Nonlinear ultrasonic inspection of friction stir welds

Friction Stir Welding (FSW) is a relatively new solid state welding procedure that was developed at the Welding Institute (TWI), in the UK in 1991, and that has particular advantages over traditional welding techniques. However, just like any other fusion welding technique, the FSW procedure is equally susceptible to a variety of welding flaws such as voids, inclusions and kissing bonds (also known as lack of penetration closed crack). Therefore, it is required to develop dedicated inspection techniques that can evaluate friction stir welds in a nondestructive manner and localize critical defects. One of the most common and hazardous defect in FSW is a kissing bond that occurs at the weld root. Due to the characteristics of the kissing bond and the one-sided access for inspection, conventional ultrasonic methods are not always able to readily detect such kissing bond defects. To solve this problem, we here propose to take advantage of a nonlinear ultrasonic method for the inspection of closed cracks (kissing bond) using a contact pitch-catch method. In the proposed technique, two out of phase (phase 0 & phase 180) chirp signals (ranging from 2 to 5MHz) are emitted by a transmitting transducer with 3.5MHz central frequency. The back wall reflections for both opposite phases are then received by a custom-made transducer with 7MHz central frequency, and the “out of phase canceled signal” (also known as the “pulse inversion signal”) is calculated by the summation of the two out of phase received signals. After normalization with respect to the total energy of the signal, the frequency spectrum of the pulse inversion signal can therefore be used as an indicator of the nonlinearity, and thus the defect severity, along the weld. This frequency spectrum analysis is applied on all A-scan signals acquired along the weld centerline. By stacking up these spectra, frequency B-scan image can be easily obtained and interpreted as a 2D heat map displaying the locations of the damage zones, as shown on the Fig 1. Horizontal and vertical axes on the 2D heat map represent the frequency and the measurement position along the weld centerline respectively.