Project Background

Friction Stir Welding (FSW) is a relatively new solid-state joining technique for aluminium alloys which offers excellent joint performance and consistent reproducibility.  The use of FSW is of particular interest to the aerospace and wider transport sectors; compared with commonly used riveting joining methods used for aerospace structures, the adoption of FSW will contribute to increased joining speed, higher stress tolerance and longer service life.  In addition, the application of FSW methods have been shown to save up to 10% weight on typical airframe structures, as well as reducing working hours by 40%.

"Kissing bonds" are a problem with FSW; as such defects features can reduce fatigue performance of joints and currently are very difficult to detect (or accurately size) using existing NDT methods. This concern limits the adoption of FSW joining in aerospace, particularly for critical components.

A kissing bond is a specific type of defect in solid-state bonding, in which two solid materials are in contact but with little or no metallurgical bonding present. One of the major causes of kissing bond defects in friction stir welds (FSW) is insufficient penetration of the tool pin into the workpiece material, causing incomplete welding. The kissing bond can significantly reduce the impact strength and fatigue life of the welds causing premature failure. This defect is particularly worrying since it is difficult to detect or accurately size using established NDT methods such as X-ray, conventional ultrasonic and phased arrays techniques.

Figure1: The cross-section view of friction stir welding zone and fatigue life of welds: (top left) defect free; (bottom left) with kissing bond; (right) fatigue life of FSW with and without kissing bonds; with 0.5mm kissing bond, rapid fatigue failure, 0.3mm kissing bond, reduced fatigue lifetime.

Effective detection of kissing bonds would open up a much wider range of applications, including safety-critical subcomponents. Being able to enable the adoption of FSW into the wider, SME dominated, aerospace component market is potentially a €43 billion opportunity. The StirScan NDT inspection system will provide a reliable and cost saving method of detecting kissing bonds.