Jan 9, 2018 | By Benedict
Researchers at MIT have developed a new way for getting sprayed metal coatings to adhere, in a study that could have big implications for metal 3D printing. The researchers used an ultra-fast camera to observe a spray coating process and discover the ideal conditions for metal bonding.
Metal surfaces after spray impact, with craters formed by melting
Melting is generally seen as a great way of bonding metal, but a group of MIT researchers recently made a discovery that throws some commonly held scientific beliefs into doubt. Their discovery could have a big impact on metal additive manufacturing.
This exciting research was made possible thanks to a high-tech imaging setup used by the researchers: a high-speed camera that uses 16 separate charged-coupled device (CCD) imaging chips and which can capture images in just three nanoseconds.
The optical kit was mainly developed by MIT postdoc David Veysset, whose setup is so fast it can perform the previously impossible task of tracking individual particles being sprayed onto a surface at supersonic velocities.
The camera can shoot up to 300 million frames per second, and the team put this incredible technology to work on capturing a spray-painting-like process that resembles metallic surface coating.
But this isn’t just a case of getting a closer look at a process scientists already understand. Since people have never been able to properly observe such a spray process before, scientists have had to make estimations about what’s really going on when particles hit their target surface based on the outcome rather than the process itself. Some believe that metal particles melt when they hit the surface; others don’t.
Now, with Veysset’s super-powerful optical rig, the researchers can finally provide some answers. Their discovery? Under some conditions, particles of metal being really do melt the surface. But this actually has a highly unexpected knock-on effect: the particles don’t adhere.
The MIT researchers witnessed that particles bounce away in less time than it takes for the surface to resolidify, which means the particles depart a surface that is still molten.
It’s a highly counterintuitive finding, but one that could totally reshape the way engineers think about bonding: at present, a lack of bonding is often “fixed” by increasing spray velocity or temperature to improve melting. This, however, might be the exact opposite of what they should be doing.
Particle melting and bouncing away (above); not melting and sticking (below)
It’s not all doom and gloom at MIT, however, because the team actually discovered a way to improve bonding, based on the revelatory high-speed events they were witnessing.
They found that bonding works best when sprayed impacting particles and surfaces remain in a solid state but “splash” outward. The particles aren’t liquid, but look like liquid, and this “solid splash” is the best way of getting metal particles to stick.
The researchers’ understanding is only going to improve, too. With the high-speed camera ready for more experiments, the MIT researchers can now work on finding a way to induce the right conditions for this kind of splash, potentially leading to better metal 3D printing processes and improved metal coating technology.
Of course, not all metal 3D printing process work in a similar manner to spray coating, during which sprayed particles impacting a surface. SLM 3D printer manufacturers, for example, can probably rest assured that their machines work just fine.
The researchers do, however, have some specific applications in mind that they think could benefit from the new discovery. They think an improved metal coating process, aided by this high-tech observation rig, could allow engineers to effectively coat engine components so that they might reuse worn parts rather than throwing them away.
“With an old engine from a large earth-moving machine, it costs a fortune to throw it away, and it costs a fortune to melt and recast it,” said MIT professor Christopher Schuh. "Instead, you can clean it off and use a spray process to renew the surface.”
In terms of improved metal 3D printing systems, which require one metal layer to bond effectively to the next, Schuh thinks the new “mathematical rather than empirical” approach can only help improve matters.
In addition to Veysset and Schuh, the research involved the work of postdoc Mostafa Hassani-Gangaraj and professor Keith Nelson.
Their work has been spread across two research papers: “In-situ observations of single micro-particle impact bonding” has been published in Scripta Materialia, while “Melting Can Hinder Impact-Induced Adhesion” has been published in Physical Review Letters.
Posted in 3D Printing Technology
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