Ultrasonic Phased Array Inspection Technology for the Evaluation of Friction Stir Welds
C R Bird
TWI, Cambridge CB1 6AL, UK
Paper published in Insight, vol.46, issue 1, January 2004.
This paper is based upon a project 'Qualistir' TM for the on-line quality control of FSW in Aluminium  . Qualistir is a collaborative project lead by R/D-Tech with co-industrial partners Vermont, Isotest, Neos Robotics and Gatwick Fusion, and research partners TWI, GKSS and TUS. The object of the project is to develop an on-line method for determining the quality of FSW in a butt-welded configuration. The inspection technique described in this paper detects conventional defects and supplies a method for determining whether the weld has been correctly forged which, in turn, safeguards against joint line remnant defects.
Friction stir welding (FSW) is a relatively new welding process compared to electron beam or arc welding. Unlike most other welding processes there is no liquid state for the weld pool. For this reason the potential defect types present within the weld are quite different. Conventional welding defects such as voids or lack of fusion can still be present, but defects such as slag or hot tearing due weld pool shrinkage cannot. But other defects more akin to those associated with resistance welding can be present.
As for all welding processes, if incorrectly designed or controlled, welding defects can be generated. As FSW is a machine/automated process, once the process has been designed and tested, defect production should not occur but, like all processes, process control can become out of limits or unforeseen circumstances can affect the quality of the weld. For this reason, good NDT to aid the quality control process is required.
This paper is based upon a project 'Qualistir' TM for the on-line quality control of FSW in Aluminium  . Qualistir is a collaborative project lead by R/D-Tech with co-industrial partners Vermont, Isotest, Neos Robotics and Gatwick Fusion, and research partners TWI, GKSS and TUS. The object of the project is to develop an on-line method for determining the quality of FSW in a butt-welded configuration. The industrial interest in the project has guided the project towards materials and geometries used in the aircraft industry. This paper concentrates on the NDT aspects of the project. The technique described in this paper detects conventional defects and supplies a method for determining whether the weld has been correctly forged which, in turn, safeguards against entrapped oxide defects.
Fig.1. Illustration of friction steel weld process
2. Friction Stir Welding
Friction stir welding (FSW) is a solid-state bonding technique, which uses the heat generated from a milling-type tool to plasticise and bond two sheets of metal ( Fig.2). This welding technique has many advantages. In particular, no melting means less contamination - this particularly applies to aluminium, where oxides are always present.
Fig.2. Macro-section through FSW containing voids, faying defects and lack of penetration
Like many welding processes, FSW can be incorrectly performed resulting in defects. The defect that is particular to FSW is entrapped oxide, which is defined below.
3. Flaw definitions
To avoid misunderstandings that have occurred in a number of previously published papers, this paper defines the different flaws described within the report and defines the term flaw and defect.
Some flaws may be acceptable to design codes in which case they could be termed an acceptable flaw. Others, depending upon size and severity, may be unacceptable, in which case the flaw could be termed a defect. Figure 1 below identifies four conventional flaws. Figure 2 below identifies an entrapped oxide defect. The following flaw definitions are used in this paper.
- Void. This is a flaw that is totally subsurface, volumetric (not planar) and contains no material. These flaws are usually aligned with the welding direction.
- Worm hole. This is a void which is aligned in the through wall direction.
- Lack of penetration (LOP). This flaw is present where the full thickness of the weld has not been forged leaving the original parent plate butting surfaces unbroken or undisturbed. This type of flaw usually has a small but finite gap between the adjacent parent plate surfaces and is primarily caused by the FSW pin not penetrating deeply enough into the weld.
- Faying surface flaw. This flaw is at the welding tool surface and is surface breaking. The flaw can contain oxide and is metallurgically similar to that of a rolling lap.
- Entrapped oxide. This flaw is the most difficult to detect and has been called a 'kissing bond'. This flaw emanates from an incorrectly broken and stirred fusion face, leaving a semi-continuous layer of oxide in a plane parallel to the weld. This flaw is fully bonded in that there is no air between adjacent surfaces and it provides some mechanical strength. The severity of this flaw depends upon its planar extent and proximity of the adjacent oxide particles. These flaws have also been termed lazy S flaws. By studying the microstructure of the sample shown in Fig.2 it can be seen that the oxide layer slopes, hence some forging has taken place. This compares with the weld in Fig.3 where the defect is vertical. Furthermore, by studying the sample at high magnification, it can be seen that the oxide is not continuous and that some of the grains cross the apparent line of the flaw. There is little disruption to direct passage of sound or electricity through the flaw and there is negligible foreign matter of a lower density that would enable X-ray detection. For this reason, direct detection with any NDT method is extremely difficult.
Fig.3. Weld T7 macro showing weld nugget structure (top) and coarse grain structure of the weld root (bottom). It can be observed that the pin penetration was ~0.8mm from the bottom surface during the process (leading to the creation of an entrapped oxide defect
4. Development strategy
From previous published work and discussions with the aircraft industry it was clear that conventional flaws, e.g. voids and lack of penetration, could be detected by current ultrasonic methods. The flaws that were evading direct detection were entrapped oxide defects. For this reason the NDT development concentrated on the detection of entrapped oxide and a method for in-line quality control.
To develop inspection methods, defects of a known size are required. Development of the process to generate controlled oxide defects was difficult, requiring a large number of destructive measurements following the controlled use of incorrect welding parameters. After many trials, a process was developed, using a specifically designed welding tool, which generated perfect joints when operated within the correct welding parameters. Then, by controlled reduction in pin length whilst maintaining the same welding forces, welds were generated with less tool penetration and an increasingly less forged root.
From early experiments it was clear that direct ultrasonic detection of these defects by back-reflected energy could not be achieved reliably. On some occasions small signals were detected in the weld roots but it was not clear whether these were as a result of the entrapped oxide or general material noise. These early trials included inspection with focused 10MHZ to 30MHz immersion probes working at exceedingly high inspection sensitivities.
Metallurgical properties affect ultrasonic transmission. Because the material bonding is weaker in the region of a flaw and contains foreign matter, it was decided to investigate other ultrasonic properties including frequency filtering and velocity changes between the parent plate, weld nugget and weld root region.
This project started by using conventional focused immersion probes. Once the basic concept had been developed, the technique was transferred to that of phased array probes, which benefits from increased in-line inspection efficiency. The phased array probe arrangement and scanning pattern is illustrated in Fig.4 above.
Fig.4. Illustration of phased array scanning pattern
5. Development results
The development results are divided into three categories:
- Conventional defect detection
- Ultrasonic velocity measurements
- Ultrasonic frequency measurements
- Ultrasonic noise distribution measurements
5.1.Conventional flaw detection
Although this paper concentrates on entrapped oxide flaws, the development programme also collected data on the more conventional but equally important flaws, as detailed earlier.
This project used 6.25mm thick aluminium 7075. GKSS deliberately generated a number of welds containing conventional defects including voids, lack of penetration and faying defects. Figure 5 below shows the micro-section of a weld containing voids and tight LOP. It further shows the clear difference between the weld nugget and the parent plate. Further it can be seen that the weld root has a similar undisturbed structure to that of the parent plate.
Figure 6 below shows an ultrasonic sectional image generated from the inspection of the GKSS weld shown in Fig.5 above. This image, generated by Tomoview software, shows the two voids and the weak signal from the LOP. High signal amplitudes are represented by red and low by pale blue, the signal amplitude scale being shown in Fig.10. In this weld the LOP was under compressive stress, allowing a high percentage of the energy to transmit through the flaw. The horizontal blue line represents the top of the plate and the horizontal red line the bottom of the plate. The dashed line surrounds the weld nugget region. This region is lighter blue (lower signal amplitude) than the parent plate.
Fig.5. Micro-section through conventional weld defects
5.2 Velocity measurements
Velocity measurements were made, both shear and longitudinal (St & Sv), through both the parent plate and the weld region. The direction and mode of these measurements are shown in Fig.6 below. Neither TUS's nor TWI's measurements revealed any measurable velocity variations due to the presence of entrapped oxide, or a significant change in velocity due to the weld. Furthermore, the measurements of the horizontal longitudinal (Sv2) velocity at several weld depth positions revealed no significant velocity variations.
Fig.6. Ultrasonic data from sectional view of GKSS weld
5.3 Ultrasonic frequency measurements
Data for all the welds was collected in both rectified and un-rectified forms. The un-rectified data was digitised at 100MHz and used for Fast Fourier Transform (FFT) analysis on the back-scattered energy. Furthermore, TUS performed through-transmission scans in both the Sv2 and St directions. Figure 7 below provides an example of the RF data collected on a good weld.
From both the TUS and TWI results it was clear that the forging of the weld nugget had refined the grain structure to such an extent that it had become highly transparent to ultrasonic frequencies up to 20MHz. This, in turn, provided little back-scattered energy or filtering of the energy by the grain structure. In contrast, the parent plate provided interference with the transmission of the ultrasound. For the two alloys examined, the mean back-scattered frequencies were approximately 8MHz. The 8MHz energy was also present in the root of incorrectly forged welds. Frequency filtering was investigated as a means of determining whether defectiveness could be determined in this way. Because the ultrasonic wavelength is of the same size as the resolution required from the technique, this approach was found not to be useful in practice. For this reason, the FFT approach was not taken forward, but again the data showed this very strong contrast between correctly forged and incorrectly forged weld roots, emphasising that UT could, in principle, be used as a measure of grain refinement. An example of the RF frequency spectrum is shown in Fig.8. Although the relationship between attenuation and grain size is well known it is rarely used as a quality control method.
Fig.8. Screen dump of RF data collection showing noise distribution and FFT analysis
5.4 Noise distribution results and analysis
During the data collection, it became clear that there was a clear noise pattern associated with the FSW nugget, as shown in Fig.9 below.
Fig.9. Sectional image through Weld T4 with some entrapped oxide
In Fig.9, the pale blue zone shows the grain scatter noise from the weld nugget, whereas the darker blue zones correspond to the parent plate. In this sample the low noise zone does not extent for the full depth of the parent plate. Additionally a high noise zone can be seen corresponding to the Thermo-Mechanically Affected Zone (TMAZ). The TMAZ generates ultrasonic back-scatter noise from the relatively large, vertically orientated grains in this zone. These relatively high amplitude signals can be above the normal aircraft industry reporting threshold, causing false defect calls. For this reason, simple amplitude crossing defect rejection criteria cannot be used for these inspections. Figure 10 below shows a macrograph of weld T4. This weld shows an entrapped oxide defect in the root of the weld with a through wall depth of 0.4mm. This corresponds to the depth of the ultrasonically higher noise zone in the weld root shown in Fig.9 above.
From consultation with TWI and other welding engineers it was determined that, where the weld nugget is correctly forged through to the weld root, entrapped oxide defects should not be present. Hence, if a method could be developed for determining the depth of the correctly stirred zone, a quality control method could be provided to ensure that the welding process was in control and there was a low probability of entrapped oxide defects being present.
Following this early idea, a programme of inspecting many welds containing entrapped oxide, plus control welds containing no defects, was undertaken to determine whether noise could reliably determine the extent of correct root forging. To provide reliable evidence, multiple weld manufacturers and materials were required to determine whether the ultrasonic properties were stable with respect to the welding process. TWI, GKSS and Alenia all manufactured welds for this project. GKSS and TWI used aluminium alloy 7075 and Alenia used 2219.
The design of the weld tool is critical to the weld quality and the grain structure within both the weld nugget and the TMAZ. All three manufacturers used different tool designs providing the required process stability test.
Figure 10 shows a macro-section through a weld containing a 0.4mm deep entrapped oxide defect. Figure 9 shows the clear contrast in back-scattered noise between the weld nugget and the parent plate. Furthermore, a high noise level can be seen in the weld root for a distance of approximately 0.4mm from the bottom of the plate. Figure 11 below provides a C-scan (plan view) of a weld in aluminium 2219 with no entrapped oxide. This linear scan was performed with a 10MHz phased array probe.
Fig.10. Macrograph of weld T4 containing entrapped oxide
Fig.11. C-Scan view of weld and parent plate at a depth of 2mm to 3mm from the weld root
To provide a quantitative inspection method, a stable measure of the mean noise level within the weld root was required. Initially the ratio of weld root noise to weld nugget noise was studied and reported  . From further study, the ratio of the parent plate noise to the weld root noise was chosen as the most stable measure, this ratio normalising all the inspection parameters to that of the parent plate material noise. The initial work was undertaken with Tomoview software, which was found to be suitable for the development work. More recently the R/D-Tech Quickview was chosen to provide a route to export the data to the FSW welding control system. Both software versions provide a tool for area noise assessment. This tool provides two numbers: average amplitude of signal amplitude within the contour on the display, and the standard deviation of the amplitudes within the contour. The accompanying picture is a display of the maximum amplitudes within a defined weld volume. In Fig.11 this is at a depth of 2 to 3mm from the weld root.
The results illustrated in Fig.12 are of four welds in aluminium alloy 2219, identified as CD2, CD5, CD7 & CD9. Weld CD2 was fully penetrated with no welding flaws detectable ultrasonically or by micro-section. The section through weld CD2 is shown inFig.13. Note that an un-forged weld root has a noise ratio of one, i.e. the same grain structure and noise as the parent plate.
Fig.12. Graph of noise ratio vs distance from weld root
Fig.13. Macro-section through weld CD2
Samples CD5 and CD9 ( Figure 14 shows the micro section of CD9), have no entrapped oxide but the weld nugget was not fully penetrated. Figure 12 clearly shows that for a defective weld the ratio of weld noise to parent plate noise is near one. Weld CD7 is noisier than the parent plate because of the presence of a void. In a good weld (CD2) the weld noise is lower than the parent plate by a factor of three. For welds CD5 & CD9 ( Fig.12) for a distance of 0.5mm from the weld root the noise approaches that of the parent plate but the noise ratio rapidly increases away from the root. A nominal ratio of 1.5 is presented on the graph, which could be used for an on-line quality control rejection level.
Fig.14. Macro-section through weld CD9 showing a 0.2mm high coarser grained zone in the root
Statistical analysis of all the data has been performed to determine the reliability for individual measurements. Part of the analysis is given in Fig.12. which shows that, for a completely forged weld (CD2), all the results fall within the estimated 80% confidence intervals (shown by the error bars in Fig.12). It also shows that at a constant depth within the weld the data has a very consistent noise ratio. Samples CD9 and CD5 show that the noise ratio progressively increases with height within the welds. As the distance from the root increases towards the centre of the weld, the degree of forging increases and the noise ratio increases. It can be seen that the confidence interval for these measurements is equally small as for the other samples.
Figure 15 shows a graph of noise ratio for 16 weld samples of differing degrees of weld penetration. It must be emphasised that for each TWI weld a different welding tool design was used. Thus, for a constant weld set-up, the variation in material noise ratio will be extremely small and the NDT method will be monitoring changes along the weld and not between deliberately different welding parameters. Figure 15 shows that the ultrasonic inspection method can clearly identify welds where the weld root has incomplete forging to a depth greater than 0.5mm. Furthermore, where weld roots are fully forged, they can be clearly identified.
Fig.15. Graph of noise ratio for 16 weld samples with varying degrees of entrapped oxide
6.1 Flaw detection
This project has concentrated on an in-line method for providing data as part of the overall quality control process for FSW, rather than an inspection method for previously manufactured welds. The inspection and data analysis method can clearly and precisely detect conventional defects with a through wall height of 0.1mm with a good signal to noise ratio.
The results given above are for individual welds with differing degrees of defectiveness and using different welding tools, i.e. each weld has a different tool design or welding parameter. Figure 16 below shows the low variation in noise ratio with respect to axial position along the weld for four welds. CD2, CD9 and T1 have no entrapped oxide. T7 has 0.5mm of entrapped oxide. For reference a 0.2mm EDM notch was machined into weld T1. This graph shows the stability of the inspection system for a given set of welding parameters.
Fig.16. Graph of noise ratio with respect to axial position
6.2 Quality control
This project has demonstrated two basic methods for determining the quality of FSW butt weld joints.
- Ultrasonic amplitude rejection, for conventional defects. The method demonstrates that voids with a through wall size of 0.1mm can be clearly detected.
- Material noise ratio analysis for determining the depth of penetration of the correctly forged weld nugget. This method reliably measures the depth of penetration to better than 0.5mm and, in turn, identifies possible entrapped oxide flaws with a through wall size greater 0.5mm and greater.
This ultrasonic inspection method is going to be applied to the Neos robotics welding machine as part of this project.
The report has concentrated on flaw detection and has not provided detailed discussion on application to the industrial environment. But all the techniques and equipment used for the data analysis are commercially available and the inspection method was applied at an inspection speed in excess of 50mm/second along the weld.
6.3 Future developments
During the next six months the outputs from work on the welding control process, headed by GKSS, will be combined with that of the ultrasonic development work to provide a prototype integrated quality control system.
Using the basic concepts developed within this project it is intended to develop the technique for further geometries and different aluminium alloys.
- An ultrasonic method has been developed for the reliable determination of forging depth within a friction stir weld.
- The project has developed a phased array method for on-line quality control of butt welded friction stir welds.
- It is believed that the concept of determining degree of forging in addition to direct defect detect provides a unique quality control method.
- The inspection speed provided by this technique is equal to that of the FSW process and is greater than 50mm/second along the weld.
- The EC for providing 50% of the development funds.
- Business partners in the Qualistir project including R/D-tech, Vermont, Isotest, Neos Robotics, and research partners GKSS and TUS. In particular, Olivier Dupuis of R/D-tech for providing technical support.
- AleniaSpacio for providing valuable weld samples.
- Airbus for providing project guidance.
- EC project, 'Qualistir' TM , PROJECT N°: CRAF-1999-70641.
- TMS conference proceedings 2003, New Developments of the Ultrasound Phased Array for the Evaluation of Friction Stir Welds, Colin R Bird TWI, Olivier Dupuis R/D-Tech, Andre Lamarre R/D-Tech.