The National Institute of Standards and Technology (NIST)’s Engineering Laboratory (EL) and Physical Measurement Laboratory (PML) have collaboratively produced the Additive Manufacturing Metrology Testbed (AMMT), a custom-made metal AM system which the institute intends to use in its research into AM processes. The institute’s goal is reportedly to study Additive Manufacturing in depth and produce tools for monitoring the process in real time.
Brandon Lane, a member of NIST’s Engineering Laboratory team, explained, “In the Additive Manufacturing realm, there was already a push in industry to start incorporating sensors and monitoring systems on their machines. So we wanted to be able to have that capability, and we also wanted a platform where we could test completely new ideas for sensors.”
The testbed system designed by NIST is reportedly about the size of a small car and uses laser powder-bed fusion. Currently, the machine is set up to use three different metals common to commercial AM machines: stainless steel, cobalt chrome, and a nickel alloy.
Unlike commercial systems, however, which usually come with proprietary software inbuilt, NIST’s testbed gives researchers complete control over the system. “Commercial systems are a little bit ‘black box,’” Lane continued. “You can command a certain laser power and velocity, but you really don’t have control over every single microsecond of the process. With our system, we can control the speed and power of the laser at 100 kilohertz – that’s every 10 microseconds.”
Of the two teams involved in the AMMT’s manufacture, the EL will now primarily be responsible for running the tests on the system, while the PML’s researchers are working to supply sensors for the measurement of processes, as well as calibrations and traceability to national standards.
Since many of the problems encountered during Additive Manufacturing occur during the melting stage of the process, a large focus of the NIST team’s efforts has been on developing a way to precisely monitor the temperature of the melt pool. The team has concluded that the best way to gauge the melt pool’s temperature is to measure the colour and brightness properties of the light emitted by it.
As of now, NIST has established a method for measuring the brightness of light emitted by the melt pool– which they say may be enough for many AM users. “They may only want relative measurements – relative observations of the melt pool fluctuations,” Lane explained. “But eventually we’ll want to get to a full temperature map of the surface over a wide range of light wavelengths, from blue visible light at about 400 nm to mid-infrared (IR) light at 10 microns, which has wavelengths that are too long for the human eye to see.”
Currently, the researchers are using a camera with a custom-designed achromatic lens to measure the pool’s brightness over some of the wavelengths they will eventually need, from reddish to near-IR light, at about 850 nm. “But at the higher and higher temperatures, it’s the bluer light – the shorter-wavelength visible light – that matters,” added Steve Grantham, of NIST’s PML team. “So we’ll actually have some different diagnostics to measure that.”
The addendum sensor system which Grantham and his PML colleagues plan to create over the next year and a half is called the Temperature and Emittance of Melts, Powders, and Solids (TEMPS) sensor. Among its many capabilities, the system will include a reflectometer in the shape of a hemisphere, which will allow the team to collect information about the light reflecting off of the melt pool. The reflectometer will, in turn, enable them to map the emittance of the melt pool as well as its changing temperatures over time. TEMPS will also include spectrographs, permitting measurement of the full visible and infrared spectrum to wavelengths of 10 microns.
In the coming years, NIST’s researchers stated that they expect to expand their capabilities further. “Right now, [we’re] looking at three basic materials, but anything that anybody’s using in Additive Manufacturing is fair game,” Grantham concluded.