Hot and Cold Isostatic Pressing of Ceramics
HIP and CIP technologies enable ceramic manufacturers to control material properties and increase productivity.
Hot isostatic pressing (HIP) and cold isostatic pressing (CIP) technology has been known for more than 50 years, and is considered today to be a standard production route for many applications. The HIP process applies high pressure (50-200 MPa) and high temperature (400-2,000°C) to the exterior surface of parts via an inert gas (e.g., argon or nitrogen). The elevated temperature and pressure cause sub-surface voids to be eliminated through a combination of plastic flow and diffusion. The challenge is to reach the highest possible theoretical density while maintaining productivity goals.
The CIP process applies an even higher pressure to metal or ceramic powders, typically 100-600 MPa at ambient temperature or slightly elevated temperatures
(< 93°C), to achieve “green” parts with enough strength to be handled, machined, and consequently sintered to final strength. Uniform rapid cooling* is a process by which thin-walled pre-stressed wire-wound HIP units increase productivity up to 70% compared with natural cooling, and increase the density to ~ 100% of theoretical density for many alloys. The added cost to reach this density is around $0.20-0.30/kg for a large production HIP system, depending on the material.
HIP technology was introduced in the early 1950s and has since gained interest for many applications. HIP is a fabrication process for the densification of castings, consolidation of powder metals (as in in metal injection molding or tool steels and high-speed steels), compaction of ceramics for dental and medical parts, additive manufacturing (3-D printing), and many more applications.
Today, about 50% of all HIP units are used for the consolidation and heat treatment of castings. Typical alloys include Ti-6Al-4V, TiAl, aluminum, stainless steel, nickel super alloys, precious metals like gold and platinum, and heavy alloys and refractories like molybdenum and tungsten. More applications will likely come rapidly due to the increased interest in the additive manufacturing of ceramics for aerospace and automotive applications.
The pressure applied in a HIP is generally between 100 and 200 MPa using pure argon gas. However, both lower and higher pressure can be used for some special applications. Other gases like nitrogen and helium are also used, while gases like hydrogen and carbon dioxide are more seldom put into use in production units. Combinations of these gases can also be used. The application determines which gas is used for which purpose, especially since helium is quite expensive compared with argon/nitrogen and hydrogen in incorrect concentrations is very explosive.
The parts to be HIPed are initially heated either at elevated pressure or in vacuum. Introducing the gas early in the process, and while heating, causes it to expand and help to build up the pressure in the HIP furnace more effectively. The material composition and suggested HIP cycle govern the startup procedure. The three main advantages of HIPing include:
Increase in Density
- Elimination of internal porosity for defect healing of castings
- Longer lifetime of HIPed parts
- Predictive lifetime
- Lighter and/or low-weight designs
Improvement of Mechanical Properties
- Fatigue life increased up to 10 times, depending on the alloy system
- Decrease in variation of properties
- Increase in ductility and toughness
- Form metallurgical bond between dissimilar materials (diffusion bonding)
More Efficient Production
- Decreased scrap/loss
- Less or no non-destructive testing (NDT)
- Freedom to choose casting methods for optimal productivity
Cold isostatic pressing (CIP) is a manufacturing process for the consolidation of metal and ceramic powders. For metals, around 100% theoretical density can be achieved, while ceramic powders that are more difficult to compress can reach about 95% of theoretical density. The CIP uses a liquid media, such as water or an oil or glycol-mixed water, to apply pressure to the powder. The powder is contained in a mold that does not change shape, but preserves the shape from the mold. The mold also protects against liquid penetrating into the powder (see Figure 1).
The very high pressure enables the voids in the powder to be smaller or eliminated, through high compaction forces that will make metal powder deform due to its ductility and ceramic powders to likely crumble somewhat so that density increases and the end-product is a “green” part that can be handled, machined and sintered (see Figure 2). Typical pressures range from 100-600 MPa, and the temperature is usually at room temperature. If an application requires elevated temperatures, a heat exchanger can bring up the temperature to about 93°C. The risk of boiling increases at higher temperatures due to the temperature increase when water is compressed, about 4°C per 100 MPa.
Common applications for CIP include the consolidation of ceramic powders, compression of graphite, refractories and electrical insulators, and other fine or advanced ceramics for dental and medical applications. Material systems include silicon nitrides, silicon carbides, boron nitride, boron carbide, titanium boride, spinels, etc. The technology is expanding into new applications such as the pressing of sputtering targets, coatings of valve parts in an engine to minimize wear of the cylinder heads, telecommunications, electronics, aerospace and automotive. The three main advantages associated with the CIPing of ceramics include:
60-95% of Theoretical Density
- Consolidation of ceramic powders
- “Green parts” for ceramics are ready for next process steps
- Elaborate designs of ceramics by CIM (ceramic injection molding)
Improved Material Properties
- Increased mechanical properties (e.g., powder compacts of alumina and electrical insulators)
- Reduced scatter of data for safest control of production
- Highest properties for minimal corrosion deterioration
Safer and More Efficient Production
- Fastest method for consolidation of ceramic powders to high density
- Lowest cost/kg
Like the cooling in a HIP, the decompression in a CIP is what sets the quality of the “green” compacts. Since the metal or ceramic powder is compacted, air is entrapped between the grains and the gas pressure increases due to the outer applied compaction pressure. A metal compact has a very high “green” strength and ductility; the productions steps that follow CIPing, like sintering or HIPing, will release the entrapped air (see Figure 3).
However, a ceramic “green” compact is more brittle due to the nature of ceramic powders. If the pressure is released too quickly and in an uncontrollable manner, the ceramic compact will likely crack where the air cannot escape. The way to avoid this is to release the applied pressure in a controllable fashion through a fine-tuned decompression system. This is especially important at lower pressure, when the applied pressure is equal to the internal gas pressure, and the entrapped air will insert internal stresses (see Figure 4).
Uniform Rapid Cooling and Quenching
The demand from industry has always been to have shorter cycle times and thereby increase the productivity for the HIP unit in order to achieve better payback for the investment. The HIP pressure vessel itself plays an important role in increasing the cooling rates for the efficient removal of the heat generated in the furnace. The wire-wound technology shown in Figure 5 provides several advantages to enhance this heat removal.
The heat removal is achieved by the thin-walled pre-stressed wire-wound cylinder; the wire-winding and an internally water-cooled liner produce an effective heat exchanger between gas-gas and gas-water. Without this thin-wall solution, the cooling rates would be significantly lower and true rapid cooling could not be achieved. An extra advantage with rapid and uniform cooling is that the wear life of the furnace is increased dramatically, thereby substantially lowering the costs associated with maintenance and spare/wear parts.
The gas cooling rates can be over 3,000°C/min, which is called uniform rapid quenching.** The pressure is controlled and maintained through the full cooling time. After cooling, the temperature is allowed to increase again to heat treat the material and enable optimal grain growth before the final part cooling is done. Pressure is maintained during the whole cycle, which aids in achieving optimal grain size and reducing the risk of cracking and spalling.
With this procedure, heat treatment becomes very advantageous in terms of softening, annealing and even tempering. The outcome is a better-quality material that results in lower costs by reducing the scrap rate and shorter lead times due to less re-work. In addition, parts can be heat treated in the same furnace, thereby eliminating separate handling and additional steps for heat treatment (e.g., heating and subsequent quenching in water, oil or salt baths). This lowers the total capital investment, as well as the running costs.
In many metal alloy systems, avoiding detrimental phases, such as the sigma phase in stainless steel or phase transformation between α- and β-phase in titanium, is crucial. By rapidly cooling the parts down into the safe region of the phase diagram, no detectable levels of these phases can be measured. Without rapid cooling, an increased level of mixed phases will negatively affect the material properties, grain growth and formation of oxides, carbides, and nitrides at the grain boundary.
Since ceramics are more fragile and far less ductile than metals, the uniform rapid cooling function is all about temperature control. For a ceramic part (e.g., nitrides, carbides, borides, spinels, etc.), having control of the cooling rate and the cooling curve is equally or more important than speed. The brittleness of the ceramic part makes it essential to make time for the cracking and re-stacking of ceramic powders in the CIP to move below 40% porosity. Sufficient time in the HIP is also necessary in order to form necking and pore closure, so the density can increase to close of the theoretical density (above 99% of theoretical density).
Productivity and Cost
Considerably shorter cycle times in a HIP can be achieved with uniform rapid cooling. The obvious target is to increase productivity, which lowers the parts’ cost by decreasing processing costs and investment depreciation time. The cycle time can be reduced by as much as 70% (see Figure 6). For example, for a small- to medium-sized HIP unit, it is possible to run two cycles per 8-hr shift, instead of one cycle per shift (with natural cooling), and have time to achieve heat treatment in the HIP.
The operating cost of HIPing has reduced drastically during recent years. This is due not only to uniform rapid cooling, but also to less maintenance-sensitive solutions, particularly of the furnace. For a large production HIP unit used for powder capsules, consolidation of large forgings and casting, the cost per kg HIPed material is around $0.20-0.30. This should be compared with the cost and time spent on NDT, X-rays, weld repairs and higher scrap rates, which to a large extent can be excluded when HIPing is used.
HIP and CIP technologies provide manufacturers with opportunities to control their material properties and increase productivity. Productivity is increased two-fold through the use of uniform rapid cooling. The quality of ceramic “green” compacts is greatly improved through the use of fine-tuned decompression systems. In addition, combining HIP and heat treatment in the same equipment results in shorter lead times, better material properties, and processing and investment cost savings.