TWI Industrial Member Report Summary 907/2008
By C Blignault
Friction stir welding (FSW) is a relatively new welding technology, which has been rapidly developed and implemented throughout the world, since its invention by TWI in 1991. Control methods for FSW have also developed rapidly during this period, and a number of different systems have been produced by various FSW machine manufacturers and process users. Such systems usually act to control one of the key welding parameters, most commonly z-axis position or force (although systems that control x-axis force, z-axis torque, or weld/tool temperature have also been demonstrated). A review of these different control approaches is timely in order to provide guidelines for process users on the most appropriate choice of system for a given FSW application.
Process management and quality control (QC) for FSW is also a developing area for this relatively new technology. FSW is a solid-phase welding technology, and as such is capable of generating relatively unusual flaws and/or defects, which can prove to be very challenging for conventional weld NDT methods. A summary of the potential flaws that can occur in FSW, and the capabilities of current NDT systems are given in the Appendix of this report. In the absence of fully established NDT procedures for FSW, significant attention has been given to process monitoring systems. A range of monitoring and process management approaches are currently applied by industrial users of the process, but relatively little published information on these systems is available. Monitoring and control systems developed by research institutions range from basic force measurement units to more sophisticated adaptive control equipment. Advanced monitoring and control of the FSW process, and the use of such data in QC are still not well understood or standardised, therefore, scope exists to review and summarise the current state-of-the-art in this area.
In addition to a review of current practices for FSW management and control this Core Research Project (CRP) will investigate the measurement and interpretation of multiple process parameters such as tool forces, temperature and torque by means of a precision in-situ monitoring system. A new advanced FSW monitoring system will be built and demonstrated as part of this programme. The intention is to use the data obtained by the new system to better explain tool performance and interactions within the weld region. Force, torque and temperature traces can be used to establish a reference against which welding parameters can be compared. These relationships can potentially be used in future by control algorithms, which will make the optimisation of process parameters, and in turn weld quality during real-time applications possible.
In summary, FSW process monitoring and QC is important to:
- Provide a degree of confidence that repeatable welds are being produced, especially for critical applications.
- Assist with the scientific understanding of process dynamics and tool performance.
- Assist with optimisation of tool designs and process parameters.
- Record data and provide feedback on the machine and process conditions.
- Aid with the specification of new FSW equipment.
- Review the possible methods for control of the FSW process and to provide technical guidance on the most appropriate approach for common FSW applications.
- Design and build an advanced process monitoring system to assist with process investigation and quality control in FSW.