Introduction

HOT ISOSTATIC PRESSING (HIP) is a process involving the use of high-pressure gas isostatically applied to a part or workpiece at an elevated temperature performed in a specially constructed pressure vessel. When consolidating metal powders in pressure-tight, sealed compacts, the HIP process plastically deforms the powder, which closes up porosity and achieves 100% theoretical density in the part. Thousands of fully dense HIP P/M compacts (equivalent to >10,000 tons) are commercially produced each year. This includes net shapes, near-net shapes, and a variety of mill forms for subsequent thermomechanical processing. Monolithic and bimetallic or clad parts are part of the near-net shapes that are supplied in many different conditions such as: HIP, HIP plus heat treated, HIP plus thermomechanically processed, and so forth. The majority of individual parts are made as near-net shapes. Other parts are made as net shapes, or with some net surfaces and other surfaces that need finishing to match mating parts or to meet some other critical parameters. The elevated temperature in HIP ranges from approximately 480 °C (896 °F) for aluminum alloy powders to 1700 °C (3092 °F) for tungsten powders. Most of the commercial HIP activity is with steel and nickel alloys, which are commonly hot isostatically pressed between 1100 °C (2012 °F) and 1205 °C (2201 °F). High-density argon gas is the most common pressure medium used in the process, although other gases such as helium or nitrogen can also be used. Pressures ranging from 20 to 300 MPa (3 to 45 ksi) are possible with 100 MPa (15 ksi) being the most common.

The HIP process was invented at Battelle Memorial Institute in 1955 as a method to diffusion bond dissimilar materials where Zircaloy was clad to uranium oxide nuclear fuel elements (Ref 1, 2). As the technology evolved, applications other than diffusion bonding were discovered, namely consolidating encapsulated powder, porosity healing inside castings, and densification of presintered components. In the 1960s, HIP technology (sometimes referred to as gas pressure forging) was used to consolidate beryllium metal powder, refractory metal powder, ceramic powder, and cemented carbides. Not until the 1970s, however, did the HIP process expand to an industrial scale. For example, high-volume hot isostatic compaction of high-speed tool steel was achieved in the United States (Ref 3) and Sweden by 1972. By the late 1970s, the United States Air Force Materials Laboratories funded work to develop HIP technology to manufacture titanium and superalloy components for aircraft engines. This application currently represents the highest technology level of the process. Powder metallurgy tool steels represent the highest production tonnage.

Some of the current applications of HIP P/M parts include:

• Tool steel billets (for mill processing) for hot and cold working tools and dies

• Nickel and titanium alloys for high-temperature components on aircraft and marine gas turbine systems

• Nickel alloys in the oil and gas and petrochemical industries for corrosion-resistant components on wellheads and piping systems

• Nickel alloys in the nuclear power industry for corrosion-resistant components in reactors

• Titanium and cobalt alloys in the biomedical industry for prosthetic implants

• Refractory metal alloys for high-temperature service

• Cemented carbides for superior wear-resistant parts

• Composite aluminum materials for lightweight structural components

Although HIP technology is commonly used for sealing porosity in castings and P/M sintered compacts, this article focuses on the consolidation of metal powder and the diffusion bonding of dissimilar materials.

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