature and high pressure, hot isostatic pressure furnace in the package sleeve softens and shrinks, squeezing the internal powder to make it move with itself. The densification process of the powder under the simultaneous action of high temperature and high pressure is quite different from that of general unpressurized sintering or room temperature pressing. It is roughly divided into the following three stages:
2 The main application areas of hot isostatic pressing technology Hot isostatic pressing technology has become mature after nearly 60 years of development, and has been widely used in the fields of nuclear materials, aerospace materials, cemented carbide, high-temperature alloys and ceramics, etc. It is an advanced production process and means to develop and process materials and improve material properties, and it has become a practical technology in the production of many high-performance materials and an indispensable technology for the development of new materials. It has become a practical technology in the production of many high-performance materials and a new technology indispensable for the development of new materials.
2.1 Densification treatment of castings hot isostatic pressing in the casting densification treatment of the application of research and development earlier, is the application of hot isostatic pressing more mature and perfect field. Hot isostatic pressing method can not only make the new casting densification, but also can be used to repair the casting in use, so that the casting in the use of reduced performance can be restored. Castings with a certain calculated life value under specified temperature and stress conditions will continue to produce microscopic defects after a period of use, and relative motion between crystals will produce defects at grain boundaries. These internal defects, which are similar to common shrinkage holes, can then be cured by hot isostatic pressing. With such a treatment method, the mechanical properties and fatigue properties of the engine parts in use can be restored to the water quality of new castings. The utility and significance of using hot isostatic pressing to treat castings can be summarized as follows: (1) hot isostatic pressing treatment can reduce the casting in the X-ray inspection and surface projection inspection of the scrap rate; (2) compared with the untreated casting, the casting by hot isostatic pressing treatment of the casting in the weld after the production of fewer cracks, thus reducing the cost of repair welding; (3) the use of hot isostatic pressing treatment can improve the casting parameter range and expand the new casting Alloy varieties; (4) improved fatigue strength and ductility of hot isostatic pressing castings can replace expensive forgings.
2.2 Thermal Isostatic Cladding and Thermal Isostatic Composite Diffusion Connection Thermal Isostatic Cladding and Thermal Isostatic Composite Diffusion Connection is another important application of thermal isostatic pressing. Thermal isostatic pressure cladding technology is the use of thermal isostatic pressure technology in a material on the preparation of a special physical or chemical properties of the cladding, in order to improve its comprehensive performance of the new technology, thermal isostatic composite diffusion connection is two or more different materials, in the high temperature and high pressure under the action of the diffusion connection of a new technology, the materials involved can be a metal metal, metal, a non-metal, non-metal non-metal, it is the development of recent years. It is a better technology that has been developed in recent years and is worth applying and popularizing especially in civil applications [12]. Compared with the general connection method, hot isostatic composite diffusion connection has some of the following advantages: (1) the connection material has the characteristics of the base material, no melting zone in the weld joint, thus eliminating is the weld joint performance attenuation of the grain growth; (2) can be connected to usually can not be connected to the dissimilar metals, in the high hot isostatic pressure can prevent the formation of Kirkendall vacancies; (3) is not subject to the limitations of the fixed mold, can be processed and processed Any complex shape of the parts; (4) can connect brittle materials or low ductility materials without fracture; (5) temperature limitations are small; (6) can connect the composite material, and very little damage to the fibers in the composite material.
2.3 Hot isostatic pressing powder consolidation
2.3.1 High-temperature alloy powder solidification of high-alloyed nickel-based high-temperature alloy ingot segregation is serious, poor hot working performance, it is difficult to deform, the traditional casting and forging technology is difficult to ensure the uniformity of the performance of large-size parts, the use of powder metallurgy process to solve such problems. Rapid solidification of pre-alloyed powder + HIP solidification technology for the production of high-alloyed nickel-based high-temperature alloy materials opened up a new direction. Powder metallurgy high temperature alloys are widely used in aviation gas turbine engines, rocket engines and ground combustion engines and other key components.3.3.2 Hot isostatic pressing of cemented carbideHot isostatic pressing treatment can effectively eliminate defects such as internal porosity in the sintered cemented carbide; hot isostatic pressing treatment temperature is lower than the sintering temperature of the cemented carbide can prevent the coarsening of the carbide grains; hot isostatic pressing treatment of large size of the cemented carbide products. After hot isostatic pressing, large-size cemented carbide products have greatly improved their comprehensive performance, and their service life and reliability have been significantly improved. Hot isostatic pressing of cemented carbide has the following advantages compared with conventional sintered carbide: (1) Participating in the almost complete elimination of voids, the density can be 99.8% of the original theoretical density to 99.999% of the theoretical density. (2) When manufacturing large or large diameter ratio products, the scrap rate is low, the surface defects are greatly reduced, and the polished surface can be obtained with very high finish. (3) Due to the products in the pore volume is significantly reduced, the elimination of the source of fracture, so that the performance and life of the products greatly improved.3.3 High-speed steel powder cemented high-speed steel is a complex chemical composition of high-alloy steel. In the traditional melt casting – forging method of production of high-speed steel, due to the ingot size, slow cooling, inevitably produce carbide segregation. This segregation organization not only to the forging, rolling and other hot processing difficulties, damage to the various properties of the product, and limit the further increase in alloy content, hindering the development of high-speed steel. The introduction of hot isostatic pressing technology, so that many high-alloy high-speed steel can take the powder metallurgy process to manufacture, so as to overcome the cast steel carbide segregation defects, the powder metallurgy technology has been successfully introduced into the dense just now and the production of alloy steel field.
2.3.4 Ceramic material powder sintering in the preparation of metal ceramics, usually solid state method and liquid method of the two major types of preparation process, the application of hot isostatic pressing process can not only be used to achieve the solid state method of molding and sintering together, but also on the liquid method of metal ceramics prepared by heat treatment, improve the performance of products. The use of hot isostatic pressing process for the preparation of metal ceramics has the following advantages: (1) greatly reduce the temperature of sintered metal ceramics, grain refinement; (2) improve the densification of the products, improve the various properties of the metal ceramics; (3) can be directly prepared for the shape of the complex and large size of the engineering components, reducing or even avoiding the costly machining; (4) reduce the residual stresses during cooling, reduce the molding of the reactive layer, control the 2.3.5 Titanium Alloy Powder Solidification Titanium alloys are widely used in aerospace, aviation, marine and chemical industries because of their high strength, high toughness, oxidation and corrosion resistance. However, the expensive price of titanium products limits its application. The main reason for the high cost of titanium products is the complexity of the traditional manufacturing process and the loss of material in secondary processing. Typically, titanium melting and processing processes include: titanium sponge pressed into electrodes; two vacuum electric arc furnace melting; precision casting or forging, rolling processing and machining. Powdered titanium alloys are considered to be the way out to further improve the performance of titanium alloys and reduce their price. Hot isostatic pressing process to produce powder titanium alloy, not only simplify the melting process and cutting process, and because of the fine powder grain is conducive to the alloy organization homogenization, so that the product performance has been improved. 2.4 Hot isostatic pressing process in new areas of application (1) in the application of porous materials: due to the use of nitrogen as a medium, the generation of nitride at high temperatures, so that the HIP on the role of porous materials; (2) HIP technology with nitriding, etc., and nitriding, etc., the HIP technology is a good solution. ) HIP technology combined with nitriding and other surface treatment to expand the HIP function; (3) HIP technology can be used in the preparation of high-purity materials by suspension melting process. Because of the increased density of high-pressure gas, the melted material can be suspended to realize crucible-free melting, thus greatly improving the melting purity. (4) HIP technology for food industry: applying high pressure to the food can make the food in the nutrition is not destroyed and keep the original color and taste of the case, to achieve the purpose of sterilization, for food processing provides a new way.
4 Conclusion With the continuous development of material science, hot isostatic pressure technology has occupied an increasing proportion in modern production technology. And people are increasingly using hot isostatic pressing technology for the preparation of new materials, such as metal-based ceramics, carbon/carbon composites, cemented carbides, tungsten and molybdenum products, rare refractory metals and other products. Hot isostatic pressing technology is also more and more penetrating into more technical fields, and by virtue of its unique advantages, it will play a broader role in the development of new materials and new energy space.
