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A better understanding of the interaction between laser and metal could give industry more control over laser welding, according to a three-year study in laser welding conducted by the National Institute of Standards and Technology (NIST). The data that NIST scientists collected is being used by computer modelers to improve simulations of laser welding processes, a step toward applying the results of the study to industry.
The goal of the NIST team is to build a firm foundation for a model for laser welding that will help manufacturers determine which combination of laser settings will produce the best weld for their application. The NIST researchers are measuring everything that a simulator will need to model the outcome of a weld — the amount of power that is hitting the metal, the amount of energy the metal is absorbing, and the amount of material that is evaporating from the metal as it is heated — all in real time.
Inside NIST’s laser welding booth, a high-power laser melts a piece of metal to form the letters “NIST.” Courtesy of Paul Williams/NIST.
“Our results are now mature enough to where academic researchers are starting to use our data to thoroughly test their computer models in a way that they just haven’t been able to do before, because this kind of data hasn’t been available,” said physicist Brian Simonds.
Many of the techniques the researchers are using to collect the data were created at NIST to measure novel aspects of welding. To gauge laser power during a weld, the researchers designed and built a device that uses the pressure of the laser light to measure the power of the laser. To sense the amount of light absorbed by the heated material as it undergoes changes, they surrounded the metal sample with a device called an integrating sphere, which was designed to capture all the light bouncing off the metal. Using this technique, the researchers discovered that the traditional method for making this measurement underestimates the amount of energy absorbed by the metal during a laser weld. The integrating sphere also allowed the data to be measured in real time.
The team also devised a way to measure the weld plume using laser-induced fluorescence (LIF) spectroscopy. To detect the tiny amounts of elements that evaporate out of the sample during welding, the researchers hit the plume with a second laser that targeted one kind of element at a time. The targeted element absorbed the second laser’s energy and then released it at a slightly shifted energy, producing a strong signal that is also a unique marker of that element. So far, the researchers have demonstrated that LIF can sense trace elements in the weld plume with 40,000× more sensitivity than traditional methods.
All of the experiments are being conducted with a type of stainless steel that is a NIST standard reference material. Use of a standard reference material ensures that experiments conducted anywhere in the world will have access to metal samples with a composition identical to those used by the NIST team.
The NIST researchers said that a multikilowatt laser beam can heat a smaller area of the metals being joined, creating a smaller, smoother seam than a conventional weld, and that laser welding is faster and more energy-efficient than conventional welding. Even with these and other advantages, laser welding makes up only a small fraction of overall welding efforts in the U.S. that could benefit from this technique. A better understanding of the process could make it easier for industries to consider investing in laser-welding infrastructure, the researchers said.
The NIST scientists are collaborating with institutes around the world to expand the data set. They will collaborate with the U.S. Department of Energy’s Argonne National Laboratory to take advantage of that lab’s ability to do high-speed x-ray imaging of the molten pool of metal in real time. Other collaborators include Graz University of Technology, Queen’s University, and the University of Utah.
The researchers are also broadening the scope of their work by directing their high-power laser beams onto metal powders. The powder studies could directly support the community of additive manufacturing, a market worth more than an estimated $7.3 billion in 2017.
The research was published in Applied Optics, a publication of OSA, The Optical Society (https://doi.org/10.1364/AO.58.001239); in Physical Review Applied (https://doi.org/10.1103/PhysRevApplied.10.044061); and in Procedia CIRP (https://doi.org/10.1016/j.procir.2018.08.072).
To learn more about laser welding, register for a free Photonics Media webinar, Laser Source Selection for Microwelding Applications, on June 25, 2019, 1 to 2 p.m. EDT.
This high-speed video shows a weld made with 360 W of focused laser power. The laser (not visible) heats the metal until it melts and forms a pool, which then solidifies. The depth of the finished weld is about 470 micrometers (a little less than half a millimeter). Courtesy of Jack Tanner/NIST.