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Laser processing firm augments peening capabilities using x-ray diffraction

LSP Technologies (LSPT), an Irish equipment manufacturer and provider of laser processing services, has invested in a new x-ray diffraction (XRD) instrument in order to improve its laser peening capabilities.

The firm will use XRD to perform in-house measurements of compressive residual stress in metals, enabling it to develop laser peening prescriptions for customer needs more quickly and accurately for complicated geometries and parts.

‘Customers are increasingly asking for the kind of precision which XRD measurements can yield,’ said Dr Micheal Kattoura, a materials research engineer for LSPT. ‘Now we can prove out the benefits of laser peening on even the most complicated geometries efficiently with XRD measurements.’

Laser peening, a cold deformation process for metal improvement, induces deep levels of compressive residual stress in metal to prevent fatigue, corrosion, cracking, flaking, and other cases of metal failures, increasing the useful lifetime of metal components by a factor of five or more.

To document the benefits of laser peening, the labs at LSPT’s Materials Science Center of Excellence provide customers with detailed reports on the depth and character of residual stress levels below the surface of metals.

Now LSPT will be able to achieve this via XRD, a process by which x-rays are directed towards a material and penetrate its surface by a few micrometres, causing them to become diffracted. By analysing the unique diffraction fingerprint, patterns of residual compressive stress can be revealed in the material. The process is particularly ideal for examining metals because the wavelength of x-rays is at the same magnitude as the distance between grain layers in metals.

The new XRD device being used by LSPT is the Xstress G3 from Stresstech, a Finnish provider of both non-destructive and destructive testing solutions for process control and quality inspection. The system is able to access small spaces and complex geometries, including many of the industrial shapes and components typically treated by LSPT’s laser peening processes.

The setup of the Xstress G3 XRD system from Stresstech. (Image: LSPT)

In most cases, according to LSPT, XRD exceeds the precision of slitting tests, formerly used by the firm’s materials lab to measure compressive residual stress. Here, a wire slitter would slice through the surface of metal parts, and electronic strain gauges would then provide data that matched a residual stress profile. Due to the strain gauges being mounted on the back of the sample, slitting was not able to capture near surface residual stresses with high certainty. The slitting process used at LSPT was designed for rectangular shaped samples, but not for complex metal shapes such as blades, gear teeth, or the interior of tubes.

‘XRD testing overcomes many of the problems with slitting, but I think the real advantages of in-house XRD testing is our ability to improve our cycle time on delivering results to customers,’ remarked Kattoura.

This is because for new alloys and shapes of components, laser peening must deliver precise levels of residual compressive stresses at precise locations. That may take some experimentation at several different levels of power density (GW/cm2), different layering of peening spots, and the altering of other parameters available with LSPT’s Procudo laser peening system – which the firm claims is the only commercially available laser peening system.

‘Now, with XRD in our toolkit, we can test quickly and accurately, seeing the stress levels we achieved in the critical locations within hours, not days, over a wide range of variables, and then we can meet customer requirements accurately and economically,’ Kattoura concluded. ‘The XRD testing capability will help us understand more about the benefits of laser peening, and that can only help customers we serve and customers we hope to serve in the future.’

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