Directional reflectance microscopy offers quicker, cheaper quality control of safety-critical metal components

By studying how light reflects from the surface of metals, engineers can now tell if manufactured parts meet quality control standards. This time- and cost-effective technology opens new avenues for quality control of safety-critical metal components, such as those used in aerospace.

Typically, metals comprise a myriad of tiny crystal grains, which differ in size, shape, and crystal lattice orientation—namely how the atoms within the grain are packed and arranged in space. The ensemble of these features is usually referred to as the metal’s “microstructure,” which can vary significantly depending on the manufacturing processes used to make the metal part (like casting or forging).

Because the properties of metals are invariably linked to their microstructure, it is paramount to assess this information as parts are produced and even as they are used in engineering applications. This then allows quality control engineers to certify metal components or make informed decisions on whether they have reached end of life and should be replaced.

The challenge, however, is that assessing the microstructure of metals requires high-end equipment and tedious procedures. The current gold standard, for instance, is a scanning electron microscopy technique, which is based on electron diffraction.

Besides the high cost of the equipment required to run these measurements, this technique prevents the direct analysis of entire parts because of the small size of the vacuum chamber. This limits microstructure analysis to small-sized, flat samples that must be extracted from the metal parts produced.

Because of the high cost and low scalability of these measurements, the industry must rely on conservative microstructure estimates to minimize safety concerns around the use of different metal components.

The alternative, innovative technique proposed by researchers at the University of Cambridge is well poised to change this paradigm forever. Using just visible light, the method—called directional reflectance microscopy (DRM)—offers the same microstructural information in an ambient environment, at a fraction of the cost, and over entire metal components.

These results are reported in the journal npj Computational Materials, where the research team showcases these capabilities on an entire turbine blade—the “heart” of jet engines in modern aviation.

“We believe that DRM could open a completely new quality control process flow, whereby metal parts can be analyzed in real time during manufacturing,” says Dr. Matteo Seita from Cambridge University Department of Engineering, who led the research. “This approach is perfectly aligned with the idea of digital manufacturing, where each part is produced as a digital passport that includes information about microstructure.

“Faster and cheaper means that more industries today will be able to check the quality of the metals we use routinely. It also means that the industries of tomorrow will be able to invent new metals more efficiently, like those which we will use to colonize space.”

Dr. Seita and his team have spent years to make DRM as low-cost and accessible as possible, while ensuring that the microstructural information acquired is precise and reliable. DRM requires a simple optical camera and a rotating source of white light, which illuminates the surface of metal parts from different directions.

After etching the metal surface using chemical reagents, the reflected light intensity measured by the optical camera is fed to special image analysis algorithms, which allow inferring the underlying crystal orientation of the grains composing the material. This information is then used to reconstruct the microstructure of the metal part.

The most impressive feature of the new study published by Dr. Seita’s team is that DRM can provide microstructural information directly from the complex, non-flat surface of life-sized metal components.

“This is a game-changer in the field of non-destructive analysis,” says Dr. Seita. “There is no need to dissect metal components into small, flat specimens so that they can fit into the electron microscope. The material’s microstructure can be imaged directly onto the curved surface of the metal part.”

To develop this feature, the research team—which was part in Cambridge and part in Singapore—studied a way to “decouple” the optical signal generated by the underlying microstructure from that which is produced by the non-flat surface of metal parts. Because the local geometry of metal components may not be known a priori, the research team carried out and presented a rigorous error analysis based on the local surface tilt using a combination of experiments and simulations.

“We believe that incorporating DRM into a digital manufacturing paradigm could create more responsive and resilient production systems,” says Dr. Seita. “As a tool that aligns with the vision of interconnected, intelligent factories, DRM represents a significant step forward towards achieving Industry 4.0’s promise of efficient, high-quality, and sustainable manufacturing.

“Indeed, being able to directly measure the microstructure of metal parts rather than relying on estimates may provide an opportunity to relax safety factors around part performance and life predictions. These benefits translate into a longer use of the metal parts produced and thus into a more efficient and sustainable use of resources.”