Hot Isostatic Pressing (HIP) was developed in the 1950s at the Battelle Memorial Institute in Columbus, Ohio, as part of the Navy Nuclear Power Program.
In those days, Battelle’s scientists had to face the problem of cladding zirconium to a zirconium-uranium alloy while maintaining strict dimensional control, to prepare a pin-type fuel element [1,2]. As conventional fabrication techniques proved unsatisfactory, such as rotary swaging and drawing prior to diffusion anneal, the idea of using an isostatic means to force the cladding into intimate contact with the core during the diffusion treatment was promoted. A gas was suggested as the best pressure media. Initial investigations revealed promising results, in particular regarding the uranium-based powder densification. This technique, called gas pressure bonding, was patented by four Battelle scientists [3].
An exciting challenge has led to discovering new technical solutions that might improve the process: it was said that, «the HIP process has always seemed to lead the availability of adequate HIP equipment» [1].
That challenge has resulted in the development of a technology that not only enabled diffusion bonding – which prior nuclear engineers had anticipated – but also removed porosity in castings and sintered products, helping to achieve 100% maximum theoretical density, and improving mechanical properties such as ductility, toughness, tensile strength, and fatigue resistance of high-performance materials.
The castings’ porosity is caused by shrinkage of the metal and a decrease in gas solubility during cooling, while in powder metallurgy the porosity is present at the green stage, depending on which processing process is used.
During HIP treatment the workload is uniformly heated while the pressure is isostatically applied to all surfaces. The pressure medium is a gas; usually an inert gas like argon or helium, but for some applications it can be nitrogen or air. A compressor provides the required gas pressure. The furnace is inserted into the pressure vessel that is protected by radiation and convection heat shields and a cooling system. For some applications, the open porosity allows for evacuation through a vacuum pump before heating starts, in order to simplify the process.
The success of the HIP process is linked to the correct choice of temperature, applied pressure, the time intervals of the temperature, and the stress application steps When the correct temperature is reached, the material starts to behave plastically: the material plastic flow allows the porosity to be closed. The phenomena leading to porosity healing are described by dynamic material models that relate deformation speed and material strength [4,5]. The HIP process allows lower densification temperatures and avoids the heterogeneous modification of the products’ geometry, so that net shape or near net shape components can be treated. Furthermore, modern HIP apparatuses perform solution heat treatments, removing carbides, nitrides and oxides precipitates.
Nowadays the HIP treatment is widely applied to parts produced by Additive Manufacturing (AM), a production process that usually leaves an unacceptable level of porosity. The microstructural quality of HIPped products is improved by several orders of magnitude.
F.I.L.M.S. SpA acquired its first HIP apparatus in the early 1980s. Since then, F.I.L.M.S. SpA has been developing relevant HIPping expertise on a wide range of different materials and products, from special steels to titanium and cast aluminium, from hard metals to diamond composite beads, from silicon nitride sealings to ceramic composites and cladded metals.
The third HIP apparatus in FILMS history is a wired wound Quintus plant that can reach 2000 bar and 2000 °C in a 250 mm diameter by 750 mm length process chamber.
References
[1] C.B. Boyer, Historical Review of HIP Equipment, in Hot Isostatic Pressing – Theory and Applications, Koizumi M. eds., Springer, Dordrecht (1992)
[2] E. Cerrai, Dispense del Corso di Tecnologie dei Materiali per Impianti Nucleari, Politecnico di Milano, Milano (1988)
[3] H.A. Saller, S.J. Paprocki, R.W. Dayton, E.S. Hodge, A Method of Bonding, U.S. Patent 687-842 (classified). H.A. Saller, E.S. Hodge, S.J. Paprocki, R.W. Dayton, Method of bonding, US Patent 4709848, 1987-12-01, US Energy
[4] H. Gegel, in Synthesis of atomistics and continuum modelling to describe microstructure, Proc. Conf. Computer Simulation in Material Science, R.J. Arsenault et al. eds., ASM International, Materials Park (OH), 1989, 291
[5] M.C. Zhang, Z.J. Luo, F.C. Zeng, J. Mater. Process. Technol. 72 (1997) 262
