Isostatic Pressing

Isostatic pressing allows more uniform density compared to uniaxial compaction in rigid dies. These methods rely on flexible molds for application of pressure in all directions, which reduces friction and allows compaction of compact shapes.

Cold Isostatic Pressing. CIP uses a flexible membrane to isolate the powder from a liquid medium that is pressurized to cause densification of the powder. Typical mold materials are latex, neoprene, urethane, polyvinyl chloride, and other elastomeric compounds. Because the mold moves with the powder as it densifies, friction effects are minimized. Also, because the pressure is applied uniformly around the mold, there is no theoretical size limit. Height-to-diameter and overall size are limited by the pressure vessel size. Often a rigid mandrel is part of he tooling; and, because powder must slide along this mandrel, it is coated with a friction reducing material.

In comparison to die pressing, cold isostatic pressing can achieve more uniform densities due to minimized friction effects. Pressure vessels are typically limited to pressures of 415 MPa (60 ksi) although units with twice this capacity have been produced. Isostatic pressing equipment can be automated (i.e., dry bag CIP units), but the production rates are lower than those of die pressing. Dimensional control is generally not as tight as with die pressing due to flexible tooling. As stated, however, rigid members can be incorporated into the flexible mold assembly to produce accurate surfaces where desired.

Hot isostatic pressing (HIP) is a versatile near-net shape process that has found niches in the production of aerospace structure and engine markets, high alloy and tool steel mill shapes and individual components, titanium hardware, and monolithic and composite alloy components for the energy industry. The process fundamentals and manufacturing steps are covered in detail in the article "Hot Isostatic Pressing of Metal Powders" in this Volume.

The aim of hot isostatic pressing is a near-net shape and full density. Powder is hermetically sealed in a container that is flexible at elevated temperatures; the "canned" powder is heated within a pressurized vessel and held for a specified time. Commercially used containers include low carbon steel sheet formed into a container, stainless steel sheet, and even glass. The pressurized medium is usually an inert gas such as argon, and pressures range between 100 and 300 MPa (15 and 45 ksi). The temperature for HIP is material dependent, of course, but typical production equipment can heat parts 1000 to 1200 °C (2000 to 2200 °F). HIP units for ceramics and carbon-base materials may heat up to 1500 °C (2700 °F). Densification mechanisms active during HIP include bulk deformation (limited amount), sintering, and creep, with the latter accounting for a significant portion of densification. Densities >98% of full density are typical, and full density is routinely achievable with care during powder sealing and strict control of time, pressure, and temperature.

The powders used in hot isostatic pressing are usually spherical in shape and very clean. The particle surfaces are free of contaminants, such as oxide films. The sphericity facilitates can loading and handling, and the particle surface cleanliness facilitates particle bonding. Powder handling and avoidance of contamination is critical to the success of the process, and considerable investment in facilities and equipment, followed up by attention to operating procedures and "good housekeeping," is required.

In comparison to hot pressing where only billet shapes are produced, hot isostatic pressing is capable of producing complex shapes. As in cold isostatic pressing, the achievable dimensional tolerances are at best near-net due to the flexible mold. Some net surfaces may be achieved if rigid members are incorporated into the mold. Figure 6 compares HIP capabilities with other compaction methods.

Density, %

Fig. 6 Application areas of HIP based on part size, complexity, and level of densification three variables that dictate the P/M approach are size, density, and performance (as a percentage of wrought). This behavior corresponds to ferrous-base P/M systems, but is representative of many P/M materials. P/S, press and sinter; reP, press, sinter and repress; P/S + F, press and sinter and forge; CIP + S, cold isostatically press and sinter; HIP, hot isostatic press; HIP + F, hot isostatic press plus forge.

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