In-process discontinuity detection during friction stir welding | Dr. Amber Shrivastava | 2019

Sabarmati Young Researcher Seminar Series
Abstract
Friction stir welding (FSW) is inherently cost-effective, however, the need for significant weld inspection can make the process cost-prohibitive. A new approach to weld inspection is required - where an in-situ inspection of welded joints can be performed, reducing the need for post-process inspection. The discontinuity detection methods developed in this work are based on force measurements during FSW. Friction stir welds with discontinuity and without discontinuity were created. Force data from the welds was analyzed. The welds without discontinuities were accompanied by higher force amplitudes at the tool rotation frequency. Also, the welds with discontinuities were accompanied, by higher force amplitudes at higher harmonics corresponding to the number of flats on the FSW tool’s probe, in addition to the tool rotation frequency. Computerized tomography scans of the welds were performed to locate and visualize the sub-surface discontinuities. The location and geometry of the discontinuities, along with the corresponding force pattern confirm the formation of a cavity at the back of the friction stir tool once per revolution. It is shown that discontinuities are formed when the cavity is not completely filled by the material moving under the influence of the friction stir tool. The relationship between the force transients and the discontinuity sizes indicate that the force measurement during friction stir welding can be effectively used for detecting discontinuities in friction stir welds. The normalized force transient data and normalized discontinuity size are correlated to develop a criterion for discontinuity detection, which is validated against additional welds. The discontinuity sizes estimated by the force measurement-based method are in good agreement with the discontinuity sizes measured by computerized tomography. These results show that the force measurement based discontinuity detection model method can be effectively used to detect discontinuities during friction stir welding.