A case study on the evaluation of friction stir welds by ultrasonic inspection technique

Friction stir welding (FSW) is an innovative process that has been successfully used in joining aluminum alloys, normally difficult to weld. Important advantages over fusion welding are better retention of baseline material properties, lower residual stresses and excellent mechanical properties. Defects that occur in FSW joints are due to improper tooling or setup. Non-destructive techniques used to inspect FSW joints are X-rays, eddy current, conventional ultrasonic testing and dye penetrant proper only for surface breaking defects. Currently there is no non-destructive technique that can guarantee absence of flaws in friction welds during manufacturing. In this work, non-destructive remote testing of friction welds, based on laser ultrasonic technique, was investigated to detect and quantify defects in the joint. The laser ultrasonic system generates ultrasonic acoustic waves by thermal expansion and detects the surface vibration by an interferometric receiver. Several specific FSW samples were inspected; some, manufactured with optimum parameters, used as calibration samples, and others, manufactured with modified settings, with defects. The results of the tests conducted on the FSW samples show that the system is able to detect the defect. A quantitative analysis of the defects was extrapolated from the ultrasonic signals acquired across the weld. Since the defect evaluation can be done in-line, during the friction weld manufacturing process, the laser ultrasonic technique plays an important role in containing the FSW production cost.