Hot isostatic pressing (HIP) of copper alloys
How extreme pressure sintering maximizes the conductivity of printed copper.
1. Introduction: The pore trap in copper
Pure copper is the holy grail for heat exchangers, inductors and electrical components. But in the Laser Powder Bed Fusion (PBF-LB) process, pure copper is extremely stubborn. Even with modern green lasers (515 nm), microscopic cavities (pores) remain in the component, which often limits the density to 98% to 99%.
Each pore is an insulator for thermal and electrical conductivity. To get the maximum performance out of 3D printed copper, the industry resorts to a brutal tool: hot isostatic pressing (HIP).
What happens during hot isostatic pressing (HIP)?
The printed copper part is placed in a high pressure chamber. The chamber is flooded with argon gas, which is brought to extreme pressure (often 1,000 to 2,000 bar) and high temperatures (often 800 °C to 950 °C for copper). The argon presses on the component absolutely evenly (isostatically) from all sides. The metal becomes soft and internal cavities are literally crushed.
2. The physics of densification
HIP is not just "flattening", but a complex creep metallurgical process.
- The component loses minimal volume (shrinkage) due to HIPing, but retains its external shape because the pressure comes from every direction at the same time.
- Internal cavities (pores) that were created in a vacuum or under protective gas during printing collapse completely. The metal walls of the pore touch each other, the copper diffuses together in the extreme heat (diffusion welding) and the pore disappears completely. The copper part reaches 99.99% density (theoretical density).
3. Open vs. closed porosity
The HIP process has a crucial physical limit with copper: it only works with closed porosity (sub-surface porosity).
If a pore has a microscopic connection to the outside of the component (open porosity / surface-connected), the high-pressure argon penetrates into the pore during the HIP process. Since there is now the same enormous gas pressure in the pore as outside, the pore cannot collapse. Therefore, when printing PBF-LB, the component must have an absolutely dense, continuous “skin” at least on the outside so that the HIP process can function in the core.
4. Effects on conductivity
The result of HIPed copper is breathtaking.
- Electrical conductivity: While PBF copper as-built (directly from the printer) often only reaches 85-90% IACS (International Annealed Copper Standard), the conductivity increases to almost 100% IACS after the HIP process. It behaves like pure cast copper.
- Thermal conductivity: Heat exchangers made of ge-HIPed copper achieve thermal conductivity values of almost 400 W/(m K), which is essential for cooling high-performance chips or battery cells.
5. Conclusion: The expensive path to perfection
A HIP cycle takes many hours and the machines are extremely expensive to operate. The costs per component increase massively due to this post-processing step. Nevertheless, there is absolutely no alternative to HIP for additively manufactured copper components in high-end applications (e.g. particle accelerators, semiconductor industry), as this is the only way to completely eliminate the microscopic defects of powder bed printing.