Process Description

A flow diagram of the PIF process is shown in Fig. 1. Beginning on the left, P/M materials are prepared in one of two forms: a preform or a loose powder. A preform is consolidated to a near-net- or net-shape workpiece by one of several approaches, including conventional die pressing, cold isostatic pressing (CIP), or metal injection molding. The green pieces are often sintered before they are introduced to the PIF process. As a preform, the workpiece is prepared for PIF processing by a range of techniques including with and without coatings, limited encapsulation, and full encapsulation. The appropriate preparation is chosen to match the material and its initial compacted characteristics. All such choices are material dependent and proprietary in nature.

Materials preparation

Fig. 1 Flow diagram of the PIF process

As an alternative to preform preparation, the free-standing powder is encapsulated in a thin-walled metal container and evacuated, typically in a simple geometric form as a cylinder or rectangular slab. The encapsulated material is then PIF densified. Various powders (including nickel compositions, copper compositions, and chromium, among others) have been processed by PIF.

After the material is prepared, it is preheated to a designated temperature. The temperature choice is selected based on the candidate material and is coordinated with a planned pressure cycle. As a typical value, the temperature choice will be 50 to 200 °C (120 to 390 °F) below a corresponding choice for a HIP transaction. Once the workpiece is in thermal equilibrium, it is transferred to the pressure system. Immediately following the transfer, the pressure system is energized to values up to 415 MPa (60,000 psi). A proprietary pressurization technique is used to increase the pressure and pressurization rate to the desired value. Once the desired pressure is achieved, it is held for periods up to 1 min. Depressurization strategy follows specific rules based on the materials being processed, but in most instances the pressure is relaxed within 1 to 5 min. Once consolidated, the workpiece can be cooled under pressure when seeking a specific desired end effect or the workpiece can be removed from the pressure chamber shortly after consolidation and cooled in a supplemental cooling station.

Densifying Materials by PIF. Because PIF is isostatic in nature and uses either argon or nitrogen gas as the pressure medium, it is often confused with hot isostatic pressing. The PIF process uses plastic deformation with high-strain-rate forces as its consolidation mechanism, whereas HIP uses very limited plastic deformation and primarily relies on diffusion and creep with low strain rate to achieve its end goals. Therefore, PIF is a true forging process utilizing a gas hammer to rapidly transfer energy to form the workpiece. Some materials cannot be forged to full density and may require the creep mechanism to achieve results, thereby requiring HIP.

Consolidation by PIF requires satisfying two critical conditions:

• The surface condition of the workpiece must be free of significant defects or holes and/or interconnected porosity where gas can be absorbed, causing a loss of pressure differential and therefore negating the effect of PIF. Experience has shown, however, that properties of high-pressure gas, particularly argon, enable full densification of preforms having densities as low as 87% of theoretical.

• In order to consolidate the workpiece to full density, its temperature must be of sufficient value to reduce the flow stress requirement of the material for forging such that the applied pressure or force "can collapse the material" in on itself or impart plastic flow. Experience indicates that some nonisostatic preform compaction techniques can produce uneven density profiles that can limit achieving full density by PIF.

Pressure/Strain Rate/Temperature/Time. The deployment of the PIF process is a careful orchestration of the process variables including pressure, strain rate, temperature, and time. Typical pressures used in the PIF process range from 310 to 415 MPa (45,000 to 60,000 psi). For each cycle, an end pressure is chosen where a pressure level is maintained for a period of 10 s to 2 min. Temperature plays the key role by reducing the flow stress requirement, thereby reducing the consolidating pressure and increasing the sensitivity of the deformation speed by means of the strain rate. Time is a factor in the preheat step where the material is heated to a homogeneous value before pressurizing. Typical temperatures include 525 °C (975 °F) for aluminum, 900 °C (1650 °F) for copper, and 1225 °C (2235 °F) for iron. A strain rate is chosen to develop the appropriate shear stress levels to consolidate the workpiece.

Dimensional Properties. A natural consequence of consolidation is the shrinkage of the workpiece. The changes in size and the character of the change are a direct result of the applied force and the prior conditions of the workpiece. Preexisting work hardening and density distributions within a workpiece significantly, if not uniquely, determine any efforts to consolidate a workpiece by PIF.

As a true isostatic compacting technology, PIF provides a uniform force in all directions to the workpiece. Therefore, the workpiece contracts in a volumetric manner. A typical consequence of nonisostatic compaction is the hourglass distortion of cylindrical forms. Fundamentally, PIF does not distort; however, properties installed in the workpiece from first-stage compaction and sintering influence the final dimensions and any distortions. It is well recognized that the constraining nature and uniaxial forces of die compaction produce density distributions in the workpiece. Sintering can amplify the effects of these density distributions. Applying supplemental treatments such as coining can further disturb the natural properties of the workpiece. None the less, PIF maintains dimensions within a few thousands of an inch from its starting condition.

Consolidation by PIF of die-compacted and sintered components tends to move the material in a proportional manner. As such, distortion is avoided when the same density distribution is maintained pre- and post-PIF. This occurs when the density distribution is uniform or when any area of the workpiece moves to the extent that it is either attaining just full density or less. Distortions during consolidation are a direct consequence of nonuniform properties. For example, isostatically or metal injected molded compactions have few, if any, density distributions; therefore, these specimens consolidate with little or no significant distortion.

The PIF process uses isostatic force to sustain dimensional properties; that is, no dies are used to constrain the workpiece. Tooling and fixturing can be used to influence the movement of the workpiece, thereby holding particular geometric positions similar to that provided by dies in first-stage compaction, coining, and powder forging. Care must be exercised to avoid bonding the workpiece to the fixture. Insulating materials and thermal properties of the fixture material are considered.

Estimates of Dimensional Shrinkage. The actual shrinkage of a workpiece by PIF is estimated using some simple relationships. Because PIF is volumetric movement in nature, a cubic variation in parameters prevails. The formula used to forecast dimensional change is:

where di and df are pre- and post-PIF dimensions (i.e., radius, length, width, or thickness), respectively, and Di and Df are pre- and post-PIF density, respectively. This empirical formula is very effective for estimating dimensional changes on geometries with a length-to-diameter ratio of 5 or less.

Some trial and error is required when applying the formula because the final density of the workpiece is a factor in the total dimensional change. Length-to-diameter ratio also influences specific dimensional changes. Materials consolidating to full density conform best to the formula estimates. Some materials and workpiece configurations do not attain full density, and therefore they must be calibrated to determine final density and the actual dimensional changes. Preforms with uniform density satisfy formula predictions very accurately.

Process Advantages. The pressure and temperature of the process are controlled and managed independently of one another, thereby providing an opportunity to control microstructure and properties of the material. As an additional

consideration, lower processing temperatures and shorter exposure time to elevated temperature reduce grain growth in many materials. The short processing time at pressure expedites material throughput and maximizes the availability of the pressure vessel, therefore reducing the capital equipment requirements. Further, the pressure vessel size requirements are reduced. Material handling and processing requirements are simplified, thereby facilitating automated actions. Overall, the process is well suited to automated transactions where sustained throughput of workpieces is desired.

Process Limitations. The PIF process is used to enhance the properties of a workpiece through forging by improving the density and attaining various mechanical and magnetic properties. As a forging process, PIF relies on its pressing media to exert sufficient force to overcome any resistance movement by the material. In short, the force must overcome the flow stress requirements of the material in order to consolidate it. If the preform has density distributions, the PIF process will not significantly alter these distributions.

As a true forging process, plastic deformation by application of stress is the consolidation technique. If the material requires diffusion or creep to attain its consolidation goals, then PIF will not satisfy those needs.

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