Thermisostatic pressing: One of the most advanced heat treatment and shaping technologies in the world
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In 2021, the North American Heat Treatment Society identified the three most promising technologies and processes globally at that time, namely hot isostatic pressing, hydrogen combustion, and additive manufacturing. Hot isostatic pressing technology is being applied in an increasing number of industry sectors and new materials thanks to its unique advantages. Not only does it significantly improve the quality of materials and products, but the economic benefits it brings also attract attention from various countries; as a result, leading companies are gradually making moves to enter the global hot isostatic pressing market. Overview of Hot Isostatic Pressing Technology: Hot Isostatic Pressing (HIP) is a process in which materials are subjected to equal pressure in all directions under high temperature (900–2000°C) and high pressure (50–200 MPa) using an inert gas medium such as argon or nitrogen, thereby achieving densification. HIP originated in the 1950s at the Battelle Institute in the United States, in the context of research on nuclear reactor materials, and it achieves material densification and defect repair through high-temperature and high-pressure conditions. In early hot isostatic pressing equipment, the connection between the end cover and the cylinder body of the high-pressure vessel relied mainly on threaded connections, which presented problems such as poor safety and size limitations. In 1965, the Swedish company ASEA utilized a prestressed steel wire-wound frame structure to manufacture high-pressure vessels, which significantly improved the pressure resistance and safety of such devices, laying the foundation for the widespread use of modern HIP technology. With the continuous improvement in the performance of hot isostatic pressing equipment, HIP has become increasingly mature and is widely used in areas such as the densification of castings, powder metallurgy sintering, and the diffusion bonding of composite materials. It represents an advanced manufacturing process and technique for producing components used in high-end equipment in aviation, aerospace, maritime, and other fields, and it is also a key technology for enhancing material properties and ensuring the reliability of such equipment.
The operational steps of the hot isostatic pressing HIP process are as follows: first, the inspected and cleaned workpiece is placed inside a high-pressure chamber, followed by vacuum pumping； Next, the pre-pressurization procedure is carried out using an inert gas such as nitrogen or argon, and pressure is gradually increased via a compressor; once the pressure stabilizes, the heating system is activated to raise the temperature of the container ； When the temperature and pressure parameters reach the preset values, it enters a constant-temperature and constant-pressure maintenance phase to ensure that the workpiece is fully heated and compressed ； After the workpiece has undergone high-temperature and high-pressure treatment, the temperature inside the furnace is gradually reduced using a temperature control system, followed by a pressure release operation to discharge the internal pressure ； Finally, the workpiece is removed and subjected to quality inspection and any necessary machining in sequence.
According to researchers from the National Key Laboratory of Advanced Equipment Casting Technology, the hot isostatic pressing equipment system relies on a highly integrated setup; its key components include high-pressure vessels, heating furnaces, compressors, vacuum pumps, gas storage tanks, cooling systems, and computer control systems. As a core component, high-pressure vessels must withstand both high temperatures (usually ≤1500°C) and extremely high pressures (≤200 MPa); their structural design has a direct impact on the safety and reliability of the process. The prestressed steel wire winding technology developed by the Swedish company AVURE achieves uniform stress distribution in the container walls through equal shear stress variable tension winding using multiple layers of high-strength steel wires, thereby avoiding the risk of fatigue failure associated with stress concentration in traditional unprestressed structures. Ultra-high pressure vessels utilize a prestressed winding structure made of high-strength steel wires; they are in a state of compressive stress even when not under load. Even when subjected to internal pressure, they remain in a compressive stress state, with a relatively low stress level. As a result, they possess high fatigue resistance and load-bearing capacity, and are not prone to damage. This technology has become an industrial standard, contributing to the development of larger and more modular hot isostatic pressing equipment.
Current status of HIP equipment manufacturing: In the field of HIP equipment manufacturing, foreign manufacturers mainly include Swedish Quintus, Belgian EPSI, American AIP, British Bodycote, and Japanese Kobe Steel. Quintus has been engaged in HIP research for nearly 80 years. Thanks to its advanced prestressed wire winding technology, it has become a leader in the field of high-temperature and high-pressure technologies worldwide. Its technology boasts two key advantages: (1) the capability to manufacture super-large equipment, having developed the world’s largest hot isostatic pressing equipment with an effective heating zone diameter of ⌀2050 mm ； (2) The Uniform Rapid Cooling technology (URC@) enables all the loads in a medium-sized HIP to be cooled from 1250°C to 300°C in less than 30 minutes, thereby reducing cycle time and increasing productivity. Sichuan Aviation Industry Western Sichuan Machinery Co., Ltd. (under the China Aviation Industry Corporation) and Gangyan Haopu Technology Co., Ltd. (under the China Iron and Steel Research Group Corporation) are key players in China’s independent innovation in hot isostatic pressing technology; the former is renowned for its military-grade quality, while the latter is famous for its collaborative innovation in materials and equipment. Other companies in China’s field of hot isostatic pressing equipment manufacturing include: Shenyang Vacuum Technology Research Institute Co., Ltd., Hunan Weishang Technology Co., Ltd., Hunan Dingli Technology Co., Ltd., and others. Although effective hot-zone 1850 mm hot isostatic pressing equipment has been developed in China, there is still a certain gap compared to foreign counterparts in key parameters such as the size of the effective hot zone, operating temperature, and operating pressure.
The \"strength\" of hot isostatic pressing technology: The HIP densification process eliminates defects such as porosity and shrinkage cavities within castings/additively manufactured parts by applying both high temperature and high pressure, thereby densifying the material. Castings/additively manufactured parts subjected to hot isostatic pressing not only experience a significant improvement in their mechanical properties, but also see enhanced fatigue and stress corrosion resistance, thereby extending their service life. In recent years, as requirements for product performance have increased, HIP densification is no longer limited to the densification of castings; it is now applied to 3D-printed components, spray-formed parts, and high-performance sintered parts. The range of materials used has expanded from titanium alloys and superalloys to magnesium alloys and aluminum alloys, while the fields of application have extended from aerospace to nuclear power, oil industry, and target materials.
Powder metallurgy hot isostatic pressing near-net shaping: This technology combines the advantages of powder metallurgy and hot isostatic pressing; by using high pressure and high temperature, it allows metal powders to be densified directly into parts that are close to their final shape, thereby reducing the need for subsequent mechanical processing. This technology not only improves material utilization but also significantly shortens the production cycle. Using the powder metallurgy hot isostatic pressing process to manufacture components has the following advantages: (1) High material utilization rate: Near-net shaping techniques reduce material waste and improve the efficiency of material use. (2) Capability to manufacture complex shapes: It is able to produce parts with complex shapes that are difficult to process using traditional methods. (3) High material properties: Through HIP treatment, powder metallurgy parts can achieve or approach the properties of forged materials. (4) Less subsequent processing required: Near-net shaping technology reduces the need for further mechanical processing, thereby lowering production costs. (5) Material diversity: Suitable for various metal and alloy powders, including superalloys, titanium alloys, etc. Thermisostatic pressing diffusion bonding technology is an advanced bonding method that enables solid-solid, solid-powder, and powder-powder metallurgical bonding between two or more metal/ceramic materials, through plastic deformation of the materials and atomic diffusion, in an environment of high temperature, high pressure, and inert gas. Compared to other joining techniques, the core advantage of this technology is: (1) a uniform microstructure at the joint site: there are no defects such as heat-affected zones, pores, or cracks associated with traditional welding, and the structure is uniform. (2) High strength at joints: This technology enables tight bonding without defects at the material interfaces, with properties at these interfaces identical to those of the base material ； When connecting materials with the same properties, no liquid phase is formed at the interface, so the interfacial bonding strength is comparable to that of the base material ； When connecting materials with different properties, a good metallurgical bond can be achieved at the interface, and the resulting properties are not lower than those of the weaker of the two materials. (3) Wide applicability: It can weld materials such as ceramics, metal compounds, and amorphous materials that are difficult to weld using traditional methods. (4) Strong geometric adaptability: Supports the joining of parts with complex shapes and enables precise dimension control. (5) Precise deformation control: Minimal macroscopic deformation of the material, making it suitable for precision assembly. It is precisely because of these many advantages that the hot isostatic pressing diffusion bonding technique has attracted the attention of numerous researchers. Currently, HIP diffusion bonding technology is used in engineering fields such as nuclear energy and aerospace, and it is particularly suitable for solving the problem of joining dissimilar materials (metals, alloys, and non-metallic materials such as ceramics).
Application areas of hot isostatic pressing include the aerospace industry. The United States has long utilized HIP technology in military and aerospace applications; this technology is widely used in the manufacturing of key components for aerospace systems, such as the engines of space shuttles (SSMEs), as well as titanium alloy turbine pumps, pump casings, and valve bodies for the Atlas-3 and Atlas-5 rockets. In the field of military aviation, the titanium alloy support rods and fuselage load-bearing struts of the F-14 fighter jet, the Ti-6Al-4V front cone of the F-15 fighter jet, and the fixing support frames for the engines of the F-18 fighter jet are all manufactured with high density and excellent performance through the hot isostatic pressing process. British engine manufacturer Rolls-Royce, in collaboration with the University of Birmingham, has developed titanium alloy compressor casings for aircraft engines using advanced computer numerical simulation combined with HIP. This is the largest-sized titanium alloy aerostructural component (with a diameter of around 600 mm) ever produced via powder metallurgy HIP, as reported in public sources.
Gas turbines: The turbine blades of gas turbines are the most critical components of heavy-duty gas turbines. Since the impellers must operate stably at high temperatures of 1400–1600°C for extended periods, this represents a working environment that demands extremely high standards regarding both the quality and performance of the materials used. Therefore, turbine blades in heavy-duty gas turbines are manufactured from superalloy materials. For nickel-based superalloys, conventional casting processes used to produce large-sized components result in internal defects such as shrinkage cavities, porosity, and gas holes. These defects significantly reduce the overall mechanical properties and service reliability of the components; therefore, the HIP process is necessary to eliminate these defects and achieve material densification. Target preparation: For both metal and ceramic targets, purity, density, and uniformity are key parameters determining the quality of the target; they directly affect the quality of the thin films, the stability of the sputtering process, and the reliability of the final products. When preparing targets using hot isostatic pressing, the combined effect of high temperature and high pressure enables the density of the target to be increased to over 99.8%, and it also eliminates residual stresses within the material, thereby preventing the target from cracking during the sputtering process. At the same time, the powder metallurgy + hot isostatic pressing near-net shaping method can directly produce targets with complex shapes, reducing the amount of subsequent mechanical processing required and minimizing waste of expensive materials. The advantages of using hot isostatic pressing in the field of preparing refractory metal targets are even more prominent. In the field of nuclear energy, the HIP diffusion bonding technology is widely used in the International Thermonuclear Fusion Experimental Reactor (ITER) project. It enables the diffusion bonding of identical materials such as low-activation martensitic CLF-1, 316L, and CLAM steel used in complex structural cooling channels, as well as the diffusion bonding of dissimilar materials such as W/CuCrZr, Be/CuCrZr, Be/HR1 steel, and WNiFe/Cu. In the field of additive manufacturing, hot isostatic pressing can be considered one of the most useful tools for the 3D printing (additive manufacturing) industry; various components produced through additive manufacturing benefit greatly from the use of this technology. During the rapid prototyping process of 3D printing, internal defects such as tiny holes and cracks inevitably occur, and the presence of these defects limits the industrial application of 3D-printed materials. As a post-processing step for 3D printing, hot isostatic pressing can eliminate the residual defects from 3D printing processes, thereby significantly improving the mechanical properties and service life of the products.
According to available data, there are approximately 1,800 or more hot isostatic pressing units in operation worldwide. The United States and Japan account for about 65% of the total, Europe accounts for about 20%, and China accounts for about 10%. Hot isostatic pressing technology is currently in a phase of rapid adoption and widespread use in China. The country places great emphasis on independent research and development; it now possesses the capability to design, develop, manufacture, operate, and maintain hot isostatic pressing equipment, and the gap with advanced foreign levels is gradually narrowing. In the future, HIP will experience rapid development in terms of larger scale, greater diversity, higher efficiency, better performance, and increased intelligence. References: Zhao Jun, et al.: Development and Applications of Large-Scale Hot Isostatic Pressing Equipment and Technologies, Shenyang Foundry Research Institute, China National Machinery Industry Corporation; Che Hongyan, et al.: Applications and Development Trends of Hot Isostatic Pressing Technology in Metal Material Processing, Antai Technology Co., Ltd.; Lu Zhoujin, et al.: Applications of Hot Isostatic Pressing Technology in the Preparation of High-Quality Refractory Metal Targets, Gangyan Haopu Technology Co., Ltd