Introduction

POWDER METAL PARTS are sometimes classified by density, where part production technologies may have low-density and high-density variants. Proponents of a particular technology for "high-density" parts may refer to the technology as a "high-density" or "full-density" process without explicit definition of the term for potential customers or comparison with other processes or materials. For the purposes of this article, "higher-density consolidation" means effectively 100% dense material that will compete with forgings in terms of mechanical properties. The most popular way to achieve 100% density in powder materials is with hot isostatic pressing, although there are a number of pseudoisostatic pressing techniques that can achieve high densities and at times at a cost savings. These are generally not important production processes at this time and are not included in this article.

Hot isostatic pressing (HIP) is a materials processing technique in which high isostatic gas pressure is applied to a powder part or compact at elevated temperatures to produce particle-to-particle bonding. This process usually results in the manufacture of a fully dense body, although partially dense bodies also can be intentionally produced. During processing, the compact is subjected to equal pressure from every side.

Elevated temperature, in reference to HIP of metal powders, ranges from approximately 480 °C (895 °F) for aluminum alloy powder to approximately 1700 °C (3090 °F) for tungsten powder. High-density argon gas is the most common medium used in the process, and pressures range from approximately 20 to 300 MPa (3 to 45 ksi), with 100 MPa (15 ksi) as the most typical pressure.

The HIP process was invented at Battelle Memorial Institute in 1955 by Saller et al. (Ref 1). Early designs utilized the "hot-wall" configuration; the furnace surrounded the pressure vessel. Material limitations precluded scale-up, and the development of a cold-wall vessel design, now used throughout industry, took place.

Hot isostatic pressing was initially used for diffusion bonding of clad nuclear fuel elements. Consolidation of beryllium metal powder "to-shape" was first carried out in 1964. High-volume hot isostatic compaction of high-speed tool steel was achieved in the United States and Sweden by 1972. The U.S. Air Force Materials Laboratories expanded HIP technology to include forging of preforms and near-net shapes of nickel-base superalloy and titanium alloy powders from 1970 to 1980. A comprehensive review of early HIP applications is given in Ref 2. During the 1970s it was also discovered that

HIP could be used to permanently heal internal porosity in complex cast shapes without distortion of major casting features. This has remained a major use for HIP technology but is not discussed here.

Current applications of HIP technology in P/M processing include near-net shapes in steel and stainless steel alloys for power turbine components and for manifolds and valves for oil-field applications; nickel-base superalloys for aircraft engine turbine disks and shafts (such shapes are "squared-off" cross sections suitable for sonic inspection); nickel-base P/M forging and rolling preforms and nickel-base P/M integral pump and turbine impeller wheels; titanium alloy P/M billets, forging preforms, and shapes; tool steel billets (for mill processing), large die blocks, and composite structures; net shapes in P/M beryllium, niobium alloys, and other refractory metals; and dispersion and fiber-strengthened P/M aluminum alloys. Small parts processed by a combination of cold compaction of metal powder, sintering, and HIP include blended elemental titanium-base alloys, tool steel shapes, rare-earth magnets, and tools, dies, rolls, wear parts, and seals manufactured in tungsten carbide/cobalt and other carbide compositions. The iron- and nickel-base parts made for valve and manifold applications represent the leading edge of the technology in terms of size and shape complexity. Nickelbase P/M aircraft engine applications represent the highest technology level of the method in terms of mechanical properties, and tool steels represent the highest production tonnage. Hot isostatic pressing of P/M tungsten carbide/cobalt parts is still employed worldwide; however, most manufacturers have gone to a combined sinter/HIP process to reduce process time and costs.

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