Transforming Welding with Comprehensive NIST Metrology

Welding has been around, in some form, for centuries. Today it enables a large percentage of the U.S. economy,* thanks to its role in the creation of a diverse range of everyday goods from furniture and cars to computers and mobile phones.

But despite its history and ubiquity, some aspects of welding are still poorly understood, which costs many industries money and time.

To aid in this effort, a collaboration between NIST's Physical Measurement Laboratory (PML) and Material Measurement Laboratory (MML) has begun an ambitious program to study the science of welding using a new facility on NIST's Boulder, Colo., campus. In particular, they are investigating welds made with high-power lasers.

"The NIST Laser Welding Program is an exciting opportunity to bring the full breadth of NIST metrology to a difficult subject that impacts our daily lives," says Marla Dowell, Chief of PML's Applied Physics Division, who is co-leading the effort with James Fekete, Chief of MML's Applied Chemicals and Materials Division. "We are applying state-of-the-art metrology to develop a fundamental understanding of the weld process, connecting process parameters to weld quality through advanced materials characterization techniques. The end goal is to develop a predictable welding process, thus avoiding costly, time-consuming destructive measurements after the fact."

Laser welding is more energy efficient than conventional welding, says PML physicist and laser applications project leader Paul Williams. Conventional welding typically uses an arc of electricity to heat and fuse materials. In contrast, a multi-kilowatt laser beam can heat a smaller area of the metals being joined, creating a smaller, smoother seam than a conventional weld, on the order of millimeters rather than centimeters.

Even with these and other advantages, laser welding makes up only a small fraction of overall welding efforts in the U.S. that might otherwise use this technique, Williams continues. A better understanding of the process could make it easier for industries to consider investing in laser welding infrastructure.

"The idea behind this new welding lab is to introduce more fundamental science into the welding process," Dowell says. "If we're successful, we'll come up with a set of tools so that you can measure the conditions while you're welding, and so that you can predict in advance whether or not it's a good weld without actually having to go and destroy those welds after the fact."

NIST is hardly the first institute to study the science of welding. However, the researchers feel their effort is unique in some ways because of its scope.

Their facility features a new welding booth – an enclosed space for the welds to take place – as well as lasers capable of producing a beam up to ten kilowatts (kW).** (A 10 kW laser can cut 1 cm of stainless steel.) A set of spectroscopy instruments will give them a measurement of the atoms leaving the metal during a weld, which will help them to understand local metal composition.***

By measuring radiation pressure – that eerie phenomenon in which massless light produces a force that can move objects – Williams and colleagues have created a system that gives them an absolute reading of their laser's power in real time. This is a first; previously, only relative measurements in real time were possible.

Meanwhile, within the same block of buildings on campus, a team of researchers led by MML's Ann Chiaramonti Debay is using a suite of tools in the NIST Precision Imaging Facility to get a three-dimensional picture of the chemical composition of a weld's cross section with resolution across seven orders of magnitude, from atomic to centimeter scale. Finally, with tools in MML's Mechanical Testing Facility, MML's Jeff Sowards is exploring weld fatigue and fracture characteristics at both microscopic and macroscopic scales and, with the information he gathers, determining how to improve the process. These measurements provide a previously unavailable window into the welding process, Dowell says.

When put together, this correlated information could potentially be used to create a database of clues that manufacturers could look for when creating their own recipes for the perfect welds for their respective systems, the researchers say. Furthermore, the team is exploiting the database infrastructure of MML's Thermodynamics Research Center to make their results accessible to the welding community.

High on the list of questions the PML/MML team hopes to address are subjects of debate within the welding community. For example, researchers know that welding can introduce gases such as nitrogen into welds while metals are joined. Welding can also deplete nitrogen from alloys where it is intentionally added to promote strength and corrosion resistance. The ability to measure and track where the nitrogen is going during the welding process could enable greatly optimized weld compositions and properties.

Also, it's understood that heating a weld area changes its chemical composition, and that this heat is so intense – roughly three orders of magnitude higher irradiance than on the surface of the sun – that stress can build up between the hotter and cooler parts of the materials. This can lead to undesired phenomena such as cracking; the new program should shed light on this process.

In addition to continuing their own tests and working with MML, the PML welding researchers are engaged in several collaborations. For example, along with MML they are partnering with the Auto/Steel Partnership (A/SP), a consortium whose members include GM, Ford, Chrysler, and several North American steel manufacturers, to do a comparison of arc and laser welding. "Joining processes are critical elements in vehicle lightweighting strategies, and the A/SP plays a key role in evaluating joining technologies, including laser welding", Fekete says. They are also looking at welding as it relates to the processing of solar cells, an effort spearheaded by PML NRC postdoc Brian Simonds with materials scientists at the University of Utah and University of New South Wales.

And the PML/MML team is working with PML's Dan Hussey on NIST's Gaithersburg campus to use the neutron scattering facility; neutrons can see "through" metals, creating images that show areas of stress and strain.

Early results include real-time power measurements during the weld process, which could profoundly change how laser welding is done. The team also has preliminary data connecting laser power to weld properties, though it's too soon to make any statements yet about their findings.

"What we're trying to understand is extreme laser-matter interactions," Dowell says. "We're looking at kilowatt levels. We're looking at tens of millions of atoms. And we're trying to understand a process where all four states of matter are present – gas, solid, liquid, and plasma. It's really very complicated."

Dowell says she hopes the collaboration will have some definitive answers within the year.

— Reported and written by Jennifer Lauren Lee

*The combined revenue of industries related to welding represented as much as one third of the U.S. GDP in 2000, according to one analysis.

** Between May and August 2014, this high-power laser lab was utilized to perform the first multi-kilowatt laser calibrations at any NMI (national metrology institute).

*** Interrogation techniques include laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF), which actively probe the weld plume with light to help identify the metal's lighter elements, which can be predictors of weld quality.