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Two years ago, the National Institute of Standards and Technology (NIST), part of the US Department of Commerce, released a report on metal powder additive manufacturing, which included a set of guidelines for powder bed metal 3D printing fusion processes. The report was compiled in an effort to improve manufacturing and promote advances in reliability and process control. NIST said that these guidelines identified important unknown facts of the AM process, so that hopefully researchers could make the methods capable of automated fine-tuning.

The market for metal 3D printing products and services has grown nearly five times since 2010, but while the technology is popular, it can be tough to maintain quality control for the products it makes. Part strength can be diminished by tiny pores in the layers, and residual stresses can increase while the layers cool, causing cracks and warping. According to Dr. Brandon Lane with NIST, two metal products that are manufactured the same way, on the same 3D printer, may not “necessarily come out with the same dimensions.”

“In the additive manufacturing realm, there was already a push in industry to start putting sensors and monitors on their AM machines. So we wanted to have those capabilities and to have a platform on which we could test new ideas for sensors,” said Dr. Lane.

NIST is still exploring metal additive manufacturing, and has built a testbed 3D printer in order to determine how to give users more control over the process. Researchers will use the custom testbed printer, known as the Additive Manufacturing Metrology Testbed (AMMT), to study and explore the metal additive manufacturing process, so that manufacturers and engineers can maintain more predictability over metal 3D printed parts. The AMMT will allow NIST to make tools that can monitor the process in real time, and answer questions like how hot the melting metal gets in each layer, and how to lower the stresses that cause warped and cracked prints.

NIST researchers Steven Grantham, Dr. Lane, Jorge Neira, Sergey Mekhontsev, Mihaela Vlasea, and Leonard Hanssen published a paper on their work, titled “Optical design and initial results from NIST’s AMMT/TEMPS facility.” They are able to completely control the additive manufacturing process with the AMMT, unlike the proprietary software that controls it on commercial systems.

Dr. Lane said, “Commercial printers are a little bit like a black box. Users can dial in a certain laser power and velocity, but they really don’t control every single microsecond of the process. With our system, we control the speed and power of the laser at 100 kilohertz—that’s every 10 microseconds.”

One aspect the AMMT, which is roughly the size of a small car, does have in common with commercial printers is its use of laser powder-bed fusion – a mechanical arm inside the printer applies a thin, even layer of metal powder to a metal plate, and a high-powered laser beam moves across the surface of the powder in the design pattern and melts the powder, which cools into a solid after a brief liquefaction. Oftentimes, process issues happen during this melting, and NIST, which needed a way to monitor the temperature of the melt pool, measured the properties of the light coming off it, as materials turn different colors depending on their temperature.

The AMMT, which can currently print with cobalt chrome, a nickel alloy, and stainless steel, is set up to measure brightness; according to Dr. Lane, many additive manufacturing users “may only want relative measurements of the melt pool fluctuations.”

The goal is to turn these relative measurements into absolute measurements, so brightness and other properties can be used to gauge the actual temperature of the melt pool. To complete this goal, NIST will have to characterize the AMMT system and make sure that the light intensity sensors “are well-calibrated with standards.”

Dr. Lane said, “Eventually we’ll want to get to a full temperature map of the surface over a wide range of light wavelengths.”

This range will be able to stretch from visible blue light, at about 400 nm, to mid-infrared (IR) light, at about 10 microns; this last has wavelengths that are far too long to be seen with the naked eye. At the moment, the team uses a camera that has a custom achromatic lens to measure the brightness of the meltpool over some of these wavelengths.

Grantham explained, “But at the higher temperatures, it’s bluer light, the shorter-wavelength visible light, that matters. So we’ll actually have some different way to measure that.”

Grantham is talking about a device called the Temperature and Emittance of Melts, Powders, and Solids (TEMPS); emittance is a value which accounts for the ability of a substance to put off light at a certain temperature, compared to how a black body emits light. A device like this could be used to study solid materials that experience extreme heat, like the wingtips of supersonic aircraft.

Solid model assembly of the AMMT/TEMPS system with the main carriage removed from the vacuum chamber. Not shown are the TEMPS module, which is positioned behind the vacuum chamber, and various cables and plumbing.

TEMPS will collect data about the light that reflects off the melt pool with a hemispheric reflectometer; this device will then allow the researchers to map the pool’s emittance and its changing temperatures. By knowing the emittance of the melt pool, the team will have a better understanding of how melted metal absorbs the light from the laser, which in turn will help them determine the pool’s actual temperature. TEMPS will also include spectrographs, so the researchers can measure the full visible and IR spectrum to wavelengths of roughly 10 microns. Discuss in the NIST forum at 3DPB.com.

[Source: Machine Design / Images: NIST]

 

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