NIST utilizes laser reflection to enhance 3D metal printing

A project at NIST has developed a new way to monitor and assess 3D printing of metals.
Finding and correcting defects inadvertently created inside a 3D printed part is one of the biggest challenges for metal printing, commented NIST. But getting a close look at the printing operation as it's underway is not easy.

As well as the toxicity of the raw materials, there can be a risk of combustion or explosion, compelling the use of sealed manufacturing platforms or inert gas atmospheres. And at the exact location where a laser is acting on metal particles to create a melt pool, sputtering clouds of cold powder and molten metal make for a dynamic environment.

Caustic diagnostic: defects spotted

"It would be very helpful to monitor how the print is going in real time," said David Deisenroth from the NIST Production Systems Group. "Is the part getting too hot? Are there any defects? We want to be able to adjust the printer to address these problems because it will lead to stronger and more consistent parts."

The NIST solution involves caustics, the everyday optical phenomenon in which light rays reflected or refracted by a curved surface are focused onto a flat surface creating a pattern of illumination. Light passing through a glass of water creates caustics visible as the patterns of focused light next to the glass. A rainbow is also a caustic, with light refracted into arcs of differing radius.

In the case of metal printing, some of the laser light reflects off the surface of the metal during the printing process creating caustics. NIST theorized that the pattern of this reflection can give information about the shape of the liquid metal’s surface.

Reflections on a curved dome

NIST made use of its Fundamentals of Laser-Matter Interaction testbed, or FLaMI, a laboratory 3D printing platform designed to allow researchers to study laser-matter interactions in additive manufacturing operations.

Deisenroth outfitted the test bed with a hollow dome about the size and shape of a basketball cut in half, originally sold as an architectural decoration. This covers the metal sample that will be melted with the laser, and has a small slit at the top where the laser can pass through.

The dome was designed to catch all the light caustics reflected by the laser, in the same way that the underside of a bridge catches the light reflected off a river. High-speed video of the inside of this dome gave NIST data about how light was reflecting off the metal melt pool.

"The biggest challenge was creating a coating for the inside of the dome that would reflect the laser light only once," noted Deisenroth. "If the dome were too reflective, the light would bounce around many times, and it would look uniform. If it wasn’t reflective enough, we wouldn't see any light at all."

In proof-of-concept trials, Deisenroth successfully used the reflected light to identify the creation of particular flaws termed keyhole pores, formed when vaporized metal at the melt pool's surface presses a pit into the metal part. The optical data also allowed an estimation of how deep the keyhole pit was.

The next steps will include increasing the video frame rate from its current 60,000 fps to a much faster level, ideally up to 825,000 fps.

"The lasers we use are invisible to the eye, and the reflections move so fast that you can only see them with a high-speed camera," commented Deisenroth. "It's amazing to think that we can capture these caustics in action and draw meaning out of them."

Source: optics.org