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Development of an in-process quality assurance system for friction stir welding

Final Report Summary - SIGNASTIR (Development of an in-process quality assurance system for friction stir welding)

Executive Summary:
Friction stir welding (FSW) is best suited to mass production where repetitive long lengths of weld are produced quickly and in a cost effective manner by automated machinery. A major limiting factor to the performance of such machinery and generally in the take-up of FSW, is the lack of an effective in-process quality assurance inspection method. As some of the defects are not visible on the weld surface defective products of low integrity could potentially be supplied to customers.

The SignaStir project has developed an effective in-process quality assurance system for the inspection of friction stir welds predominantly used in the manufacture of aluminium rolling stock and marine vessels. SignaStir will also give added confidence to those companies giving consideration to use of the relatively new FSW process in their products due to the availability of a technically viable and commercially sound approach to guaranteeing weld quality and limiting post process inspection and re-work requirements.

The SignaStir project has produced a novel and innovative in-process quality assurance system for the inspection of FSW in real-time. The system detects critical defects in welds through an unique and holistic weld quality ‘signature’ and reports the presence and location of defects either to the machine operator or to the machine controller. The focus of the project was dedicated to the design and building of an integrated optical head, and the conception of user-friendly inspection software. The conception of the inspection technique and its validation for off-line inspection had been undertaken previously.

The SignaStir system inspects FSW joints in-process immediately after the welding tool has passed by. The inspection hardware is mounted using special purpose fittings and manipulation hardware so that it follows the welding tool along its processing path. The welding tool travels along the joint line at a predetermined rate; the inspection hardware follows the tool scanning the weld at the same rate. It is anticipated that the SignaStir system will be used by manufacturers who have existing welding machines. In this case the welding machine operator may not have the skills or experience to interpret the inspection data. Interface software has been developed specifically to present the inspection data in a clear unambiguous format which the welding machine operator will understand and be able to act upon. It is also planned that the SignaStir system may be integrated into new build FSW machines to provide closed loop feedback to the machine control system such that automatic corrections may be made to the welding machine settings.

The development of the SignaStir system has required the creation of new inspection technology and know-how. The key outputs of the project include a new understanding of the application of non-contact inspection systems for the inspection of FSWs and the definition of the capability of non-contact inspection systems for inspection of FSWs. All developed and manufactured hardware parts and software modules of the SignaStir system have been integrated. The optical elements and laser components (interferometer, generation and detection lasers) were connected and the laser beams aligned. The inspection equipment is operated through a virtual desktop; the software modules were integrated and tested. Good quality linear welds and linear welds containing intentional defects were produced. A first test showed that installation of the inspection equipment on a large gantry production welding machine at OTH partner SAPA had no impact on the operation of the welding machine. However, while a high quality laser signal was confirmed off-line, the signal repeatedly deteriorated once the fully assembled unit was mounted on the welding machine and real-time correlation between inspection data and weld quality was not possible. The demonstration trials gave very valuable knowledge about how the system design should be improved to be functional in a production environment. Design improvements have now been identified to alleviate the issues identified during the demonstration trials. Final laboratory measurements on the linear weld produced during the demonstration have verified that the SignaStir system is able to detect the size of voids of interest to FSW users.

Project Context and Objectives:
Friction stir welding (FSW) is best suited to mass production where repetitive long lengths of weld are produced quickly and in a cost effective manner by automated machinery. A major limiting factor to the performance of such machinery and generally in the take-up of FSW, is the lack of an effective in-process quality assurance inspection method. As FSW machines are set and operated by humans there is always a possibility of errors being made. Once set, a FSW machine is able to make many tens of metres of weld per hour with minimal human intervention. This level of productivity is impressive but, without appropriate weld quality monitoring, has the potential to go very wrong and result in either major rework or scrap issues for the manufacturer. As some of the defects are not visible on the weld surface defective products of low integrity could potentially be supplied to customers. The prototype SignaStir system is thought to be the World’s first application of effective in-process quality assurance for FSW.

The SignaStir project has developed an effective in-process quality assurance system based on laser ultrasound (L-UT) for the inspection of friction stir welds predominantly used in the manufacture of aluminium rolling stock and marine vessels. The focus of the project was dedicated to the design and building of an integrated optical head, and the conception of user-friendly inspection software. The conception of the inspection technique and its validation for off-line inspection had been undertaken previously

The SignaStir project has produced a novel and innovative in-process quality assurance system for the inspection of FSW in real-time. The system detects critical defects in welds such as buried volumetric defects. The system determines the unique and holistic weld quality ‘signature’ and reports the presence and location of defects either to the machine operator or to the machine controller. No other inspection system available today can provide this service.

The SignaStir system inspects FSW joints in-process immediately after the welding tool has passed by. The inspection hardware is mounted using special purpose fittings and manipulation hardware so that it follows the welding tool along its processing path. The welding tool travels along the joint line at a predetermined rate; the inspection hardware follows the tool scanning the weld at the same rate.

It is anticipated that the SignaStir system will be used by manufacturers who have existing welding machines. In this case, the welding machine operator will not have the skills or experience to interpret the inspection data. Interface software has been developed specifically to present the inspection data in a clear unambiguous format which the welding machine operator will understand and be able to act upon. It is also planned that the SignaStir system may be integrated into new build FSW machines to provide closed loop feedback to the machine control system such that automatic corrections may be made to the welding machine settings. This approach has been explored and implemented as far as practically possible.

The development of the SignaStir system has required the creation of new inspection technology and know-how. The key outputs of the project include a new understanding of the application of non-contact inspection systems for the inspection of friction stir welds and the definition of the capability of non-contact inspection systems for inspection of friction stir welds.

The objectives of the SignaStir project are to make a number of technological advances combined into a prototype that enable on-line real-time non-destructive testing of friction stir welds at welding speeds in excess of 500mm/min. Key elements of the inspection system have been specified, designed and assembled to give a prototype with the ability to detect the location of all significant defects to within +/- 2mm.

Project Results:
Assessment of L-UT capability
The propagation of ultrasound through aluminium FSW samples has been modelled using the CIVA ultrasonics module (version 10). Both 2-D and 3-D simulation models were constructed. Root defects produced significant signal amplitudes due to their ideal position on the weld centreline. Cavities were modelled as side drilled holes; flaws of different diameters and position within the weld were simulated. Cavity detection is strongly dependent upon the size of the flaw and its position relative to the inspection system.

A user’s perspective of FSW inspection was provided by SAPA and Rubitech, this information is being used to guide the specification and development of the SignaStir system. A programme of practical welding trials was developed in discussion with both Rubitech and SAPA. The matrix seeks to generate both good quality welds and welds containing representative weld flaws through appropriate adjustment of welding conditions. The matrix enables the effect of material thickness, alloy composition and joint geometry to be determined.

Work at ISPL focused on the determination of the quality of the friction stir welded joints. Assessment included tensile and fatigue testing, hardness measurements, microscopic examination (macro and microscopic examination), non-destructive tests by radiographic and conventional ultrasonic techniques. L-UT evaluation was undertaken by KIM. The experimental results were in good agreement with the capability predicted in the modelling activity. The L-UT technique is able to detect voids down to 200micron, and can differentiate between small, medium and large voids. It has not been possible to detect root flaws of the level that leave a witness following bend testing. This is principally due to the vertical form of the defect and the absence of any width/volume.

Inspection system development
A full technical specification of the L-UT equipment including laser capability, detection and receiver positioning are now available. Information regarding the dimensional constraints for current FSW equipment has been provided by the process users to determine the space available for the SignaStir system. The design of the prototype sub-system is based on the optical equipment used by RTD KIM. The prototype design was developed by SME Inspecta following a site visit to OTH partner SAPA. The layout of the optical system is designed to be compatible with the access restrictions on SAPA’s production FSW machine, which will be used for the final demonstration in work package 7. The laser equipment is packaged with a small amount of adjustment and also incorporates a dedicated camera and additional illumination. Alignment of the optical elements and off-line testing was also completed.

The functional requirements of the mounting system have been identified via a survey of available free space surrounding the spindle on partner FSW equipment. Dimension specifications in terms of the physical dimensions of the L-UT hardware, the required positioning of the L-UT sensor(s) relative to the work piece, and possible mechanical interface for the FSW machine all require consideration. The mounting and manipulation sub-system has been designed and manufactured to enable installation of the inspection hardware on SAPA’s production FSW equipment.

Lasers are classified according to power; L-UT uses Class 4 lasers which means they are damaging to both eyes and skin. A laser shielding device had been designed to accommodate a range of vertical distances between fixing location and work piece. The laser beams are contained within a column, the flat base of which slides along the surface of the work piece.

The prototype SignaStir inspection system includes laser power units, analysis equipment and items of computer hardware. It is advantageous for these items to be packaged together. Additionally it is good practice to minimise the distance between the laser and its power source. An equipment frame has been designed to house the ancillary equipment. Where the SignaStir system is fitted to a gantry-type machine, the equipment frame will be fixed to the top of the gantry. Where the SignaStir system is fitted to a smaller machine or a robot, the equipment frame will be floor mounted.

The end users have supported the specification of the user interface. The inspection system produces data which must be displayed in an easily interpretable format. The expectations of the end users have guided the development of the user interface. The user interface uses commercially available systems in combination with newly developed modules to deliver the required functionality. Off line testing was realised using a specific generator software to produce a representative stream of test dat. Modification of the L-UT analysis software has been undertaken to generate a ‘quality index’ (QI), which is used to distinguish a good weld from a bad weld.

Integration with new-build equipment
Integration of the SignaStir system with new equipment provides three opportunities:
1) an integrated interface & mounting system;
2) closed loop feedback control capability;
3) independent movement of the L-UT system relative to the welding head.

The variety of welding machines available on the market and further opportunity to tailor the basic design to a client’s specific requirements indicate a true plug-and-play system with integrated interface and mounting system is unlikely. The mounting system developed incorporates significant flexibility in order to minimise the distance between the FSW tool and inspection location. Work has been undertaken to develop the hardware and software required to use the SignaStir system on commercial machines.

The SignaStir hardware can be interfaced with commercial machines via the ‘ScanStir’- control system. Seam tracking capability is required to inspect 2-D weld paths. It is important that the L-UT system can both follow the weld path, and move relative to the welding head to maintain the correct orientation of the laser relative to the joint-line. Options for the implementation of feedback control have been considered.

Integration & Demonstration:
All developed and manufactured hardware parts and software modules of the SignaStir system have been integrated constituting the SignaStir prototype. The optical elements and laser components (interferometer, generation and detection lasers) were connected and the laser beams aligned. The inspection equipment is operated through a virtual desktop; the software modules were integrated and tested. Good quality linear welds and linear welds containing intentional defects were produced.

A first test showed that installation of the inspection equipment on a large gantry production welding machine at OTH partner SAPA had no impact on the operation of the welding machine. However, while a high quality laser signal was confirmed off-line, the signal repeatedly deteriorated once the fully assembled unit was mounted on the welding machine and real-time correlation between inspection data and weld quality was not possible. The demonstration trials gave very valuable knowledge about how the system design should be improved to be functional in a production environment. Design improvements have now been identified to alleviate the issues identified during the demonstration trials. Final laboratory measurements on the linear weld produced during the demonstration have verified that the SignaStir system is able to detect the size of voids of interest to FSW users.
Potential Impact:
The SignaStir project has made significant technological progress in the challenging area of FSW inspection. The project has developed a system for real-time FSW quality assurance. The SignaStir system is the first of its kind for this application and has significant scientific data to support its operational characteristics. A number of possible alternative applications have been identified including use of the system for in-process inspection of other weld types, use of the system for in-process inspection of other hot working process products and use as part of a closed loop control system.

The SignaStir project has demonstrated a novel and innovative in-process quality assurance system for friction stir welds in real-time. The system will detect critical defects in welds and report them to the machine operator. The project work has resulted in significant improvements beyond the state-of-the-art and led to the following project innovations:
1) Creation of a prototype standalone FSW integrity monitoring system.
2) An integration strategy allowing the monitoring system to be incorporated into new-build FSW machines.
3) Development of a user-interface and supporting algorithms to present weld quality data to the operator.

The SignaStir system will find application across many industry sectors where FSW can be used to add value and quality to aluminium fabrications. The expected applications will span a number of key industry sectors including aerospace, rail, marine and general purpose fabrication. All these sectors use FSW on key components. It is also expected that more components will become candidates for FSW as the SignaStir system will make the process more reliable to apply. This will put the SME partners in a strong position as providers of the technology to industry.

For the SMEs participating in the project, there is an impact at both European and International level. Although FSW was invented in Europe, the industrial development of the technique and commercial exploitation have been more significant in the Far East and North America. The Global rail manufacturing industry has accepted FSW as an important process in terms of economical manufacture and production of crashworthy rolling stock. Production of low integrity FSW joints in rail vehicles is unacceptable due to the reduction in critical joint performance during a collision in service. As such, manufacturers of rolling stock must currently invest considerable time and finance in post weld inspection which will be avoided through the use of a SignaStir system.

Increased use of FSW will bring about significant economic advantages for its users but also major environmental and safety benefits. Use of the SignaStir system to facilitate FSW in place of conventional fusion welding techniques will reduce power consumption, pollution (in terms of particulate fume, gases and noise) and improved safety for the operator who will no longer be exposed to dangerous UV radiation, extreme high temperatures and intense noise at close quarters.

The results of the project provide useful information regarding the creation of NDT protocols for the FSW process. FSW is a relatively new process and to date no agreed European Standards or Codes of Practice have been issued governing weld inspection, although ISO 25239 does state maximum permissible defect sizes.

This project has developed data linking L-UT assessment data with the presence of defects, and also determined the effect of the detected flaws on weld performance. Both these outcomes are important elements in moving toward such codes of practice. A number of partners are represented on national & international welding committees and are ideally placed to disseminate the capability of L-UT for real time inspection of friction stir weld.

Exploitation & dissemination:
Promotion of the project idea and dissemination of project results started during the project and will continue as part of business development activities performed by the four SMEs after the project end.
A first project brochure has been professionally produced and provided to the partners to distribute to interested parties. The brochure was also translated into Polish for distribution at a seminar held at ISPL.
In addition to this SME partner HBSE presented a paper at the 9th International FSW Symposium in June 2012, which was well received. A paper discussing the correlation between simulated L-UT results and those obtained experimentally has been prepared by KIM & TWI for presentation at ‘LU2013 Laser Ultrasonics and advanced sensing’ in July 2013.

The prototype system was tested under real production conditions at OTH partner SAPA at the end of the project. Mounting and testing the system could be realised without significant interruptions of the production, safety issues could be handled in cooperation with SAPA.
Although the results of the test are very promising some system weaknesses have been detected during the work with system. One possible improvement detected is the easier handling of lens adjustments, not requiring dismounting the entire system from the FSW spindle.

The system is now in a status proving the concept without the need for further academic research work. Based on the documentation of the partners involved in the final tests the SMEs will assess the status of the prototype and the effort for improving the system. Here the market situation of 2013 has to be monitored, too. An enhanced system concept considering the lessons learned and the feedback from the end users will be worked out by the SMEs after the project.

The SMEs agreed to develop the system further. Each partner will bring in its individual expertise and will concentrate on applications/components they are specialised in. The combined effort will result in the upcoming SignaStir system.

To separate the SignaStir project from the future business related matters, the SME decided to set up a new website under their supervision where potential customers are provided with a first set of information. It is very likely that potential customers are willing to see the system in action. Therefore machines available at one SME could be used in the future. Equipment not directly owned can be rent on a fairly basis from the RTD partners.

List of Websites:
A project website was set up to act as a communication port between the partners and disseminate the project http://www.signastir.eu. A product focussed website has been created to promote and market the commercialised SignaStir inspection system http://www.signastir.com.

Project
For general enquiries regarding the SignaStir project please contact Dr Kathryn Beamish, kathryn.beamish@twi.co.uk at TWI Ltd, Granta Park, Great Abington, Cambridge, CB21 6AL, UK. Tel: +44 (0)1223 899000, Website: www.twi.co.uk

System
All enquiries regarding the SignaStir inspection system should be addressed to Dr Henrik Schmidt, hs@hbs-engineering.dk at HBS Engineering, HBS Engineering ApS, Erdalsvej 1, 2600 Glostrup, Denmark. Tel: +45 21766440, Website: www.hbse.dk or www.signastir.com.