Hot isostatic pressing (hipping) can be used for upgrading castings, densifying presintered components, consolidating powders, and interfacial bonding. It involves the simultaneous application of a high pressure and elevated temperature in a specially constructed vessel. The pressure is applied with a gas (usually inert) and, so, is isostatic. Under these conditions of heat and pressure, internal pores or defects within a solid body collapse and diffusion bond. Encapsulated powder and sintered components alike are densified to give improved mechanical properties and a reduction in the scatter band of properties. In this article, the basic science of sintering and hipping is summarized and contrasted. The current state of understanding and modeling of hipping is then reviewed. Models can be classified either as microscopic or macroscopic in their approach. In the microscopic approach, the various mechanisms of densification are analyzed in terms of a single particle and its surroundings. In the macroscopic approach, the compact is treated as a continuous medium. In hipping, although the pressure is isostatic, shrinkage is not generally isotropic, particularly if containment is used. However, the shrinkage can now be well predicted, provided that the material and container properties are accurately known.

Fundamental aspects of hot isostatic pressing: An overview

The changes in microstructure induced by plastic deformation in hot isostatically pressed (HIPed) P/M Rene 95 under isothermal conditions are discussed. Results of the constant true strain rate compression tests are presented for initially fine (7 μm) and coarse (50 μm) grained compacts deformed at temperatures of 1050 °C, 1075 °C and 1100 °C and at strain rates in the range from 10 −4  s −1 to 1 s −1 . Under these test conditions, both the fine and coarse-grained compacts recrystallize and their grain size are refined during flow. This grain refinement gives rise to softening in both materials. Ultimately, their microstructures transform into the same equiaxed fine-grained microduplex structure at which point their flow strength becomes identical. Continued deformation at that point produces no further change in grain size or flow strength. Under this steady state regime of deformation, the microduplex grain size and flow strength are independent of the original microstructure but are conditioned by the strain rate at a given temperature. The steady state grain size increases whereas the steady flow strength decreases with a decrease in strain rate and/or an increase in temperature.

Change in grain size and flow strength in P/M Rene 95 under isothermal forging conditions

The changes in microstructure induced by forging and their influence on flow strength of hot isostatically pressed P/M Rene 95 as revealed by constant strain rate compression tests under simulated isothermal forging conditions are discussed. Results are presented for initially fine (7 μm) and coarse (50 μm) grained compacts tested at temperatures of 1050, 1075 and 1100 °C and at strain rates in the range from 10−4 to 1 s−1. Under these test conditions, both the fine and coarse-grained compacts recrystallize and their grain size is refined during plastic deformation. This grain refinement gives rise to softening in both materials. Ultimately, their microstructures transform into the same equiaxed fine-grained microduplex structure at which point their flow strength becomes identical. Continued deformation at that point produces no further change in grain size or flow strength. In this regime of deformation, the microduplex grain size and flow strength are independent of the original microstructure but are conditioned by the strain rate at a given temperature. The steady state grain size increases whereas the steady flow strength decreases with a decrease in strain rate and/or an increase in temperature. It is shown how changes in microstructure and flow strength during isothermal forging can be modelled in P/M Rene 95 compacts by means of established deformation models for predicting peak flow strength, using the steady state deformation data as a boundary condition for the evolution of microstructure and flow strength and a model for deformation-induced recrystallization during forging that has been recently developed for this class of materials.

Modelling of deformation and microstructural changes in P/M Rene 95 under isothermal forging conditions

The stress-strain curves and microstructure properties of superalloy GH4169 was tested by thermal simulation experiment with different parameters, which were deformation temperature and strain rate and strain and original grain size. The influence of technology parameters on crystal grain size of dynamic recrystallization (DRC) was analyzed. The kinematics model of superalloy GH4169 was established, in which the relation between grain size of dynamic recrystallization and function Z (Zener-Hollomon) and parameters was described. The dynamics model of superalloy GH4169 was put forward, which described the relation between the quantity of dynamic recrystallization and function Z and parameters. The research results showed that the grain size of dynamic recrystallization increased with increasing the temperature and decreasing the strain rate. And the grain size of DRC bore no relationship to original grain size. And the quantity of dynamic recrystallization decreased with increasing the original grain size.

Kinematics and Dynamics Model of GH4169 Alloy for Thermal Deformation

The morphology evolution of lamellar δ phase in Inconel 718 during Delta Processing was characterized by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy A spheroidization mechanism is proposed for transformation of the lamellar δ phase The results showed that lamellar δ phase hindered the movement of dislocations, which led to a dislocation aggregation zone around the fracture location of the δ phase Strain in the region thus increased, which promoted the occurrence of dynamic recrystallization and fracture of the lamellar δ phase A diffusion channel for Nb atoms was simultaneously provided by the high-density dislocations, which led to dissolution and reduction in the amount of δ phase Under the combined action of deformation, fracture, and dissolution, the lamellar δ phase gradually transformed into granular short rods and spheroids

F.C. Zeng

Papers

Dynamic behavior modeling for P/M superalloys during hot isostatic pressing

An effective technique for producing high performance IN718 forgings using hammers