HIP Technology

Whether it is consolidating powder in an encapsulated compact, bonding two surfaces together, or healing a casting or sintered compact, HIP is a process to remove porosity. The following describes the mechanisms and stages of pore closure for metal powders and HIP diffusion bonding of two surfaces.

Consolidation of Encapsulated Powder. As will be described in more detail later, powder particles are randomly packed inside a container that encapsulates the powder during the HIP process. Powder used for HIP is typically spherical with a broad particle size distribution. This is better for achieving high packing density when compared to irregularly shaped particles and/or monosized particles. With monosized particles or powder with narrow particle size distributions, there are no small particles to fill the interstices, thus decreasing packing density. However, even with the ideal size distribution, there will always be some percentage of void space due to the spherical morphology (Fig. 4) of the powder and possibly the presence of hollow particles (Fig. 5) that may have formed during the powder-making process.

Fig. 4 Pore with varying radii of curvature around the surface. The broken curve shows the pore surface after some spheroidization has taken place by redistribution of material from convex surfaces to concave. Source: Ref 1

Fig. 5 Pores in -80 mesh Astroloy powder particles produced by argon gas atomization. Courtesy of Industrial Materials Technology, Inc.

To describe the consolidation of metal powders and thus the removal of pores, it is generally believed that the HIP process is similar to sintering where three physical mechanisms (Ref 11) are present, namely:

Powder consolidation Neck growth

Final densification

Powder in compacts prior to HIP is packed at a relatively low density (e.g., 60 to 80% of theoretical density). During the powder consolidation phase, powder is considered a cohesionless granular material where particles can slide freely without particle deformation. Particle rearrangement occurs, and some macroscopic deformation is observed. Once a particular density is attained and particle rearrangement discontinues, necks grow at the contacts between particles. At this stage, particles are beginning to bond, but the porosity is still interconnected. Various mechanisms describe this phenomenon as follows:

• Plastic deformation of the particles by dislocation

• Power-law creep or dislocation creep

• Nabarro-Herring creep or volume diffusional creep

• Coble creep or grain boundary diffusion creep

During final densification, the material may be considered a solid containing isolated pores connected by grain boundaries. The mechanisms controlling the final densification stage is the same as those controlling neck growth (Ref 11). A more detailed description of the mathematics is given in the article "Principles and Process Modeling of Higher-Density Consolidation" in this Volume and elsewhere (Ref 11, 12, 13, 14).

Interface/Diffusion Bonding. Hot isostatic pressing technology was initially developed as a method to diffusion bond two dissimilar materials. Unlike fusion methods (e.g., welding, brazing, etc.), diffusion bonding depends on atomic transport across two mating surfaces to remove bond line pores. Fusion processes involve melting, which can cause segregation or solidification cracking. Unlike consolidation of a powder compact, components undergoing HIP diffusion bonding deform very little macroscopically. However, microscopically there is localized plastic flow across the bondline with the size of the initial pore being a function of the surface roughness (Fig. 6). Depending on the mismatch in physical and mechanical properties of the two materials, pores at the interface will spheroidize due to surface energy considerations. Afterward, previously described mechanisms take over until final densification is reached (Ref 1).

Fig. 6 (a) Magnified view of a region where two materials come into contact showing surface roughness. (b) Magnified view of the same region after microplastic deformation. (Material B is harder than material A.) Source: Ref 1

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