Simulation Study and Development of Ultrasonic Inspection Technique for Cu-W Monoblock Divertor Assembly

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1 More Info at Open Access Database Simulation Study and Development of Ultrasonic Inspection Technique for Cu-W Monoblock Divertor Assembly Kedar Bhope 1, Mayur Mehta 1, M.S.Khan 1 and S.S.Khirwadkar 1 1 Institute for Plasma Research, Bhat, Gandhinagar , India kedar@ipr.res.in, mayur@ipr.res.in, khan@ipr.res.in Keywords: Ultrasonic Testing, Cu-W Monoblock, CIVA, Phase-reversal, de-bond, PFC. Abstract: Monoblock type Divertor targets are the Plasma Facing Components (PFC), primarily responsible for effective heat transfer from the divertor system. The design of these components is such that it will sustain high heat load (10-20 MW/m 2 ). The actively cooled Cu-W monoblock types PFC are made up of Tungsten (W) to CuCrZr joints and performance of this component is mainly depend on the Cu-W bonding. One of the quality control steps during fabrication of these components is the ultrasonic testing of bonded region between Tungsten and Copper alloy tube i.e. lying within 3mm depth. Towards this, ultrasonic testing of Cu-W monoblock assembly is carried out using side looking normal beam immersion technique to detect de-bond at the interfaces. This paper highlight the successful application of ultrasonic simulation study using CIVA 10.1 [1] for selection of test parameters to characterize shallow bonding defects viz., de-bonds between Cu alloy tube, OFHC (Oxygen Free High Conducting) copper and W tile using phase-reversal phenomena. Further simulation studies are also been carried out for sensitivity criteria to characterize smallest dissimilar Cu-W metal joints as well as response of defect present at the interface. The results of defect response are compared with experimental results to validate and to analyze the uniform sensitivity throughout the volume, keeping in view of lateral resolution and sizing accuracy for 0.5mm Flat Bottom Hole (FBH) at Cu-W interface. Copper tube containing different diameter of FBH and slots at different depth has been used for calibration. Based on the simulation studies, suitable test parameters have been optimized for ultrasonic examination of Cu-W Monoblock assembly. This paper presents in detail the experimental methodologies and the results of the investigation. Introduction In nuclear fusion power program, a divertor is a device that allows safe handling and removal of particles and energy escaping out from core plasma region along the scrape off layer. Divertor mainly consists of two parts viz. Divertor Targets & Dome (Heat Removal System) and Divertor Cassette Body (Support structure for Divertor Targets). Divertor targets are the PFCs, which are responsible for effective removal of heat from the divertor system. The monoblock type geometry is a most potential design for plasma facing components of divertor. Tungsten monoblock type mock-ups are developed by various joining techniques likewise Hot Isostatic Pressing (HIPping), Hot Radial Pressing (HRP), diffusion bonding, active brazing etc [2].During such manufacturing processes blow holes, delimitations, cracks type of defects will be incorporated at joint interlayer of the mock-up. One of the quality control requirements during manufacture of monoblock divertor assembly is to detect de-bonding between OFHC Cu-W tile and between Cu-Cu alloy tubes. Ultrasonic Technique provides a fast, nondestructive way to detect defects present at joint interface of monoblock assembly. As per the ultrasonic pulse echo principle, a high frequency of ultrasound transmitted through copper heat sink tube and reflected energy from de-bond at interface detected as a time of flight. The purpose of this paper is to establish ultrasonic technique for testing monoblock assembly using CIVA simulation.

2 Mock up description Tungsten monoblock type mock-up was developed by high temperature high pressure diffusion bonding process at NFTDC, Hyderabad. The monoblock divertor assembly consists of Tungsten tiles of dimension ( ) mm 3 thick with a central hole of diameter 17 mm. OFHC Cu casted in 17 mm hole of W tile to create a hollow Cu tube of diameter 15 mm and 1.0 mm thickness. Subsequently this Cu is bonded with a CuCrZr alloy of inner diameter (ID) of 12 mm and 1.5 mm thickness using Hot Radial pressing. All the dimensions of a mock-up are as shown in figure 1. The materials and the method used for manufacturing of Cu-W Monoblock assembly are presented in Table Ø17 30 Ø15 Ø12 Figure1. Drawing of monoblock mock-up with 5 tiles [3] Table1: Material and methods used in manufacturing of Cu-W Monoblock assembly Material Component Monoblock Manufacturing method W Armour, Monoblock Machining of W tiles OFHC Copper Intermediate layer Casting of OFHC Copper to W tiles CuCrZr tube Cooling tube Hot radial pressing of CuCrZr tube to OFHC Copper Ultrasonic testing In ultrasonic testing, the flaw detection is based on reflection of the incident sound wave from the discontinuity/flaw. The reflection is strong if the impedance mismatch across the interface is high. In Cu-W monoblock assembly, when looking from inside there are two interfaces; one is between Cu alloy tube and OFHC Copper while the second one is between the OFHC Copper and W alloy tile. Acoustically, OFHC Cu and CuCrZr are similar and hence the interface separating these two will not reflect the incident sound wave if the bonding is good. If there is de bond, then there will be strong reflection since the interface is between Cu and Air. The second interface separates OFHC Copper from W tile which are acoustically very different materials. Phase reversal technique [4] can be used. In this method, when the sound beam falls on good bonding region, phase of echo is in phase with water interface echo as small amplitude of impedance mismatch in un-rectified (RF) mode and when there is de-bonding, phase of echo is out of phase with water interface echo. Hence the reflection based method is useful for Cu-Cu alloy de-bond detection and phase reversal technique is used to detect de-bond between Cu and W. CIVA simulation has been performed to select the most suitable test parameters for detection and characterization of bonding defects in terms of size and its location with region of interest 3 mm using imaging.

3 CIVA Simulation studies CIVA is a powerful and a versatile NDE simulation package developed by CEA, France. It offers simulation capabilities for ultrasonic, eddy current and radiography inspections. The ultrasonic simulation part of CIVA has two separate modules: Beam Computation and Defect Response. The beam computation module gives the results in the form of beam profile in the component, while the defect response module gives the results in the form of, B-scan images or an A-scan response from a simulated defect in the component [1, 5]. Geometry of Cu-W monoblock assembly enables to carry out the examination from the inside surface of the Cu tube. This necessitated the use of side looking immersion transducer of suitable frequency to resolve low thickness as well as phase reversal phenomena for Cu-W bonding. Single W tiles of monoblock assembly were generated in defect response module with defining material properties like sound velocity, density, sound attenuation. Four rectangular defects of size 1 1 mm (each 2mm radialy separated) are taken as reference study to simulate four case studies (two on de-bonds defects between Cu- W with phase reversal and other two defects are in OFHC Copper & Cu-Cu alloy de-bonds). (a) (b) (c) (d) Figure 2.Ultrasonic simulation obtained from ultrasonic probe for selected frequencies (MHz): (a) 5 (b) 10, (c) 15 & (d) 20. To resolve phase reversal phenomena, the simulation was conducted by different probe frequencies viz., 5, 10, 15 and 20 MHz s. Figure 2(a-d) shows A and B scan results obtained for these test frequencies. The A &B- Scan results for 5 MHz probe indicates that signal from Cu- Cu alloy de-bond and Cu-W de-bond are not clearly distinguished. Hence use of 5 MHz probe is not suitable for this geometry. At 10 & 15 MHz, although the Cu-Cu alloy and Cu-W de-bond signals are resolved but the resolution is poor for detection of shallower casting defects of OFHC Copper viz. 0.2 mm and 0.4 mm deep. The side looking of 20 MHz immersion probe shows optimum resolution and a good sensitivity for de-bond detection of Cu-W interface using phase reversal technique and thus it is chosen for the present study. Interface defect response studies: Investigation of interface defect response is necessary to study two purposes: (1) To detect the smallest Cu-W de-bond by optimizing parameter and (2) to study its response by varying water path using 20MHz focused sound beam. A single tungsten tile model is generated using 2D CAD feature of CIVA 10.1 and the smallest 0.5 mm FBH has been introduced to Cu-W interface. Side looking 20 MHz focused (F =12.5 mm) probe used to detect the smallest de-bond as well as to study the response in defect response module. For Cu-W Monoblock, region of interested depth is Cu-W interface. Water path distance is found out 4.7 mm as per the following equation. V M W p = F M p (1) VW

4 Where Wp is water path, F is focal length in water, M p is metal depth where beam is to be focused, V M is longitudinal velocity in metal and V W is the longitudinal velocity in water [6]. is the plan view of scanning area which is obtained by scanning over the object in raster pattern. Defect sizing by is possible only by optimizing the parameters like scan and index such as to obtain good resolution. In present case, main aim is to obtain better scan step and its index step for sizing the 0.5 mm FBH in terms of shape of defect as well as its location. Scan is set in rotational direction and index is in perpendicular direction to get raster pattern as shown in figure 3. Simulation has been performed for several series of scan and index parameter with scan region of 90 4 mm. images obtained for 1 1mm, 0.7 x 0.7mm, mm and 0.1 x 0.1mm steps in scan and index direction respectively. (a) (b) (c) (d) (e) B-scan A-scan Figure 3.Ultrasonic simulation results obtained from (a) 1 x1mm, (b) 0.7 x0.7mm, (c) mm, (d) 0.2 x0.2mm and (e) 0.1 x0.1mm scan parameters. Comparing the C-Scans for different scan steps and index steps, it is found that shape of the defect is not clearly identified with 1 x 1 mm, 0.7 x 0.7 mm, 0.5 x 0.5 mm and 0.2 x0.2 mm resolution with focusing at depth of 2.5 mm. However as shown in figure 3 (e) simulated A- Scan, B-Scan and C-Scan represents that 0.1 x0.1 mm resolution gives clear defect size and it is lowest step for scan and index. Thus optimized 0.1 x 0.1 mm C-Scan parameters are chosen for inspection to size the smallest defect. It is also necessary to study the variation of focusing on defect detection and its response for inspection. This is being simulated by pointing the probe on center of 0.5 mm FBH and measuring the amplitude of reflected echo in terms of percentage of full screen height (FSH). From ultrasonic point of view, both OFHC Copper and Cu alloy tube exhibit negligible sound attenuation and similar sound velocities which results in good signal to noise ratio. Reflected echo amplitudes recorded for water path mm. Graphical representation of CIVA simulated results in figure 4 shows that maximum amplitude response of 0.5 mm FBH obtain in between 4 to 5.5 mm. It confirmed that focusing of ultrasonic beam takes place at Cu-W interface with calculated 4.7 mm focal point region. Validation of simulated results: In order to validate the simulation results as well as calibration of system, a calibrated 0.5 mm diameter of FBH has been drilled at center up to Cu- W interface in single. Defect response study also been performed experimentally and amplitude

5 response recorded with varying water path for 3 to 10.5 mm. The depth of the 0.5 mm FBH is estimated from A-Scan & B-Scan images using time-of-flight and it found to be 2.4 mm from inner tube diameter as shown in figure5. Water Path[mm] CIVA Simulated amplitude [%FSH] Expt. amplitude [%FSH] Figure 4.Comparison of 0.5mm defect response amplitudes 0.5mm FBH (a) (b) Figure 5.Cu-W Monoblock tile with 0.5mm FBH at Cu-W interface (a) A-scan (b) B-scan results Results obtained from CIVA simulation and experimental amplitude responses for 0.5 mm FBH at Cu-W interface with variation in water path was compared [7]; Figure 5 shows the comparison between CIVA simulated and experimental amplitudes which represents that detection of smallest de-bond can be possible with good signal to noise ratio and it is matches with simulated results, it conclude that reduction in reflected amplitude showing de-focusing phenomena. At calculated water path for focusing at Cu-W interface (2.4mm), amplitude reaches maximum and it remains constant for 1.5 mm of water path change. Thus it is concluded that variation in water path up to 1.5 mm for required focusing depth does not make appreciable change in defect detection. (a) (b) Figure6. (a) CIVA Simulated & (b) experimental images of 0.5mm FBH at Cu-W interface using 0.1º 0.1mm parameter

6 To validate optimize C-Scan parameter as well as to size the smallest defect; ultrasonic imaging of 0.5 mm FBH have been carried out using optimized water path and parameter. These images reveal how optimized inspection parameters are effective in getting very good resolution due to focusing effect. Apart from the resolution, focusing of sound beam improves sensitivity which implies one can able to detect smaller defects. Thus it concludes that simulated scanning parameter 0.1 (step) 0.1 mm (index) is the best C-Scan parameters to carry out the inspection of Cu-W Monoblock assembly with sensitivity of 0.5 mm. Hence ultrasonic CIVA simulation studies have been validated to detect and size the smallest Cu-W de-bond by optimizing C-Scan parameter and its response by varying water path using 20 MHz sound beam. Experimental setup Experimental set-up for ultrasonic inspection mainly consisting of two main parts: immersion tank with high-accuracy manipulators and an ultrasonic unit (OMNISCAN MX) in association with TOMOVIEW for data acquisition, control and imaging. The schematic of ultrasonic system is depicted in figure 7. Ultrasonic Data Acquisition Motion Control system Omniscan MX PC Probe Tank Sample Rotating chuck Figure7. Schematic of Experimental setup The set-up for Monoblock divertor assembly placed in an immersion tank on a circular rotating table is shown in figure 7. Ultrasonic 20MHz focused (F =12.5 mm) side looking probe with extension rod is inserted vertically through index axis (Z-axis) inside the monoblock to detect the de-bonding. 4.7mm water path distance is maintained so that ultrasonic beam focused on interface. Scanning of straight monoblock assembly has been performed by rotating the table by 360º and indexing in Z- direction to generate the C-Scan image. Calibration The calibration is necessary to distinguish the response caused by a defect compared to normal response due to presence of an interface between different materials. The second type of analysis allows for determining the distance of a defect or an interface with respect to the ultrasonic probe viz., its position with respect to depth [8]. Figure 8 shows reference tube made up of pure copper and is utilized in determining the probe focal distance inside the material and also to study defect of Cu-Cu alloy interface. The ID of sample is 12 mm same as ID of Cu-W Monoblock tube and having thickness of 3 mm.

7 Figure 8.Copper reference tube for calibration Reference Cu tube sample contains 8 FBHs with 3 slots. Four FBH of diameter 1 to 4 mm are drilled at same depth of 1.5 mm (which is Cu-Cu alloy tube interface).another four FBHs are made at different depth starting from 0.75 to 2.55 mm with step of 0.5 mm. Three slots of 6 mm width machined with steps of 0.75 mm in half area of tube outer diameter. In order to calibrate the system, ultrasonic testing of reference tube is carried out using optimized parameters. Figure 9 shows the result of the ultrasonic scanning of the reference sample. is the map of the reflected signal amplitudes which identifies the probe sensitivity at different depths. Calibrated FBHs and slots in reference tube are identified in terms of size and reflected amplitude responses. The scale in D-Scan shows the actual distance (in mm) of the reflectors and obtained depths are matching with actual depth of calibrated defects. This method completes the calibration of the system in terms of sizing the defect and to get actual depth information for inspection of Cu-W Monoblock assembly. Results D-scan D-scan Figure 9. and D-scan imaging of Copper calibration tube Ultrasonic testing parameters have been optimized and validate based on 0.5 mm FBH defect. In order to precisely identify location of the defect, W tile assembly has been divided in four different faces viz. Face 1, Face 2, Face 3 and Face 4. Each face contained an angle ranges from 0-90, , , respectively. Ultrasonic testing of the Cu-W Monoblock assembly was performed using optimized parameters and echo amplitude of 0.5 mm FBH was set 80% of FSH as sensitivity reference. image was obtained by gating at a depth of interface shown in the figure 10. Defects are clearly identified by location and size. Analysis of A-scan and B-scan images of defected region gives idea of defect depth and phasereversal at Cu-W interface.

8 0 0 Face Face Face Face Tile 5 Tile 4 Tile 3 Tile 2 Tile 1 Summary and Conclusion Figure 10.Ultrasonic imaging of Cu-W Monoblock assembly Ultrasonic technique for assessment of de-bonding in Cu-W Monoblock divertor assembly has been investigated in details using CIVA simulation. Studies on different inspection parameters viz. probe of suitable frequency and size, C-Scan parameters for imaging and defect response study by water path variation has also been carried out using CIVA simulation. The simulated results are validated against experiments on W tile mock-up regarding interface defect as well as sensitivity criteria for inspection. On the basis of these studies, ultrasonic testing parameters have been optimized and used for inspection. Calibration of ultrasonic system has been carried out using copper reference tube. Presence of defect location and its area in Cu-W Monoblock assembly are obtained from C-Scan image (Fig.10) which gives an idea on quality of the Monoblock assembly and its acceptance/rejection for fusion application. Acknowledgement The authors would like to thank Mr. Paritosh Nanekar (QAD, BARC) for scientific discussions and valuable suggestions about implementation of Ultrasonic Technique for testing the Monoblock Divertor assembly. The authors would also like to thank scientists, engineers and technical staff of NFTDC, Hyderabad, India for development of monoblock type of mock-ups. References [1] P. Calmon, S. Mahaut, S. Chatillon, R. Raillon, Ultrasonics 44, 2006, p [2] J Linke, et al. Phys. Scr. T123, pp (2006). [3] Y Patil, et al. Proc. of National Seminar and Exhibition on NDE, pp (2011). [4] G. Tie and Y. Takahashi, Ultrasonic echo signal features of dissimilar material bonding joint, Trans. Nonferrous Mett. Society china, 60(6), (2004) , [5] S. Mahaut, S. Lonné, S. Chatillon and G. Cattiaux, Validation of Civa simulation Tools for UT Examination, France. [6] ASTM E Standard Practice for Measuring Ultrasonic Velocity in Materials [7] E Ginzel,D Stewart, CIVA Modeling for Pipeline Zonal Discrimination, NDT.net April 2011 Issue. [8] S. Roccella, G. Burrasca, E. Cacciotti, A. Castillo, A. Mancini, A. Pizzuto, A. Tatì and E. Visca, Non-destructive methods for the defect detection in the ITER high heat flux Components, Fusion Engineering and Design, 86, (2011)