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FrictionHarmonics: Improving FSW Adoption for Industry

TWI has been working as part of a collaborative group of partners on the FrictionHarmonics Project, to develop a non-linear ultrasonic testing (NLUT) solution that will aid adoption of friction stir welding (FSW) by the aerospace and automotive industries.

The project, which is funded by Horizon 2020, commenced in October 2018 and is due to run until February 2021. TWI is working with Vermon, I KNOW-HOW, Coşkunöz Holding and RI.SE to refine the NLUT system, which will be released under the name KiScan.

KiScan has the ability to detect kissing bonds as small as 0.3mm in diameter, enabling end-users to decrease production times by using FSW for parts and components that are currently joined by riveting.


Friction Stir Welding and the Need for FrictionHarmonics

Developed at TWI in 1991, FSW is a material joining technique that offers substantial advantages compared to other joining methods, particularly for the welding of aluminium alloys. With minimum pre-weld setup and no requirement for post-weld heat treatment, FSW is three to four times faster than gas metal arc welding and consumes less energy while also generating no noise, sparks or fumes.  FrictionHarmonics and subsequently the KiScan system will also be of benefit for the environment as the weight reduction afforded by FSW will lead to lower fuel consumption and thereby a reduction in emissions.

However, FSW is not without its problems. In some instances, disbands can appear between the two joint faces – an effect known as ‘kissing bonds.’ These defects can initiate cracking in-service, thereby undermining the resilience of the join and potentially leading to the failure of the structure. While small, these kissing bonds can have a large impact on the rate of adoption of FSW by manufacturers. Furthermore, the kissing bonds are difficult to detect or size accurately using existing non-destructive testing methods. Research by the FrictionHarmonics consortium has found that the fatigue life of friction stir welds is greatly reduced if kissing bonds exist. For example, a 0.67mm kissing bond reduces fatigue life by 91% compared to the base metal itself, as well as being responsible for crack initiation.

While FSW is widely used by many manufacturers to complete thousands of kilometres of welding each year (with an annual worth of €750m), this still accounts for only 10% of aerospace industry welding.

About FrictionHarmonics

FrictionHarmonics and subsequently the KiScan system adopts a non-linear ultrasonic technique, which has been customised for the inspection of kissing bonds in friction stir welds. The technique uses finite amplitude or acoustic-elasticity to investigate defects in materials. The finite amplitude harmonics measure the acoustic harmonics generated by the interaction of a sound wave with nonlinearities in the material. If finite amplitude ultrasound passes through a region of non-linear properties, it generates higher order acoustic harmonics signals. The harmonic of the fundamental wave is generated when the mechanical wave interacts with an imperfect interface. Since kissing bonds are very tight defects they lead to ‘clapping and rubbing’ when they are excited by the input signal. This generates higher harmonics of the input signal. You can see a schematic of the behaviour of the input pulse after interaction with a kissing bond defect in Figure 1.


Current Progress

TWI has built upon a wealth of experience in order to mature the technique of nonlinear ultrasonics into a fully functional detection system that has been validated on plate structures at TWI’s NDE laboratory in Middlesbrough. This has involved the design and optimisation of new hardware into the system to meet operational and market requirements for dimensions, weight and manufacturing cost.

TWI has built on our past experience in related areas to mature the NLUT scanner into a fully-functional FrictionHarmonics prototype that has been validated on plate structures in a laboratory at TWI’s Friction Stir Welding facilities near Sheffield. This work has included adding and optimising new hardware into the system so that it can meet operational and market requirements for dimensions, weight and manufacturing cost.

The NLUT scanner has been operational for over 18 months, which has allowed for new hardware and software to be commissioned to ensure robustness ahead of environmental testing. Environmental testing was carried out the friction stir welding facility at TWI Yorkshire to determine how the scanner performed in an industrial environment. The detection system has successfully demonstrated good accuracy and repeatability (see Figure 2) and now achieved TRL7 (Technology readiness level). (see Figure 2).

Preparation for Commercial Launch

The next phase of the project will include further refinement of the KiScan software to integrate an operator friendly graphical user interface (GUI). The final stage of the project will be to obtain certification for the safe use of the system to ensure it is market ready. The system will be commercially launched as KiScan from February 2021 with invitations for demonstration issued to industry in due course.

If you are interested in a demonstration of the KiScan please contact us, below.

Figure 1.
Figure 1.
Figure 2.
Figure 2.