NIST develops laser stirring method for metal AM alloys

Researchers at the US National Institute of Standards and Technology (NIST) have reported developing a new metal Additive Manufacturing approach that actively mixes molten metal during processing, enabling the production of complex alloys that have traditionally been difficult to manufacture.

The team also developed an advanced X-ray diffraction method that allows researchers to observe atomic-scale changes in metals in real time as they melt and solidify during the Additive Manufacturing process.

The research focused on overcoming a major challenge in metal AM: achieving uniform mixing of multiple metallic elements within the melt pool. This issue is particularly significant for high-entropy alloys (HEAs), a class of materials containing multiple principal elements in near-equal proportions that can offer enhanced high-temperature performance for applications such as aerospace propulsion and nuclear energy systems.

“HEAs need to be mixed down to the atomic level,” explained Fan Zhang, a physicist at NIST and co-leader of the project. “It takes extra effort to get metals to blend together in those ratios.”

Unlike conventional alloys, which are typically based on a single primary metal with smaller additions of alloying elements, HEAs contain several metals in similar proportions. Differences in density, melting temperature and surface tension can cause these elements to separate during solidification, resulting in chemical inhomogeneity and degraded material performance.

According to the researchers, metal AM offers a potential solution to this challenge. In work published in Additive Manufacturing, the NIST-led team demonstrated a method of improving elemental mixing during Laser Beam Powder Bed Fusion (PBF-LB) Additive Manufacturing by modifying the motion of the laser beam.

Rather than scanning in conventional straight-line patterns, the researchers programmed the laser to follow looping trajectories, creating a stirring effect within the molten metal pool and promoting more uniform mixing.

“Commercial 3D printer software can’t make these patterns,” stated Ho Yeung, a NIST researcher involved in the project. “They are very limited in how the laser’s path can be adjusted, so we had to write the software from scratch.”

Because the approach relies on software rather than new hardware, the researchers believe it could potentially be implemented on existing metal PBF-LB Additive Manufacturing machines.

To validate the technique, the team combined a refractory high-entropy alloy known as RHEA-19 with a titanium alloy, materials that are typically difficult to blend effectively. By applying the looping laser strategy across the interface between the two materials, the researchers produced a new mixed alloy region.

Verification of the process required observation of the material as it transformed from liquid to solid in less than a second. To achieve this, NIST partnered with Argonne National Laboratory’s Advanced Photon Source (APS), a synchrotron facility near Chicago capable of producing extremely bright X-ray beams.

“The APS is one of the few photon sources in the world powerful enough to allow us to perform this type of measurement,” said Zhang.

Using high-speed X-ray diffraction, the researchers tracked atomic-scale structural changes as the alloy solidified. The diffraction measurements were complemented by electron microscopy analysis of the final material, confirming that the modified laser strategy successfully improved mixing between the alloy systems.

The team believes the technique could eventually enable more flexible alloy development and production within metal AM machines. Rather than requiring a dedicated powder feedstock for each alloy composition, elemental or simple alloy powders could potentially be combined and mixed during the build process to create customised materials on demand.

According to the researchers, the approach may also enable the production of functionally graded components with composition changes across a part, reducing the need for joining processes such as welding.

“We want to accelerate alloy making,” Yeung concluded. “Metal 3D printing has the potential to make parts that used to be impossible.”

‘Laser stirring with elliptical scanning enables on-demand alloying in additive manufacturing’ is available here.

www.nist.gov