University of Cambridge surface imaging offers improved quality control

12 Dec 2024

Directional reflectance microscopy assesses microstructures of safety-critical components.

Reflected light is a valuable tool for studying the quality of manufactured metal parts for quality control purposes, with the nature of the surface's crystal grains affecting the patterns of returned light.

However, this examination and assessment requires high-end equipment and tedious procedures, according to a project at the University of Cambridge that has now developed an alternative approach.

"The current gold standard is a scanning electron microscopy technique which is based on electron diffraction," commented the team. "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."

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

The Cambridge solution is a technique based on visible light, directional reflectance microscopy (DRM), intended to offer the same microstructural information in ambient environment, at a fraction of the cost, and over entire metal components.

As reported in npj Computational Materials, the new method highlights the potential for part-specific quality control in the context of digital manufacturing, according to the team.

"We believe that DRM could open a completely new quality control process flow, whereby metal parts can be analyzed in real time during manufacturing," said Matteo Seita from Cambridge University's Additive Microstructure Engineering Lab (AddME). "This approach is perfectly aligned with the idea of digital manufacturing, where each part produced has a 'digital passport' that includes information about part microstructure."

Game-changer in non-destructive analysis

DRM involves measuring the directional reflectance of surface grains to reconstruct the local surface topography of a material, and in turn map the orientation of the underlying crystal lattice across the sample surface.

It captures a series of optical micrographs from an etched crystalline solid under varying illumination angles, compiling the acquired directional reflectance into directional reflectance profiles. There are then analysed using either analytical or machine learning models depending on the complexity of the signal.

The most impressive feature of the new study, said the researchers, is that DRM can provide microstructural information directly from the complex, non-flat surface of life-size metal components. In trials, DRM was applied to the airfoil of an aerospace turbine blade known to have a faulty bi-crystalline microstructure, and revealed the exact crystallographic orientation in a quantitative manner more suited to digital quality control methods than visual inspection.

"This is a game-changer in the field of non-destructive analysis," commented Matteo 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."

Cambridge believes that incorporating DRM into a digital manufacturing paradigm could create more responsive and resilient production systems, a significant step forward towards achieving the promise of digital-driven "Industry 4.0" manufacturing.

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