The need for monitoring laser material processes israpidly growing in the manufacturing industry.This technology has begun to be introduced in pro-duction lines, but for laser welding processes thereare still some problems of reliability because of thecomplex physical interactions involved and the largenumber of parameters that influence the process.At present it is necessary to perform a series of post-process analyses, mostly destructive, to certificate awelding procedure. Furthermore, even the use of awell-tested set of parameters does not always guar-antee acceptable weld quality because there are localprocess instabilities; then on-line quality controlturns out to be required for detection of defects andprevention of their repetition.
Various kinds of sensor have been developed forthis purpose: thermal sensing of the weldingpool1–4; CCD vision of the seam1–4; charge sensors todetect the electric potential between the welding noz-zle and the workpiece5; acoustic monitoring of the
waves generated by keyhole plasma6; and photosen-sors that detect the UV–visible radiation emitted bythe plasma plume generated over the laser–metalinteraction zone.7,8 Owing to the complex nature ofthe process, the information that these sensors give isoften qualitative, and, even when they give quanti-tative results these sensors are able to control only asingle parameter, such as penetration depth9 or focusposition, by electrical10 or optical means.11
Particularly promising are sensors that analyzethe behavior of the laser-generated plasma plumethat plays an important role in the welding processbecause it partially absorbs the incident laser radia-tion, influencing the energy transfer to the weldingmetal. Therefore perturbations that lead to the for-mation of weld defects are strongly correlated toplasma instabilities. A commercial system that ex-ploits plasma optical emission in only the UV and thenear-IR ranges has been also developed.12
Several papers have been devoted to describing thekinetics of laser-induced plasmas by theoreticalmodels13–15 and to experimentally investigating theplasmas’ electron temperature and density by usingspectroscopic techniques.16–23
We present here a study of the CO2 laser-inducedplasma’s optical signal emitted during welding ofAISI 304 stainless steel. For the first time, electrontemperatures of the various chemical species thatcompose the plasma plume have been determinedsimultaneously, by use of related emission lines.The results show a nonuniform temperature distri-
The authors are with the Dipartimento InterAteneo di Fisica,Universita degli Studie Politecnico di Bari and Istituto Nazionaleper la Fisica della Materia; Unita di Ricera di Bari via Orabona 4,I-70126 Bari, Italy. A. Ancona’s e-mail address is [email protected].
Received 10 April 2001; revised manuscript received 6 August2001.
bution that leads to nonuniform radiation of the var-ious chemical species inside the plume.
We have determined the plasma’s electron temper-ature under several operating conditions, i.e., byvarying the laser incident power, the shielding gas,the gas flux, and the beam focus position. A corre-lation has been found between the temperature sig-nal and the quality of the welding seam. Thiscorrelation has allowed us to develop an innovativeoptical monitoring system that is able to detect weld-ing defects in real time.
2. Theory of Plasma Spectroscopy
Spectroscopic studies of the welding plasma’s opticalemission in the UV–visible and near-infrared rangescan give information on such plume characteristics ascomposition, electron temperature, electron density,and absorption coefficient.24 These kinds of laser-induced plasma are generally assumed to be opticallythin and in local thermal equilibrium; this meansthat particles have Maxwellian energy distributionsand collisional processes dominate over radiativeones. The critical electron density necessary to ful-fill this criterion25 is given by
Ne � 1.6 � 1012Te1�2��E�3, (1)
where Ne is the electron density �in inverse cubiccentimeters�, Te is the electron temperature, and �Eis the largest energy gap in the atomic energy levelsystem �e.g., for Fe �I� it is 0.74 eV�; for the casesinvestigated, we obtained Ne � 6.4 1013 cm�3. Thetypical electron densities in a plasma plume over ametal surface, estimated under laser-welding condi-tions, exceed this threshold by 2 orders of magni-tude,20,26 so the local thermal equilibrium is assumedto be valid.
We used the Boltzmann plot method to calculatethe plasma’s electron temperature by measuring therelative intensities of the several emission lines froma chosen element that compose the welding metal,the shielding gas, or both in a given ionization state.The intensity Imn of a plasma emission line associ-ated with the transition from level m to level n isgiven by
Imn � Nm Amnh�mn, (2)
where h�mn is the energy of the transition, Amn is thetransition probability, and Nm is the population of theupper state. The population of the excited state, m,is given by the Boltzmann distribution
Nm � �N�Z� gm exp � Em�kT, (3)
where N is the total density of the state, gm is thestatistical weight, and Z is the partition function.Equation �2� becomes
ln�Imn�mn
Amngm� � ln�Nhc
Z � �Em
kT. (4)
By plotting the first term of Eq. �4� versus Em forseveral lines, one can estimate the electron temper-ature, which is related to the slope of the linear fit.
The electron temperature has been also estimatedby use of the intensity ratio of a couple of emissionlines �labeled �1� and �2� in the following equations�that correspond to the same chemical species:
I�1�
I�2��
A�1� g�1���1�
A�2� g�2���2�exp��
Em�1� � Em�2�
kTe� . (5)
Extracting Te from Eq. �5�, we obtain
Te �Em�2� � Em�1�
k ln�I�1� A�2� gm�2���1�
I�2� A�1� gm�1���2��. (6)
This method is advantageous because, as the re-lated emission line parameters are known, it does notrequire too many calculations; therefore it can easilybe implemented in suitable software for real-timetemperature measurement. Selected emission linesare used for this purpose; because lines that belong tothe same multiplet state must not be chosen, we ver-ify the selected lines to fulfill the criterion Em�1� �Em�2� kT on the upper energy levels. The linesmust also be free from self-absorption; one can provethat this condition has been fulfilled by verifying thatthe optical depth21 of the plasma for the selectedspectral lines is � 0.1.
3. Experimental Setup
A DC025 Rofin Sinar CO2 laser with output power ofas much as 2500 W was used. It can operate in apulsed or in a continuous-wave regime, in the TEM00mode with a small TEM01 component. The 25-mm-diameter exit beam had a divergence of 0.5 mrad. Itwas focused on the workpiece by means of a parabolicfocusing mirror of 200-mm focal length, which re-sulted in a spot size of 0.2-mm diameter and a laserdensity power of �107 W�cm2 on the workpiece. Ei-ther argon or helium gas was used as a shielding gas�30-L�min maximum flow rate� to prevent oxidationof the weld seam. The gas was ejected from twopipes placed sideways in the laser beam. The dis-tance between the pipes and the workpiece was 6mm. Samples were moved perpendicularly to thelaser beam by use of a four-axis motorized table,which was controlled by a personal computer througha numeric control program.
A quartz collimator of 6-mm focal length was fixedto the focalizing head and pointed at the laser–metalinteraction zone to collect plasma-emitted radiation.We used a 50- m diameter optical fiber to transmitthe signal to the 10- m entrance slit of a miniaturespectrometer equipped with a CCD detector array;the spectral range detected was 390–575 nm, and theoptical resolution was 0.3 nm �Fig. 1�. The spec-trometer was interfaced to a personal computer bymeans of an acquisition card. We acquired the in-tensity of the selected pairs of emission lines and, by
using a suitable algorithm to implement Eq. �6�, wecalculated the electron temperature in real time.
4. Experimental Results and Discussion
A series of fully penetrated bead-on-plate weldingtests was performed on AISI 304 120-mm-long,3-mm-thick stainless steel plates, at a translationspeed of 10 mm�s, with an incident laser power of asmuch as 2000 W. The welding plasma’s opticalemission was acquired and analyzed for an average of1500 acquisitions, each one with a 3-ms exposuretime. More than 200 iron, chromium, and manga-nese emission lines were identified.27 Electron tem-peratures were calculated by the Boltzmann plotmethod; we chose three different sets of atomic iron,chromium, and manganese emission lines, whose pa-rameters are listed in Table 1. All selected lineswere verified to be free from self-absorption and tofulfill the criterion for the upper energy levels ex-plained in Section 2.
Figure 2 shows the Boltzmann plots and the elec-tron temperatures evaluated from the three differentsets of lines for the same welding process. The dif-ference �as much as 2300 K� between values of tem-perature estimated for the various atomic species canbe explained in terms of nonuniform temperaturedistribution inside the plasma plume. This differ-ence was previously observed for atomic and ioniciron lines23 and was ascribed to the variation in theemission coefficients of the selected lines as a func-tion of temperature: A nonuniform temperaturedistribution causes a nonuniform radiation behaviorof the chemical species inside the plume. Thus itcan be supposed that the bulk of the radiation fromFe �I�, Cr �I�, and Mn �I� lines is emitted from differ-ent regions of the plume, i.e., at increasing tempera-tures, from the outer to the inner region of theplasma. Thus, depending on the set of emissionlines selected, it is possible to investigate spatiallymore-or-less energetic regions at different distancesfrom the laser–material interaction zone.
We used these results to develop a real-time mon-itoring system for the welding process. We calcu-lated electron temperatures on line by selecting threecouples of lines from the emission spectrum that be-longed to different chemical species. The selectedlines are shown in Fig. 3.
Suitable software performed the calculations basedon Eq. �6� and recorded the plasma temperature sig-nals at a rate of 50 Hz. From an analysis of theelectron temperature signal, the size of weld defectscan be determined when the welding speed is known;at a welding speed of 10 mm�s the optical sensorreaches a spatial resolution of 0.2 mm.
Figure 4 shows the temperature signals for a weld-ing process that has used optimal process parame-ters. The result is a welding bead without defectsand a stable mean value of the plasma temperature;the observed small oscillations are due to plasmafluctuations.
Experimental results presented in Fig. 5 clearly
Fig. 1. Experimental setup of the optical sensor.
Table 1. Spectroscopic Parameters of the Emission Lines Used forCalculations of Electron Temperaturea
show that the standard deviation of the plasma’stemperature signal is strongly correlated to thequality, of the welded joint and one can use it to findthe best process parameters. Several weldingtests were performed. The average plasma tem-perature’s mean value and its standard deviationfor the three different chemical species were ac-quired as a function of laser incident power, gasflow rate, and beam focus position. Electron tem-perature signals are most stable for a 1400-W inci-dent power, an argon flow rate of 30 L�min, and abeam focus position 1 mm inside the plate; these arethe optimal process parameters and lead to a fullypenetrated welding bead. Figure 5�a� shows that,once the gas flow rate and the beam focus positionare fixed, a lower or higher incident power causesinstability in the molten pool and accordingly in the
plasma plume, resulting in higher standard devia-tions of the temperature signals and in lack of pen-etration or in perforations in the welded joints.Operating at fixed laser incident power and gas fluxand varying the laser beam’s focal position yieldeda lower standard-deviation value when the focalposition was 1 mm inside the plate �see Fig. 5�b��,corresponding to the best-quality welded joint.Figure 5�c� shows the temperature signal for weld-ing tests carried out at variable gas flow rates andfixed laser incident power and beam focus position.We observed that an increase in the gas flux in-duced a reduction in the volume of the plasmaplume resulting in a more stable temperature dis-tribution inside the plume, which in turn, caused adecrease in the signal’s standard deviation. Simi-
Fig. 2. Boltzmann plots of atomic Fe, Cr, and Mn. The selected line parameters are listed in Table 1. Electron temperatures wereextracted from the linear fits of the data, as explained in the text.
Fig. 3. Plasma-emission spectrum acquired for welding per-formed at 1400-W incident laser power and 10-mm�s translationspeed at a 30-L�min argon flow rate. Arrows indicate the Fe �I�,Cr �I�, and Mn �I� emission lines used for calculation of electrontemperature. 1 Å � 0.1 nm.
Fig. 4. Electron temperature signals acquired for a welding beadwithout defects, obtained by operation at 1400-W incident power,a 30-L�min argon flow rate, and a 10-mm�s translation speed.The resultant welding bead is shown in the photo.
lar results were obtained with a helium shieldinggas.
In all cases shown in Fig. 5 the electron tempera-ture’s mean value appears to be less sensitive to vari-ations in process parameters with respect to thesignal standard deviation value. Standard deviationanalysis can thus be considered a valuable method foroptimizing welding process parameters.
We tested the sensitivity of our optical monitoringsystem by simulating various localized weld defects
and recording the three electron temperature signalsas a function of position on the welded joint.
Figure 6�a� shows the response of the optical sensorfor laser welding performed with 2000 W of incidentpower, during which material spatter and crater for-mation on the welding bead were produced. In fact,excessive incident laser power usually causes insta-bility in the welding pool owing to increasing volumeof the molten metal; this instability can result inentrapment of vapor bubbles, producing porosity in
Fig. 5. Mean value and standard deviation of plasma’s electron temperature measured for Fe �I� �light-gray bars�, Cr �I� �dark-gray bars�,and Mn �I� �white bars� averaged over five welding processes: �a� 30-L�min argon flow rate, beam focus position 1 mm inside the plate,at different incident laser powers; �b� 30-L�min argon flow rate, 1400-W incident laser power, at different beam focus positions; the zeroin the focal shift axes corresponds to a beam focus positioned 1 mm inside the plate; �c� 1400-W incident laser power, beam focus position1 mm inside the plate, at different argon flow rates.
Fig. 6. Electron temperature signals correlated to the welded joints shown in the photos, for various weld defects: �a� crater formation,�b� weld disruptions, �c� plate undulations, �d� gas interruptions.
the welded joint or ejection of molten material fromthe welding seam. There is also a decrease in allthree temperature signals connected with the welddefect. An analogous response of the optical sensorwas observed in the case of weld disruptions causedby localized imperfections intentionally produced onthe metal surface �Fig. 6�b��. The decreases in tem-perature signal were due to the collapse of the plasmaplume generated at sites that weld defects. In bothcases the optical monitoring system had been able todetect surface imperfections of dimensions as smallas 1 mm.
Figure 6�c� shows the behavior of the electron tem-perature signal recorded for welding carried out on abent steel plate. Undulations of the welding metalproduced defocusing of the incident laser beam on thesurface, leading to a decrease in the incident powerdensity and consequently in the thermal input to thematerial. This condition caused a different temper-ature distribution inside the plasma plume, and amodulation of the electron temperature signals thatfollow plate undulations was detected; the electrontemperature was lower at the maximum beam defo-cus, where the thermal input was reduced.
It is worth noting that, in this case, the three elec-tron temperature signals show different levels of sen-sitivity to detection of weld defects. The plasmatemperature signal that arises from Fe �I� emissionlines seems to be less sensitive to decreases in inten-sity. The plasma temperature’s mean value sug-gests that the bulk of the radiation from the Fe �I�selected lines comes from the less-energetic part ofthe plasma plume, i.e., the outer part. Thus we cansuppose that this temperature signal gives informa-tion on the most peripheral portion of the plasmaplume that is less perturbed by instabilities inducedby the generation of defects on the metal surface.However, the temperature signals that arise fromCr �I� and Mn �I� lines are strictly related to the plas-ma’s core temperature distribution, which is moreinfluenced by perturbations in the welding process.
The last weld defect simulated was oxidation of thewelding seam induced by local interruptions in theflow rate of the shielding gas. As can be seen fromFig. 6�d�, a decrease in the electron temperature sig-nal’s mean value calculated from Fe �I� and Cr �I�relative emission lines was observed when in-processargon flux interruptions occurred. The temperaturesignal estimated from Mn �I� emission lines was lesssensitive to the process perturbation in terms of av-erage value, but an increase in signal oscillation wasstill noticed. A possible explanation for this result isthat gas interruptions introduce a more intense per-turbation in the peripheral part of the plasma’splume than in its core.
5. Conclusions
The development of an optical sensor for real-timemonitoring of laser welding quality has been re-ported. The sensor is based on measurement of thewelding plasma’s electron temperature through aspectroscopic analysis of the plasma’s optical emis-
sion. Non-uniform radiation behavior of the chemi-cal species that compose the plasma plume, caused bya nonuniform temperature distribution, has been ob-served.
A real-time calculation of electron temperaturewas made for the first time simultaneously for threedifferent channels by use of line-emission parametersthat belong to three different chemical species:Fe �I�, Cr �I�, and Mn �I�. It has been shown that, byvarying some process parameters such as laser inci-dent power, gas flow rate, and beam focus position,one obtains the best welding results with operatingconditions that give the lowest values of signal stan-dard deviations.
It has been clearly demonstrated that local pertur-bations or decreases in the temperature signals arerelated to welding defects. The introduction of sev-eral channels for temperature calculation makes ouroptical sensor capable of detecting either plasma coreinstabilities or perturbations in the peripheral part ofthe plume induced by weld defects. A wide range ofweld defects, such as lack of penetration, weld dis-ruptions, crater formation in or oxidation of the weld-ing bead, and plate undulations, have been detected.
Our sensor is nonintrusive in the process and suit-able for use with all welding metals once their chem-ical composition is known and most plasma emissionlines are identified. Because of its small size, it caneasily be implemented in automated production lines.
Preliminary tests for the application of the sensorto monitoring arc welding have been successfully car-ried out, with the aim of introducing the sensor intomanual arc welding procedures, where a high per-centage of defective the welding joints, the result ofusing unskilled manual labor, is usually found.
This research was supported by the Istituto Nazio-nale per la Fisica della Materia �INFM� Sud Project.The authors thank M. Sibilano, M. Tamma, D.Dell’Olio, and all the staff of the mechanical machineshop of the Department of Physics, INFM, for knowl-edgeable technical help.
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