Cold Metal Transfer (CMT) in the WAAM process
Cold, spatter-free and precise: How oscillating welding wires tame large-format printing.
1. Introduction: Welding at the coldest point
Wire Arc Additive Manufacturing (WAAM) is the undisputed master when it comes to printing huge metal parts (e.g. 10 meter long bridge girders or ship propellers) extremely quickly and cheaply. A normal MIG/MAG welding robot melts wire layer by layer. The problem: Conventional arc welding brings a brutal amount of uncontrolled heat into the component. The metal melts away, warps extremely (warping) and splashes everywhere (spatter).
The Fronius company developed the solution for precise 3D printing using arcs: Cold Metal Transfer (CMT).
Is CMT really “cold”?
No, of course metal doesn't melt when cold. “Cold” here just means “significantly colder than normal”. The heat input is extremely minimized in the CMT process and is physically decoupled from the droplet detachment, making WAAM suddenly suitable for thin-walled 3D printing structures.
2. The Secret of the Wire Retraction Movement
In classic welding, the wire feed stubbornly drives the wire forward into the weld pool. The arc burns constantly and massively heats up the workpiece.
The CMT process, on the other hand, is highly regulated (up to 90 times per second!):
- The wire advances. An arc ignites and melts the end of the wire.
- As soon as the liquid drop touches the melt pool (short circuit), the machine drastically switches off the power. There is no longer an arc, so there is no heat effect (cold phase).
- At the same time, the wire feed motor physically pulls the wire back a bit! As a result of this retraction, the liquid drop is "deposited" into the melt pool mechanically (and not by electromagnetic forces as with MAG).
3. The advantages of WAAM 3D printing
This oscillating back-and-forth motion makes CMT the ultimate WAAM tool.
- Almost spatter-free: Since the drop is deposited gently and is not thrown away by a burning arc, the surrounding area remains clean. This is important in 3D printing as spatter would ruin later layers.
- Wall thickness control: Since the melt pool immediately becomes tough again due to the extreme cold phases (rapid solidification), you can print precise, thin walls directly on top of each other without the metal running down the sides like wax.
4. Multi-material and aluminum
Aluminum dissipates heat extremely well and is difficult to control in traditional arc welding (often fails). The minimal heat input of the CMT process makes printing aluminum structures (e.g. for aviation) using WAAM economically possible.
In addition, CMT technology allows extremely dissimilar metals to be joined (e.g. aluminum on galvanized steel), as the aluminum melts, but the steel underneath is only wetted (brazing effect) without melting.
5. Conclusion: The standard for large format AM
When start-ups today “print” rocket tanks from sheet steel, they almost always use a variant of the CMT process. He transforms the wild, imprecise welding robot into a precise scalpel that deposits layer by layer in a controlled manner, thus unlocking the gigantic economic potential of the WAAM process for heavy industry.