Application Note: X-Ray Diffraction for Weld Assessment

The problem: Residual stress in welding

A university needed to develop modified parameters for use in friction stir welding as applied to common aerospace materials. Specifically, the group was focused on creating weld recipes which result in increased fatigue life. One approach was to reduce the steep gradient in residual stress induced during the joining procedure.

The testing method: X-ray diffraction

X-ray diffraction was used to measure line profiles of surface residual stress across sample weld zones. Stress was measured every 5mm spanning the weld and surrounding “heat-affected zones” for a total distance of 70mm. Stress was measured in two directions, parallel and adjacent to the weld.

Diagram of the sample residual stress measurement location. — Figure 1: Diagram of the sample measurement location.

Non-destructive surface measurements allowed for nearly overlapping measurements and a high-resolution characterization of the residual stress variation across the weld zone. The results ultimately helped the research team understand the stress and thus performance effects related to specific changes in process parameters.

The results

For simplicity, the results from a single sample are shown below in Figure 2. In this sample, there is a significant difference between the stresses parallel to and transverse to the weld. The results correlated well to the variation in fatigue life when tested in orthogonal loading conditions for the sample.

Figure 2: 2-Direction residual stress vs. location from weld center.

Instruments used for this application:

Xstress G2R

X-ray diffractometer Xstress G2R represents advances in design and construction, which provide enhanced reliability and function in a truly portable residual stress and retained austenite analyzer. Bi-axial and tri-axial stress state analysis is effortless and automated with rotating diffractometer.