Final Report Summary - STIRSCAN (Detection of kissing bonds in friction stir welds in aero structures)
Executive Summary:
This project addresses the challenging problem of detecting kissing bonds in friction stir welds. 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 sector. Compared with the commonly used riveting joining methods for aerospace structures, the adoption of FSW offers opportunities for increased joining speed, higher stress tolerance, and longer service life. In addition, the application of FSW methods has been shown to save up to 10% weight on typical airframe structures, as well as reducing production time by 40%.
However, “kissing bonds” are a concern with FSW. 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 is insufficient penetration of the tool pin into the workpiece material, causing incomplete welding. Such features reduce fatigue performance of joints and currently are very difficult to detect or accurately size using existing NDT methods. The fatigue life of friction stir welds is significantly reduced if kissing bonds exist, as has been demonstrated by mechanical testing of welded coupons with defects of known size.
The StirScan project developed innovative ultrasonic based non-destructive testing (NDT) techniques to enable the detection of kissing bond defects below 0.3mm in size in friction stir welded components. A novel non-linear ultrasonic technique, and an oblique incidence high frequency ultrasonic technique, have been used for the assessment. The technique will lead to a more sensitive measurement of interfacial defects and will detect small kissing bond flaws.
Effective detection of kissing bonds would open up a much wider range of applications, including safety-critical subcomponents. Enabling the adoption of FSW into the wide, SME dominated, aerospace component market is potentially a €43 billion opportunity.
StirScan is a collaborative project comprising the following organisations:
SMEs:
• Vermon, (France)
• Theta Technologies, (UK)
• Innora Ltd, (Greece)
• ABIS Sp. Z o.o. Spolka Komandytowa (Poland)
• Saint Jean Wheels, (Norway)
RTDs
• TWI Ltd (UK)
• Katholieke Universiteit Leuven, (Belgium)
• VZLU, (Czech Republic)
The project is co-ordinated and managed by Vermon and is partly funded by the EC under the Research for the benefits of SMEs Project Ref: FP7-SME-2012-315436.
The consortium utilised the ‘Research for the benefit of SMEs’ initiative to build and enhance their internal capability and achieve the unique position to exploit the opportunity identified. Through successful completion of the StirScan project, the SME NDT equipment suppliers and FSW End-Users will be in a unique (and IP protected) market position to be able to exploit the growing (and SME dominated) aerospace and automotive component supply industry.
Project Context and Objectives:
The StirScan project has developed an ultrasonic testing NDT prototype system in order to address the challenging problem of detecting kissing bonds in friction stir welds. The friction stir welding technique is a high-performance joining technique mainly for aluminium alloys. It offers excellent joint performance and reproducibility, which is of significant interest to sectors such as aerospace and automotive, where fatigue performance is particularly crucial.
Friction stir welded structures are used by many aerospace OEMs, including EADS, Boeing, Embraer, Eclipse, NASA, Lockheed Martin, United Launch Alliance and SpaceX for the production of thousands of kilometres of welds, at a cost of approximately €750m each year. However, “kissing bonds” are a concern with friction Stir welding (FSW) as such features reduce fatigue performance of joints, and currently are very difficult to detect or accurately size using existing NDT methods. As fatigue performance is particularly crucial, this limits the adoption of FSW joining in aerospace, particularly for critical components. Until there is a reliable method of detecting kissing bonds, the use and associated benefits of FSW will be restricted to non-safety critical components.
The StirScan project has developed a new and innovative NDT methodology to use two combined ultrasonic techniques, the linear high frequency ultrasonic technique (HFUT), and the non-linear ultrasonic technique (NLUT). The developed techniques are able to detect kissing bond defects below 0.3mm in size in aerospace structures such as: fuselage and wing skins (top and bottom) and automotive components such as aluminium alloy wheels.
The kissing bond is closed or partially closed parts of a crack. In friction stir welds, a crack contains both open and closed parts. The HFUT will be developed to detect the open part of a crack and the NLUT will be developed to detect partially or fully closed part of cracks.
The high frequency ultrasonic technique relies on a methodology based on reflection, transmission and scattering of waves at the defect location (due to a local impedance mismatch). Theoretically, the size of a defect that can be detected by reflection relies on its size being larger than a half of the ultrasonic wavelength. It is for this reason that using high frequency sound waves would potentially result in the detection of smaller flaws. When there is a discontinuity (such as an open crack or kissing bonds) in the wave path, part of the energy will be reflected back from the defect. In contrast to the linear ultrasonic technique, non-linear ultrasonic techniques do not primarily rely on the reflection and scattering of sound from a defect. Sometimes it is difficult to detect kissing bonds using conventional ultrasonic techniques because the resulting change in amplitude is often very small, and therefore not easily noticeable. NLUT is based on the generation of harmonic frequency components (spectrum broadening) during wave propagation through a defect zone (due to local nonlinear friction effects). Both techniques have certain advantages over conventional methods. In this project, both ultrasonic techniques were developed with oblique incidence emission of sound waves.
StirScan has developed two advanced ultrasonic techniques:
• A non-linear ultrasonic technique and
• A high frequency ultrasonic technique for the detection of ‘kissing bonds’ in friction stir welded joints.
StirScan non-destructive testing prototype hardware and software (non-linear and high frequency) contain:
• Three customised ultrasonic probes
• Two multi-axis scanners
• An electronic pulser-receiver instrument to implement both developed techniques and
• Data acquisition and analysis software
By developing the StirScan NDT inspection system, a number of pan-European benefits will be generated through the research collaboration. The developed StirScan system will:
• allow the assessment of high performance aluminium friction stir welded joints for aerospace structures and automotive components;
• enable increased adoption of FSW in the aerospace and automotive industries which will lead to low welding operation costs, clean and safe welding process and new opportunities for product design;
• open up a new inspection market place, targeting on detection of kissing bonds within friction stir welded joints, for the SME inspection equipment and service suppliers;
• enable manufacturers to supply more high performance and good quality friction stir welded components to be able to increase revenue.
To ensure that the work of the RTD performers leads to the development of prototype StirScan NDT system, a number of project objectives were defined throughout the project:
• Develop different models for ultrasonic wave interaction with open parts and partially closed parts (kissing bonds) of cracks using high frequency and nonlinear ultrasonic techniques and to enable development of suitable interpretation algorithms and probe designs.
• Design and develop nonlinear and high frequency ultrasonic probes for the inspection of friction stir welded wheels and plates.
• Establish a detailed specification of defect acceptance criteria related to the effect of the defects on mechanical behaviour and performance.
• Design and development of a prototype pulser-receiver instrument that incorporates a pulser generator capable of implementing both high frequency and nonlinear ultrasonic techniques.
• Development of data analysis algorithm and data collection and processing software.
• Demonstrate functionality of a prototype NDT system with the ability to detect kissing bond defects.
• Design and development of a probe holder and scanner for implementation of both techniques on friction stir welded components.
Project Results:
Samples:
Final friction stir welded plates, with and without kissing bonds, have been manufactured by TWI. The height of some kissing bonds produced within the plates has been confirmed to be below 0.3mm using metallography examination. TWI has also produced some friction stir welded plates containing wormhole defects. This is to show that the developed NDT system is able to detect other types of defects as well. StJean has fabricated friction stir welded wheels with and without lack of tool penetration. The developed high frequency ultrasonic (HFUT) technique has been implemented on both plates and wheels. Due to the complex geometry of wheels, it is extremely difficult to use nonlinear ultrasonic (NLUT) to carry out inspection. Therefore, following discussions within the whole consortium, this technique has been decided to be utilised for the inspection of plates only.
Ultrasonic instrument design and development:
In order to assess the novel inspection technique to detect kissing bonds, an ultrasound instrument with the required capabilities was developed through extensive simulation and experimentation. The ultrasound instrument is capable of implementing both NLUT and HFUT and so is more complicated than conventional ultrasonic instruments. Two transmit channels are necessary and have been developed: one to generate a high power ultrasonic pulse, and the other to generate a high frequency ultrasonic wave. While two transmitters are required, reception of the ultrasound signals is essentially identical, although different frequency ranges are used by each technique. As a result, the instrument contains only one receive channel, one high speed analogue-to-digital converter, and one high speed memory circuit.
To be able to analyse the data obtained when subjecting the material under test to the aforementioned variety of ultrasound inspection techniques, the instrument has been developed to be able to communicate directly with a PC. This requires high speed communications, the development of protocols to facilitate this, and an interface to allow the user to interact with the instrument. Therefore, a control system has been developed to mediate all data transfers to and from the PC. To be effective in an industrial environment, the control system was developed to be capable of taking multiple successive measurements in a short space of time. While there are many interfaces available on a PC, there is a trade-off between data transfer rate and complexity of design. The USB protocol allows a moderate data transfer rate and is relatively simple to implement. Because of this, communications via a USB port was chosen to achieve the level of data throughput required while maintaining a moderate level of design simplicity. In changing applications, the required output signal strength and the received signal strength are controlled by the user, and the digital to analogue converters (DACs) are able to receive instructions from the host PC. In addition to these functionalities the circuitry required for the position sensing capability has been added to provide the option for real time readouts of the current position of the sensors with respect to the material under test.
The developed instrument consists of several printed circuit boards (PCBs) each containing one or more of these modules. As a result, the successful development of instrument covers microcontroller, computer interface, test software, receiver amplifier, digitiser and memory, HFUT transmitter and NLUT frequency generation.
Ultrasonic modelling and simulation
The HFUT technique relies on a methodology based on reflection, transmission and scattering of waves at the defect location (due to a local impedance mismatch). In the case of small defects especially, it is known that shear wave propagation potentially leads to a more sensitive measurement of interfacial defects than the normal incidence longitudinal wave technique. Theoretically the size of a defect that can be detected by reflection relies on its size being larger than a half of the ultrasonic wavelength. This is why the use of high frequency shear sound waves would potentially result in the detection of smaller flaws. When there is a discontinuity (such as an open crack or kissing bonds) in the wave path, part of the energy will be reflected back from the defect. In order to detect kissing bonds in friction stir welded components the required frequency should be higher than 20MHz for the linear high frequency ultrasonic technique. This frequency will provide an ultrasonic wave of which the wavelength (longitudinal wavelength 0.32mm shear wavelength of 0.16mm) is similar or smaller than the defect size (0.3mm). Therefore, 25MHz shear wave was selected. In aluminium the longitudinal wavelength is 0.25mm and shear wavelength is 0.12mm.
In contrast to the linear ultrasonic techniques, non-linear ultrasonic techniques do not primarily rely on the reflection and scattering of sound from a defect. Sometimes it is difficult to detect kissing bonds using conventional ultrasonic techniques because the resulting change in amplitude is often very small, and therefore not easily noticeable. Nonlinear ultrasonic techniques are based on the analysis of spectral broadening (creation of harmonics) at the defect location. Several case studies have shown that the sensitivity of these techniques to incipient damage is far better than conventional ultrasonics. The hypothesis is that a ‘good’ friction stir welded bond (without kissing bonds) behaves almost linearly with no spectral broadening and, as the severity of kissing bonds increases, it behaves more non-linearly with a significant generation of harmonics and sub-harmonics. The degree of harmonic generation provides information about the extent to which the defect behaves non-linearly. As such, this method requires high excitation amplitudes generated by focussing transducers.
Both techniques have certain advantages over conventional methods. In this project, both ultrasonic techniques will be used with oblique incidence emission of sound waves.
Extensive modelling and simulation studies were performed by TWI and KUL to define the most appropriate configurations of ultrasonic transducers. These studies included many exchanges with Vermon to assess the feasibility of the defined probes. The final configurations are finalised below. All of the transducers have been fabricated by Vermon.
• 25MHz high frequency ultrasonic (HFUT) probe with elliptical shape (6mm vertical length, 4mm transverse length, flat in vertical length direction and geometrical focal radius 50mm in transverse length direction). This probe is used to inspect friction stir welded plates.
• 25MHz high frequency ultrasonic (HFUT) probe with circular shape (6mm diameter and spherical focus geometry of 40mm radius). This probe is used to inspect friction stir welded wheels.
• A nonlinear ultrasonic (NLUT) probe with Fermat surface profile containing an inner reception transducer (7MHz) with an area of 25% of the total transducer area and an outer transmission transducer (3.5MHz) thus a relative area of 75%. This probe is designed to inspect friction stir welded plates.
Software and algorithm development
TWI and KUL have developed HFUT and NLUT algorithms which are used to analyse the acquired ultrasonic data. When using a high frequency ultrasonic technique, the technique basically relies on time-domain analysis of measured signals from back-wall reflection to detect any amplitude variations in the received signals. For the non-linear ultrasonic technique, the algorithms are based on either frequency or time analysis of such back-wall reflection signals to obtain the information of the spectral broadening (harmonics and intermodulation frequencies) or signal scalability and polarity, in order to obtain a quantifiable signature of the local non-linearity at the damage location. Innora and Theta have received the algorithms together with a set of simulated signals in order to assist with the developments of the software and testing.
The software developed has been successful in acquiring data from HFUT and NLUT scans. The software is able to select each technique and use the developed algorithm (NLUT or HFUT by KUL and TWI) to post process the collected data. Data measured from both techniques has been used to size (length) and locate defects. Under the NLUT technique the user has the option to select a wide variety of signal types, including frequency amplitude and bandwidth. This, coupled with commercial standard processing techniques, make this piece of equipment, in terms of both hardware and software, a complete package ready to detect a wide range of flaws in welded materials. It has been stated by KUL that the system performs, in terms of data acquisition speed and file saving, at an approximately equivalent level to commercially available equipment. The developed StirScan equipment, in terms of both hardware and software, is a prototype package. However, it has already shown a good performance compared to ‘off the shelf’ equivalents.
Manually driven encoded scanners
As part of the StirScan project to inspect friction stir welds containing kissing bonds, two scanners have been developed by ABIS; one for the inspection of plate samples, which will use both the HFUT probe and the NLUT probe; and the other for the inspection of a wheel sample using just the HFUT probe. The full specifications were agreed by the project partners, and ABIS was tasked with the design and manufacturing of the scanners.
Flaw acceptance criteria
In order to establish the flaw acceptance criteria for kissing bonds, VZLU has devised an extensive mechanical testing programme for the friction stir welded plates manufactured by TWI. The material includes both AA7475 and AA2024. Note that no kissing bond is acceptable for aerospace constructions and, at the same time, the presence of kissing bond cannot be excluded during serial production. Therefore, a flaw size criterion must be established to define the requirements for detectability when using non-destructive testing methods. Testing on two materials, categorised into eight series of welds with various kissing bond sizes proved that NDT must be able to detect a kissing bond size of at least 315μm and 300μm for AA7475 and AA2024, respectively.
Summary of Results
By using the developed StirScan prototype system, the defects in friction stir welded wheels and plates have been successfully detected. The main results developed within StirScan project are as follows:
• StirScan electronic instrument which is able to drive both HFUT and NLUT, including microcontroller, computer interface and test software, receiver amplifier, digitiser and memory
• HFUT transmitter, NLUT frequency generation, NLUT transmitter amplifier.
• Software - data acquisition module for both HFUT and NLUT.
• Dual frequency (3.5MHz and 7MHz) NLUT probe with Fermat surface used for friction stir welded plate inspection.
• Elliptical geometry and spherically shaped HFUT probe used for friction stir welded plate inspection.
• Circular geometry and spherically shaped HFUT probe used for friction stir welded plate inspection.
• Software including post processor module for both HFUT and NLUT.
• The developed HFUT post processing algorithm and inspection methodology to inspect both friction stir welded plates and wheels.
• The developed NLUT post processing algorithm and inspection methodology to inspect friction stir welded plates.
• Manually driven plate encoded scanner used to implement both HFUT and NLUT to inspect friction stir welded plates.
• Manually driven wheel scanner used to implement HFUT to inspect friction stir welded wheels.
A project website was set up at the start of the project to facilitate and act as a communication tool for the consortium. The website consists of two main areas: one accessible to the public, and one only accessible by the members of the consortium. The project website address is: www.stirscan.eu. The website is used also for dissemination of the project results.
A final plan for use and dissemination has been developed giving a clear indication of the exploitation and dissemination activities already carried out and those planned.
Potential Impact:
A number of project results were generated in order to satisfy the innovation needs of the SMEs in the consortium.
• Innovative ultrasonic transducers used for both NLUT and HFUT are available at the end of project.
• Ultrasonic procedures utilising both NLUT and HFUT have been developed.
• Innovative hardware and instrument have been developed in order to implement NLUT and HFUT to inspect friction stir welded components.
• Advanced signal processing algorithms and software to analyse data acquired from non-linear and high frequency ultrasonic methods are available to use with developed hardware and instrument.
• A defect acceptance criterion has been established using mechanical testing results.
• A flat encoded scanner has been developed in order to implement both HFUT and NLUT to inspect friction stir welded plates.
• A wheel scanner has been developed in order to implement HFUT inspection of friction stir welded wheels.
The StirScan project has the potential to create significant impact and economic value for the consortium SMEs through development of an innovative inspection technology for testing the friction stir welded components used in aerospace and automotive industries. The innovative design of the probes, software and instrument will give SMEs a competitive edge in the aerospace and automotive industries’ inspection business. Current commercially available NDT systems have significantly limited capability to detect kissing bonds in friction stir welded components.
The use of the developed StirScan system will allow the extension of the FSW process to application on structural components, or structures that would currently be manufactured using traditional aerospace joining technologies. The developed system will increase the value and quality of manufactured aluminium parts (the European aircraft structure market is worth >10 billion Euros per annum). The adoption of FSW will provide a lighter, high-quality, structure to the aerospace industry that will result in the reduction of carbon emission, fuel consumption and operation cost. This will also assist in the global efforts to reduce carbon emissions. Other sectors which utilise FSW as the joining process, and which will see benefit from the StirScan project, are high performance automotive, rail and shipping.
The NDT inspection system developed in StirScan will provide improved root flaw detection and with great potential to increase inspection rates. Therefore it will enable increased adoption of FSW in transport structures where all structures must comply with safety standards. The advantages of increased FSW adoption include:
• ability to increase the size of commercially available aluminium sheets by welding them before forming.
• enabling continuous, good quality and better surface finish joints due to lack of welding cap.
• production of excellent aerodynamic and aesthetic finishes
• minimum maintenance
• ease of manufacture
• better in-service performance
This project aims to increase the adoption of FSW as a joining method for aircraft structures, especially for fuselage and wing skins (upper and lower). Aside from adopting FSW in aircraft, it could also be used for shuttle rockets, drones (UAVs), etc. With FSW, it will enhance research for other materials (which are strong but light) and complex joint geometries that could not benefit from the riveting technique. Furthermore, the developed StirScan system will benefit other transport industries such as shipping, rail, and high performance automotive in terms of making longer and lighter panels.
Dissemination:
All StirScan partners have been active in dissemination. This included dissemination of project activities and results through scientific journal and conference publications. Also, dissemination activities were conducted via flyers, banners, workshops and online-survey. The StirScan website was regularly updated. A video about the StirScan project has been made and uploaded onto YouTube. The video has also been embedded onto the front page of the StirScan website. A final plan for the use and dissemination has been developed giving a clear indication of past and future dissemination and exploitation activities.
All publications (including websites) contain the following sentence:
‘The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement no 315436. Copyright StirScan © 2014. All Rights Reserved’.
A market survey was set up and went live online in July 2013, and the data collected has been analysed by Theta and TWI. The survey explained the scope of the StirScan project and sought information on end-users’ specific needs. The level of response was not statistically significant but provided general guidance on the potential market. Respondents were predominantly in the field of aerospace (73%), automotive (13%) and rail (13%). Just under half were prime manufacturers (OEM), the remainder being component manufacturers or firms in testing, consultancy or research. The provisional conclusion is that reliability in detection is paramount, whereas recording the results, speed, portability etc. are less significant.
A public video was filmed during the final field and demonstration trials of the developed StirScan system. The whole consortium has participated in the final demonstration, and RTDs have demonstrated the use of the StirScan NDT prototype system to inspect both friction stir welded plates and wheels. The NDT system is able to select the required techniques: high frequency and nonlinear ultrasonic techniques. This video, with duration of 7.16 minutes, went ‘live’ on the 19 Jan 2015, at the following sources:
YouTube - https://www.youtube.com/watch?v=ltVKViYO578&feature=youtu.be
StirScan website - http://www.stirscan.eu/
Exploitation
The results generated from StirScan have been reviewed and some of the results (e.g. Fermat surface dual-frequency transducer, pulser-receiver) potentially could be patented. SMEs in StirScan are currently investigating the applicability of potential patents generated from StirScan. Patents can be used to protect the IPR of a specific product or technical solution. A patent provides the best protection against copies of the product or technology.
A thorough publication and patent search has been carried out in order to investigate if the foreground generated from the project can be patented. The search results show that:
• Information regarding non-destructive testing of friction stir welded components has been published. However, none of the publications have used the combination of nonlinear ultrasonic and high frequency ultrasonic to 100% cover inspection in order to detect kissing bonds. In addition, none of the publications have developed a high frequency ultrasonic technique with an innovative aluminium wedge to inspect friction stir welded wheels with a complex geometry.
• The list of patents contains documents detailing automatic inspection and on-line monitoring of friction stir welding. The developed techniques of those patents are not sufficiently sensitive to detect kissing bonds.
Therefore, the developed StirScan system could be patented based on the publication and patent search results.
List of Websites:
A project website was set up at the start of the project to facilitate and act as a communication tool for the consortium. The website consists of two main areas: one accessible to the public and one only accessible by the members of the consortium. The project website address is: www.stirscan.eu
For general enquiry please contact via StirScan website.
This project addresses the challenging problem of detecting kissing bonds in friction stir welds. 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 sector. Compared with the commonly used riveting joining methods for aerospace structures, the adoption of FSW offers opportunities for increased joining speed, higher stress tolerance, and longer service life. In addition, the application of FSW methods has been shown to save up to 10% weight on typical airframe structures, as well as reducing production time by 40%.
However, “kissing bonds” are a concern with FSW. 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 is insufficient penetration of the tool pin into the workpiece material, causing incomplete welding. Such features reduce fatigue performance of joints and currently are very difficult to detect or accurately size using existing NDT methods. The fatigue life of friction stir welds is significantly reduced if kissing bonds exist, as has been demonstrated by mechanical testing of welded coupons with defects of known size.
The StirScan project developed innovative ultrasonic based non-destructive testing (NDT) techniques to enable the detection of kissing bond defects below 0.3mm in size in friction stir welded components. A novel non-linear ultrasonic technique, and an oblique incidence high frequency ultrasonic technique, have been used for the assessment. The technique will lead to a more sensitive measurement of interfacial defects and will detect small kissing bond flaws.
Effective detection of kissing bonds would open up a much wider range of applications, including safety-critical subcomponents. Enabling the adoption of FSW into the wide, SME dominated, aerospace component market is potentially a €43 billion opportunity.
StirScan is a collaborative project comprising the following organisations:
SMEs:
• Vermon, (France)
• Theta Technologies, (UK)
• Innora Ltd, (Greece)
• ABIS Sp. Z o.o. Spolka Komandytowa (Poland)
• Saint Jean Wheels, (Norway)
RTDs
• TWI Ltd (UK)
• Katholieke Universiteit Leuven, (Belgium)
• VZLU, (Czech Republic)
The project is co-ordinated and managed by Vermon and is partly funded by the EC under the Research for the benefits of SMEs Project Ref: FP7-SME-2012-315436.
The consortium utilised the ‘Research for the benefit of SMEs’ initiative to build and enhance their internal capability and achieve the unique position to exploit the opportunity identified. Through successful completion of the StirScan project, the SME NDT equipment suppliers and FSW End-Users will be in a unique (and IP protected) market position to be able to exploit the growing (and SME dominated) aerospace and automotive component supply industry.
Project Context and Objectives:
The StirScan project has developed an ultrasonic testing NDT prototype system in order to address the challenging problem of detecting kissing bonds in friction stir welds. The friction stir welding technique is a high-performance joining technique mainly for aluminium alloys. It offers excellent joint performance and reproducibility, which is of significant interest to sectors such as aerospace and automotive, where fatigue performance is particularly crucial.
Friction stir welded structures are used by many aerospace OEMs, including EADS, Boeing, Embraer, Eclipse, NASA, Lockheed Martin, United Launch Alliance and SpaceX for the production of thousands of kilometres of welds, at a cost of approximately €750m each year. However, “kissing bonds” are a concern with friction Stir welding (FSW) as such features reduce fatigue performance of joints, and currently are very difficult to detect or accurately size using existing NDT methods. As fatigue performance is particularly crucial, this limits the adoption of FSW joining in aerospace, particularly for critical components. Until there is a reliable method of detecting kissing bonds, the use and associated benefits of FSW will be restricted to non-safety critical components.
The StirScan project has developed a new and innovative NDT methodology to use two combined ultrasonic techniques, the linear high frequency ultrasonic technique (HFUT), and the non-linear ultrasonic technique (NLUT). The developed techniques are able to detect kissing bond defects below 0.3mm in size in aerospace structures such as: fuselage and wing skins (top and bottom) and automotive components such as aluminium alloy wheels.
The kissing bond is closed or partially closed parts of a crack. In friction stir welds, a crack contains both open and closed parts. The HFUT will be developed to detect the open part of a crack and the NLUT will be developed to detect partially or fully closed part of cracks.
The high frequency ultrasonic technique relies on a methodology based on reflection, transmission and scattering of waves at the defect location (due to a local impedance mismatch). Theoretically, the size of a defect that can be detected by reflection relies on its size being larger than a half of the ultrasonic wavelength. It is for this reason that using high frequency sound waves would potentially result in the detection of smaller flaws. When there is a discontinuity (such as an open crack or kissing bonds) in the wave path, part of the energy will be reflected back from the defect. In contrast to the linear ultrasonic technique, non-linear ultrasonic techniques do not primarily rely on the reflection and scattering of sound from a defect. Sometimes it is difficult to detect kissing bonds using conventional ultrasonic techniques because the resulting change in amplitude is often very small, and therefore not easily noticeable. NLUT is based on the generation of harmonic frequency components (spectrum broadening) during wave propagation through a defect zone (due to local nonlinear friction effects). Both techniques have certain advantages over conventional methods. In this project, both ultrasonic techniques were developed with oblique incidence emission of sound waves.
StirScan has developed two advanced ultrasonic techniques:
• A non-linear ultrasonic technique and
• A high frequency ultrasonic technique for the detection of ‘kissing bonds’ in friction stir welded joints.
StirScan non-destructive testing prototype hardware and software (non-linear and high frequency) contain:
• Three customised ultrasonic probes
• Two multi-axis scanners
• An electronic pulser-receiver instrument to implement both developed techniques and
• Data acquisition and analysis software
By developing the StirScan NDT inspection system, a number of pan-European benefits will be generated through the research collaboration. The developed StirScan system will:
• allow the assessment of high performance aluminium friction stir welded joints for aerospace structures and automotive components;
• enable increased adoption of FSW in the aerospace and automotive industries which will lead to low welding operation costs, clean and safe welding process and new opportunities for product design;
• open up a new inspection market place, targeting on detection of kissing bonds within friction stir welded joints, for the SME inspection equipment and service suppliers;
• enable manufacturers to supply more high performance and good quality friction stir welded components to be able to increase revenue.
To ensure that the work of the RTD performers leads to the development of prototype StirScan NDT system, a number of project objectives were defined throughout the project:
• Develop different models for ultrasonic wave interaction with open parts and partially closed parts (kissing bonds) of cracks using high frequency and nonlinear ultrasonic techniques and to enable development of suitable interpretation algorithms and probe designs.
• Design and develop nonlinear and high frequency ultrasonic probes for the inspection of friction stir welded wheels and plates.
• Establish a detailed specification of defect acceptance criteria related to the effect of the defects on mechanical behaviour and performance.
• Design and development of a prototype pulser-receiver instrument that incorporates a pulser generator capable of implementing both high frequency and nonlinear ultrasonic techniques.
• Development of data analysis algorithm and data collection and processing software.
• Demonstrate functionality of a prototype NDT system with the ability to detect kissing bond defects.
• Design and development of a probe holder and scanner for implementation of both techniques on friction stir welded components.
Project Results:
Samples:
Final friction stir welded plates, with and without kissing bonds, have been manufactured by TWI. The height of some kissing bonds produced within the plates has been confirmed to be below 0.3mm using metallography examination. TWI has also produced some friction stir welded plates containing wormhole defects. This is to show that the developed NDT system is able to detect other types of defects as well. StJean has fabricated friction stir welded wheels with and without lack of tool penetration. The developed high frequency ultrasonic (HFUT) technique has been implemented on both plates and wheels. Due to the complex geometry of wheels, it is extremely difficult to use nonlinear ultrasonic (NLUT) to carry out inspection. Therefore, following discussions within the whole consortium, this technique has been decided to be utilised for the inspection of plates only.
Ultrasonic instrument design and development:
In order to assess the novel inspection technique to detect kissing bonds, an ultrasound instrument with the required capabilities was developed through extensive simulation and experimentation. The ultrasound instrument is capable of implementing both NLUT and HFUT and so is more complicated than conventional ultrasonic instruments. Two transmit channels are necessary and have been developed: one to generate a high power ultrasonic pulse, and the other to generate a high frequency ultrasonic wave. While two transmitters are required, reception of the ultrasound signals is essentially identical, although different frequency ranges are used by each technique. As a result, the instrument contains only one receive channel, one high speed analogue-to-digital converter, and one high speed memory circuit.
To be able to analyse the data obtained when subjecting the material under test to the aforementioned variety of ultrasound inspection techniques, the instrument has been developed to be able to communicate directly with a PC. This requires high speed communications, the development of protocols to facilitate this, and an interface to allow the user to interact with the instrument. Therefore, a control system has been developed to mediate all data transfers to and from the PC. To be effective in an industrial environment, the control system was developed to be capable of taking multiple successive measurements in a short space of time. While there are many interfaces available on a PC, there is a trade-off between data transfer rate and complexity of design. The USB protocol allows a moderate data transfer rate and is relatively simple to implement. Because of this, communications via a USB port was chosen to achieve the level of data throughput required while maintaining a moderate level of design simplicity. In changing applications, the required output signal strength and the received signal strength are controlled by the user, and the digital to analogue converters (DACs) are able to receive instructions from the host PC. In addition to these functionalities the circuitry required for the position sensing capability has been added to provide the option for real time readouts of the current position of the sensors with respect to the material under test.
The developed instrument consists of several printed circuit boards (PCBs) each containing one or more of these modules. As a result, the successful development of instrument covers microcontroller, computer interface, test software, receiver amplifier, digitiser and memory, HFUT transmitter and NLUT frequency generation.
Ultrasonic modelling and simulation
The HFUT technique relies on a methodology based on reflection, transmission and scattering of waves at the defect location (due to a local impedance mismatch). In the case of small defects especially, it is known that shear wave propagation potentially leads to a more sensitive measurement of interfacial defects than the normal incidence longitudinal wave technique. Theoretically the size of a defect that can be detected by reflection relies on its size being larger than a half of the ultrasonic wavelength. This is why the use of high frequency shear sound waves would potentially result in the detection of smaller flaws. When there is a discontinuity (such as an open crack or kissing bonds) in the wave path, part of the energy will be reflected back from the defect. In order to detect kissing bonds in friction stir welded components the required frequency should be higher than 20MHz for the linear high frequency ultrasonic technique. This frequency will provide an ultrasonic wave of which the wavelength (longitudinal wavelength 0.32mm shear wavelength of 0.16mm) is similar or smaller than the defect size (0.3mm). Therefore, 25MHz shear wave was selected. In aluminium the longitudinal wavelength is 0.25mm and shear wavelength is 0.12mm.
In contrast to the linear ultrasonic techniques, non-linear ultrasonic techniques do not primarily rely on the reflection and scattering of sound from a defect. Sometimes it is difficult to detect kissing bonds using conventional ultrasonic techniques because the resulting change in amplitude is often very small, and therefore not easily noticeable. Nonlinear ultrasonic techniques are based on the analysis of spectral broadening (creation of harmonics) at the defect location. Several case studies have shown that the sensitivity of these techniques to incipient damage is far better than conventional ultrasonics. The hypothesis is that a ‘good’ friction stir welded bond (without kissing bonds) behaves almost linearly with no spectral broadening and, as the severity of kissing bonds increases, it behaves more non-linearly with a significant generation of harmonics and sub-harmonics. The degree of harmonic generation provides information about the extent to which the defect behaves non-linearly. As such, this method requires high excitation amplitudes generated by focussing transducers.
Both techniques have certain advantages over conventional methods. In this project, both ultrasonic techniques will be used with oblique incidence emission of sound waves.
Extensive modelling and simulation studies were performed by TWI and KUL to define the most appropriate configurations of ultrasonic transducers. These studies included many exchanges with Vermon to assess the feasibility of the defined probes. The final configurations are finalised below. All of the transducers have been fabricated by Vermon.
• 25MHz high frequency ultrasonic (HFUT) probe with elliptical shape (6mm vertical length, 4mm transverse length, flat in vertical length direction and geometrical focal radius 50mm in transverse length direction). This probe is used to inspect friction stir welded plates.
• 25MHz high frequency ultrasonic (HFUT) probe with circular shape (6mm diameter and spherical focus geometry of 40mm radius). This probe is used to inspect friction stir welded wheels.
• A nonlinear ultrasonic (NLUT) probe with Fermat surface profile containing an inner reception transducer (7MHz) with an area of 25% of the total transducer area and an outer transmission transducer (3.5MHz) thus a relative area of 75%. This probe is designed to inspect friction stir welded plates.
Software and algorithm development
TWI and KUL have developed HFUT and NLUT algorithms which are used to analyse the acquired ultrasonic data. When using a high frequency ultrasonic technique, the technique basically relies on time-domain analysis of measured signals from back-wall reflection to detect any amplitude variations in the received signals. For the non-linear ultrasonic technique, the algorithms are based on either frequency or time analysis of such back-wall reflection signals to obtain the information of the spectral broadening (harmonics and intermodulation frequencies) or signal scalability and polarity, in order to obtain a quantifiable signature of the local non-linearity at the damage location. Innora and Theta have received the algorithms together with a set of simulated signals in order to assist with the developments of the software and testing.
The software developed has been successful in acquiring data from HFUT and NLUT scans. The software is able to select each technique and use the developed algorithm (NLUT or HFUT by KUL and TWI) to post process the collected data. Data measured from both techniques has been used to size (length) and locate defects. Under the NLUT technique the user has the option to select a wide variety of signal types, including frequency amplitude and bandwidth. This, coupled with commercial standard processing techniques, make this piece of equipment, in terms of both hardware and software, a complete package ready to detect a wide range of flaws in welded materials. It has been stated by KUL that the system performs, in terms of data acquisition speed and file saving, at an approximately equivalent level to commercially available equipment. The developed StirScan equipment, in terms of both hardware and software, is a prototype package. However, it has already shown a good performance compared to ‘off the shelf’ equivalents.
Manually driven encoded scanners
As part of the StirScan project to inspect friction stir welds containing kissing bonds, two scanners have been developed by ABIS; one for the inspection of plate samples, which will use both the HFUT probe and the NLUT probe; and the other for the inspection of a wheel sample using just the HFUT probe. The full specifications were agreed by the project partners, and ABIS was tasked with the design and manufacturing of the scanners.
Flaw acceptance criteria
In order to establish the flaw acceptance criteria for kissing bonds, VZLU has devised an extensive mechanical testing programme for the friction stir welded plates manufactured by TWI. The material includes both AA7475 and AA2024. Note that no kissing bond is acceptable for aerospace constructions and, at the same time, the presence of kissing bond cannot be excluded during serial production. Therefore, a flaw size criterion must be established to define the requirements for detectability when using non-destructive testing methods. Testing on two materials, categorised into eight series of welds with various kissing bond sizes proved that NDT must be able to detect a kissing bond size of at least 315μm and 300μm for AA7475 and AA2024, respectively.
Summary of Results
By using the developed StirScan prototype system, the defects in friction stir welded wheels and plates have been successfully detected. The main results developed within StirScan project are as follows:
• StirScan electronic instrument which is able to drive both HFUT and NLUT, including microcontroller, computer interface and test software, receiver amplifier, digitiser and memory
• HFUT transmitter, NLUT frequency generation, NLUT transmitter amplifier.
• Software - data acquisition module for both HFUT and NLUT.
• Dual frequency (3.5MHz and 7MHz) NLUT probe with Fermat surface used for friction stir welded plate inspection.
• Elliptical geometry and spherically shaped HFUT probe used for friction stir welded plate inspection.
• Circular geometry and spherically shaped HFUT probe used for friction stir welded plate inspection.
• Software including post processor module for both HFUT and NLUT.
• The developed HFUT post processing algorithm and inspection methodology to inspect both friction stir welded plates and wheels.
• The developed NLUT post processing algorithm and inspection methodology to inspect friction stir welded plates.
• Manually driven plate encoded scanner used to implement both HFUT and NLUT to inspect friction stir welded plates.
• Manually driven wheel scanner used to implement HFUT to inspect friction stir welded wheels.
A project website was set up at the start of the project to facilitate and act as a communication tool for the consortium. The website consists of two main areas: one accessible to the public, and one only accessible by the members of the consortium. The project website address is: www.stirscan.eu. The website is used also for dissemination of the project results.
A final plan for use and dissemination has been developed giving a clear indication of the exploitation and dissemination activities already carried out and those planned.
Potential Impact:
A number of project results were generated in order to satisfy the innovation needs of the SMEs in the consortium.
• Innovative ultrasonic transducers used for both NLUT and HFUT are available at the end of project.
• Ultrasonic procedures utilising both NLUT and HFUT have been developed.
• Innovative hardware and instrument have been developed in order to implement NLUT and HFUT to inspect friction stir welded components.
• Advanced signal processing algorithms and software to analyse data acquired from non-linear and high frequency ultrasonic methods are available to use with developed hardware and instrument.
• A defect acceptance criterion has been established using mechanical testing results.
• A flat encoded scanner has been developed in order to implement both HFUT and NLUT to inspect friction stir welded plates.
• A wheel scanner has been developed in order to implement HFUT inspection of friction stir welded wheels.
The StirScan project has the potential to create significant impact and economic value for the consortium SMEs through development of an innovative inspection technology for testing the friction stir welded components used in aerospace and automotive industries. The innovative design of the probes, software and instrument will give SMEs a competitive edge in the aerospace and automotive industries’ inspection business. Current commercially available NDT systems have significantly limited capability to detect kissing bonds in friction stir welded components.
The use of the developed StirScan system will allow the extension of the FSW process to application on structural components, or structures that would currently be manufactured using traditional aerospace joining technologies. The developed system will increase the value and quality of manufactured aluminium parts (the European aircraft structure market is worth >10 billion Euros per annum). The adoption of FSW will provide a lighter, high-quality, structure to the aerospace industry that will result in the reduction of carbon emission, fuel consumption and operation cost. This will also assist in the global efforts to reduce carbon emissions. Other sectors which utilise FSW as the joining process, and which will see benefit from the StirScan project, are high performance automotive, rail and shipping.
The NDT inspection system developed in StirScan will provide improved root flaw detection and with great potential to increase inspection rates. Therefore it will enable increased adoption of FSW in transport structures where all structures must comply with safety standards. The advantages of increased FSW adoption include:
• ability to increase the size of commercially available aluminium sheets by welding them before forming.
• enabling continuous, good quality and better surface finish joints due to lack of welding cap.
• production of excellent aerodynamic and aesthetic finishes
• minimum maintenance
• ease of manufacture
• better in-service performance
This project aims to increase the adoption of FSW as a joining method for aircraft structures, especially for fuselage and wing skins (upper and lower). Aside from adopting FSW in aircraft, it could also be used for shuttle rockets, drones (UAVs), etc. With FSW, it will enhance research for other materials (which are strong but light) and complex joint geometries that could not benefit from the riveting technique. Furthermore, the developed StirScan system will benefit other transport industries such as shipping, rail, and high performance automotive in terms of making longer and lighter panels.
Dissemination:
All StirScan partners have been active in dissemination. This included dissemination of project activities and results through scientific journal and conference publications. Also, dissemination activities were conducted via flyers, banners, workshops and online-survey. The StirScan website was regularly updated. A video about the StirScan project has been made and uploaded onto YouTube. The video has also been embedded onto the front page of the StirScan website. A final plan for the use and dissemination has been developed giving a clear indication of past and future dissemination and exploitation activities.
All publications (including websites) contain the following sentence:
‘The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement no 315436. Copyright StirScan © 2014. All Rights Reserved’.
A market survey was set up and went live online in July 2013, and the data collected has been analysed by Theta and TWI. The survey explained the scope of the StirScan project and sought information on end-users’ specific needs. The level of response was not statistically significant but provided general guidance on the potential market. Respondents were predominantly in the field of aerospace (73%), automotive (13%) and rail (13%). Just under half were prime manufacturers (OEM), the remainder being component manufacturers or firms in testing, consultancy or research. The provisional conclusion is that reliability in detection is paramount, whereas recording the results, speed, portability etc. are less significant.
A public video was filmed during the final field and demonstration trials of the developed StirScan system. The whole consortium has participated in the final demonstration, and RTDs have demonstrated the use of the StirScan NDT prototype system to inspect both friction stir welded plates and wheels. The NDT system is able to select the required techniques: high frequency and nonlinear ultrasonic techniques. This video, with duration of 7.16 minutes, went ‘live’ on the 19 Jan 2015, at the following sources:
YouTube - https://www.youtube.com/watch?v=ltVKViYO578&feature=youtu.be
StirScan website - http://www.stirscan.eu/
Exploitation
The results generated from StirScan have been reviewed and some of the results (e.g. Fermat surface dual-frequency transducer, pulser-receiver) potentially could be patented. SMEs in StirScan are currently investigating the applicability of potential patents generated from StirScan. Patents can be used to protect the IPR of a specific product or technical solution. A patent provides the best protection against copies of the product or technology.
A thorough publication and patent search has been carried out in order to investigate if the foreground generated from the project can be patented. The search results show that:
• Information regarding non-destructive testing of friction stir welded components has been published. However, none of the publications have used the combination of nonlinear ultrasonic and high frequency ultrasonic to 100% cover inspection in order to detect kissing bonds. In addition, none of the publications have developed a high frequency ultrasonic technique with an innovative aluminium wedge to inspect friction stir welded wheels with a complex geometry.
• The list of patents contains documents detailing automatic inspection and on-line monitoring of friction stir welding. The developed techniques of those patents are not sufficiently sensitive to detect kissing bonds.
Therefore, the developed StirScan system could be patented based on the publication and patent search results.
List of Websites:
A project website was set up at the start of the project to facilitate and act as a communication tool for the consortium. The website consists of two main areas: one accessible to the public and one only accessible by the members of the consortium. The project website address is: www.stirscan.eu
For general enquiry please contact via StirScan website.