Aluminium alloys - welded with integrity
'Friction stir welding' (FSW) is a relatively new technique for joining aluminium sheets of varying thickness, which is particularly useful when producing aircraft and automobile components. Unfortunately, such welds are prone to a type of defect called kissing bond, which limits the use of FSW. The EU-funded project STIRSCAN (Detection of kissing bonds in friction stir welds in aero structures) was initiated to develop a non-destructive testing system to detect kissing bonds in aerostructures and automotive components. The aim was the detection of defects smaller than 0.3 mm in length. STIRSCAN project partners have developed an innovative methodology to combine a non-linear ultrasonic technique and an oblique incidence high-frequency ultrasonic technique. This allows for sensitive measurement of a wide variety of defects and detection of small kissing bonds. The high-frequency ultrasonic technique relies on the reflection of two out-of-phase chirp signals emitted by a transmitting transducer with a frequency of ~ 3.5 MHz. With the use of sound waves of a relatively higher frequency, smaller welding defects can be detected. The non-linear ultrasonic technique is based on harmonic frequency components that are generated during wave propagation through a defect. Several case studies have confirmed that the sensitivity of both techniques is far better than conventional methods. The effective detection of kissing bonds will open up a much wider range of applications for FSW, including safety-critical components. Enabling widespread adoption of FSW, the small- and medium-sized enterprise dominated aerospace and automotive component market is potentially worth billions of euros.
Keywords
Aluminium alloys, non-destructive testing, friction stir welding, kissing bonds, non-linear ultrasonic technique
