NDT quality - signal processing for stir welding
TWI Bulletin, January - February 2004
The dual disciplines of friction stir welding and phased array ultrasonic inspection are combined in a truly international quality assurance project.
Davide Kleiner joined TWI in 2002 as an NDT Technical Project Leader following a student placement programme with the company in 2001. He is a UMIST graduate and his final year project was focused on Phased Array programming, which was carried out at TWI. Currently, he is involved in a number of European sponsored projects run by the company involving the application of Phased Array techniques.
Colin Bird joined TWI in 2000 having acquired some twenty years' experience in the nuclear power generation industry, developing remote inspection methods. He has been largely responsible for developing ultrasonic phased array methods for critical defect sizing and methods for detecting defects in friction stir welds. Whilst at TWI he has also been closely involved in the development of ultrasonic methods for measuring the material thickness of the inner layer of a coaxial stainless steel component.
TWI is currently running the Qualistir project; its objective is the development of novel NDT techniques and integrated in-line process monitoring for robotic and flexible FSW systems. Qualistir is a CRAFT project sponsored by the European Union under the 'Competitive and Sustainable Growth' programme. The project co-ordinators are R/D Tech (France) and the other partners in the consortium are Vermon (France), Isotest (Italy), SMT Tricept (Sweden), Forward Precision Engineering (UK), GKSS (Germany), the Technical University of Sofia (Bulgaria) and TWI (UK). As Davide Kleiner and Colin Bird report the project concentrates on 6mm thick aluminium 7075 FSW butt welds. The Qualistir project has now been successfully completed including intergration of the robotic welding and NDT developments.
Background
This project involves two recently developed techniques both in the welding field and in the NDT field. The former is friction stir welding and the latter is ultrasonic phased array inspection.
Friction stir welding
Friction stir welding is a continuous process that involves transversing a specially shaped rotating tool between the abutting faces of a joint. The relative motion between the tool and the substrate generates frictional heat which creates a plasticised region of material around the immersed portion of the FSW tool. The tool is moved along the joint line, forcing the plasticised material to coalesce behind the tool to form a solid-phase joint. The FSW process is a highly repeatable welding method that requires relatively simple machine tool systems and little operator training. As such, the FSW process, when correctly applied commonly produces high quality solid-phase welds. However when incorrectly applied the FSW process can produce unique types of weld flaws and hence new inspection techniques are required to find them.
Previously published work and discussions with the aircraft industry indicated that any conventional weld flaws in FSW samples, eg voids and lack of penetration, could be reliably detected by current ultrasonic methods. The flaws evading direct detection were joint line remnants (entrapped oxide) defects. For this reason the development concentrated on the detection of joint line remnants. This flaw has been called a 'kissing bond' and is caused by incorrectly broken and stirred joint faces creating a semi-linear layer of oxide in a line parallel to the weld. This defect can be bonded in that there is no air between adjacent surfaces. Hence detection with any NDT method is extremely difficult (see Fig.1).
Fig.1. Macro-section showing weld nugget strucutre (top) and coarse grain structure of the weld root (bottom)
Phased array ultrasonic inspection
Phased array equipment was recently developed for the nuclear industry. It involves the use of a very specialised probe containing a large number of miniature crystals. The ultrasonic beam angle and focal distance are controlled by a sophisticated electronic system, which is computer programmable (see Fig.2). This enables a large volume of data to be stored and analysed.
Fig.2. Ultrasonic beam manipulation by phased array using constructive interference principles: beam sweep (left) and focusing (right)
The benefits of the system over normal A-scan techniques are:
- Improved sizing capability
- Pictorial presentation of the data
- Shorter inspection times through electronic scanning
- Permanently recorded results
- Post acquisition data manipulation (eg statistical analysis)
Inspection technique development
The phased array probe arrangement and scanning pattern are illustrated in Fig.3.
Fig.3. Phased array probe arrangement and scanning pattern
Experiments with focused 10MHz to 30MHz immersion probes working at high inspection sensitivities showed that direct ultrasonic detection of these defects by back reflected energy could not be achieved reliably. On some occasions the inspection detected small signals in the weld root but it was not clear whether these were present as a result of the entrapped oxide.
During the data collection a noise pattern associated with the FSW nugget was observed as shown in Fig.4. This is normally viewed as a colour image. In Fig.4 dark to light grey indicates increasing signal amplitude. The outline of the weld nugget has been superimposed in white.
Fig.4. B-scan (end-view) of correctly forged FSW butt weld at 10MHz UT
It can be seen in this sample that the low noise zone extends for the full depth of the parent plate except for the spark eroded notch. This gives a first indication that the nugget has been forged correctly. High noise can be seen in the Thermo Mechanically Affected Zone (TMAZ). The TMAZ generates ultrasonic back scatter noise. Where the weld nugget is correctly forged through to the root, entrapped oxide defects will not be present. Hence, capacity to determine the depth of the correctly stirred zone would enable implementation of a weld quality control method.
Results show that the forging of the weld nugget in FSW refines the grain structure to such an extent that this area becomes highly transparent to ultrasonic frequencies up to 20MHz. This in turn causes little back scattered energy or filtering of the energy by the grain structure. Where the parent plates provided interference with the transmission of the ultrasound, frequency filtering was investigated: the data showed a very strong contrast between correctly forged and incorrectly forged weld roots. TWI developed an algorithm to measure the noise ratio in different parts of the weld, which in turn predicts the presence or absence of entrapped oxide defects.
Signal processing
Figures 5 and 6 show the data collected from the samples after statistical signal processing. The algorithm imports 3D volumes of data out of the raw inspection data from the specified areas within the weld and parent plate. It then performs a volumetric moving average type function to smooth out the individual noise amplitude values. This method also minimises the effect of spurious indications in the scan data, which generally appear singularly and not in clusters. The algorithm then compares the manipulated noise data of the weld root with that of the parent plate. The ratio of the two is a quantitative indicator of the weld quality.
Fig.5. Weld shown in Figure 4 after statistical signal processing
Fig.6. Incorrectly forged FSW butt weld after statistical signal processing
Figure 5 shows a weld where the noise level in the parent plate is much higher than that at the root of the weld nugget, hence the weld quality indicator lies between 0 and 0.5. The spark eroded notch however (located ~230mm along the weld) reaches a value greater than 1.2. Figure 6 shows a weld the quality indicator of which lies almost entirely over the value of one, indicating a high noise level at the weld root and hence an incorrectly forged nugget.
The inspection technique is flexible and can be modified and optimised for different series of aluminium and different weld geometries. TWI is currently running a number of projects looking at aluminium 2000, 5000, 6000 and 7000 series FSW butt welds, lap welds, T welds and box welds.
Conclusion
The inspection method for aluminium 7075 FSW butt welds developed by the Qualistir consortium was successful. The choice of frequency and equipment set-up was optimised for the specified sample types and the technique is able to produce an ultrasonic image of the FSW sample, where parent plate, weld nugget and weld root can all be confidently identified. The statistical signal processing algorithms developed give a reliable quantitative indicator of the quality of the FSW process and hence of the likelihood of entrapped oxide flaws being present.
Acknowledgements
We would like to thank all the members of the Qualistir consortium for the technical expertise and effort they have invested in this project.
