Inspection of Friction Stir Welds using Triple Array Methods

Transcription

1 More Info at Open Access Database Inspection of Friction Stir Welds using Triple Array Methods Barend van den Bos 1, Sara Axelsson 1, Ulf Ronneteg 2, Thomas Grybäck 2 1 Exova Materials Technology AB, Box 1340, Linköping, Sweden Phone: , Fax: ; barend.vandenbos@exova.com, sara.axelsson@husqvarnagroup.com 2 SKB Swedish Nuclear Fuel and Waste Management Co, Oskarshamn, Sweden; ulf.ronneteg@skb.se, thomas.gryback@skb.se Abstract SKB, the Swedish Nuclear Fuel and Waste Management Co, is currently developing a system for disposal of spent nuclear fuel from the Swedish nuclear reactors, the KBS-3 system. The KBS-3 design is based on encapsulation of the fuel in canisters consisting of a load bearing nodular cast iron insert and an outer corrosion shield of copper. The canisters will be embedded in bentonite clay and disposed in crystalline bedrock at a depth of about 500 m. To verify that the canisters fulfil the extraordinary requirements, an extensive programme for quality control is developed. In this programme the use of non-destructive testing (NDT) is vital and therefore it is very important to develop reliable NDT methods. The copper shield consists of a 5 cm thick tube with base and lid welded to it. The welds are performed using friction stir welding (FSW). For inspection of the welds, a combination of ultrasonics, eddy current and digital X-ray methods are used with array techniques to achieve coverage and detectability. Development of the inspection techniques has been performed both as a gradual optimisation of preliminary techniques and development of new techniques, using practical experiments as well as simulations. Inspection procedures have been developed using input from research on human factors to make them reliable and user friendly. Finally, the resulting techniques are verified on full scale components. Keywords: friction stir welding, ultrasonic phased array, eddy current array, digital X-ray, copper 1. Introduction SKB, the Swedish Nuclear Fuel and Waste Management Co, is responsible to take care of all radioactive waste produced in Sweden. For low- and medium radiating waste a system is in place and functional. For the high radiating waste (the spent fuel from the nuclear reactors) a system is under development. The red arrows in figure 1 show you the principals of the system. Figure 1. SKB s system for taking care of radioactive waste.

2 The spent nuclear fuel is transported from the reactor to an interim storage (currently operational). From the interim storage the spent nuclear fuel will be transported to an encapsulation plant, where it will be placed in a canister. After sealing of the canister, it will be transported to the final repository. In the final repository, the canisters will be placed in holes, drilled in the rock, at a depth of about 500m. The repository itself will be sealed after placement of the canisters. The principals of the final repository and its components have been decided, but development is still ongoing concerning detailed design- and production processes. 2. The canister The canister, shown in figure 2, consists of a load bearing insert (grey in figure 2) made of nodular cast iron and a copper shell which serves as a corrosion barrier. The total length of the canister is about 5m. The outer diameter is approximately 1m. Figure 2. The canister and its components. First, a copper bottom is welded to the copper tube by friction stir welding. Then the insert is mounted in the copper tube. The spent nuclear fuel is then placed in the channels of the insert. After that, the lid of the insert is mounted. Finally, the canister is sealed by welding a copper lid to the tube using friction stir welding. 3. Friction stir welding The focus of this paper is the inspection of the friction stir weld. The principle of friction stir welding of the base and lid is shown in figure 3. Figure 3. The principle of the friction stir welding process. A hole is drilled in the lid. In this hole, a rotating tool is placed. The rotation of the tool creates heat which causes the material around the tool to soften. The rotation also causes the softened material to be stirred. When the tool is moved along the joint line according to the left arrow in figure 3, the softened material will be stirred to create a weld with good properties.

3 Figure 4 shows a cross section of the weld. The weld zone is shown as the darker colour. One advantage of friction stir welding is that the structure in the weld zone will be very fine grained, facilitating ultrasonic inspection. Figure 4. Cross section of the weld. 4. Inspection requirements Possible defects have been identified both in the root of the weld and close to the surface. The main types are visualised in figure 5. Figure 5. Defects in the friction stir weld. Cavities have been found close to the surface spread over the weld (darker in figure 4). The orientation of the cavities is more or less random. They can be clustered or singularities. Cavities can occur if the weld is performed at a too low temperature. Joint line hooking is a crack-like root defect that can occur if the tool penetrates too far into to the material.

4 Remaining joint is also a crack-like root defect that can occur if the tool does not penetrate far enough into the material. 5. Inspection strategy An inspection strategy was adopted for the development of techniques based on the following fundamentals: The inspection has to be mechanised and has to have the potential to be controlled remotely, as the lid weld will be inspected with the radioactive fuel loaded in the canister. Ultrasonics is the main inspection method for root defects. Other inspection techniques have to be used as well to complement the ultrasonic inspection, as the detectability, by ultrasonics, of the irregularly shaped cavities might be insufficient. 6. Development of inspection techniques for the friction stir weld As the title of this paper points out, three methods using array technologies have been deployed for the inspection of the weld, ultrasonic phased array, eddy current array and digital X-ray. Ultrasonics is the main technique for detection of root defects and all three methods are used for detection of cavities. The inspection techniques are described in the following sub sections. 6.1 Ultrasonic phased array The ultrasonic inspection is performed from the top of the lid, as shown in figure 6. This way an optimal angle for detection of root defects is achieved. A 3.5MHz linear array transducer with 128 elements is used. The size of the array permits a complete coverage of the weld without having to move the transducer in radial direction. Figure 6. Schematic image of the ultrasonic inspection. Inspection channels were defined based on the available information on weld geometry and defect formation. One set of channels were developed for inspection of the root section and one for the remaining weld. The development of the ultrasonic inspection technique was performed using laboratory trials on both side-drilled holes and real defects supported by sound field simulations using the Civa software. The real defects were created by purposely varying different welding parameters; for example, the cavities were created by welding at a temperature that was too low and

5 various root defects were created by either an excessively deep or excessively shallow welding depth. Based on the fact that the root defects, with their expected orientation, only can be detected by ultrasonic inspection, five different inspection angles were defined. As the root area is rather narrow, the beams were focussed as much as the lid geometry allowed. The choice of inspection angles (0, 12 and 20 ) was based on the signals from the real defects and the expected defect geometry. For the remaining volume of the weld, which includes the outer region where the cavities can be formed, the lid geometry limits the degrees of freedom for possible inspection angles. To cover the entire weld, which is wider closer to the surface two focussed ultrasonic channels were created for each inspection angle. Through an analysis of the signals from a number of real cavities and the geometric inspection limitations, the angles +25 and +35, as well as a 0 inspection channel, were chosen. Based on experiments, the final focal laws, gain and gate settings, were developed. For the inspection performed with the array in contact with a thin water film, the welded tube is rotated, and the array is fixed on top of the lid surface. 6.2 Eddy current array The purpose of the eddy current inspection is to detect defects open or very close to the surface with a ligament of up to 2mm. The eddy current inspection is performed from the envelope surface, using an array probe consisting of 32 coils. The coils are wired in a transmitter/receiver arrangement with two transmitting coils and one receiving coil, as shown in figure 7. Figure 7. The principal arrangement of coils in the array and positioning of the array probe on the weld surface (right). As the expected defects do not have a known primary extension it was important to have a high detectability in all directions. Therefore the coils are connected in circumferential direction, shown to the left in figure 7, and axial direction, shown to the right in figure 7, to

6 create two inspection channels. The axial channel is sensitive to defects with a primary extension in the axial direction while the circumferential channel is sensitive for defects with a primary extension circumferentially. Using these two channels, detectability is maximised in all directions. The copper has a very high electrical conductivity, which limits the penetration depth of the eddy currents. Tests were performed to find a good inspection frequency. The tests were performed using artificially made defects consisting of flat bottom holes with different diameter and ligament. For the available array probe 1.1kHz showed to give the best signalto-noise ratio for holes with a ligament of 2mm. To get a good definition of defects open to the surface a higher frequency was also chosen, 10kHz. 6.3 Digital X-ray The purpose of the X-ray inspection is to be a complement to ultrasonics for the detection of cavities. The X-ray inspection is performed using an accelerator placed on one side of the lid, and a collimated line array detector on the other side of the lid, as shown in figure 8. Figure 8. X-ray equipment with accelerator to the left and detector to the right. The radial propagation is the most critical for defects in the welds, whereas the exact radial position is of minor interest. Thus, the incidence angle has been kept as low as possible for two reasons; to minimize the projection effect and to reduce the penetrated wall thickness and thereby obtaining higher X-ray sensitivity. Due to the geometry of the lid and weld region, an incidence angle of 8 was selected (see Figure 9). The inspection is performed by rotating the welded canister slowly while the accelerator transmits a pulsed X-ray beam and the detector collects data every 0.4 mm in the circumferential direction and in the axial direction by averaging eight elements in the detector array.

7 Figure 9. Cross section showing the path of the X-rays through the weld. 7. Inspection results Several welds have been inspected using the three methods described previously. In this section some inspection results will be presented. 7.1 Root defect A typical ultrasonic indication of a joint line hooking root defect is shown in figure 10. The upper row of images shows C-scans for the different inspection angles and the lower row of images shows B-scans. It can be observed that the amplitude increases as the inspection angle becomes perpendicular to the defect angle; the highest amplitude is achieved at an inspection angle of -20. Figure 10. Results from inspection of a joint line hooking. The images on the left show, with the weld surface to the left, the C-scans (top) and B-scans (bottom) for inspection angles of 20, 12, 0, -12 and In the right image, a macrograph of a joint line hooking is shown. Figure 11 shows the ultrasonic results from a remaining joint. A micrograph can also be seen in figure 11. Typical for remaining joints larger than 1 mm, is that the amplitude is approximately similar for all inspection channels.

8 Figure 11. Results from inspection of a remaining joint. The images on the left show, with the weld surface to the left, the C-scans (top) and B-scans (bottom) for inspection angles of 20, 12, 0, -12 and -20. In the right image, a macrograph of a remaining joint is shown (circled in red). 7.2 Cavities Figures 12 to 14 show results obtained on intermittent welds. The entire circumference does not consist of one weld but of several, which is one way of testing more weld parameters while limiting the amount of test material. In such cases, the scan results will show sections that are unwelded, and, marking the end of each specific weld, a hole. These sections are specifically marked in figures Other visible indications are relevant indications from cavities. Figure 12 shows the inspection result from a large row of cavities. The results are shown, from left to right, for ultrasonics (C-scan), X-ray inspection, eddy current inspection at 1,1kHz and eddy current inspection at 10kHz. Figure 12. Results from inspection of a row of cavities. The images show, from left to right, ultrasonic C- scan, X-rays, eddy current 1,1kHz, eddy current 10kHz.

9 Figure 13 shows another example of inspection of a section containing a cavity, detectable with all three methods. The impedance planes to the right follow the vertical cursor for 1,1kHz (top) and 10kHz (bottom). Figure 13. Results from inspection of a row of cavities. The images show, from left to right, ultrasonic C- scan, X-rays, eddy current 1,1kHz, eddy current 10kHz. 7.3 Cavity/scratch, at the circumferential surface Figure 14 shows inspection results from a section with two defect indications, first an indication only detectable with eddy currents (blue spot in the upper part of the image), and second a root indication only detectable with ultrasonics. The photo to the right is from the surface at the ET indication. A few barely visible scratches were discovered. The two impedance plane presentations at the bottom are following the vertical cursor in the ET scan images. Figure 14. Results from inspection of a section with two indications, a surface indication only detectable with ET to the top and a root indication only detectable with ultrasonics to the bottom.

10 8. Prospects Still, some work remains. Focus has up to now been directed towards development and implementation of the NDT methods addressed in this paper. Now when several welds have been inspected, a validation of the results needs to be done. Interesting sections need to be cut and examined microscopically to verify the indications detected with NDT. Computerised tomography of samples to get a three dimensional view of the indications is also likely to be performed. 3D printing of defects as a means of visualization is also currently being investigated. For cavities open to the surface, penetrant inspection is planned to be performed as a complement. For the eddy current inspection optimization of the array probe will be performed to increase the spatial sensitivity. The possibilities to merge data from the different inspections to make the interpretation of the results easier will be investigated. 9. Conclusions For inspection of the friction stir weld joining tube and lid/base three different NDT methods using linear array techniques have been developed and implemented. Inspection results from several welds show that the UT, RT and ET inspections complement each other to achieve an inspection that fulfils detectability requirements. References This paper is to a large extent built on the following documents, though not specifically referred to: U. Ronneteg, T. Grybäck, Non-destructive testing of canister components and welds, SKB report , Stockholm: Svensk Kärnbränslehantering AB. SKB, RD&D Programme Programme for research, development and demonstration of methods for the management and disposal of nuclear waste, SKB TR-13-18, Stockholm: Svensk Kärnbränslehantering AB.