Nickel-based superalloys, which remain strong and corrosion-resistant at high temperatures, are popular for making aerospace components such as turbine engines. They can be machined with laser melting, both for welding and additive manufacturing, but the process can sometimes lead to cracking. Now a group of researchers using the Advanced Photon Source at Argonne National Laboratory has shown how in-situ X-ray diffraction can explain thermomechanical behaviors in such alloys during laser processing.
The team examined the Ni alloy 718, a widely used alloy that primarily contains nickel, chromium, molybdenum, and iron. Laser processing of the material creates a melt pool consisting of two areas: the fusion zone where the material fully melts and the heat-affected zone, where the temperature rises high enough for the metal to undergo changes without melting. Additionally, a laser with high enough energy and penetration can vaporize the material, creating a temporary cavity called a keyhole within the fusion zone.
The researchers found that mechanical changes in the structure varied between these zones and depended on the depth of the laser penetration and the direction in which it moved across the alloy. They found slight local variations within the melt pool, including regions in the keyhole that either fully or partially resolidified. They then compared residual elastic strains in the keyhole across different laser powers and scan speeds. and looked at different depths in the sample. Researchers found that varying laser conditions had little effect on the strain within the keyhole. In conduction mode processing, which created a melt pool with no keyhole, residual strains were greater and increased with the speed of the laser scan.
The researchers attributed this difference in strain to the longer time that material in the keyhole remained melted. That leads to slower solidification of the alloy and more relaxation of the stress. Additionally, the unstable keyhole mode scans are known to lead to a high density of pores, which can alleviate the stress, while micro-cracks in laser-processed areas can allow the strain to relax.
The X-ray diffraction studies that the researchers performed produced spotty diffraction patterns. These corresponded to images from a scanning electron microscope that showed that different elements within the alloy had separated and collected into tree-like dendrites, altering the mechanical qualities of the alloy. The concentrations of chromium and iron decreased in the space between the dendrites, while the concentrations of niobium and molybdenum increased.
To verify that their estimation of the thermal behaviors in the laser processed region was correct, the researchers performed finite element simulation, using a computer to model heat flow. Although the model came up with somewhat different numbers than the diffraction data for the temperature and magnitude of strain, the results were in reasonable agreement with the experimental data, and the scientists said they shored up their explanation for the material behavior they observed.
The researchers performed in situ X-ray diffraction at Beamline 1-ID-E at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory. That beamline provides high-energy X-ray beams that can be focused to a small spot, allowing scientists to examine the different heat-affected areas under laser scanning using a high-speed detector.
Future work could apply these same techniques to other materials used in additive manufacturing, such as titanium or aluminum alloys. Scientists could also test a wider range of laser conditions to deepen their understanding. – Neil Savage
See: S. Oh1, R. Lim2, A.C. Chuang3, B. Gould3, A. Bucsek4, A. Rollett1, “Multiscale investigation of thermomechanical and compositional developments in Ni alloy 718 under laser processing,” Acta Mater 294, 121145 (2025) 10.1016/j.actamat.2025.121145.
Author affiliations: 1Carnegie Mellon University; 2Lawrence Livermore National Laboratory; 3Argonne National Laboratory; 4University of Michigan.
This work was supported by the National Nuclear Security Administration under grant number DE-NA0003915. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No DE-AC02–06CH11357. The apparatus for rapid laser heating was originally developed by Prof. Tao Sun (Northwestern Univ.). The authors acknowledge use of the Materials Characterization Facility (MCF) at Carnegie Mellon University supported by grant MCF-677785.
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