Non-destructive ultrasonic examination of root defects in friction stir welded butt-joints

• An empirical positioning and time gating technique is applied for root defect examination of welded butt joints.
• The backscattered energy of the time gated echo depends on grain morphology and on the presence of micro-defects.
• Identification and localization of root defects is realized by analyzing their unique backscatter energy C-scan fingerprint.
• Alternative ultrasonic inspection technique for AL alloys without the habitual need for high frequency analysis.

As a solid-state welding process, Friction Stir Welding (FSW) offers a variety of advantages over traditional welding processes. Problems that are typically occurring due to the cooling of the liquid phase, such as solidification cracking and formation of porosity, generally do not occur in FSW. Nevertheless, as a result of suboptimal settings of the welding process parameters and certain uncontrollable conditions, FSWs are still associated with a number of specific flaws, e.g. root flaws and wormholes.Ultrasonic non-destructive testing and evaluation techniques (NDT&E) can be used for quality assessment of friction stir welded joints. In this paper, a novel approach for the detection of root flaws is proposed using an immersion ultrasonic testing method in oblique incidence and backscatter mode. The backscattered energy C-scan images obtained after an empirical positioning and proper time gating can be straightforwardly interpreted by direct comparison with typical ‘flaw’ patterns, allowing for identification and localization of the root flaws in the weld. The method is illustrated for FSW butt joints of the AlZnMgCu (7XXX series) alloy.

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