Researchers at the Massachusetts Institute of Technology (MIT) have found that in some situations, metal melting can inhibit metal bonding rather than promote it. The surprising and counterintuitive finding could have serious implications for the design of certain coating processes or for three-dimensional (3-D) printing, which require materials to bond together and stay that way.

When bonding two pieces of metal, either the metals must melt a bit where they meet or some molten metal must be introduced between the pieces. A solid bond then forms when the metal solidifies again.

The research indicates, however, that under some conditions, molten particles bounce away in much less time than it takes for the surface to resolidify, and leave the surface while it is still molten.

Revolutionary Optical Technology

A revolutionary advance in the technology for observing extremely high-speed interactions, along with high-speed imaging, reveals that melting induced by the impact of metal particles can impede bonding. The optical setup, primarily developed by MIT postdoctoral researcher David Veysset, includes a high-speed camera that uses 16 individual charged-coupled device (CCD) imaging chips and can record images in three nanoseconds.

The camera is so fast that it can track individual particles being sprayed onto a surface at supersonic velocities, an accomplishment that previously was not possible. The team used this camera, which can shoot up to 300 million frames per second, to observe a spray painting type of process similar to those used to apply a metallic coating to surfaces.

While such processes are widely used, their characteristics have been determined empirically until now, since the process itself is so fast “you can’t see it, you can’t tell what’s happening, and no one has ever been able to watch the moment when a particle impacts and sticks,” said Christopher Schuh, a MIT professor and member of the research team.

As a result, there has been ongoing controversy about whether the metal particles actually melt as they strike the surface to be coated. The new technology enables the researchers to observe what is occurring and study it, he says.

If engineers find that a coating material isn’t bonding well, they may be inclined to increase the spray velocity or temperature to increase the chances of melting. However, the new results show that melting should be avoided.

Most Effective Bonding

The most effective bonding occurs when the impacting particles and impacted surfaces remain in a solid state but “splash” outward in a way that looks like liquid. According to Schuh, it was an eye-opening observation. For a metal to bond to metal, the splash needs to be solid — a liquid splash doesn’t bond. That phenomenon was found in a variety of these metal-processing methods, he said. With the new ability to observe the process, the researchers note that the conditions needed to induce that bond can now be identified.

The findings could be relevant for processes used to coat components. In addition to coatings, the new information could also help in the design of some metal-based additive manufacturing systems, known as 3-D printing. There, as with coatings, it is critical to ensure that one layer of the printing material adheres solidly to the previous layer.

By determining the optimal conditions to ensure a solid bond, Schuh said, this work provides an accurate and mathematical approach rather than an empirical approach for improving the bond of sprayed metal coatings.

The work was supported by the U.S. Army through MIT’s Institute for Soldier Nanotechnologies, the U.S. Army Research Office, and the U.S. Office of Naval Research.

For more information, contact: MIT, (617) 253-2700, www.mit.edu