Additive manufacturing of tungsten (tungsten)
Printing X-ray collimators: The fight against the brittleness of the most heat-resistant metal.
1. Introduction: The most stubborn metal on earth
Tungsten (tungsten) is a metal of superlatives. It has the highest melting point of all pure metals (3,422 °C), is extremely dense (almost as dense as gold) and absorbs radioactive radiation excellently. It is the dream material for X-ray tubes, radiation protection and high-temperature ovens.
At the same time, it is an absolute nightmare for engineers: it is so extremely brittle and hard at room temperature that it can hardly be shaped into complex shapes using conventional methods (turning, milling). With 3D printing (PBF-LB), the industry is now trying to force this stubborn metal into shape.
Why doesn't the printer melt itself?
A fiber laser in the PBF printer has enough energy (due to the extremely focused spot of 50 µm, for example) to locally reach over 3,500 °C and melt the tungsten grain at specific points. The surrounding titanium or aluminum components in the chamber remain cool as the heat flows off into the surrounding powder bed at lightning speed.
2. The microcrack epidemic
The biggest obstacle to melting tungsten powder (PBF-LB) is the extreme temperature shock. Tungsten has a very high ductile-brittle transition temperature (DBTT), which for pure tungsten is often between 200 and 400 °C.
If the 3,500 °C hot melt pool cools down in the millisecond range, the material inevitably cracks microscopically. Almost every laser-printed component made of pure tungsten resembles a cracked ice sheet under the microscope (micro-cracking). Pure PBF tungsten is currently often unsuitable for structurally supporting parts.
3. Collimators: When cracks don't matter
Fortunately, cracks do not interfere with every application. In medical technology (computed tomography) and nuclear technology, highly complex tungsten structures are used to bundle and shield X-rays or gamma radiation: so-called anti-scatter grids (collimators).
Collimators consist of extremely thin walls and complex honeycomb structures. They don't have to carry weight, they just have to be solid and dense. For the first time, 3D printing makes it possible to produce honeycomb collimators made of tungsten that filter radiation with micrometer precision. The unavoidable microcracks do not interfere with radiation protection in the slightest.
4. Solutions: Tungsten heavy alloys and binder jetting
In order to get the brittleness of pure tungsten in laser melting under control, the industry is pursuing alternative routes:
- Tungsten heavy alloys (WHA): Tungsten is mixed with nickel, iron or copper (e.g. W-Ni-Fe). The addition of these softer metals ("binder metals") drastically reduces the tendency to crack and makes the material processable.
- Binder Jetting: Since laser heat destroys tungsten, the tungsten powder is printed cold (with an adhesive using an inkjet head). The “green part” is then slowly sintered in the oven at extremely high temperatures (often with infiltration of bronze or copper). This is currently the most promising route for dense, complex tungsten components.
5. Conclusion: A fight against physics
The additive manufacturing of pure tungsten remains one of the greatest challenges in modern metallurgy. But for highly specialized radiation protection elements and nuclear fusion components (Tungsten Divertors), which are otherwise impossible to produce, 3D printing (especially via the detour of binder jetting) is the only way into the future.