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Real-time arc welding defect detection technique by means of plasma spectrum optical analysis
NDT&E International 39 (2006) 356–360 www.elsevier.com/locate/ndteint
Real-time arc welding defect detection technique by means of plasma spectrum optical analysis J. Mirapeix a,*, A. Cobo a, O.M. Conde a, C. Jau´regui b, J.M. Lo´pez-Higuera a a
Grupo de Ingenierı´a Foto´nica, Dept. T.E.I.S.A. Universidad de Cantabria, Avda. Los Castros s/n, 39005 Santander, Cantabria, Spain b Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK Received 22 September 2005; accepted 5 October 2005 Available online 22 November 2005
Abstract An optimized technique for real-time spectral analysis of thermal plasmas, with application in the monitoring and defect detection of industrial welding processes, particularly arc-welding, is presented in this paper. The calculation of the plasma electronic temperature by means of a subpixel algorithm permits on-line quality assessment of the welds, allowing the detection of common defects to be found in the welding seam, such as oxidation due to insufficient shielding gas flux or lack of penetration caused by current fluctuations of the welding power source. The proposed technique has been successfully checked in a real-time arc-welding monitoring system, and experimental results of stainless-steel welds are also reported. q 2005 Elsevier Ltd. All rights reserved. Keywords: Spectroscopy; Plasma emission; Arc-welding; Real-time processing
1. Introduction The spectroscopic analysis of plasmas is a widely used technique, with applications in many fields, like chemical characterization or industrial process monitoring. Plasma emission spectrum is characterized by the presence of several emission lines, whose examination offers relevant information about the process responsible of the plasma formation [1]. In particular, arc and laser welding are industrial processes that can be monitored by means of a spectroscopic sensor system. In some critical applications (e.g. the aerospace sector), an early, real-time detection of common defects in the weld seam, like oxidation, porosity or lack of penetration, is highly desirable as it can reduce traditional extensive non-destructive testing (NDT) procedures routinely performed to every welded piece. Several different approaches have been proposed for the analysis of welding processes, including the monitoring and real-time control of the focal point in laser welding [2], the measurement of the charge voltage induced by the plasma on the welding nozzle [3], acoustic emissions [4], imaging in * Corresponding author. Tel./fax: C34 942200877. E-mail address: [email protected] (J. Mirapeix).
0963-8695/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2005.10.004
different spectral bands (including infrared thermography [5]), and single-point optical methods, with photodiodes [6] or optical fibre capture of light [7]. In general, electrical and acoustic sensors suffer from the noise generated by the welding process, while most of the optical techniques are intrusive in the sense that part of the sensor system must be placed close and around the weld pool. This can be a serious problem, especially in the production environment or for the welding of complex shapes. Although several other parameters can be obtained from the plasma using the captured spectra, the most widely employed variables in spectroscopic analysis are the electronic temperate and density of the plasma. It has been demonstrated that there is a relationship between those parameters and the resulting quality of the weld seam [8– 10]. The reliable acquisition and subsequent analysis of those variables allow the identification of defects which can appear in the weld seam as a result of a wide variety of causes: intensity fluctuations, shielding gas flux variations, irregularities in the width of the welded materials and many others, not to mention those defects generated by human operators. For those parameters to be obtained from the plasma spectrum, a precise analysis of the emission peaks from the various species contributing to the plasma must be carried out, involving central peak wavelength, amplitude and peak width. Some solutions have been developed by using iterative
J. Mirapeix et al. / NDT&E International 39 (2006) 356–360
techniques to perform a peak fitting to previously established known functions, but these strategies are specially time consuming. For a proper design of a real sensor system, that should be able not only to detect defects but to actuate over the process to assure an optimal result, real-time operation is required. A new real-time technique being able to process the spectrum obtained from the plasma emission, to identify the spectral lines within the range of interest and finally to detect even small discontinuities in the weld seam for arc welding processes is presented in this paper.
which is required in order to prevent the selection of lines with similar upper level energies. Taking for granted that, as it has been considered previously, the plasma is optically thin and the lines are free from self absorption, the ratio of the line intensities is described by: Ið1Þ Að1Þ gð1Þ lð2Þ Emð2Þ KEmð1Þ Z exp : (5) Ið2Þ Að2Þ gð2Þ lð1Þ kTe Reaching finally the expression which allows the calculation of the electron temperature Te: Te Z
2. Theoretical background In this work, the detection of defects in the weld seam is accomplished by spectroscopic analysis of the thermal plasma generated by the arc welding process, trying to find a correlation between the weld quality and the electronic temperature of the plasma. This parameter is obtained from the relative intensities of some emission lines belonging to a particular atomic species within those participating in the plasma [1]. Previously, some conditions about the plasma have to be assumed in order to perform a correct analysis. The main hypothesis is that the plasma is in local thermal equilibrium (LTE), which implies that the various particles within the plasma have Maxwellian energy distributions and collisional processes exceed radiative ones. This assumption is fulfilled when the electron density is high enough so that: Ne R 1:6 !1012 Te ðDEÞ3 ;
(1)
where Ne is the electronic density, Te the electronic temperature and DE is the largest energy gap in the atomic energy level system). When LTE is satisfied, the population of an excited level is determined via the Boltzmann equation: N KEm Nm Z gm exp ; (2) Z kTe where N is the population density of the state m, Z the partition function, gm the statistical weight, Em the excitation energy, k the Boltzmann constant and Te the electron temperature. In optically thin plasmas, the intensity of a given spectral line Imn, induced by a transition from the level m to the level n, can be related to the population density of the upper level Nm through Imn Z Nm Amn hgmn ;
(3)
being Amn the transition probability, and hgm the energy of such transition. By combining Eqs. (2) and (3) with a pair of spectral lines (of intensities I(1) and I(2)) of the same element, and satisfying the criterion: Emð1Þ KEmð2Þ O kT;
(4)
357
Emð2Þ KEmð1Þ I Að2Þ gð2Þ lð1Þ k In Ið1Þ A g l ð2Þ ð1Þ ð1Þ ð2Þ
(6)
As it could be derived from Eq. (6), the estimation of the peak intensities and an unambiguous identification of the spectral lines via their corresponding wavelengths are key steps to perform a correct estimation of the electronic temperature. As real-time requirements are involved, fast and efficient algorithms are required to develop this task. As the use of conventional peak-fitting approaches, like the Levenberg–Marquardt technique [11], are computationally inefficient, a new technique based on the LPO (linear phase operator) sub-pixel algorithm has been proposed [12,13]. The use of this operator allows a precise and fast calculation of the central peak wavelength associated to each spectral emission line under analysis. This step is of main relevance for the whole process, as it will permit the determination of the chemical species corresponding to those emission lines. 3. Experimental setup and results To verify the validity of the developed technique, the system schematically described in Fig. 1 was designed. To perform TIG welds, the ‘Kemppi Mastertig 2200’ power source was selected (with an output intensity ranging from 5 to 220 A). While the torch in our system was fixed, the plates used for the tests were controlled by a high-precision positioning system, formed by the controller (Newport MM4005) and two linear stages (Newport MTM100PP1), with a resolution of 1 mm in both axes. The end of the tungsten rod electrode (1 mm diameter) was placed at 2 mm from the plates. In our experiments only AISI-304 stainless steel plates were used, although the material to be welded does not imposed any restriction or calibration requirement in the proposed system. The optical instrumentation of the system was formed by a collimating lens receiving the light emitted from the plasma and a solarization resistant optical fiber (Ocean Optics P400UV-SR) used to guide the light from the collimator to the spectrometer. This low-cost, 2048 linear CCD spectrometer, (Ocean Optics USB2000), has a spectral resolution of 0.3 nm and spectral range from 195 to 535 nm. Argon was used as shielding gas, with a typical flow rate of 12 L/min. Initial analyses of spectra performed in weld tests offered spectral information apt to be analyzed in the spectral range from 350 to 510 nm, identifying spectral lines from Ar, Fe, Mn
358
J. Mirapeix et al. / NDT&E International 39 (2006) 356–360
Fig. 1. Experimental setup.
and Cr species. After some considerations, Ar II lines located between 455 and 490 nm (Fig. 2) were chosen as the most suitable ones for further analyses. Nonetheless, other lines outside the indicated region were also used in some of the weld tests. Although the final objective of the proposed system is to detect defects in weld seams, it was considered necessary to previously obtain a reference weld without defects. In Fig. 3 an example of a weld seam performed with a constant intensity of 65 A and a constant gas flux of 12 L/min is shown. In Fig. 3(a) the resulting reference weld seam (top) can be observed. The Te signal depicted in Fig. 3(b) was calculated considering two Ar II lines located at 461 and 485 nm, respectively. After a typical initial transient state due to the instability at the beginning of the welding process, the Te signal shows no relevant fluctuations as no visible defects are to be found. It is worth noting that this kind of reference test is not necessary in our system, not even when different materials were to be welded. An example of the ability of the proposed technique to detect small defects is presented in Fig. 4. In Fig. 4(a), where the bottom of the weld seam is shown, the defect, caused by a reduction in the gas flux from 12 to 6 L/min, is located at xz5 cm. Although it was mentioned before that mainly Ar lines were to be used in our experiments, in Fig. 4(b) it is depicted the result obtained when Fe I lines with central wavelengths 373 and 393 nm are considered. In Fig. 4(b) the defect is clearly identified. Besides, when considering Fe lines, the shortening of gas implies an increment in the Te signal, although it could be expected to be the same with the Ar lines, in Fig. 4(c) the same defect appears as a sharp reduction in the Te signal. This is due to the fact that when the flux of Ar gas is lowered, less concentration of Ar is expected within the plasma. A combination of both signals, which can be easily performed in real-time with our proposed technique, allows the
improvement in the sensitivity of defects detection. Fig. 4(c) presents the result of computing Te by calculating the mean value of all the possible combinations of Ar II lines identified for each capture during the process. The ability of the designed system to detect various defects in a single weld seam is shown in Fig. 5. The plate used in this test had a thickness of 3 mm, but it presented two parallel incisions, orthogonal to the weld seam longitudinal axis, of width equal to 1 cm and 2 mm depth at xZ1 and 7 cm (Fig. 5(a)). While a constant intensity of 102 A was chosen, the initial gas flux of 12 L/min was reduced to approximately 4 L/ min, producing a notorious defect in the seam (xZ5 cm). In Fig. 5(b) the result of considering all the possible Ar II lines combinations is presented. There are three mayor events in the Te evolution coinciding with the position of the incisions in the plate and with the cut of the shielding gas. The value of Te is
Fig. 2. Ar II spectral lines.
J. Mirapeix et al. / NDT&E International 39 (2006) 356–360
359
Fig. 5. Detection of various defects along a single weld seam. Fig. 3. Example of a reference weld seam without defects.
not stable up to xZ1 cm, due to the lack of stability in the beginning of the welding process. Other tests were performed trying to generate more subtle defects. During the weld shown in Fig. 6, performed on a plate with a thickness of 2 mm, a reduction in the welding intensity from 52 to 48 A was performed at xz3 cm. A more obvious defect caused by a reduction in the gas flux from 12 L/min to approximately 2 L/min at xz4.5 cm is also displayed. The resulting weld seam is shown in Fig. 6(a). Fig. 6(b) and (c) presents the result of calculating Te with all the possible combinations of Fe I and Ar II pairs of lines, respectively. It can be observed that in Fig. 6(b) and (c) the apparent defect
Fig. 4. Identification of a defect with different species.
created by the reduction in the gas flux at the end of the weld seam is clearly identified. The presented results confirm the ability of the presented technique to detect, in real-time and, therefore, on-line, common weld defects generated by a variety of causes. Although some subtle defects seem to be hard to identify within the depicted Te profiles, as in the case of the intensity reduction at xz3 cm presented in Fig. 6, increased defect sensitivity based on the Boltzmann-plot technique is currently being considered. Further research is being conducted at present in order to collect the light emitted from the plasma with an optical fiber embedded in the welding torch, thus allowing a non invasive
Fig. 6. Detection of defect in weld seam.
360
J. Mirapeix et al. / NDT&E International 39 (2006) 356–360
monitoring system more appropriate for a production environment. Field tests will be also presented and discussed. 4. Conclusions A new real-time technique for the analysis of thermal plasmas, specially those generated in arc or laser welding processes has been presented. The discussed technique is able to ensure a specific weld quality in real-time, and consequently to reduce the costs associated to the extensive NDT procedures that are typically carried out in some critical applications, as those belonging to the aerospace or automotive sectors. The use of the LPO sub-pixel operator allows fast and accurate determination of the central wavelengths of the spectral emission lines, thus enabling our system to perform a realtime, on-line defect detection during the welding process. Several tests confirming the validity of the proposed technique have been presented. Acknowledgements Authors thank the Spanish Government under the Ministry of Science and Technology for its support through the EOAMOP (TIC’2002-01259) R&D project. References [1] Griem HR. Principles of plasma spectroscopy. Cambridge: Cambridge University Press; 1997.
[2] Bardin F, Cobo A, Lopez-Higuera JM, Collin O, Aubry P, Dubois T, et al. Optical techniques for real-time penetration monitoring for laser welding. Appl Opt 2005;44(19):3869–76. [3] Li L, Brookfield DJ, Steen WM. Plasma charge sensor for in-process, noncontact monitoring of the laser welding process. Meas Sci Technol 1996; 7(4):615–26. [4] Gu HP, Duley WW. Resonant acoustic emission during laser welding of metals. J Phys D Appl Phys 1996;29(3):550–5. [5] Grabas B, Dardthuret J, Laurent M. Observation of penetration with infrared thermography in laser welding. J Phys IV 1994;4(C4):139–42. [6] Cheol-Jung K, Sung-Hoon B, Min-Suk K, Chin-Man C. Auto-focus control and weld process monitoring of laser welding using chromatic filtering of thermal radiation. Laser processing of materials and industrial applications II, Beijing, China; 1998, p. 388–98. [7] Haran FM, Hand DP, Peteres C, Jones JDC. Focus control system for laser welding. Appl Opt 1997;36:5246–51. [8] Sforza P, de Blasiis D. On-line optical monitoring system for arc welding. NDT E Int 2002;35:37–43. [9] Ancona A, Spagnolo V, Lugara PM, Ferrara M. Optical sensor for realtime monitoring of CO2 laser welding process. Appl Opt 2001;40(33): 6019–25. [10] Ferrara M, Ancona A, Lugara PM, Sibilano M. On-line quality monitoring of welding processes by means of plasma optical spectroscopy. Proc SPIE 2000;3888. [11] Robinson AW, Gardner P, Stampfl APJ, Martin R, Nyberg G. Error analysis in the fitting of photoemission lineshapes using the LevenbergMarquardt method. J Electron Spectrosc Relat Phenom 1998;94(1–2): 97–105. [12] Zeh T, Schweizer H, Meixner A, Purde A, Koch AW. Enhancement of detection accuracy of fiber Bragg grating sensors. Proc SPIE 2004;5502: 540–3. [13] Mirapeix JM, Cobo A, Conde O, Quintela MA, Lo´pez-Higuera JM. Spectroscopic analysis technique for arc-welding process control. Proc SPIE; 2005 5948.
Real-time arc welding defect detection technique by means of plasma spectrum optical analysis J. Mirapeix a,*, A. Cobo a, O.M. Conde a, C. Jau´regui b, J.M. Lo´pez-Higuera a a
Grupo de Ingenierı´a Foto´nica, Dept. T.E.I.S.A. Universidad de Cantabria, Avda. Los Castros s/n, 39005 Santander, Cantabria, Spain b Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK Received 22 September 2005; accepted 5 October 2005 Available online 22 November 2005
Abstract An optimized technique for real-time spectral analysis of thermal plasmas, with application in the monitoring and defect detection of industrial welding processes, particularly arc-welding, is presented in this paper. The calculation of the plasma electronic temperature by means of a subpixel algorithm permits on-line quality assessment of the welds, allowing the detection of common defects to be found in the welding seam, such as oxidation due to insufficient shielding gas flux or lack of penetration caused by current fluctuations of the welding power source. The proposed technique has been successfully checked in a real-time arc-welding monitoring system, and experimental results of stainless-steel welds are also reported. q 2005 Elsevier Ltd. All rights reserved. Keywords: Spectroscopy; Plasma emission; Arc-welding; Real-time processing
1. Introduction The spectroscopic analysis of plasmas is a widely used technique, with applications in many fields, like chemical characterization or industrial process monitoring. Plasma emission spectrum is characterized by the presence of several emission lines, whose examination offers relevant information about the process responsible of the plasma formation [1]. In particular, arc and laser welding are industrial processes that can be monitored by means of a spectroscopic sensor system. In some critical applications (e.g. the aerospace sector), an early, real-time detection of common defects in the weld seam, like oxidation, porosity or lack of penetration, is highly desirable as it can reduce traditional extensive non-destructive testing (NDT) procedures routinely performed to every welded piece. Several different approaches have been proposed for the analysis of welding processes, including the monitoring and real-time control of the focal point in laser welding [2], the measurement of the charge voltage induced by the plasma on the welding nozzle [3], acoustic emissions [4], imaging in * Corresponding author. Tel./fax: C34 942200877. E-mail address: [email protected] (J. Mirapeix).
0963-8695/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2005.10.004
different spectral bands (including infrared thermography [5]), and single-point optical methods, with photodiodes [6] or optical fibre capture of light [7]. In general, electrical and acoustic sensors suffer from the noise generated by the welding process, while most of the optical techniques are intrusive in the sense that part of the sensor system must be placed close and around the weld pool. This can be a serious problem, especially in the production environment or for the welding of complex shapes. Although several other parameters can be obtained from the plasma using the captured spectra, the most widely employed variables in spectroscopic analysis are the electronic temperate and density of the plasma. It has been demonstrated that there is a relationship between those parameters and the resulting quality of the weld seam [8– 10]. The reliable acquisition and subsequent analysis of those variables allow the identification of defects which can appear in the weld seam as a result of a wide variety of causes: intensity fluctuations, shielding gas flux variations, irregularities in the width of the welded materials and many others, not to mention those defects generated by human operators. For those parameters to be obtained from the plasma spectrum, a precise analysis of the emission peaks from the various species contributing to the plasma must be carried out, involving central peak wavelength, amplitude and peak width. Some solutions have been developed by using iterative
J. Mirapeix et al. / NDT&E International 39 (2006) 356–360
techniques to perform a peak fitting to previously established known functions, but these strategies are specially time consuming. For a proper design of a real sensor system, that should be able not only to detect defects but to actuate over the process to assure an optimal result, real-time operation is required. A new real-time technique being able to process the spectrum obtained from the plasma emission, to identify the spectral lines within the range of interest and finally to detect even small discontinuities in the weld seam for arc welding processes is presented in this paper.
which is required in order to prevent the selection of lines with similar upper level energies. Taking for granted that, as it has been considered previously, the plasma is optically thin and the lines are free from self absorption, the ratio of the line intensities is described by: Ið1Þ Að1Þ gð1Þ lð2Þ Emð2Þ KEmð1Þ Z exp : (5) Ið2Þ Að2Þ gð2Þ lð1Þ kTe Reaching finally the expression which allows the calculation of the electron temperature Te: Te Z
2. Theoretical background In this work, the detection of defects in the weld seam is accomplished by spectroscopic analysis of the thermal plasma generated by the arc welding process, trying to find a correlation between the weld quality and the electronic temperature of the plasma. This parameter is obtained from the relative intensities of some emission lines belonging to a particular atomic species within those participating in the plasma [1]. Previously, some conditions about the plasma have to be assumed in order to perform a correct analysis. The main hypothesis is that the plasma is in local thermal equilibrium (LTE), which implies that the various particles within the plasma have Maxwellian energy distributions and collisional processes exceed radiative ones. This assumption is fulfilled when the electron density is high enough so that: Ne R 1:6 !1012 Te ðDEÞ3 ;
(1)
where Ne is the electronic density, Te the electronic temperature and DE is the largest energy gap in the atomic energy level system). When LTE is satisfied, the population of an excited level is determined via the Boltzmann equation: N KEm Nm Z gm exp ; (2) Z kTe where N is the population density of the state m, Z the partition function, gm the statistical weight, Em the excitation energy, k the Boltzmann constant and Te the electron temperature. In optically thin plasmas, the intensity of a given spectral line Imn, induced by a transition from the level m to the level n, can be related to the population density of the upper level Nm through Imn Z Nm Amn hgmn ;
(3)
being Amn the transition probability, and hgm the energy of such transition. By combining Eqs. (2) and (3) with a pair of spectral lines (of intensities I(1) and I(2)) of the same element, and satisfying the criterion: Emð1Þ KEmð2Þ O kT;
(4)
357
Emð2Þ KEmð1Þ I Að2Þ gð2Þ lð1Þ k In Ið1Þ A g l ð2Þ ð1Þ ð1Þ ð2Þ
(6)
As it could be derived from Eq. (6), the estimation of the peak intensities and an unambiguous identification of the spectral lines via their corresponding wavelengths are key steps to perform a correct estimation of the electronic temperature. As real-time requirements are involved, fast and efficient algorithms are required to develop this task. As the use of conventional peak-fitting approaches, like the Levenberg–Marquardt technique [11], are computationally inefficient, a new technique based on the LPO (linear phase operator) sub-pixel algorithm has been proposed [12,13]. The use of this operator allows a precise and fast calculation of the central peak wavelength associated to each spectral emission line under analysis. This step is of main relevance for the whole process, as it will permit the determination of the chemical species corresponding to those emission lines. 3. Experimental setup and results To verify the validity of the developed technique, the system schematically described in Fig. 1 was designed. To perform TIG welds, the ‘Kemppi Mastertig 2200’ power source was selected (with an output intensity ranging from 5 to 220 A). While the torch in our system was fixed, the plates used for the tests were controlled by a high-precision positioning system, formed by the controller (Newport MM4005) and two linear stages (Newport MTM100PP1), with a resolution of 1 mm in both axes. The end of the tungsten rod electrode (1 mm diameter) was placed at 2 mm from the plates. In our experiments only AISI-304 stainless steel plates were used, although the material to be welded does not imposed any restriction or calibration requirement in the proposed system. The optical instrumentation of the system was formed by a collimating lens receiving the light emitted from the plasma and a solarization resistant optical fiber (Ocean Optics P400UV-SR) used to guide the light from the collimator to the spectrometer. This low-cost, 2048 linear CCD spectrometer, (Ocean Optics USB2000), has a spectral resolution of 0.3 nm and spectral range from 195 to 535 nm. Argon was used as shielding gas, with a typical flow rate of 12 L/min. Initial analyses of spectra performed in weld tests offered spectral information apt to be analyzed in the spectral range from 350 to 510 nm, identifying spectral lines from Ar, Fe, Mn
358
J. Mirapeix et al. / NDT&E International 39 (2006) 356–360
Fig. 1. Experimental setup.
and Cr species. After some considerations, Ar II lines located between 455 and 490 nm (Fig. 2) were chosen as the most suitable ones for further analyses. Nonetheless, other lines outside the indicated region were also used in some of the weld tests. Although the final objective of the proposed system is to detect defects in weld seams, it was considered necessary to previously obtain a reference weld without defects. In Fig. 3 an example of a weld seam performed with a constant intensity of 65 A and a constant gas flux of 12 L/min is shown. In Fig. 3(a) the resulting reference weld seam (top) can be observed. The Te signal depicted in Fig. 3(b) was calculated considering two Ar II lines located at 461 and 485 nm, respectively. After a typical initial transient state due to the instability at the beginning of the welding process, the Te signal shows no relevant fluctuations as no visible defects are to be found. It is worth noting that this kind of reference test is not necessary in our system, not even when different materials were to be welded. An example of the ability of the proposed technique to detect small defects is presented in Fig. 4. In Fig. 4(a), where the bottom of the weld seam is shown, the defect, caused by a reduction in the gas flux from 12 to 6 L/min, is located at xz5 cm. Although it was mentioned before that mainly Ar lines were to be used in our experiments, in Fig. 4(b) it is depicted the result obtained when Fe I lines with central wavelengths 373 and 393 nm are considered. In Fig. 4(b) the defect is clearly identified. Besides, when considering Fe lines, the shortening of gas implies an increment in the Te signal, although it could be expected to be the same with the Ar lines, in Fig. 4(c) the same defect appears as a sharp reduction in the Te signal. This is due to the fact that when the flux of Ar gas is lowered, less concentration of Ar is expected within the plasma. A combination of both signals, which can be easily performed in real-time with our proposed technique, allows the
improvement in the sensitivity of defects detection. Fig. 4(c) presents the result of computing Te by calculating the mean value of all the possible combinations of Ar II lines identified for each capture during the process. The ability of the designed system to detect various defects in a single weld seam is shown in Fig. 5. The plate used in this test had a thickness of 3 mm, but it presented two parallel incisions, orthogonal to the weld seam longitudinal axis, of width equal to 1 cm and 2 mm depth at xZ1 and 7 cm (Fig. 5(a)). While a constant intensity of 102 A was chosen, the initial gas flux of 12 L/min was reduced to approximately 4 L/ min, producing a notorious defect in the seam (xZ5 cm). In Fig. 5(b) the result of considering all the possible Ar II lines combinations is presented. There are three mayor events in the Te evolution coinciding with the position of the incisions in the plate and with the cut of the shielding gas. The value of Te is
Fig. 2. Ar II spectral lines.
J. Mirapeix et al. / NDT&E International 39 (2006) 356–360
359
Fig. 5. Detection of various defects along a single weld seam. Fig. 3. Example of a reference weld seam without defects.
not stable up to xZ1 cm, due to the lack of stability in the beginning of the welding process. Other tests were performed trying to generate more subtle defects. During the weld shown in Fig. 6, performed on a plate with a thickness of 2 mm, a reduction in the welding intensity from 52 to 48 A was performed at xz3 cm. A more obvious defect caused by a reduction in the gas flux from 12 L/min to approximately 2 L/min at xz4.5 cm is also displayed. The resulting weld seam is shown in Fig. 6(a). Fig. 6(b) and (c) presents the result of calculating Te with all the possible combinations of Fe I and Ar II pairs of lines, respectively. It can be observed that in Fig. 6(b) and (c) the apparent defect
Fig. 4. Identification of a defect with different species.
created by the reduction in the gas flux at the end of the weld seam is clearly identified. The presented results confirm the ability of the presented technique to detect, in real-time and, therefore, on-line, common weld defects generated by a variety of causes. Although some subtle defects seem to be hard to identify within the depicted Te profiles, as in the case of the intensity reduction at xz3 cm presented in Fig. 6, increased defect sensitivity based on the Boltzmann-plot technique is currently being considered. Further research is being conducted at present in order to collect the light emitted from the plasma with an optical fiber embedded in the welding torch, thus allowing a non invasive
Fig. 6. Detection of defect in weld seam.
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J. Mirapeix et al. / NDT&E International 39 (2006) 356–360
monitoring system more appropriate for a production environment. Field tests will be also presented and discussed. 4. Conclusions A new real-time technique for the analysis of thermal plasmas, specially those generated in arc or laser welding processes has been presented. The discussed technique is able to ensure a specific weld quality in real-time, and consequently to reduce the costs associated to the extensive NDT procedures that are typically carried out in some critical applications, as those belonging to the aerospace or automotive sectors. The use of the LPO sub-pixel operator allows fast and accurate determination of the central wavelengths of the spectral emission lines, thus enabling our system to perform a realtime, on-line defect detection during the welding process. Several tests confirming the validity of the proposed technique have been presented. Acknowledgements Authors thank the Spanish Government under the Ministry of Science and Technology for its support through the EOAMOP (TIC’2002-01259) R&D project. References [1] Griem HR. Principles of plasma spectroscopy. Cambridge: Cambridge University Press; 1997.
[2] Bardin F, Cobo A, Lopez-Higuera JM, Collin O, Aubry P, Dubois T, et al. Optical techniques for real-time penetration monitoring for laser welding. Appl Opt 2005;44(19):3869–76. [3] Li L, Brookfield DJ, Steen WM. Plasma charge sensor for in-process, noncontact monitoring of the laser welding process. Meas Sci Technol 1996; 7(4):615–26. [4] Gu HP, Duley WW. Resonant acoustic emission during laser welding of metals. J Phys D Appl Phys 1996;29(3):550–5. [5] Grabas B, Dardthuret J, Laurent M. Observation of penetration with infrared thermography in laser welding. J Phys IV 1994;4(C4):139–42. [6] Cheol-Jung K, Sung-Hoon B, Min-Suk K, Chin-Man C. Auto-focus control and weld process monitoring of laser welding using chromatic filtering of thermal radiation. Laser processing of materials and industrial applications II, Beijing, China; 1998, p. 388–98. [7] Haran FM, Hand DP, Peteres C, Jones JDC. Focus control system for laser welding. Appl Opt 1997;36:5246–51. [8] Sforza P, de Blasiis D. On-line optical monitoring system for arc welding. NDT E Int 2002;35:37–43. [9] Ancona A, Spagnolo V, Lugara PM, Ferrara M. Optical sensor for realtime monitoring of CO2 laser welding process. Appl Opt 2001;40(33): 6019–25. [10] Ferrara M, Ancona A, Lugara PM, Sibilano M. On-line quality monitoring of welding processes by means of plasma optical spectroscopy. Proc SPIE 2000;3888. [11] Robinson AW, Gardner P, Stampfl APJ, Martin R, Nyberg G. Error analysis in the fitting of photoemission lineshapes using the LevenbergMarquardt method. J Electron Spectrosc Relat Phenom 1998;94(1–2): 97–105. [12] Zeh T, Schweizer H, Meixner A, Purde A, Koch AW. Enhancement of detection accuracy of fiber Bragg grating sensors. Proc SPIE 2004;5502: 540–3. [13] Mirapeix JM, Cobo A, Conde O, Quintela MA, Lo´pez-Higuera JM. Spectroscopic analysis technique for arc-welding process control. Proc SPIE; 2005 5948.