Hot isostatic pressing (HIP)
The essential post-processing step to eliminate pores and maximize fatigue strength.
1. Introduction: Perfection under extreme pressure
Metal 3D printed parts almost always have microscopically small cavities (pores) and enormous internal stresses after printing (as-built). If components are intended for use in engines, as chassis components or under high cyclic loads, a microscopic pore inside is a critical weak point (predetermined breaking point).
The solution to this problem is hot isostatic pressing (HIP for short). Without HIP, the certification of additively manufactured high-performance components in aerospace is practically unthinkable.
What happens during HIP?
The printed component is placed in a massive high-pressure chamber (an autoclave). There it is exposed to an extreme combination of heat (often just below the melting point of the metal, e.g. 1,200 °C) and enormous pressure (up to 2,000 bar) from an inert gas (usually argon). The component becomes "soft" and compressed from all sides (isostatically).
2. Elimination of residual porosity
The main goal of the HIP process is to eliminate internal defects. Pores, "lack of fusion" (bonding defects) or tiny voids that were created during laser melting are literally squeezed together by the enormous isostatic pressure.
As the material is very hot, the atoms diffuse and weld the edges of the pores together (diffusion welding). The result: The porosity of the component drops to extremely close to 0%. The component achieves a density of almost 100% (full density).
3. Huge increase in fatigue resistance
The effects of HIP on the mechanical properties are enormous.
- Fatigue Strength: Under changing loads (e.g. in a rotating turbine), a microscopic pore acts like a wedge in which a crack occurs. By closing these pores using HIP, the cyclic fatigue strength of the component increases many times over.
- Internal stresses: The internal stresses caused by the extreme heating and rapid cooling in the 3D printing process are completely reduced in the HIP process.
- Structure optimization: HIPen simultaneously acts as a heat treatment, which stabilizes and homogenizes the fine-grain crystal structure that is created in 3D printing.
4. The challenges: costs and shrinkage
Despite its indispensability for aerospace and high-end medical parts, HIP brings challenges:
- Shrinkage: Since closing the internal cavities costs volume, the component shrinks minimally during the HIP process. This shrinkage must be precisely taken into account in advance in the CAD design (measurement) in order to achieve the final dimensions.
- Cost and Time: HIP is extremely expensive. The process takes many hours and the high-pressure systems are capital goods worth millions. For less critical components (e.g. brackets without dynamic load) a simple, cheaper stress-relieving annealing in a normal oven is often sufficient.
5. Conclusion: The obligatory step for high-end components
Hot isostatic pressing transforms a “very good” 3D printed part into an absolutely reliable component for critical applications. For additive manufacturing in aerospace, medical technology and racing, HIP is not an optional additional step, but an integral part of the process chain to meet the highest certification standards.