Future Developments

Applications using HIP technology have evolved from diffusion bonding of dissimilar materials to consolidating encapsulated powder and sealing microporosity in castings. Hot isostatic pressing technology is continuing to grow with diversification into new areas. These areas include equipment improvements, mechanistic modeling of material undergoing HIP, and new applications of HIP.

Refinements of Batch Processing. One equipment refinement that is generating interest is "quick cool" or "HIP quenching." After the HIP cycle hold, furnace cooling on a cold-walled vessel can take several hours with cooling rates of about 100 °C to 200 °C/h depending on the vessel and size of the load. By utilizing a flow device (Ref 15) and the introduction of cold gas into the hot gas, the convective cooling is dramatically increased. One portion of the gas is forced to the outside of the thermal barrier for cooling while the other portion is circulating inside. To achieve the desired cooling rate, the proportion of the hot and cold gas can be computer controlled. The major driver of this technological improvement is to increase productivity, which ultimately increases capacity and decreases costs. In addition, there may be metallurgical enhancements of some materials, thus potentially eliminating some downstream processing steps.

HIP Modeling and Microstructure Prediction. As described in the article "Principles and Process Modeling of Higher Density Consolidation" in this Volume, there has been much work devoted in the 1990s (Ref 13) to predicting dimensional changes during hip via continuum mechanics/finite element modeling. To predict shrinkage changes, an understanding is needed of the anisotropy of consolidation brought about by the complex interrelationships between the properties of the P/M and container materials as a function of temperature, density, and part geometry. With the development of the constitutive equations for the particles and powder aggregates to predict shrinkage, the underlying mathematics now exist to also predict microstructure of the HIP product (Ref 11, 12, 35). With computational power continually increasing at an affordable rate and material property characterization available from hot triaxial compaction tests (Ref 36), the ability to predict grain size (Ref 37) and other microstructural features (Ref 12, 38) may soon be possible.

HIP Modeling and Closing Porosity in Spray Formed Billets. To compete with ring-rolled products, there has been some interest in producing large nickel-base superalloy rings via spray forming followed by HIP (Ref 39, 40). For this process, metal is nitrogen-gas-atomized onto a low-carbon steel substrate to form a partially dense preform (typically, >90%). The resulting microstructure is determined by amount of liquid in the spray before impact and amount of liquid on the top surface of the deposit. As the amount of liquid is increased, an increase in deposit yield is observed (i.e., atomizing into a swamp); however, these slower solidification rates typically lead to a coarser grain size. If a finer grain size is required, the amount of liquid is decreased, but this typically increases the amount of unusable overspray that cannot be recycled due to increased nitrogen content concerns. Hot isostatic pressing of the preform increases the density to nearly 100% density with some interconnected surface porosity present.

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