Innovative Eddy Current Probe for Micro Defects

INNOVATIVE EDDY CURRENT PROBE FOR MICRO DEFECTS Telmo G. Santos, Pedro Vilaça, Jorge dos Santos, Luísa Quintino, and Luís Rosado Citation: AIP Conf. Proc. 1211, 377 (2010); doi: 10.1063/1.3362418 View online: http://dx.doi.org/10.1063/1.3362418 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1211&Issue=1 Published by the AIP Publishing LLC. Additional information on AIP Conf. Proc. Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors Downloaded 18 Jul 2013 to 193.136.124.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_permissions INNOVATIVE EDDY CURRENT PROBE FOR MICRO DEFECTS Telmo G. Santos1, Pedro Vilaça1, Jorge dos Santos2, Luísa Quintino,1 and Luís Rosado3 1 2 3 IDMEC, Av. Rovisco Pais, 1049-001 Lisbon, Portugal GKSS, Max-Planck-Street 1, D-21502 Geesthacht, Germany IST, UTL, Av. Rovisco Pais, 1049-001 Lisbon, Portugal ABSTRACT. This paper reports the development of an innovative eddy current (EC) probe, and its application to micro-defects on the root of the Friction Stir Welding (FSW). The new EC probe presents innovative concept issues, allowing 3D induced current in the material, and a lift-off independence. Validation experiments were performed on aluminium alloys processed by FSW. The results clearly show that the new EC probe is able to detect and sizing surface defects about 60 microns depth. Keywords: Eddy Currents, Probe, Friction Stir Welding, Micro-defects PACS: 81.70.Ex INTRODUCTION Friction Stir Welding (FSW) and Friction Stir Spot Welding (FSpW) are solid state welding processes, which proceed below the melting point of the weld material, i.e., there is no bulk melting in the region of the joint as occurs for conventional fusion welding processes. In FSW the workpieces are joined by the interaction of a rotating nonconsumable tool, which is plunged and traversed along the joint line [1]. The principles behind the technology have been patented and the increase of industrial applications of FSW, namely aerospace, aeronautic, naval and automotive sectors, has been asserting the importance of this process in the scope of joining technologies [2]. In spite of the good quality of FSW beads there are some defects that may arise, namely in butt welds, which is the most frequent joint arrangement (Figure 1). The most demanding FSW defects in terms of NDT are the micro-defects at the root of the weld bead [3]. This type of root flaws may exhibit a particular morphology, characterized by i) very low size (typical < 100 µm), ii) no physical material discontinuities, iii) very low energy reflection effect, and also iv) electric conductivity changes due to the FSW process itself, even with no defects [4]. Although of this morphology, the micro-defects at the root of the weld bead leads to a significant loss of the mechanical properties of the welding joints under fatigue loading. Therefore these defects are the target defects on NDT of FSW. Furthermore their particular morphology leads to a very difficult detection when using the existent eddy current (EC) probes [5]. In fact, the lift-off effect of conventional EC probes CP1211, Review of Quantitative Nondestructive Evaluation Vol.29, edited by D. O. Thompson and D. E. Chimenti © 2010 American Institute of Physics 978-0-7354-0748-0/10/$30.00 377 Downloaded 18 Jul 2013 to 193.136.124.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_p FIGURE 1. Macrograph of the typical defects on butt joint FSW configuration. a) Flash formation, b) Voids, c) Oxides alignment, d) Thickness reduction, e) Root defects, f) Oxides alignment in the root bead. introduces signal disturbances which mask the signal of this small FSW root defects, making their detection difficult or even impossible. Additionally, the low sensitivity of common EC probes may not enable to distinguish between defective and non defective weld conditions. Catalin Mandache et al. [6] applied PEC in FSW, but even in the presence of significant size root defects the inspection accuracy is low. PEC variant has the capability to detect deeper defects, nevertheless micro-defects in FSW can hardly be detected. Neil Goldfine et al. [7] have developed a MWM® probe which is able to detect some FSW defects, but only quite large (typically above 150 micron). A. Lamarre et al. [8] was one of the firsts to apply phased array ultrasonic on FSW, and P. H. Johnston et al. [9] has shown the difficulties in detecting the oxides alignment defects on the nugget of the FSW using this NDT technique. In order to improve the reliability in FSW nondestructive inspection a new NDT EC probe was developed and tested in different FSW defects conditions. The new EC probe allows a 3D induced eddy currents in the material; deeper penetration; independence of the deviation between the probe and the material surface; and easy interpretation of the output signal based on a comprehensible qualitative change [10]. PRODUCTION AND CHARACTERIZATION OF FSW DEFECTS SAMPLES Three different root defect conditions were produced in AA2024-T351 plates with 3.8 mm thickness: Type 0, Type I and Type II (Figure 2). Defect Type 0 is characterized by some residual particles alignment in an intermittent path along ≈ 150 m. This condition is typically considered a non defective weld. Defect Type I is characterized by a weak or intermittent welding since the materials are in close contact, under severe plastic deformation, but with no chemical or mechanical bond along ≈ 50 m. Defect Type II is characterized by ≈ 200 m non welded zone, followed by particles alignment in an intermittent path. The three different conditions present a consecutive increase of the defect intensity, suitable for a reliability analysis of a NDT system. 378 Downloaded 18 Jul 2013 to 193.136.124.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_p 60 µm 130 µm 200 µm 300 µm Type 0 Type I Type II FIGURE 2. Transversal macrographs of three different FSW root defects conditions. Defect Type 0: particles alignment, Defect Type I: ≈ 60 m, Defect Type II: ≈ 200 m. The existence of a stired zone, even with no imperfections causes material conductivity changes in the welded zone, even without any imperfection. These conductivity changes are a consequence of the microstructure changes introduced by the FSW process. Hence, the electric impedance of the EC probe changes are mainly due to the presence of the welded material, instead the presence of a imperfections. Therefore an accurate discrimination between this two diverse conductivity changes (due to the welded material and due to the imperfections) must be considered in order to detect and sizing defects. Conventional helicoidally and planar spiral absolute EC probes can hardly distinguish both cases. Figure 3 allows understanding the generic conductivity fields involved in a FSW joint with and without root defect. The dashed line represents the conductivity field across a FSW weld bead with no imperfections: a spreaded increase of conductivity occurs centered in the weld nugget. The continuous line represents the conductivity of a FSW weld bead with root imperfection: it is similar to the conductivity of a FSW with no imperfections, but it has a local and dip decrease in the center due to the root imperfection. Experimental results have shown that the typical FSW conductivity field in the vicinity of a joint is properly described by the Equation 1. It is a probability density function of the Gaussian distribution, with a standard deviation ( ) typically near to 1/3 of the shoulder diameter of the FSW tool. A factor K is used to normalize the curve. σ ( x) = K ⋅ ⎛ x2 ⎞ 1 ⋅ exp⎜⎜ − 2 ⎟⎟; λ 2π ⎝ 2λ ⎠ λ≈ φShoulder 3 (1) FIGURE 3. Typical electric conductivity fields across the FSW bead. 379 Downloaded 18 Jul 2013 to 193.136.124.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_p DEVELOPMENT OF A NEW EDDY CURRENT PROBE A new EC probe was developed and called IOnic probe [10] (Figure 4). This new patented design allows: a 3D induced eddy currents in the material; deeper penetration; an independence of the deviation between the probe and the material surface (planar lift-off); easy interpretation of the output signal based not on the absolute value but on a comprehensible characteristic change. In addiction this probe can be manufactured on flexible subtracts, allowing non-planar surfaces inspection. The IOnic probe is constituted by one excitation filament, in the middle of two sensitive planar coils, in a symmetric configuration. Due to this layout the operation of the IOnic probe is based on an integration effect along each sensitive coil, and simultaneous, on a differential effect between the two coils. The magnetic stimulus is generated by the current flow, Ī, on the excitation filament, and the detection of the defects is based in the induced voltage (Ūout) on the remaining terminals of the sensitive coils. The probe was manufactured on 1.6 mm dual layer FR4 PCB subtract with an external diameter of 11 mm. The two sensitive coils are formed by tracks of 100 m width separated by same dimension gaps. One particular feature of the IOnic probe compared to the conventional axissymmetric EC probes is the eddy current field display inside the material to be inspected. Figure 5 show the qualitative eddy current display in both cases. Conventional axissymmetric EC probes induces a magnetic flux perpendicular to the surface of the material, causing an eddy current flux parallel to the surface and coaxial to the excitation coils. The IOnic probe, in contrast, has an excitation filament, which induces a non confined magnetic flux all around the filament. As a consequence, the eddy current flux flows parallel to the excitation filament in the surface plane, but it flows also in other non parallel planes, collinear with the axis of the excitation filament, in a radial arrangement. FIGURE 4. The IOnic Probe prototype (left) and a conventional planar circular spiral eddy current probe (right) with 20 coils and an outside diameter of 9 mm. FIGURE 5. Eddy currents and magnetic flux display in IOnic Probe (left) and conventional axis-symmetric EC probes (right). 380 Downloaded 18 Jul 2013 to 193.136.124.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_p RESULTS The IOnic EC probe was tested on the FSW defects condition described in Figure 2. The data S(x) = Im{Ūout/Ī} and S(x) = Re{Ūout/Ī} was acquired from the root side, along a sweep on the transversal direction to the weld joint, with the excitation filament of the IOnic probe parallel to weld joint. The starting point of the tests was set to 25 mm before the weld bead, and 50 mm long segments were characterized. In Figure 6 it is presented the obtained results. In this figure the line 1, 2 and 3 correspond to defect condition Type 0, Type I and Type III, respectively. Column 1, 2 and 3 correspond to the inspection frequencies 50 kHz, 100 kHz and 250 kHz, respectively. As FSW process causes material conductivity changes, even without imperfections, the weld bead is responsible for the large curve on the imaginary part. The presence of imperfections creates a small perturbation observed on the middle of the joint, highlighted with a box. This behavior can be understood taking into account the conductivity fields implicated in the FSW processed material, previously mentioned in Figure 3. The large trend of S(x), with a minimum in x ≈ –7 mm and a maximum in x ≈ +7 mm, concerns to the spreaded increase of the electric conductivity field (σ(x)) due to the FSW joint itself, independent of the existence or absence of defects. Thus, this trend is almost the same for the three defect conditions. The small perturbation observed at the middle of the joint concerns to the suddenly decrease of conductivity due to the local root defect of each defect condition. These results show that IOnic probe is able to identify the three different types of defects. It also becomes clear that there is a very good proportionality between the defect dimension and the previously mentioned perturbation on S(x) = Ūout/Ī. The same three defect conditions were tested under the same operating conditions previously described for the IOnic probe tests. However, in this second NDT tests it was used a conventional planar circular spiral EC probe with 20 coils (Figure 4). In Figure 7 it is presented the obtained results S(x) = Re{Z} @ f = 100 kHz, 250 kHz and 750 kHz. The three curves concerning to the three defects conditions present a very similar trend between them. In fact, unlike the IOnic probe there is no distinctive signal feature that can allow to distinct between each defect condition. Indeed, the absolute planar spiral probe can only reproduce the global spreaded increase of conductivity field (s(x)) due to the FSW bead (correspondent to the dashed line in Figure 3). Such probes are not able to distinguish smal l suddenly variations of conductivity (correspondent to the continuous line in Figure 3), caused by a local root defect with small size as those tested. These results compared to the IOnic probe results illustrate the difficulty of NDT of FSW when using conventional EC probes. 381 Downloaded 18 Jul 2013 to 193.136.124.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_p FIGURE 6. Results for the FSW joint with defect types 0, I and II @ f = 50, 100 and 250 kHz using the IOnic Probe. FIGURE 7. Results for the FSW joints with defect types 0, I and II @ f = 100, 250 and 750 kHz using a planar circular spiral probe with 20 coils. 382 Downloaded 18 Jul 2013 to 193.136.124.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/righ CONCLUSIONS In this paper a new EC probe was introduced and tested on FSW of AA2024-T351 with root defects. The results were compared with a planar spiral EC probe. Conventional axis-symmetry EC probes such as planar circular spiral probes are not able to distinguish small local variations of conductivity, caused by typical FSW root defects with depth below 200 m. The experimental results shown that the IOnic probe is able to identify different levels of FSW root defects, through a qualitative perturbation on the output signal. It was also shown that exist a good proportionality between the defects size and this signal perturbation. ACKNOWLEDGEMENTS The authors would like to acknowledge the Portuguese Fundação para a Ciência e a Tecnologia (FCT) for its financial support via the PhD scholarship FCT – SFRH/BD/29004/2006. REFERENCES 1. W. M. Thomas, International patent No. PCT/GB92/02203 (December, 1991). 2. H. Assler and J. Telgkamp, “Design of aircraft structures under special consideration of NDT”, in 9th European Conference on NDT - ECNDT, Berlin, 2006. 3. D.G. Kinchen and E. Aldahir, “NDE of friction stir welds in aerospace applications”, Inspection Trends (AWS), 5, (2002). 4. T. Santos, P. Vilaça, L. Quintino, Welding in the World, 52, 30-37 (2008). 5. T. Santos, P. Vilaça, L. Reis, L. Quintino, M. de Freitas, “Advances in NDT Techniques for Friction Stir Welding Joints of AA2024”, in The Minerals, Metals & Materials Society (TMS), Vol. 3, TMS, New Orleans, 2008, pp. 27-32. 6. C. Mandache, L. Dubourg, A. Merati, Materials Evaluation, 4, 382-386 (2008). 7. D. Grundy, V. Zilberstein, N. Goldfine, “MWM-Array Inspection for Quality Control of Friction Stir Welded Extrusions”, in ASM 7th International Conference on Trends in Welding Research, Pine Mountain GA, 2006, pp. 16-20. 8. A. Lamarre, M. Michael, “Phased array ultrasonic inspection of FS Weldments”, in Review of Progress in Quantitative Nondestructive Evaluation, AIP Conference Proceedings, Vol. 509, American Institute of Physics, 2000, pp. 1333-1340. 383 Downloaded 18 Jul 2013 to 193.136.124.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/righ 9. P. H. Johnston, “Addressing the Limit of Detectability of Residual Oxide Discontinuities in Friction Stir Butt Welds of Aluminum Using Phased Array Ultrasound”, NASA Technical Report, 2008. 10. T. Santos, “END por Correntes Induzidas: Desenvolvimento e Aplicação à SFL”, (in Portuguese), Ph.D. Thesis, Technical University of Lisbon (2009). 384 Downloaded 18 Jul 2013 to 193.136.124.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_p