Aerospace Develops New Method to Detect 3-D Printing Mistakes
Additive manufacturing (also known as 3-D printing) can create parts with complex geometries, building them up one layer at a time. One common version uses a laser to fuse or sinter powdered composites. The technique has been gaining popularity, in part because it can rapidly produce unique components with complex shapes that have customized and graded material properties. In many cases, multiple parts can be combined without the need for subsequent assembly and bonding.
Space system designers are eager to adopt the technology; however, certifying the processes and products remains a challenge. Variations in equipment, the way the tooling is used, and the source-material attributes can affect the final component quality and reliability. When errors and defects do creep in, they are typically not detected until well after production, using conventional testing techniques.
Dr. Henry Helvajian, senior scientist with the Center for Laser Material Processing (part of the Space Materials Laboratory), has been looking for ways to detect manufacturing defects during the manufacturing process itself. He and his team (Dr. Anthony Manzo, William Hansen, who retired this summer, and Lee Steffeney) have developed a laser-based ultrasound system to measure temperature and surface finish at the build location, with pinpoint accuracy.
“We rely on the fact that the speed of sound in materials depends on the temperature of the material that it travels through,” Helvajian explained. The technique uses a pulsed laser to generate ultrasound waves (greater than 10 megahertz) and monitors a specific wave mode that travels along the surface, typically called a Rayleigh wave. By measuring the arrival time of this Rayleigh wave, it is possible to discern the temperature of the zone through which it travelled.
“To sense the arrival time, we had to develop a surface-displacement sensor that could measure down to several angstroms, since acoustic waves do not displace the surface very much,” Helvajian said. “We applied the technology used in FM radios, which is heterodyning, and put it into the optical domain, so that with two lasers (the ultrasound source and the sensor source) placed in very close proximity about the build zone, we can measure the local temperature in between.” Also, by analyzing the frequency content of the sensed-wave packet, the system can discern which waves were scattered (i.e., never reached the sensor), and this provides information regarding the surface roughness of the sampled area.
The team has tested the technique using both a calibrated heat source and a laser-beam heat source to verify the ability to measure relative temperature changes and a commercial surface profilometer to verify the surface roughness results. “We have shown that the technique works on flat and curved surfaces and are now exploring how to image embedded defects,” Helvajian said. “We have also built logic circuits that give the temperature information on a time scale that allows us to make corrections, if necessary.”
The experimental results have recently been accepted for publication in the peer-reviewed Journal of Laser Applications. “Pulsed Laser Ultrasonic Excitation and Heterodyne Detection for In Situ Process Control in Laser 3D Manufacturing,” by Anthony J. Manzo and Henry Helvajian will be published later this year.
One long-term goal is to design a system that will not only detect anomalies and defects, but correct them as part of the process. In this case, signals from the sensors would activate a tertiary laser to repair the build. A detailed record, correlated to the 3-D tool-path for that build, would indicate the exact locations of temperature anomalies and subsequent repairs to aid in qualification and certification. This would require circuitry fast enough to keep up with high-speed additive manufacturing tools. “This may require the development of specific DSP [digital signal processor] chips, which will require that we collaborate with electronic circuit developers,” Helvajian said.
The technology can be adapted to all types of 3-D processing (e.g., laser or electron beam) as well as conventional processing (e.g., machining where surface roughness information might be desired). It has already spawned two patent applications and has been awarded two Small Business Technology Transfer Research grants from the U.S. Navy to spur further development. “We also have the interest of a laser company that wants to collaborate with us to help build the next generation of 3-D printers,” Helvajian said.