An overview of hot isostatic pressing

Journal of Materials Processing Technology, 30 (1992) 45-65

45

Elsevier

An overview of hot isostatic pressing N.L. Loh and K.Y. Sia School of Mechanical Production Engineering, Nanyang Technological University, Singapore (Received October 29, 1990; accepted in revised form February 13, 1991 )

Industrial Summary Hot isostatic pressing (HIP) is a manufacturingprocess that involves simultaneous application of high temperature and pressure. It was invented in 1955 for diffusion-bondingapplications in the nuclear industry and has since found numerous applications in other fields. This paper provides an overview of hot isostatic pressing, looking into the various HIP process steps, temperature and pressure cycles, and the equipment used. Consideration is also given to the application of HIP to the healing of castings, diffusion bonding, powder metallurgy and ceramics. Finally, the latest trends in equipment, research and application are discussed.

1. Introduction Hot isostatic pressing (HIP) involves the simultaneous application of isostatic pressure and elevated temperature to a workpiece, which results in the workpiece (usually powder) becoming consolidated. The pressure medium used is an inert gas such as argon or nitrogen, which is pumped into a pressure vessel and pressurised to up to 200 MPa, whilst a furnace in the vessel produces temperatures of up to 2000 oC. The workpiece is usually encapsulated in an evacuated capsule of sheet metal, ceramic or glass. Care must be taken in the design of the capsule and during the filling operation, to avoid distortion under compression. In the case of castings, the surface of the workpiece serves as its own capsule: encapsulation is therefore not required. A typical HIP cycle is shown in Fig. 1 [1]. A HIP cycle takes into consideration the material being HIPped, the desired results and the capability of the equipment. For a specific material, the process parameters need to be optimised. HIP processing offers several advantages as compared to conventional processes [2 ], many of which are a result of the isostatic nature of the applied pressure: (i) powders are consolidated to higher densities at lower temperatures; (ii) highly complex shapes can be processed; (iii) HIPped parts have homogeneous density;

0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

46 15001

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(iv)

the high gas density facilitates heat transfer, resulting in rapid heating and shorter cycle times; and (v) brittle materials can be processed because of the more uniform heating. Major applications of HIP are in the areas of powder metallurgy, ceramics, healing of castings and diffusion bonding. 2. The HIP process

2.1. HIP cycles HIP involves the application of high temperature and pressure cycles, of which there are four basic types [3], as shown in Fig. 2. Cycle 1. A cold-loading cycle, where the temperature is increased some time after the pressure, with both reaching their peak at the same time: this gives good geometric control in sheet metal encapsulation. Cycle 2. A hot-loading cycle, where the pressure is applied after the temperature has reached its desired value: this is particularly important where glass encapsulated products are concerned, as early application of pressure will crack the brittle glass encapsulation. Cycle 3. Where the temperature is raised only after the pressure reaches its desired value, such that the recrystallisation of powder particles is enhanced through plastic deformation, enabling the use of a lower temperature. Cycle 4. A cost-effective cycle, where the pressure and the temperature are increased simultaneously, to reduce the processing time.

2.2. Process tooling In the HIP process, the tooling required is simply the capsule, which must possess the following characteristics [4 ]: (i) weldability - if made of sheet metal - to facilitate sealing of the capsule;

47 Cycle 1

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(ii) compatibility with the work and furnace materials; (iii) deformability at H I P temperatures; (iv) oxidation resistant when hot-loaded; and (v) easily separated from the work after HIPping. Materials commonly used to fabricate the capsule are sheet metal, ceramic or glass. If mild steel is used, the capsule is usually deep drawn or pressed, but when made from several components, the parts can be TIG-welded. Care is needed in the design of the capsule to avoid the occurrence of distortion during pressurisation. Ceramic investment moulds are used sometimes, but costs may prove to be prohibitive. Glass capsules ae used mainly for ceramic applications, the glass particles being coated onto the green body and sintered during the HIP cycle: this is a flexible method for encapsulating green bodies formed to near-net shape.

2.3. The process [or powder metallurgy The process flow for powder metallurgy involves powder filling, encapsulation, HIP and capsule removal. Powder filling is usually performed in a vacuum or in an inert atmosphere. The capsule is vibrated to improve the fill density, which latter should achieve at least 68% of theoretical density to facilitate uniform shrinkage. After filling, the capsule is vacuum-sealed: this is necessary as HIP relies on differential pressures to consolidate the powders or materials to be processed.

48 Additionally, vacuum sealing removes air and water vapour which may lead to oxide formation and hindering of the sintering process. To ensure a good seal, leak testing is usually performed, normally done with vacuum in the capsule and helium gas as a tracer. Failure to achieve a good seal may result in the pressure medium entering the container during pressurisation, which will affect the densification process adversely. Leaks may also cause workpiece contamination from exposure to, and reaction with, the pressure medium. Finally, after depressurisation and cooling, the workpiece is removed from the H I P vessel and the capsule is removed, removal being achievable by machining, pickling or sand blasting.

2.4. The process [or castings For castings, H I P is applied to remove internal porosity. In this application, no capsule is required, as the outer surface of the casting acts as its own capsule. Where the pores are contiguous to the surface, welding can be employed to seal the surface before HIPping. 3. HIP equipment

The first HIP units were of the hot-wall type designed with the furnace surrounding the vessel. Owing to creep of the heated vessel, both the temperature and the pressure were limited, to maxima of 700°C and 200 MPa, respectively. To overcome these limitations, a cold-walled system was developed subsequently, with the furnace inside the vessel. A heat-shield placed between the furnace and vessel limited the pressure vessel components to a temperature of 80 °C to 100 ° C. Presently, typical H I P units operate from 500 °C to 2200 ° C, with pressures from vacuum up to 200 MPa, and generally consist of a pressure vessel, a furnace, a gas system, a power supply, instrumentation and controls and auxiliary systems. Figure 3 [5] shows the general layout of a H I P system.

3.1. Pressure vessel In the design of the pressure vessel, the most important considerations are the maximum allowable stress for the designed service life, the fatigue life and the creep life. There are 3 basic types of vessels [4,6]: (i) monolithic with threaded end-closures; (ii) monolithic with non-threaded end-closures and a separate yoke frame; and (iii) prestressed wire-wound with non-threaded end-closures and a separate yoke frame. Monolithic vessels with threaded end-closures (Fig. 4 ) [ 7 ]. The threaded endclosures mean that consideration must be given to the end loads, which are

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proportional to the square of the vessel diameter. Of particular concern is the design of the thread: these have to be designed such as to avoid stress concentration on the first two threads and at the thread grooves. Problems with galling and fatigue in these regions have been observed in some designs. An elegant solution is the use of a resilient thread insert made from a long helical spring wound on a steel core-rod. Monolithic vessels with non-threaded end-closures and a separate yoke frame. These vessels eliminate the problems associated with threaded end-closures by using an external yoke frame to contain the axial forces. This method simplifies cover handling and minimises stresses by there not being threads on the inner wall of the vessel. Pre-stressed wire wound with non-threaded end-closures and a separate yoke frame (Fig. 5) [6]. This design keeps the cylinder under compression, even at the top operating load, thereby reducing fatigue risks. If the vessel should burst, the windings would help to contain the blast and fragments. Inspection of the

50

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Fig. 4. Monolithic pressure vessel with threaded end-closures.

vessel is also simpler and the metallurgical quality of the components is also easier to control.

3.2. Furnace During HIPping, a controllable hot work-zone is established and maintained by the furnace. The furnace provides the heat required either by convection or radiation. Convection is the primary means of heat transfer, with radiation becoming significant at elevated temperatures. A heat shield protects the vessel from the furnace heat and prevents contact between the inner vessel wall and the hot gas.

3.2.1. Types of furnace Figure 6 shows, schematically, the three basic types of furnace normally used in HIP [5,7]. Radiation furnaces. These are multi-level, multi-zone type furnaces, with the heating element surrounding the workpiece. Natural convection furnaces. These heat the gas in the furnace element area, the gas then being convected into the work zone, a convection liner helping to

51

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52

create a path for the flow of gas. Gas circulation continues until the temperature equalises. The advantage of convection furnaces is that the workpiece is not subjected to direct radiation. In this design, for a given vessel diameter, a larger work zone would also be available. Forced-convection furnaces. These are similar to the natural convection type, except that the gas flow, and hence heating/cooling rate, is accelerated by a fan.

3.2.2. Furnace elements There are three major types of furnace element, each with different operating temperatures and properties that determine the cold- or hot-loading capability of the unit. Kanthal A-l, Hoskins 875 ( < 1230°C). Kanthal A-1 (Fe - 22-23Cr - 4.55.7A1 - 0.5-7.0Co ) or Hoskins 875 (Fe - 22.5Cr - 5.5A1 - 0.5Si) has the advantage that it may be exposed to air at elevated temperatures. Its use as a furnace element can reduce H I P cycle times significantly by allowing hot loading and unloading. Molybdenum ( < 1600 ° C ). Molybdenum oxidises rapidly on exposure to air at elevated temperatures and it can be used only in cold-loading systems. Embrittlement at ambient temperatures can occur during extended use, due to recrystallisation. Graphite ( < 2200 oC ). Graphite has excellent strength and dimensional stability at elevated temperatures. W h e n fibre-reinforced, it can be extremely rugged, reducing damage during load/unloading operations. It can also be used

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Fig. 8. HIP system configurations (after [7] ). for sinter-HIP processing and run on nitrogen gas. The disadvantage is that it generates carbon monoxide by reaction with water vapour, which results in carburization of the workpieces.

3.3. Gas-handling system Argon is perhaps the gas that is most commonly used as a pressurising medium, but helium and nitrogen have been employed also. More recently, argon-

54 oxygen mixtures have been used for the HIPping of oxide ceramics. Figure 7 [ 7 ] shows a typical gas-handling system. The latter must deliver the gas quickly at the desired pressure (100-400 MPa) and must also be of high purity to protect the work from contamination: gas dialysis equipment may be required to maintain the purity. The design of the compressor is an important consideration also. Diaphragm compressors are commonly used for up to 100 MPa, providing a very clean operation and allowing the gas to be re-used with 5% loss for each cycle, but gas purity still needs to be monitored with an on-line gas chromatograph. Non-lubricated piston compressors are used up to 400 MPa at a fairly high rate, without contamination. Liquid argon pumps are also available, with the advantage of supplying very pure argon from liquid storage. Unfortunately, the gas cannot be re-liquified easily and thus is completely lost, for which reason liquid argon pumps are used only where gas purity is of paramount importance.

3.4. Control system The control system integrates all the components into a functional unit. The heart of the control system may simply be a relay logic, a programmable controller, or a mini- or micro-computer: these allow for different levels of control. At the lowest level, a continuous operator interface is required to initiate each step of the HIP cycle, whilst the highest level, the HIP cycle is completely automated. Additional tasks which may be performed by the computer are data logging, error diagnostics, process parameter programming and maintenance scheduling. 3.5. Auxiliary systems Among the auxiliary systems are the cooling system, the vacuum system and the material-handling system. The cooling system keeps the pressure vessel within limits, whilst the vacuum system provides a means for removing the atmospheric contaminants from the furnace or vessel. Figure 8 [7] summarises the various possible HIP system configurations. 4. Applications HIP was first applied to pressure bond or diffusion bond fuel elements. However, its potential in a number of unrelated areas was recognised quickly, and it was applied subsequently to the following major areas: (i) Castings. Removal of internal casting defects and rejuvenation of service-damaged castings; (ii) Diffusion bonding. Bonding of similar and dissimilar materials; (iii) Ceramics. Sintering and densification of ceramics;

55

(iv) (v)

Powder metallurgy. Sintering and densification of cemented carbides, metal powders and the production of near-net-shape components; and Other applications. Manufacture of carbon-carbon composites and posttreatment of thermal spray coatings.

4.1. Castings 4.1.1. Removal of internal casting-defects The fabrication of investment castings is still very prone to the development of residual internal defects despite advances in casting technology, such defects resulting in a reduction and increased scatter in high-cycle fatigue strength and creep resistance. The statistical distribution of mechanical properties of a casting is shown in Fig. 9 [ 2 ]. HIPping of the casting can increase the average level of the properties significantly and reduce the statistical spread. HIP is now used extensively for the full densification of investment castings. The closure of voids by HIP can, in most cases, be carried out successfully given the correct conditions of temperature, pressure and dwell time, these being decided on the basis of experiment and available high-temperature creep data. In HIP cycles for castings, the restrictions on the temperature range are influenced by the strength of the material (minimum level) and the occurrence of incipient melting (maximum level). If the pores are vacuous, healing takes place by void closure, followed by bonding of the two surfaces. If the pores are filled with a single gas or a mixture of gases, then there are several possibilities: if soluble, the gas may dissolve in the metal; if insoluble, it may react with the metal to form a non-metallic inclusion; or the gas may also simply be compressed without reaction.

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Fig. 9. Statistical distribution of mechanical properties: HIPped castings versus conventional castings (after [2] ).

56

Effective void-closure has been demonstrated in low-alloy steels [8], titanium [9], superalloys [10] and aluminium alloys [11 ].

4.1.2. Rejuvenation of serviceparts A significant factor in the cost of ownership of industrial, marine and aircraft gas-turbine engines is the high price of replacement for parts that have reached the limit of their original design life. Many parts, particularly those operating in the hot sections of gas turbine engines, will be replaced on a routine basis, even though they may have many safe operating hours left. Gas-turbine blades operating at high temperatures and stresses incur damage that can reduce their mechanical properties and their reliability. In the case of marine gas-turbines, exposure to salty atmospheres increases the problems associated with oxidation and sulphidation. On the other hand, landbased turbines operate on relatively clean fuel and salt-free atmospheres: they tend to operate for longer times under relatively stable conditions and problems due to thermal fatigue are usually less severe. Generally, the damage incurred may be classified as external or internal, the former including hot corrosion, oxidation/erosion, thermal fatigue cracking, and impact or foreign-object damage. Microstructural damage and structural discontinuities are classified as internal damages. With HIP, it is possible to give service parts a further heat-treatment under pressure so that the precipitate structures are restored and internal damage is repaired. Some success has been claimed also for the repair of fatigue-induced damage. Creep damage. It has been established that HIP is capable of healing casting defects such as shrinkage cavities, hot tears or micropores, hence it is expected that HIP processing can also close creep voids in service-exposed parts: this has been documented extensively by many researchers [12-15]. Nickel-based superalloys are strengthened by the presence of precipitated Ni3A1 (7') in the grain interiors and the intermetallic 7'-phase and carbides are also precipitated along grain boundaries, thereby raising creep resistance. Elevated service temperatures over long duration, however, can alter the microstructure, the 7' -particles coarsening and becoming elongated in the direction of loading. Grain-boundary carbides also agglomerate, forming continuous brittle films. Creep-induced cavities may also form at grain boundaries of transverse orientation with respect to the direction of applied stress, the linking of creep voids then leading to microcrack formation. These changes may be reversed by a combination of HIP and further heattreatment. The HIP pressure heals the structural discontinuities in a manner similar to that for the healing of castings. The selection of the HIP temperature and further heat-treatment cycle must take into consideration the formation of serrated grain boundaries, carbide precipitate morphology, and the size and shape of the 7'-particles.

57 HIP Blade set Service hours A 45000 B 43000 C 59OOO D 59000 E 35000

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58 Figure 10 [ 16 ] shows the increase in stress rupture properties of Inconel X750 blades after HIP re-heat treatment, the extension in creep life under different stress/temperature combinations being demonstrated in Fig. 11 [16]. Fatigue damage. In the area of fatigue damage, HIP has been attempted also on Ti-6A1-4V fatigue bars pre-damaged above and below the crack initiation point [2], the samples being coated by physical vapour deposition with Ti6A1-4V and submitted to a HIP cycle. Although a statistically significant increase in fatigue life was demonstrated, full fatigue life was not restored, metallographic examination showing that there was partial healing only.

4.2. Diffusion bonding The first application of HIP was the gas pressure bonding of fuel elements, but HIP has been used also to bond metal-matrix composites [ 17 ], corrosionresistant sheets to turbine blades, air foils, and wear-resistant layers to valves and cylinders [ 18 ]. Its chief advantages are: (i) stresses on the structure are compressive and balanced; (ii) complex-shaped components can be bonded without distortion problems; (iii) adequate metallurgical bonds can be achieved; (iv) close dimensional control is possible; (v) many similar and dissimilar materials can be joined, usually in a singlestep operation. The surface area to be joined is not crucial, the main limitation being the capacity of the HIP unit; (vi) brittle materials can be joined without fracture, which may occur with conventional techniques; and (vii) fabrication costs are lower than for conventional processes. Surfaces to be bonded first need to be prepared, the surfaces generally having to be machined flat and degreased to remove surface films (oxide, organic or aqueous etc). Machining produces a cold-worked layer which will lower the recrystallisation temperature at the interface. When the two surfaces are brought together under pressure and high temperature, plastic flow of the surface asperities will take place. Metallic bonding may occur and surface films are seriously disrupted. If the contact time is extended, further deformation may take place by creep. Finally, a diffusion-controlled elimination of the original interface will occur, which may involve recrystallisation, surface diffusion, atomic transport and surface-film or oxide dissolution by the base metal. In some cases, the inter-diffusion of the dissimilar materials changes the interface composition such that a liquid phase forms: this is known as diffusion brazing. Diffusion bonding can be performed either directly or with an intermediate material between the two materials to be bonded. Intermediate materials serve to increase diffusivity at lower temperatures, promote plastic flow at lower pressures, prevent inter-metallic compound formation and ensure clean surfaces.

59

4.3. Ceramics

The application of pressure at the sintering temperature accelerates the kinetics of densification by increasing the contact stress between particles and by rearranging the positions of particles to improve packing. It has been established that the energy available for densification is increased by the application of pressure during sintering, which yields the following advantages: (i) it reduces densification time; (ii) it can reduce the temperature required for densification, and hence limit grain growth; (iii) it minimises residual porosity; and (iv) it results in higher strength due to minimisation of porosity and grain growth. The above-features were achieved traditionally by hot pressing. HIP, however, has been used increasingly with additional benefits due to the isostatic nature of the pressure applied, which makes net-shape forming possible and results in greater material uniformity, eliminating preferred orientation. In addition, much greater pressures and temperatures can be used, resulting in greater densification and greater flexibility in the composition of the material. Ceramic HIPping has been applied to lead zirconate-titanate (PZT), yttria partially stabilised zirconia, alumina, silicon carbide, silicon nitride and other important ceramics. The HIPping of ceramics can be performed in a number of ways, as shown in Fig. 12. In the first method, ceramic powder with additives is formed into the required shape by cold isostatic pressing with machining, injection moulding, extrusion or slip casting. To achieve good tolerances in the final part, precision and repeatability of the density distribution is necessary. Forming additives such as plasticisers and binders are then removed from the green body before encapsulation. Glass particles are then applied over the body. Hot evacuation is carried out and the temperature subsequently raised to seal the glass: this

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60 can be done in a vacuum furnace followed by hot transfer to a HIP unit. The glass capsule cracks on cooling and the remnants are removed by sand blasting. This method has the advantage of requiring only a small amount of sintering aids, which normally do not allow for good densification with other methods. Low encapsulating temperatures also reduce decomposition and weight loss. The silicon nitride green body is protected during HIPping from reactions with gas impurities. The second method does not employ any encapsulation. Sintering is done after the green body is formed so as to remove interconnected pores, if necessary the body being machined to remove surface-connected pores. The green body is then HIPped. This method tends to result in significant weight loss affecting composition and tolerances. Nitrogen gas has been used for silicon nitride with some success to suppress decomposition and oxygen partial pressures have been used successfully also for oxide ceramics. In this method, encapsulation costs are saved. 4.4. Powder metallurgy Until the late 1960s, powder metallurgy products were considered to be brittle and thus unsuitable for structural parts. However, as improved powder fabrication methods resulted in purer powders, powder metallurgy found greater applications. Powder metallurgy using HIP processing has the following benefits [ 19 ]. (i) homogeneous material; (ii) isotropic properties; (iii) no segregation; (iv) fine grained and tough structure; (v) complicated parts can be made; and (vi) cost reductions in some applications. The first major production application of HIP was the manufacture of highspeed steel, being made originally by casting. The main problem with this was the presence of carbide stringers and other segregated structures, even in the final rolled or forged product. The use of powder metallurgy techniques reduced segregation. However, compaction and sintering alone produced steel with poor density and strength. Hot extrusion or hot forging was used, therefore, to overcome this. Performance was comparable to the conventional steel except when used in intermittent-cutting operations, which was attributed to the high oxide-content of the powder. Current methods use powder atomised in inert gas to prevent oxidation. This is then consolidated into a billet by HIP followed by hot deformation processes to fabricate the final product. Extremely high standards of surface integrity are required by the cemented carbide industry. High stress concentrations at pores result in low strength with large lot-to-lot variations.

61

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1

Fig. 13. Superalloy processing.

Cemented carbides do not generally require encapsulation before HIPping. Above the solidus, they become pasty masses with a large volume of liquid binder phase. Residual pores are sealed and the part surface acts as its own capsule. HIPping results in a significant decrease of porosity, accompanied by improvement in the average transverse rupture strength by up to 80% [20]. It also exhibits a general decrease in scatter as a result of enhanced homogeneity. Possibly the largest application of HIP and powder metallurgy is in the production of superalloys. H I P was used initially to produce billets for subsequent extrusion or rotary forming followed by isothermal forging to net shape (Fig. 13 ). The improved microstructural homogeneity reduced the number of forging steps required traditionally, thereby cutting down costs, and there was also the advantage of flexibility in the shape of H I P preforms. One of the advantages of the as-HIPped product is the ability to alter strength levels by changing the cooling rate. With forged products, control of the amount of warm work retained in complex-shaped forgings is difficult, if not impossible, so that strength control is therefore not practical. In addition to high-speed steel, cemented carbides and superalloys, HIP has also been applied to titanium and beryllium alloys. 4.5. Other applications 4.5.1. Sinter-HIP This is a pressure-assisted sintering process [21 ] which can be used to process powder metallurgy materials such as cemented carbides, which use liquidphase constituents (usually Co or Ni) as a lubricant or binder during compaction. In the sinter-HIP process, pre-compacted parts are first heated to a low temperature to remove the binders, the evaporated binder substances being con-

62 densed and trapped by a gas-handling system. The parts are then degassed in vacuum at a greater temperature before gases are introduced at a partial-pressure setting. The pre-sintering and sintering steps are then completed. After the peak temperature for sintering, the temperature is reduced and isostatic gas pressure is increased to achieve densification in-situ. The additional steps of cooling the parts in a sintering furnace and transferring to a separate HIP press for subsequent heating and densification are thus eliminated. In this way the two processes of sinter and HIP are combined, which eliminates one major piece of equipment, reduces material handling, provides greater control of the process and saves energy and labour costs. The risk of abnormal grain growth due to the re-heat of a product to liquidous temperature a second time is eliminated also.

4.5.2. Hot isostatic pressure impregnation carbonization (HIPIC) Carbon-carbon composites are light-weight structural materials in which both the matrix and fibres are carbons. They possess both the refractory properties of structural ceramics and the strength, stiffness and toughness of fibrereinforced composites. HIPIC applications include the heat shields of space shuttles, missile nose tips and rocket nozzles. Carbon fibres act as the primary phase, with the interstices filled with a temporary binder material acting as a precursor to the secondary carbon formed during the carbonization process. When pitch is used as the matrix precursor, carbon yields at atmospheric pressure are 50%. Use of HIP pressure to impregnate and densify the composite can increase carbon yield to 90% at 100 MPa [22], this process being known as hot isostatic pressure impregnation carbonization. 4.5.3. Post treatment of thermal spray coatings HIP has been used to treat thermal spray coatings to promote diffusion at the substrate-coating interfaces and internal pores have also been collapsed in the densification process, which has resulted in desirable changes in the coating properties. Improvement in Vickers hardness and bonding strength have been reported in ZrO2 coatings [23] and greater coating ductility, strength and impact resistance have been noted [24]: however, reduction in thermal shock resistance has been reported [25 ] also. 5. R e c e n t a d v a n c e s and the f u t u r e t r e n d o f H I P

There has been a rapid increase in the number of HIP installations over the last few years. In Japan alone, HIP equipment installations have doubled in the three years from 1985 to 1988, with great growth in research and development activities [ 26 ], which latter is likely to continue with increased research into ceramics and composite materials. Areas under development are the con-

63 solidation of Ti-Ni shape memory alloys, densification of glassy carbon, diffusion bonding of fibre-reinforced metals, and pressure impregnations of graphite glass. The recent advent of small laboratory-scale HIP equipment will certainly provide additional impetus to this growth. The high cost of HIPping has been a major obstacle to its wider use and this will continue to spur efforts to decrease processing costs whilst retaining the benefits of HIP. Researchers and equipment manufacturers have in general taken three approaches to this problem. Firstly, processes have been combined to save capital equipment and process costs. An example of this is the development of the sinter-HIP method. The second approach has been to increase heating and cooling rates, hence reducing cycle times. A new system known as HIP-quenching [27] introduces a rapid cooling method which enables densification and heat treatment to be combined into one operation. A cooling rate of 500 ° C / m i n can be achieved under computer control. The high heat-transfer coefficient of the pressurised gas medium ensures small temperature differentials between the gas and the surface of the part. The resulting uniform cooling gives less distortion and better material structure. This system opens new possibilities for rejuvenation applications. A third approach uses cheaper unconventional means of applying the H I P principles. A case in point is the liquid-HIP or quick-HIP method [28] which significantly reduces cycle times. This method compresses pre-heated capsules in a visco-plastic pressure medium such as oil or grease. A hydraulic press generates pressures of up to 5000 kgf/cm 2 (490 MPa). This pseudo-HIP process reduces the holding time needed to consolidate tool-steel powders from 60 min at pressure and temperature to only 1 min. No significant microstructural differences were reported between quick-HIPped material and those conventionally HIPped. With research demands requiring HIP equipment with extended capabilities, measurement technology has been hard pressed to keep pace. The upper limit of reliability for thermocouples has been quoted at 1700°C, due to recrystallisation of the thermocouple wire and degradation of electromotive force resulting from reaction between the thermocouple wire and the insulation. Spurred by the need for higher temperatures in ceramic applications, two HIP manufacturers have announced new measurement facilities. One method uses optical means to measure the temperature of a black body, the temperature of which corresponds to that of the pressure medium [29 ]: a measurement capability of up to 2600 ° C has been claimed. The other method extends the thermocouple performance to 2700 °C with an accuracy comparable to that of tungsten rhenium thermocouples [27]. These methods will be useful for the manufacture of high-temperature, high-strength ceramics because of the poor sinterability of the latter. High temperature H I P will result

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also in greater strengths through decreased use of sintering aids and reduction of internal flaws. 6. Conclusions HIP has come a long way since its invention in the 1950s. It has found many diverse applications in a wide range of industries. A number of noteworthy adaptations have enabled special applications, such as HIPIC. As equipment capability is extended to greater temperatures and pressures, research into the HIPping of new composites and materials will continue to advance rapidly. The major obstacles to its wider application are the high capital and operational costs. With innovations combining several processes in one piece of equipment, the barriers to commercial application are slowly reducing. For the moment, HIP remains largely a technology for expensive and exotic materials that would be difficult if not impossible to fabricate by any other means.

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P.J. James, Hot isostatic pressing: An economic route to powder components, Met. Mater., (November 1977), pp. 27-31. H.D. Hanes, Hot isostatic processing, in: K.W. Timmerhaus and M.S. Barber (Eds.), High Pressure Science and Technology, (Sixth AIRAPT Int. High Pressure Conf., Boulder, CO, July 25-29, 1977), Vol 2: Applications and Mechanical Properties, Plenum Press, New York, 1979, pp. 633-650. L. Buekenhout and P. Alt, Hot isostatic pressing of metal powders, Key Eng. Mater., 29-31 (1989) 207-224. H. Fischmeister, Isostatic hot compaction - A review, Powder Metall. Int., 10{3) (1978) 119-123. F.X. Zimmerman and W.H. Walker, Hot isostatic pressing equipment development, Proc. 2nd Int. Con[. on Isostatic Pressing, (A Metal Powder Report Conference, Hilton International, Stratford-upon-Avon, UK, 21-23 September 1982), Vol. 2, MPR Publishing Service, 1983, pp. 22: 1-23. U. Odebo, New generation hot isostatic press, ibid., pp. 25: 1-26. F.X. Zimmerman and W.H. Walker, Hot isostatic pressing: Processes and equipment, in P.J. James (Ed.), Isostatic Pressing Technology, Applied Science Publishers, London, 1983, pp. 169-201. B.A. Rickinson, HIPping brings new horizons for casting manufacture, Met. Mater., February 1985, pp. 104-107. M.S. Misra, S. Lemmeshewsky and D. Bolstad, The effects of weld-repair and hot isostatic pressing on the fracture properties of Ti-5A1-2.5Sn ELI castings, in: D.F. Hasson and C.H. Hamilton (Eds.) Advanced Processing Methods for Titanium, The Metallurgical Society of AIME, 1982, pp. 161-174. H. Burt, J.P. Dennison, I.C. Elliot and B. Wilshire, The effect of hot isostatic pressing on the creep and fracture behaviour of the cast superalloy Mar M002, Mater. Sci. Eng., 53 ( 1982 ) 245-250. E.L. Rooy, Improving casting properties and integrity with hot isostatic processing, Mod. Casting, (December 1983) 18-20.

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