Paper presented at NDT 2003. British Institute of Non-Destructive Testing Annual Conference, 16-18 September 2003, Bransford, Worcester.
Recipient of the Award for Best Paper by a Young Author
This paper is based upon the project 'Qualistir' TM for the on-line quality control of FSW in aluminium. Friction stir welding (FSW) is a solid-state bonding technique. Because it is an automated technique it is a controlled and reliable process. However, with changes in material condition, dimensions or welding parameters, flaws can be generated. These flaws can be conventional ones (lack of penetration or voids), or entrapped oxide defects, which are unique to this welding technique. Thelatter flaw has been called a 'kissing bond' and is extremely difficult to detect with any non-destructive testing (NDT) method due to its sub-millimetre size and transparency to NDT. Hence direct ultrasonic detection of these defects by back-reflected energy cannot be achieved reliably. Analysis of ultrasonic noise distribution, however, has provided a means to assess the quality of the weld root, where the entrapped oxide defects can cause the greatest reduction in mechanical properties. A mathematical algorithm comparing the noise level within the weld root to that within the parent metal was developed to provide a quantitative inspection method. Signal processing software imports 3D volumes of the data from the specified areas within the weld and parent plate; it then processes the acquired noise amplitude data to obtain a weld quality indicator. Hence, instead of detecting the defect directly, the procedure essentially quantifies the quality of the weld, thereby determining whether the weld nugget has been correctly forged.
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 ofSofia (Bulgaria) and TWI (UK). The inspection technique being developed in the project concentrates on 6mm thick Al7000 series FSW butt welds. The project has now successfully passed the three-quarterly review and integration of the various work packages is now successfully underway. Development of an NDT inspection technique for FSW butt welds, and design and manufacture of an ultrasonic phased array sensor assembly are complete. Initial practical trials indicated the need for statistical signal processing to be applied to the collected data.
The 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.
2.1. Friction Stir Welding
Friction stir welding is a continuous process that involves plunging a portion of a specially shaped rotating tool between the abutting faces of the joint. The relative motion between the tool and the substrate generates frictional heat that creates a plasticised region around the immersed portion of the tool. The tool is moved along the joint line, forcing the plasticised material to coalesce behind the tool to form a solid-phase joint. As the weld is a solid phase weld, different types of weld flaws can be present and hence novel inspection techniques are required to find them.
Previous published work and discussions with the aircraft industry indicated that conventional flaws, e.g. voids and lack of penetration, could be detected by current ultrasonic methods. The flaws evading direct detection were entrapped oxide defects. For this reason the development concentrated on the detection of entrapped oxide. This flaw has been called a 'kissing bond' and is caused by incorrectly broken and stirred fusion faces creating a semi-linear layer of oxide in a line parallel to the weld. This defect is fully bonded in that there is no air between adjacent surfaces. Hence detection with any NDT method is extremely difficult (see Figure 1).
Fig. 1. Macro-section showing weld nugget structure (top) and coarse grain structure of the weld root (bottom)
2.2. 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 Figure 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 focussing (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 (e.g. statistical analysis).
3. Inspection Technique Development
The phased array probe arrangement and scanning pattern are illustrated in Figure 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 Figure 4. Colours (from pale blue to dark blue, green, yellow and then red) indicate increasing signal amplitude. The weld nugget is circled in black.
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. A high noise zone can be seen at the Mechanically Affected Zone (MAZ). The MAZ generates ultrasonic back scatter noise. Where the weld nugget is correctly forged through to the root, entrapped oxide defects should not be present. Hence, capacity to determine the depth of the correctly stirred zone would enable implementation of a weld quality control method.
Results from both the Technical University of Sofia and TWI show that the forging of the weld nugget refines the grain structure to such an extent that this 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.
4. Signal Processing
Fig. 5. Weld shown in Figure 4 after statistical signal processing
Fig. 6. Incorrectly forged FSW butt weld after statistical signal processing
Figure 5 and Figure 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.
Figure 5 shows a weld where the noise level in the parent plate is much higher of 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 ~230mmalong 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 1, indicating a high noise level at the weld root and hence an incorrectly forged nugget.
5. Future Development
Final integration of the system is taking place before the end of November. 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 Al2000, 5000, 6000 and 7000 series FSW butt welds, lap welds, T welds and box welds.
The inspection method for Al7000 series 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.
We would like to thank all the members of the Qualistir consortium for the technical expertise and effort they have invested in this project.
References and Footnotes
- Qualistir Project, 'Development of Novel Non Destructive Testing Techniques and Integrated In-line Process Monitoring for Robotic and Flexible Friction Stir Welding Systems. Work Programme Version 4.0', CRAFT-1999-70641, September 2002.