Refractory metals in 3D printing: Tungsten & Molybdenum
Processing metals with extremely high melting points for extreme applications.
1. Introduction: The heat giants of the periodic table
Refractory metals are base metals of the 4th, 5th and 6th subgroups, which are characterized by extremely high melting points (over 2,000 °C) and enormous density. The most important representatives include tungsten, molybdenum, tantalum and niobium. Conventionally, these metals are considered extremely difficult or impossible to process - they are brittle at room temperature and require the highest melting energies.
Metal 3D printing, which can generate immense local energy, is considered the most promising method for producing complex components from refractory metals.
Why use refractory metals?
Components made of tungsten or molybdenum withstand temperatures at which steel has long since evaporated. They absorb ionizing radiation extremely well (X-ray screens) and have excellent creep resistance at high temperatures. This makes them indispensable for medical technology, the nuclear industry and high-temperature furnace technology.
2. The extreme demands on the printing process
The printing of tungsten (melting point 3,422 °C) or molybdenum (2,623 °C) in Laser Powder Bed Fusion (PBF-LB) pushes machines to their limits.
- Laser power: In order to fully melt these metals, very high energy densities (high-power lasers) are required. At the same time, the high thermal conductivity of the materials means that the heat immediately flows into the building board.
- Cracking (Micro-Cracking): This is the biggest problem. The extreme temperature gradient (from over 3000°C to room temperature in milliseconds) creates enormous internal stresses. The brittle nature of the metals inevitably leads to microcracks in the component.
3. Approaches: Preheating and alloying
To get cracking under control, the industry is currently pursuing two primary strategies.
- High-temperature building panels: PBF machines and EBM (electron beam melting) systems are modified to preheat the entire powder bed to temperatures from 1,000 °C to over 1,500 °C. This reduces the temperature gradient and significantly reduces thermal stresses.
- Alloy development: Pure tungsten can hardly be printed without cracks. By adding more ductile metals such as tantalum or rhenium (e.g. W-Ta alloys), toughness is increased without compromising the high-temperature properties too much.
4. Applications: From medicine to fusion
Where are printed refractory metals used?
- Medical technology (X-ray & CT): Tungsten is an excellent radiation protector. 3D printing can be used to produce high-precision collimators (structures that precisely direct X-rays) in a honeycomb structure.
- Furnace and vacuum technology: Heating elements and heat shields for high vacuum furnaces made of tantalum or molybdenum.
- Nuclear fusion: Future fusion reactors (like ITER) will require extremely heat-resistant "divertor" components (baffles for the plasma) made of pure tungsten.
5. Conclusion: A breakthrough is imminent
The printing of refractory metals is still largely the subject of intensive research and development. But advances in high-temperature printing systems (particularly in the area of EBM) show that crack-free additive manufacturing of tungsten and molybdenum will soon become a reality for industrial series production.