Isostatic-pressing

FIGURE 13.41 Schematic of dry bag isostatic mold for an automotive spark plug. Radial normal stress is used to compress the powder against the metal conductor placed at the central axis of the mold. 

In all these isostatic pressing methods, the pressure is applied uniformly to the surface of the green body because the rubber mold deforms to follow the compaction of the powder. In addition there is little or no wall friction between the powder and the rubber mold. As a result, the force balance given by A t = 0 for (ylindrical coordinates gives, for the radial component. 

At a particular point, this force balance shows that the radial normal stress, Trr, or applied pressiure is related to the angular normal stress, Tgg, and the two radial shear stresses, and r g. Under load, the particles in a volume element will density, if the Coulombs yield criterion Tij = Tii tan 8 + c has been exceeded. Therefore, we find that 

This constant radial normal stress is also a constant for all radii and therefore results in amazingly uniform packing density of isostatic 

FIGURE 13. Comparison of the uniformity of green density of a thin walled crucibU made by (a) isostatic pressing and (b) uniaxial die pressing. The numbers indicated are the densities (gm/cc) of the various zones. Data taken from Gill and Bryne [90]. 

When the economic merit of isostatic pressing is evaluated, the final product must be considered and not the shaping operation alone. 

the advantages are in a better product with more uniform structure and/or reduction in final machining requirements (production of near-net-shape parts). 

The first isostatic pressing technique was described by Madden in a US patent assigned to the Westinghouse Lamp Co. It was developed to overcome the limitations of billets compacted in dies. Madden claimed that isostatically pressed billets were uniformly compacted, devoid of strata, and possessed sufficient green strength to permit handling. 

Further patents were taken out on the isostatic pressing of refractory metal powders by Coolidge in 1917 (for tubes of tungsten and molybdenum) and by Pfanstiehp82 in 1919, while Fehse described the wet bag isostatic pressing of tungsten tubes in 1928. However, very little further development appears to have followed. 

By 1942 most of the reasons for the preference of isostatic pressing for certain applications had been recognized and the basic principles, which are still in common use today, had been established. Materials that had been pressed included ceramics, metals, and cermets. 

Many of the difficulties encountered in dry-pressing can be avoided by some form of isostatic-pressing. Ideally, this simply involves the application of hydrostatic pressure to powder in a flexible container. Powder movement is minimal and side-walls are absent. In practice shapes are often produced by the use of rigid mandrels as illustrated in Fig. 3.4. Powder is weighed into a rubber bag with a rigid mouth and a mandrel is then inserted and makes a seal with the mouth. Pressures of 20-280 MPa ( 200-2800 atm) can be applied through either liquid or gas media. The pressure must be released slowly since the air originally 

Cn combined with the green machining method can be applied to fabricate tubular membranes with one dead-end [27]. The uniform membrane precursor is first pressed into green cylindrical rods. A carbide bit is used to drill the rods into dead-end geometry. The dead-end tubes are then sintered and annealed at elevated temperatures into dense membranes. 

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Ewsuk K G 1986 Finai stage densification of aiumina during hot isostatic pressing PhD Thesis The Pennsyivania State University... 

Ewsuk K G and Messing G L 1986 A theoreticai and experimentai anaiysis of finai-stage densification of aiumina during hot isostatic pressing Hot IsostatIc Pressing Theories and Applications ed R J Schaefer and M Linzer (Materiais Park, OH ASM internationai) pp 23-33... 

Cold isostatic pressing Cold press molding Cold solvent cleaning Cold War... 

More complex shapes can be made by cold isostatic pressing (CIP). CIP uses deformable mbber molds of the required shape to contain the powder. The appHcation of isostatic pressure to the mold suspended in a pressure transfer media, such as oil, compacts the powder. CIP is not as easily automated as uniaxial pressing, but has found wide appHcation in the preparation of more complex shapes such as spark plug insulators (26). 

Quick Opening Devices. Breech block, tapered or interrupted thread, or pinned closures are often used when an end cover has to be removed quickly, as with some isostatic presses (126,136), or to enable the end cover to be removed easily after the vessel has been heated to high temperatures. 

P. J. James, ed.. Isostatic Pressing Technology, AppHed Science PubHshers, London, 1983, p. 214. 

In contrast to the cold isostatic pressing process, the hot process can readily employ powders having spherical or noninterlocking particles. The powder is simply poured in and vibration packed into a container of desired shape. The powder mass is then simultaneously compacted and bonded during the treatment. 

P/MForging. Even after conventional repressing of a P/M component, it is stiU difficult to increase density above 95%. However, hiU density in a P/M part improves its properties. Hot isostatic pressing in autoclaves works weU, especiaUy for titanium and superaUoy components, but the capital equipment is expensive and production rates are slow. 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Isostatic pressing gives a highly uniform product, although the production rate is somewhat low. It typically contains very small grains and Uttle or no porosity. In this process, a mbber sock or bag of the desked shape is filled with the refractory mix. The sock is then subjected to extremely high pressure in a hydrauUc pressure chamber. 

Cold-isostatic-pressing foUowed by vacuum sintering or HIP is also used to manufacture smaller intricate shapes. In this instance beryUium powder is loaded into shaped mbber bags and pressed isostaticaUy in a pressure chamber up to 410 MPa (60,000 psi). After the pressing operation the mbber bag is stripped from the part which is then vacuum sintered to about 99% of theoretical density at about 1200°C. If full theoretical density is required, the sintered part may be simply given a HIP cycle because there is no open porosity after vacuum sintering. In a similar manner, conventional axial cold-pressing... 

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