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Inspection of thick welded joints using laser-ultrasonic SAFTD. Lvesquea, Y. Asaumib, M. Lorda, C. Besconda, H. Hatanakab, M. Tagamib, J.-P. MonchalinaaNational Research Council Canada, 75 de Mortagne Blvd., Boucherville, Quebec J4B 6Y4, CanadabIHI Corporation, 1 Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japana r t i c l ei n f oArticle history:Received 18 November 2015Received in revised form 1 April 2016Accepted 1 April 2016Available online 2 April 2016Keywords:Thick weld inspectionLaser ultrasonicsSynthetic aperture focusing techniquea b s t r a c tThe detection of defects in thick butt joints in the early phase of multi-pass arc welding would be veryvaluable to reduce cost and time in the necessity of reworking. As a non-contact method, the laser-ultrasonic technique (LUT) has the potential for the automated inspection of welds, ultimately onlineduring manufacturing. In this study, testing has been carried out using LUT combined with the syntheticaperture focusing technique (SAFT) on 25 and 50 mm thick butt welded joints of steel both completedand partially welded. EDM slits of 2 or 3 mm height were inserted at different depths in the multi-pass welding process to simulate a lack of fusion. Line scans transverse to the weld are performed withthe generation and detection laser spots superimposed directly on the surface of the weld bead. A CCDline camera is used to simultaneously acquire the surface profile for correction in the SAFT processing.All artificial defects but also real defects are visualized in the investigated thick butt weld specimens,either completed or partially welded after a given number of passes. The results obtained clearly showthe potential of using the LUT with SAFT for the automated inspection of arc welds or hybrid laser-arcwelds during manufacturing.Crown Copyright ? 2016 Published by Elsevier B.V. All rights reserved.1. IntroductionArc welding thick metal plates in a butt joint configurationrequires in practice several passes to fill completely the gapbetween the plates. Defects, like porosity and lack of fusion, mayoccur at any depth during the welding process. Ultimately, thedetection and visualization of such defects in the early phase aftera small number of passes may reduce cost and time in the neces-sity of reworking. As a non-contact method, the laser-ultrasonictechnique (LUT) has the potential for the automated inspectionof welds, ultimately online during manufacturing. LUT uses lasersfor the generation and detection of ultrasound 1. It has the capa-bility for measurement at high temperature and the advantage ofbroadband ultrasound generation and detection, as well as easilymodifiable laser spots to achieve adequate resolution 2. More-over, LUT can be combined with the synthetic aperture focusingtechnique (SAFT) to allow detection of small discontinuities suchas cracks, inclusions and porosities 36.Recently, LUT combined with SAFT was investigated for thedefect detection in friction stir welds (FSW) made of aluminumsheets of thickness less than 3 mm thick in the lap joint and buttjoint configurations 7,8. More recently, the technique wasemployed to examine the possibility of inspecting internal flawslocated within the 9 mm thick gauge section of the hybrid laser-arc welded (HLAW) steel plates 9. Preliminary results indicatedthat the LUT-SAFT inspection can successfully detect and visualizethe presence of porosity, lack of fusion and internal crack defects.However in this study, the weld root back surface was used forinspection with the basic SAFT technique with generation anddetection superimposed. More recently, LUT with a modified ver-sion of SAFT (m-SAFT) was employed to inspect 150 mm thickwelded pipe during welding process with temperature of morethan 200 ?C 10. Ultrasound generation was performed on theweld bead while scanning the detection in the transverse directionaway from the weld on the parent materials surface. By using m-SAFT, an actual weld defect of 1.5 mm in diameter at 106 mmdepth in the specimen was observed. However, this approach relieson detecting small diffracted signals far from the defect and thedepth of the weld bead at the generation point needs to be knownfor SAFT reconstruction.In this work, testing is carried out using LUT still with the SAFTapproach on 25 and 50 mm thick butt welded joints of steel bothcompleted and partially welded. Line scans transverse to the weldare performed with the generation and detection superimposeddirectly on the surface of the weld bead. A CCD line camera is usedto simultaneously acquire the surface profile for correction in theSAFT processing. With generation and detection superimposed,/10.1016/j.ultras.2016.04.0010041-624X/Crown Copyright ? 2016 Published by Elsevier B.V. All rights reserved.Corresponding author.E-mail address: daniel.levesquecnrc-nrc.gc.ca (D. Lvesque).Ultrasonics 69 (2016) 236242Contents lists available at ScienceDirectUltrasonicsjournal homepage: /locate/ultrasthere is no need of surface preparation even in the presence ofoxide. EDM slits are inserted at different depths in the multi-passwelding process to simulate a lack of fusion. The specimens andthe laser-ultrasonic setup used are first described along with theSAFT approach. Then the results are presented and discussed. Allartificial defects but also real defects are visualized in the investi-gated thick butt weld specimens, either completed or partiallywelded after a given number of passes.2. Specimens and inspection techniqueFour thick butt welded joints of steel, with thicknesses T = 25and 50 mm, both complete and partially welded (underway), wereprepared. In each specimen, EDM slits of 2 or 3 mm height wereinserted at different locations and depths in the multi-pass weld-ing process to simulate a lack of fusion. Figs. 1 and 2 show sche-matic diagrams of the weld specimens with the identificationnumber for each slit. The slits have the height and length dimen-sions indicated on the figures. Fig. 3 shows photos of all four weldspecimens. It is noted that for thick specimens the multi-passwelding process may involve filling the V-grooves present on bothsides. In these situations, access to one or two sides for inspectionmay be acceptable or desirable.Laser-ultrasonic setup used for non-contact off-line inspectionof the test specimens was made with the intent of future onlineimplementation in the process. Line scans transverse to the weldwas performed with the generation and detection superimposeddirectly on the surface of the weld bead. Ultrasound generationwas made in the slight ablation regime (less than 1lm) with ashort pulse Nd:YAG laser in its 2nd harmonic (532 nm wavelength,6 ns pulse duration) to achieve high frequencies. For detection, along pulse Nd:YAG laser (1064 nm wavelength, 60ls pulse dura-tion) and a small spot size of about 0.5 mm were used. The phasedemodulator was a 1-m long confocal Fabry-Perot interferometerin reflection mode. Frequency content up to 30 MHz in steel wassuccessfully generated and detected in the above test specimens.Also it is noted that with generation and detection superimposed,there is no need of any surface preparation even in the presence ofoxide. Mechanical scanning along single line up to 100 mm longwas performed for data acquisition of the waveforms with a stepsize of 0.25 mm. Also, a CCD line camera was used to simultane-ously acquire the surface profile for correction in the SAFTprocessing.For numerical focusing, SAFT processing is used for synchro-nization of the ultrasonic signals scattered back in different direc-tions from each point in the weld region. While maintaining thedepth resolution, SAFT reconstruction improves the lateral resolu-tion as well as the signal-to-noise ratio (SNR) 4. The depth andlateral resolutions are estimated using:Dx ?zavDt ?v2tghfmaxDz ?12vDt ?v4fmax1wherevis the ultrasonic wave velocity,Dt is the ultrasonic pulseduration, a is the dimension of the synthetic aperture, h is the aper-ture angle from surface normal and fmaxis the maximum frequencydetected. To evaluate the SNR, an amplitude profile is extractedfrom the B-scan image and the SNR is evaluated thereafter using:SNR 20log10A ?l?r2r?2where A is the maximum amplitude at the defect location,lis thebackground level andris the noise standard deviation near thedefect. When scanning on the weld bead, a SAFT algorithm thataccounts for the surface variations on an irregular surface shouldbe used 5. For synchronization in the time-domain SAFT version,the time-of-flight of a signal acquired at location (xi, zi) for a defectto be imaged at location (x, z) is simply:ti 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffix ? xi2 z ? zi2q=vL3where zi 0 stands for the surface height at the generationdetec-tion point with reference to the parent materials surface at z = 0,andvLis the velocity of the longitudinal mode. SAFT reconstructionFig. 1. Schematic diagram of the 25 mm thick weld specimens, (a) complete and (b) underway. Dimensions are in mm.D. Lvesque et al./Ultrasonics 69 (2016) 236242237Fig. 2. Schematic diagram of the 50 mm thick weld specimens, (a) complete and (b) underway. Dimensions are in mm.Fig. 3. Photos of the weld specimens complete and underway of thicknesses (a) and (b) 25 mm, (c) and (d) 50 mm.238D. Lvesque et al./Ultrasonics 69 (2016) 236242was performed with an aperture angle of 30? and a longitudinalwave velocity of 6.0 mm/ls. With the above setup, the spatial res-olution is estimated to be better than 0.2 mm.Fig. 4 shows the performance of SAFT reconstruction for a singleline scan over 100 mm on a 50 mm thick reference block of steelwith three side-drilled holes one above the other at differentdepths on a vertical line. The synthetic aperture with such angularrange allows reconstruction of the top of each hole to be recon-structed from the hyperbolas observed in the raw signals. SAFTprocessing clearly improves the SNR for defect detection and reso-lution for defect sizing. From top to bottom, the SNR for the 3 holesafter SAFT reconstruction are 34, 27 and 24 dB, respectively.3. Laser-ultrasonic inspection resultsThe thick butt welded joints described in previous section, withthicknesses T = 25 and 50 mm, both complete and partially welded(underway), were tested with the LUT-SAFT approach. In each case,a few line scans transverse to the weld are performed directly onthe surface of the weld bead in the region where EDM slits areexpected to be present.3.1. Specimens of thickness T = 25 mmFig. 5 shows the top surface profile acquired using the CCD linecamera during laser-ultrasonic testing of the 25 mm thick buttwelded joint complete. Such surface profile is used for correctionin the SAFT processing using Eq. (3). As an illustration, Fig. 6 showsthe B-scan images on the weld specimen complete without correc-tion for the top profile for a line scan over 50 mm. While the slit32C1 is detected, no improvement in resolution is obtained afterSAFT reconstruction, in addition to a prominent indication on theback wall partly due to the curvature on top surface. Fig. 7 showsthe results after correction with slits 32C1 and 32D1 well detectedand visualized with a SNR of 21 and 13 dB respectively, at a depthof about 11 mm and 22 mm from the flat surface on top (seeFig. 1a). Not shown here, repeatability was demonstrated withsimilar results found for slits 32C2 and 32D2.Regarding the weld specimen T = 25 mm underway, Fig. 8 showsthe B-scan images with correction for the top profile and a line scanof 13 mm for both slits 42C1 and 42D1, respectively at a depth ofabout 11 mm and 22 mm with respect to the flat surface on top(see Fig. 1b). Simulating a lack of fusion, both EDM slits 42C1 and42D1 are clearly visualized with a SNR of 16 and 34 dB respectively,using LUT-SAFT testing directly on the weld bead. Not shown here,repeatability was demonstrated with similar results found for slits42C2 and 42D2. These results show the potential for online inspec-tion during welding after a given number of passes.3.2. Specimens of thickness T = 50 mmFig. 9 shows the surface profiles acquired using the CCD linecamera during laser-ultrasonic testing on top and bottom of the50 mm thick butt welded joint complete. Again it is noted thatfor such thick specimen, inspection from either side may beacceptable or desirable since the multi-pass welding processinvolves two V-grooves. Again such surface profiles are used forcorrection in the SAFT processing using Eq. (3). Fig. 10 shows theresults after correction over a line of 50 mm for both slit 5E1 and5F1 with a SNR of 17 and 20 dB respectively, at a depth of about22 mm and 35 mm with respect to the flat surface on top (seeFig. 2a). In Fig. 10a, a spurious linear indication is present nearthe back wall which is attributable to reflection of surface waveon top of the weld specimen of finite dimensions. Fig. 11 alsoshows the results from testing on the back surface visualizing bothedges of the EDM slit 5F1 with a SNR of 21 dB, as well as a realdefect about 7 mm along the weld away from the center of the sliton the upper left of the B-scan image in Fig. 11b.Regarding the weld specimen T = 50 mm underway, Fig. 12shows the B-scan images with correction for the top profile for aline scan of 13 mm for both slit 62E1 and 62F1 respectively at adepth of about 22 mm and 35 mm with respect to the flat surfaceon top (see Fig. 2b). Simulating a lack of fusion, both EDM slits62E1 and 62F1 are clearly visualized with a SNR of 21 and 20 dBrespectively, using LUT-SAFT testing directly on the weld bead.This is in addition to a real defect present on top of the B-scanimage in Fig. 12b. Not shown here, the slit 62F1 can also be visual-ized from inspection on the back surface. Again, repeatability wasdemonstrated with the similar results found for slits 62E2 and62F2. These results show the potential for online inspection duringwelding after a given number of passes.Fig. 4. B-scans on a 50 mm thick reference block of steel with three side-drilled holes, (a) raw data and (b) after SAFT reconstruction.Fig. 5. Top surface profile for weld specimen T = 25 mm complete.D. Lvesque et al./Ultrasonics 69 (2016) 236242239Fig. 8. B-scans on weld specimen T = 25 mm underway over 13 mm after SAFT reconstruction with correction for top profile at (a) slit 42C1 (nominal depth of 11 mm fromtop) and (b) slit 42D1 (nominal depth of 22 mm from top).Fig. 9. Surface profile for weld specimen T = 50 mm complete for inspection from (a) top and (b) bottom.Fig. 7. B-scans on weld specimen T = 25 mm complete over 50 mm after SAFT reconstruction with correction for top profile at (a) slit 32C1 (nominal depth of 11 mm) and (b)slit 32D1 (nominal depth of 22 mm).Fig. 6. B-scans on weld specimen T = 25 mm complete over 50 mm at slit 32C1 (nominal depth of 11 mm from top); (a) raw data and (b) after SAFT reconstruction withoutcorrection for top profile.240D. Lvesque et al./Ultrasonics 69 (2016) 2362424. ConclusionLaser-ultrasonic inspection combined with SAFT was performedon butt weld specimens of thicknesses up to 50 mm. Line scanstransverse to the weld are performed with the generation anddetection laser spots superimposed directly on the surface of theweld bead without any surface preparation. A CCD line camera isused to simultaneously acquire the surface profile for correctionin the SAFT processing.EDM slits of 2 or 3 mm height were inserted at differentlocations and depths in the multi-pass welding process to simulatea lack of fusion. All artificial defects but also real defects areFig. 10. B-scans on weld specimen T = 50 mm complete over 50 mm after SAFT reconstruction with correction for profile at (a) slit 5E1 (nominal depth of 22 mm) and (b) slit5F1 (nominal depth of 35 mm).Fig. 11. B-scans on weld specimen T = 50 mm complete from back surface over 50 mm after SAFT reconstruction with correction at (a) center of slit 5F1 and (b) 7 mm beyondalong the weld.Fig. 12. B-scans on weld specimen T = 50 mm underway over 13 mm after SAFT reconstruction with correction for top profile at (a) slit 62E1 (nominal depth of 22 mm fromtop) and (b) slit 62F1 (nominal depth of 35 mm from top).D. Lvesque et al./Ultrasonics 69 (2016) 236242241visualized in the investigated thick butt weld specimens, eithercompleted or partially welded after a given number of passes.The results obtained clearly show the potential of using the LUTwith SAFT for the automated inspection of arc welds or hybridlaser-arc welds during manufacturing. A simple calculation showsthat a 1 kHz laser repetition rate would allow real time inspectionat a speed of typically 1 cm/s along the weld.References1 C.B. Scruby, L.E. Drain, Laser ultrasonics: Techniques and Applications, AdamHilger, Bristol, 1990.2 J.-P. Monchalin, Laser-ultrasonics: from the laboratory to i