When bonding metals, the metals must melt where they meet or some molten metal must be introduced between the pieces. When the metal solidifies, a solid bond forms. However, researchers at the Massachusetts Institute of Technology (MIT) found that melting can inhibit metal bonding rather than promote it.

The counterintuitive finding could impact the design of coating processes or 3D-printing systems that require materials sticking together.

Post-doctoral researchers Mostafa Hassani-Gangaraj and David Veysset and professors Keith Nelson and Christopher Schuh reported their findings in the journals Physical Review Letters and Scripta Materialia.

High-speed imaging reveals that melting induced by impacting particles of metal can impede bonding. The optical setup, with a high-speed camera that uses 16 charged-coupled device (CCD) imaging chips and records images in 3ns, can track individual particles being sprayed onto a surface at supersonic velocities, a feat previously impossible. Shooting up to 300 million images per second allowed researchers to observe a spray-painting-like process similar to ones that apply metallic coatings to surfaces in many industries.

Until now, these proces characteristics have been determined empirically, since the process itself is so fast “you can’t see it, you can’t tell what’s happening, and no one has been able to watch the moment when a particle impacts and sticks,” says Schuh, the Danae and Vasilis Salapatas Professor of Metallurgy and head of MIT’s Department of Materials Science and Engineering.

The top row of photos shows a particle that melts the surface on impact and bounces away without sticking. The bottom row shows a similar particle that does not melt and sticks to the surface. Arrows show impact sprays that look like liquid, but are solid particles. (Photos courtesy of the researchers.)

That uncertainty has led to questions about whether the metal particles actually melt as they strike the surface. The new images make it clear that under some conditions, metal particles sprayed at a surface do melt the surface – and that, unexpectedly, prevents them from sticking. The particles bounce away long before the surface resolidifies, leaving the surface still molten.

When engineers find that a coating material isn’t bonding well, they often increase spray velocity or temperature to increase melting chances. The new results reveal the opposite: melting should be avoided.

The best bonding happens when the impacting particles and impacted surfaces remain in a solid state but splash outward in a way that looks like liquid.

Schuh says, “To stick metal to metal, we need to make a splash without liquid. A solid splash sticks, and a liquid one doesn’t.”

The findings could be relevant for processes used to coat engine components in order to reuse worn parts rather than relegating them to the scrap-metal bin.

In addition to coatings, the new information could also help in the design of some metal-based additive manufacturing (AM) systems, where it is critical to ensure that a layer of printing material adheres solidly to the previous layer.

Massachusetts Institute of Technology