In-situ alloying in metal 3D printing
How completely new high-entropy alloys are created by directly mixing element powders during printing.
1. Introduction: The Alchemy of the 21st Century
The classic way in additive manufacturing is to use pre-alloyed powder: For example, you buy Inconel 718 powder and print an Inconel component. However, the production of specialized, pre-alloyed powders (through atomization) is extremely expensive and only economical when purchasing large quantities.
This is where "In-Situ Alloying" comes in: Instead of using a finished alloy powder, pure element powders (e.g. iron, chromium, nickel) are melted together directly in the melt pool of the 3D printer (DED or PBF) during the printing process to create a completely new alloy in real time.
The biggest advantage of in-situ alloying
Speed in material development (material screening). By simply adjusting the powder mixture, scientists and engineers can test dozens of new alloy variants within a few hours without having to mix 100 kilograms of expensive special powder.
2. Technologies for in-situ alloying
Depending on the printing process, the method is implemented differently:
- Directed Energy Deposition (LMD/DED-P): Several separate powder conveyors are used here. For example, pure titanium powder is fed in via nozzle 1 and tantalum powder via nozzle 2. The mixing rate can be controlled live during printing via software. This enables “Functionally Graded Materials” – a component that smoothly transitions from material A to material B.
- Laser Powder Bed Fusion (PBF-LB): Here the pure element powders are mechanically mixed in advance in the desired ratio (blended powder) and then doctored normally into the powder bed. The laser melts the particles together.
3. The metallurgical challenge
The pure heat of the laser is often not enough to create an absolutely homogeneous material. The melt pool only exists for milliseconds.
- Melting point differences: If powder A melts at 1,500 °C and powder B only melts at 3,000 °C (e.g. when adding tungsten), it can happen that powder B only melts incompletely and remains in the structure as "chunks" (unmelted particles).
- Marangoni convection: In order to achieve good mixing, the flows that arise from temperature differences in the melt pool are used (Marangoni effect). The laser often has to be passed over the same spot several times (re-melting) in order to completely alloy the elements.
4. Development of new high-entropy alloys (HEA)
One of the most exciting applications of in-situ alloying is the research into high-entropy alloys. These do not consist of one main metal (like iron in steel), but of five or more metals in almost equal proportions (e.g. Co-Cr-Fe-Ni-Mn).
They have completely absurd properties (e.g. they become stronger instead of brittle in extreme cold in space). 3D printing using in-situ alloying is the only process with which these high-entropy alloys can be produced and tested efficiently, locally and in complex geometries.
5. Conclusion: A laboratory in the laser head
In-situ alloying turns the 3D printer into a high-performance metallurgical laboratory. The process is often too unstable for series production (since certification and quality assurance are extremely complex). However, in the research and development of completely new, application-related special alloys (custom alloys), in-situ alloying is the absolute spearhead of materials science.