Application of Hot Isostatic Pressing in the Preparation of Rare Refractory Metal Products
Release time:
2026-01-14
Hot isostatic pressing (HIP) is a technology that applies isostatic pressure to powder metallurgy products or compacts using high-pressure gas at elevated temperatures. This process eliminates internal defects and voids within the product while simultaneously inducing grain boundary diffusion bonding at high temperatures. This improves the mechanical properties of the product and achieves complete densification. The HIP process was first invented in 1955 by Saller et al. at Battelle Memorial Institute in Ohio, USA. In 1964, this technology was applied to the consolidation of beryllium metal powder. In 1967, Kennametal manufactured the first HIP press with a chamber size of 277 mm × 1,270 mm, operating at 1,500°C and capable of exerting a maximum pressure of 140 MPa, successfully applying HIP technology to the production of cemented carbides [1]. From 1972 to 1980, the U.S. Air Force Materials Laboratory extended HISP to the preforming and final-shape forging of nickel-based high-temperature alloys and titanium alloy powders, further broadening the technology's application scope. In 1963, China's first HIP test apparatus was developed at the Shenyang Institute of Metal Research, Chinese Academy of Sciences. Featuring a furnace chamber size of ϕ 65 mm, operating temperature of 850°C, and pressure of 98 MPa, it was primarily used for thermal diffusion bonding of nuclear materials. In 1977, the Beijing Iron and Steel Research Institute put into operation a small-to-medium-sized HIP press.
Through years of development, HSP technology and equipment have matured significantly, with applications expanding to include: - Final forming of nickel-based high-temperature alloy turbine discs for aircraft engines - Preforms for nickel-based powder metallurgy forging and rolling - Powder metallurgy billets for titanium alloys - Preforms and irregularly shaped components for forging - Large modular composite structures - Powder metallurgy beryllium, and other refractory metals, dispersion-strengthened and fiber-reinforced powder metallurgy products and components, tool steel irregular parts, cemented carbide cutting tools and gears, and cermets [2-8]. For certain large powder metallurgy semi-finished products or near-net-shape powder metallurgy components, hot isostatic pressing has become the sole manufacturing method. This paper primarily introduces the application of hot isostatic pressing in rare and refractory metal products.
1. Hot Isostatic Pressing Equipment
Hot isostatic pressing equipment primarily consists of a pressure vessel, heating system, gas storage and delivery system, power supply, and control system, as shown in Figure 1. During the hot isostatic pressing process, the equipment primarily utilizes convection and radiation for heat transfer. At low temperatures, gas convection dominates, while at high temperatures, radiation heat transfer prevails. For heating temperatures below 1230°C, FeCrAl is typically used as the heating element. Molybdenum heating elements can operate between 500–1600°C, while graphite-heated HSP furnaces can reach temperatures up to 2200°C.
Table 1 lists the sizes and operating parameters of major domestic hot isostatic pressing (HIP) equipment. As shown, the maximum operating temperature reaches 2000°C, with peak pressure attaining 200 MPa. Notably, the large-scale HIP equipment (ϕ 850 mm × 2,500 mm) introduced by Antai Technology in 2012 from Avure Technologies AB (a Swedish subsidiary) features advanced rapid cooling capabilities, a comprehensive and effective temperature control system, and an automated operating system. These enhancements improve process performance, significantly shorten production cycles, and markedly increase production efficiency.
2. Sleeve Fabrication and Numerical Simulation
The HIP process typically requires powder or powder metallurgy components to be placed within a sleeve for compaction. Consequently, the design, material selection, and airtightness of the sleeve are critical factors in ensuring product quality.
The dimensions and shape of the sleeve depend on the design requirements of the part being produced, plus anticipated shrinkage and machining allowances. For irregularly shaped parts, control of the external form must also be considered. Common sleeve materials include glass, low-carbon steel, stainless steel, pure titanium, and copper. Before selecting a sleeve material, it is essential to determine or test whether the powder reacts with the sleeve material at the operating temperature. For rare refractory metals like tungsten, molybdenum, tantalum, and niobium, which possess high melting points, industrial-grade pure titanium is typically used as the sleeve material. During hot isostatic pressing at 1400–1650°C, no harmful solid-state diffusion reactions or formation of low-melting-point compounds occur between the sleeve and powder. Within the HIP furnace, high-pressure gas transmits force to the components through the sleeve. The pressure differential between the sleeve's interior and exterior ensures effective force transfer. Therefore, the sleeve must maintain a tight seal to prevent high-pressure gas ingress—a critical prerequisite for successful HIP. Consequently, all sleeves undergo leak testing post-fabrication. After loading components or powder, vacuum sealing and welding are required.
As a near-net-shape forming technology, HIP has traditionally relied on empirical trial-and-error methods to establish processes, incorporating substantial allowances at critical locations followed by subsequent machining to achieve powder metallurgy products. This approach is not only time-consuming and labor-intensive but also prone to dimensional shortfalls when allowances are too small, while excessive allowances lead to material waste, increased costs, and loss of net-shape advantages. In recent years, advancements in computer technology have enabled full-process HEP simulation using software like MSC. Marc, providing robust support for predicting shrinkage in various HEP components and critical dimensions [9-10].
Hou Zhiqiang et al. [11] simulated the HEP process for 316 stainless steel powder using MSC.Marc finite element analysis and compared it with experimental results. The experimental and simulated results differed by only 1.86% and 0.65% in the radial and axial directions, respectively. YUAN et al. [12] simulated and experimentally studied the HSP fabrication of scaled-down TC4 titanium alloy engine casings. Results showed dimensional deviations between simulated and actual specimens remained within 2%.
Finite element numerical simulation optimizes HSP process design, enabling precise control over post-HSP product dimensions. This achieves true near-net-shape or even net-shape forming, effectively reducing design costs and subsequent machining expenses. As workpiece geometry complexity increases, the design and fabrication of sleeves and dies become major cost drivers in HIP processes. Combining finite element numerical simulation with batch production of workpieces can significantly reduce both material and process costs for HIP [13].
3. Application and Related Research of HIP in Rare Refractory Metals
3.1 Titanium
The application of hot isostatic pressing (HIP) technology in titanium alloys began in the 1970s and 1980s, initially used primarily for producing preforms for titanium alloy powders. It was later gradually extended to diffusion bonding of titanium alloys, the fabrication of crucible components, and the production of complex-shaped parts. Parts produced via this method exhibit high density, excellent mechanical properties, good isotropic performance, and low cost. It enables the fabrication of complex geometries difficult to achieve through other methods, making it the primary production approach for powder metallurgy components in titanium alloys today.
After years of development, HIP technology has achieved widespread application in titanium alloy part production. With robust equipment, process expertise, and technical accumulation, it can fabricate titanium alloy plates, bars, tubes, semi-finished products, and various other shapes across different grades and dimensions. Currently, combined with simulation software, HIP processes for titanium alloys can be modeled [9-11, 14-15], significantly shortening R&D cycles and enhancing production efficiency. It can be said that the application of HIP technology in titanium alloy manufacturing has reached a high level of maturity, and further elaboration here is unnecessary. This section primarily introduces research and applications of HIP technology in several other refractory metals.
Refractory metals (tungsten, molybdenum, tantalum, niobium, rhenium, etc.) possess high melting points. For producing plates, bars, tubes, and uniformly shaped billets, the melting and forging method is generally suitable. However, for fabricating large-sized, thick-walled, ultra-thin-walled, and irregularly shaped components, hot isostatic pressing demonstrates distinct advantages. It not only simplifies the manufacturing process but also minimizes machining requirements and reduces raw material costs.
3.2 Tungsten
Tungsten and tungsten alloy targets produced via hot isostatic pressing achieve large dimensions exceeding 1 m², high density, fine grain size, and excellent uniformity, making it the primary method for tungsten target production today.
W-Ni-Fe alloys, used as armor-piercing materials, require high density, strength, and toughness. Due to the large differences in melting points among their components, they are typically prepared via powder metallurgy followed by pressure processing. However, the coarse grain structure of sintered tungsten causes uneven stress distribution between the surface and core during deformation, leading to cracking and low yield rates [16]. Hot isostatic pressing (HIP) enables rapid densification of W-Ni-Fe alloys, eliminating stress non-uniformity between surface and core and overcoming grain structure anisotropy caused by pressure processing, thereby improving yield rates.
In accelerator-driven subcritical systems, tungsten is typically used as the neutron-producing target material, coated with a corrosion- and radiation-resistant metal cladding. To extend target lifespan and facilitate heat dissipation, the cladding must bond tightly to the tungsten target body without any gaps. Xu Yongli et al. [17] employed stainless steel and zirconium as cladding materials for tungsten targets, utilizing hot isostatic pressing (HIP) to produce tungsten-stainless steel and tungsten-zirconium cladding tubes. Results demonstrated a tightly bonded interface between the target body and cladding, achieving diffusion welding levels with diffusion layer depths reaching 6–13 μm.
W-Cu alloys are typically prepared via powder sintering or infiltration methods. However, as pseudo-alloys, their relative density generally only reaches 96%–98%. After HIP treatment, their density significantly increases, and both mechanical and electrical properties are improved. Lü Daming et al. [18] subjected W-30Cu alloy to 2 hours of HEP at 1000–1050°C and 100 MPa pressure, achieving a relative density of 99.4%, a 31% increase in flexural strength, and an electrical conductivity rise from 31.6 MS/m to 34.6 MS/m.
3.3 Molybdenum
HIP technology has been extensively applied in the preparation of molybdenum sputtering targets in recent years. Antai Technology Co., Ltd. has produced large quantities of molybdenum and its alloys (Mo-10Nb, Mo-10Ta, Mo-10Cr, Mo-33Ti, Mo-30W, etc.) with planar dimensions reaching 1300 mm × 850 mm. These targets exhibit flat plate profiles, uniform and fine microstructures, entirely equiaxed grains, relative densities no less than 99%, and flexural strengths exceeding 800 MPa. The large capacity of the hot isostatic pressing cylinder enables production of oversized target materials with high single-batch output, significantly boosting production efficiency and reducing costs [19].
Guo Zhijun et al. [20] investigated the mechanical properties of pure molybdenum after hot isostatic pressing (HIP). At 1300°C and 100–110 MPa, pure molybdenum exhibited high tensile strength alongside excellent ductility, with elongation reaching 20–25%—5 to 6 times that of samples treated at other temperatures. As the HSP temperature increased further, molybdenum grain growth occurred, leading to a corresponding decline in properties.
Beyond molybdenum target material preparation and mechanical property enhancement, HSP technology is also applied in the densification of molybdenum coatings. Xie et al. [21] prepared a 5 mm thick molybdenum metal layer by plasma spraying onto a graphite core. After removing the core mold, the molybdenum product underwent 1.5 hours of HIP treatment at both low pressure (10 MPa) and high pressure (125 MPa). The study revealed that after high-pressure treatment, the layered structure of the molybdenum deposition layer completely disappeared, with molybdenum particles tightly bonded. The relative density increased from 89.7% to 97.3%, tensile strength rose from 44 MPa to 110 MPa, and microhardness even doubled. This also provides a feasible method for improving the properties of complex-shaped molybdenum products prepared by plasma spraying.
3.4 Tantalum
Tantalum and its alloys exhibit high sintering temperatures and are costly materials. Manufacturing complex tantalum alloy components via ingot casting consumes substantial raw materials and poses machining challenges, significantly increasing production costs. Consequently, hot isostatic pressing (HIP) has emerged as an effective forming method for tantalum alloys. Zhang Xiaoming et al. [22] employed HIP to produce a Ta-W-Hf alloy with a relative density of 99.5%, investigating its microstructure and mechanical properties. Findings revealed that the alloy achieved a tensile strength of 1,300 MPa in the HIP-treated state. However, powder metallurgy components typically exhibit high impurity gas content and poor plasticity. However, after high-temperature annealing at 2200°C, the tensile strength decreased to approximately 900 MPa, while plasticity improved significantly, with elongation reaching 28%.
The author has also conducted recent research on the hot isostatic pressing of tantalum alloys. Using pure titanium as the cladding, tantalum-tungsten alloy powder was hot isostatically pressed to produce stepped tantalum-tungsten alloy barrel-shaped (Figure 2) and hemispherical irregular components. with finished product weights exceeding 100 kg and a relative density of 99.7%. These represent the largest tantalum-tungsten alloy irregularly shaped components currently produced via powder metallurgy in China, achieving geometries difficult to realize through conventional melting and forging processes. Mechanical property studies revealed that while densification occurred post-HIP, the grains retained powder morphology with incomplete metallurgical bonding. High-temperature annealing promoted grain equiaxialization and enhanced interfacial bonding (Fig. 3). Moreover, in the HSP state, the alloy exhibits high strength but poor plasticity due to oxygen content. After high-temperature annealing, the oxygen content decreases, significantly improving plasticity with an elongation reaching 30%. Since HSP uses powder as raw material, the oxygen content in tantalum alloy components inevitably exceeds that of cast alloys. Therefore, while controlling raw material oxygen content is essential, high-temperature annealing is indispensable for HSP-produced tantalum alloy components.
Additionally, hot isostatic pressing can be employed for densification of tantalum coatings. Wei Shaohong et al. [23] plasma-sprayed a tantalum layer onto a tungsten substrate. After HEP treatment at 1400°C/100 MPa/2 h, a 0.5 mm thick tantalum coating was produced, significantly enhancing the substrate's corrosion resistance. After HIP treatment, the relative density of the sprayed tantalum coating increased from 91.2% to 98.8%, and the surface bonding state between tungsten and tantalum improved. The tensile strength of the tantalum coating rose from 19.4 MPa to 22.5 MPa.
3.5 Niobium
The application of hot isostatic pressing (HIP) technology in niobium and its alloys, similar to other refractory metals, is primarily used for the preparation of sputtering targets and other complex-shaped components. Quxuanhui et al. [24] employed an injection molding + HIP method to fabricate small-sized, thin-walled Nb-W-Mo-Zr alloy parts featuring stepped and curved internal structures. This method involves mixing metal powder with a binder, injection molding followed by debinding, high-temperature sintering for shaping, and subsequent HIP treatment. The resulting parts achieve a relative density exceeding 99%, overcoming the limitations of traditional niobium alloy fabrication—low material utilization, significant impurity contamination, difficulty in producing complex shapes, and low production efficiency. This technique is highly suitable for the mass production of small-sized, complex-shaped niobium and its alloy components.
Wang Feng et al. [25] prepared low-density Nb-Ti-Al-V-Zr alloy ingots using hydrogenation-dehydrogenation-treated niobium alloy powder as raw material via hot isostatic pressing (HIP), followed by hot rolling and cold rolling to obtain alloy plates. Compared to forging and rolling methods, this approach significantly improves the yield of niobium alloys, reduces costs, and can also be applied to the preparation and production of large-sized niobium alloy sputtering targets.
3.6 Rhenium
Due to their high melting points, high cost, and significant volatilization losses during melting, rhenium and its alloys are predominantly prepared using powder metallurgy methods. For rhenium alloy components, achieving high density requires hot isostatic pressing (HIP). Shi Gang et al. [26] used HIP to produce pure rhenium material and studied its microstructure and mechanical properties. Results showed that HIP-treated pure rhenium achieved a relative density exceeding 99%, with a fine equiaxed grain structure predominantly ranging from 2 to 8 μm. The grains exhibited tight bonding. After high-temperature annealing at 1800°C, the elongation increased significantly, while the tensile strength rose from 1000 MPa to 1180 MPa.
Reference [27] indicates that American Rhenium Alloys has applied HEP technology to the production of rhenium alloy products, successfully fabricating thin-walled components with a wall thickness of 4 mm through high-temperature sintering and HEP.
4. Conclusion
As a near-net-shape manufacturing method, hot isostatic pressing technology has matured significantly over the years, with equipment capabilities steadily improving. In recent years, integration with numerical simulation software has not only significantly shortened production cycles but also enabled precise dimensional control of complex components, reducing subsequent machining allowances. This makes it highly suitable for batch production of high-melting-point, high-value products like rare and refractory metals, substantially lowering costs. Particularly, HESP demonstrates distinct advantages in manufacturing complex-shaped products from rare and refractory metals. Through further research and development, its applications in this field will become increasingly widespread.
References: Powder Metallurgy Industry Vol. 27 No. 3; Application of Hot Isostatic Pressing in the Preparation of Rare Refractory Metal Products; Lin Xiaohui, Li Laiping, Li Bin, Liang Jing, Xue Jianrong, Zhang Xiaoming
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