Invar 36 in 3D printing
The measure of all things: almost zero thermal expansion for precision optics and CFRP tools.
1. Introduction: When metals stop growing
All known metals expand when hot and shrink when cold. This thermal expansion is a massive problem for high-precision tools in space travel, optics or in the production of carbon fiber components (CFRP). Fractions of a millimeter of distortion make satellite cameras blind or carbon wings unusable.
The solution is called Invar 36 – an iron-nickel alloy (64% Fe, 36% Ni) that has a thermal expansion coefficient (CTE) of almost zero. The 3D printing of Invar 36 opens up completely new dimensions in tool and precision component construction.
Why is it called "Invar"?
The name stands for "invariable" (unchangeable) because the volume of the metal remains extremely constant over wide temperature ranges (from -100 °C to approx. +200 °C). This “Invar effect” was discovered at the end of the 19th century, for which the Nobel Prize was later awarded.
2. The 3D printing of Invar tools
Conventionally, Invar is machined (milled) from huge blocks. Since the material is tough and difficult to machine, this is expensive and produces gigantic amounts of scrap (buy-to-fly ratio often over 10:1).
- CFRP laminating tools: When carbon fiber (CFRP) hardens in an autoclave (oven) at 180 °C, the CFRP hardly expands. If the mold underneath was made of aluminum, it would expand and tear the component. Printed Invar shapes expand just as little as the CFRP itself. Thanks to 3D printing (usually DED/LMD), these massive tools can be printed near-net shape in a resource-saving manner and provided with internal cooling channels.
3. Laser Powder Bed Fusion (PBF-LB) for precision mechanics
In the powder bed, Invar 36 is used for highly sensitive optical and electronic components.
- Space optics: Mounts for mirrors in space telescopes. In orbit, temperatures alternate between extreme cold in the shade and enormous heat in the sun. PBF printed Invar mounts guarantee that mirrors stay in place with micron precision.
- Laser systems: Resonators in industrial lasers must not change their length when heated during operation, otherwise the wavelength of the emitted light changes.
4. Metallurgical challenges in 3D printing
The Invar effect is a delicate balance of magnetostriction and thermal lattice expansion. It depends 100% on the exact chemical composition.
In the PBF and DED process, there is a risk that alloying elements (particularly nickel) will evaporate minimally during melting or that the extremely rapid cooling will cause the nickel not to be evenly distributed in the matrix (microsegregation). In order to achieve the CTE of exactly zero, complex two-stage heat treatments after printing are often necessary to homogenize the structure.
5. Conclusion: Expensive, but no alternative
Invar 36 is heavy, extremely expensive and challenging to mill. But where absolute dimensional stability is required under temperature fluctuations, there is no substitute material. Additive manufacturing reduces the manufacturing costs for Invar tools enormously through near-net shape and the installation of conformal cooling channels and makes the material usable for broader industrial applications for the first time.