Post-processing of AM metal components: HIPing

HIPing stands for Hot Isostatic Press-ing. Hot isostatic pressing is a process in which components are subjected to the simultaneous application of heat and high pressure in an inert gas medium. Because the pressure is applied by a gas, it is uniform in all directions, or isostatic [1]. HIP is on one hand a forming technique, on the other hand a densifying technique [3].

HIP applications

Commercial uses of HIP can be grouped into six main categories. These are [1, 2]:

• porosity elimination (and simultaneous heat treatment), 
• rejuvenation of fatigue or creep-induced damage in components, 
• compaction of metal powder,
• post-densification of sintered ceramics or metal parts, 
• joining of materials and infiltration combined with densification.

Clearly HIP can also be applied to metal components built using additive manufacturing technologies such as selective manufacturing. HIP post-processing eliminates virtually all trace of gas porosity, cracks, decreases hot tearing susceptibility and improves fatigue behaviour.

How does HIPing work?

The simultaneous application of heat and pressure combine to collapse voids and porosity by creep mechanisms, plastic deformation, and diffusion bonding the void surfaces together with minimum distortion. Components of complex geometry can be treated without complex or expensive tooling. The many investigations conducted on several classes of materials show that HIP treatment provides startling improvements in mechanical properties, such as higher strength, enhanced toughness, improved fatigue resistance, and longer creep life [1-4]. 

Any increase in porosity content reduces fatigue capability. The elimination of porosity with HIPing increases fatigue life and raises tensile properties. Of high significance is the marked reduction in the statistical spread or scatter usually associated with cast and AM material properties. Minimum observed values are usually increased, resulting in improved reliability and efficiency of materials utilization. Commonly HIPed materials are nickel alloys, high speed steels, stainless steels, titanium alloys, aluminium alloys, polymers, ceramics and some composites materials.

HIP cycles [2]

A HIP cycle consists of several distinct phases, namely: loading, vacuum degassing, heat-up and pressurising, followed by a heat soak (creep-controlled step), cooling, depressurisation and unloading. Even of importance are heating and cooling speed.

HIP parameters and their optimisation [3]

1. Temperature
The general rule [4] says that the HIP temperature should be 0.8xTM, TM is the melting temperature. When HIP is used as a densifying post-treatment, the lowest temperature ever possible in regard to HIP-mechanisms should be choosen for avoiding defects in the material.
2. Pressure
For good densification, the pressure of the more plastic material of the composite should lie something around the stress of the Young's modulus at HIP-temperature.
3. Time
The third parameter, the time, allows to control diffusion effects and grain growth. Even controlled phase transformations are time dependent. In general the time for HIP densification is not longer than two/three hours.

To sum up

For AM-ed metal components, HIP can be used as a production technique (net shape HIPing) as well as post-treatment (densification). The HIP-parameters have to be applied individually to the materials in accordance to the desired results of the HIP-treatment. HIP treatment provides higher strength, longer creep life, greater fatigue resistance and enhanced toughness [1-4].

References
[1] Barre C., Hot Isostatic Pressing, Advanced Materials & Processes. 155.3 (Mar. 1999): p47. Copyright 1999 ASM International
[2] Richter D., Haour G. & Richon D., Hot Isostatic Pressing, Materials & Design, Vol. 6, Dec. 1985, p303-306
[3] H.-D. Steffens, J. Lebkuchner-Neugebauer and B. Wielage, Hot Isostatic Pressing, Mat.-wiss. u. Werkstofftech. 21, 28-31 (1990)
[4] E. Arzt, M. F. Ashby, K. E. Easterling, 1983, Met. Trans. A,Vol. 14A, p. 211.