spectroscopic diagnostics on cw-laser welding plasmas of aluminum alloys

Ž .Spectrochimica Acta Part B 56 2001 651�659

Spectroscopic diagnostics on CW-laser welding plasmasof aluminum alloys �

S. Palancoa, M. Klassenb, J. Skupinb, K. Hansenb, E. Schubertb,G. Sepoldb, J.J. Lasernaa,�

aDepartment of Analytical Chemistry, Faculty of Sciences, Uni�ersity of Malaga, E-29071 Malaga, Spain´ ´bBremer Institut fur Angewandte Strahltechnik, Klagenfurterstrasse 2, D-28359 Bremen, Germany¨

Received 14 November 2000; accepted 23 March 2001

Abstract

In-process diagnostics intended to correlate spectrometric measurements to the occurrence of laser weldingdefects such as notches and blowholes have been carried out. The plasma light emitted during high-power CO laser2welding of a 6013 aluminum alloy was guided to an imaging spectrograph and the dispersed light was detected with aCCD system. A transient recorder was used to record the signal from a fast laser-power monitor and the sync signalsfrom the CCD and a fast speed video camera. Accuracy of the measurements are discussed in relation to the low and

Ž �1 3 �1 .fast acquisition rate approaches 58 spectra s and 4�10 spectra s , respectively used in the experiments.Spectroscopic measurements at fast acquisition rates showed both an increase in the intensity of the overall spectralemission and the growth of lines corresponding to ionic aluminum species taking place right before the occurrence ofweld defects. � 2001 Elsevier Science B.V. All rights reserved.

Keywords: In-process diagnostics; Spectrometric measurements; Laser welding defects

1. Introduction

Laser welding of aluminum alloys is a challeng-ing process for both the automotive and aero-

� �space industries 1,2 . Aluminum structures ob-

� This paper was presented at the 1st International Congresson Laser Induced Plasma Spectroscopy and Applications, Pisa,Italy, October 2000, and is published in the Special Issue ofSpectrochimica Acta Part B, dedicated to that conference.

� Corresponding author. Tel.: �34-952131881.Ž .E-mail address: [email protected] J.J. Laserna .

tained with this technique are lighter than theircounterparts made from iron-based materials andstill present excellent mechanical properties.However, the complexity of the aluminum laser-welding process induces a number of weld defectsthat prevents a wider implementation of the tech-nology at the factory level. In the continuous-waveŽ .cw keyhole laser welding technique, a high-power cw-laser beam is focused to a spot approxi-

� �mately tens of millimeters in diameter 3�5 .Temperature rapidly rises in the close vicinity ofthe spot inducing a sudden vaporization and ion-

0584-8547�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 5 8 4 - 8 5 4 7 0 1 0 0 2 1 2 - 9

( )S. Palanco et al. � Spectrochimica Acta Part B: Atomic Spectroscopy 56 2001 651�659652

ization of the material along a capillary channel� the keyhole � which forms inside the samplein the prolongation of the beam axis. A layer ofmolten material surrounds the keyhole withoutcollapsing it depending on a delicate dynamicequilibrium between the vapor phases and the

� �liquid metal 6,7 . In a previous work, Klassen et� �al. 8 reported on the dynamics of the aluminum

welding process showing that although instabili-ties continuously occur, they do not necessarilylead to the generation of defects. It was foundthat low boiling points of the aluminum alloycomponents � leading to high evaporation rates� in conjunction with low viscosity of thealuminum melt gave raise to strong and fast fluc-tuations in the plasma and the melt pool, whichmay cause imperfections in the weld seam such asnotches and blowholes, which represent the mostsevere and common defects.

Consequently, real-time monitoring of theplasma reveals valuable information for this kindof welding techniques. In this sense, a number ofworks have been devoted to the study of plasmasgenerated by high-power industrial lasers on dif-ferent types of samples. Steel alloys have received

� �most of the attention 9�15 , whilst the amount ofpapers on the spectroscopy of aluminum alloysamples under welding conditions is scarce� �16�18 . A great majority of the latter focusesmainly on thermodynamic properties of the plas-mas, leaving a minor extension to the monitoringof the welding process itself. Moreover, to theauthors’ knowledge, a spectrometric study speci-fically devoted to the elucidation of instabilitiescausing weld defects on the mentioned alloys hasnot been published so far.

In the present work, plasma emission duringlaser welding of an aluminum alloy has beenstudied under different light collection setups andacquisition rates. Laser power modulation hasbeen used to reproduce the weld defects andspectrometric diagnostics at high acquisition rateshave been performed which correlate certainspectral features to the occurrence of weld de-fects.

2. Experimental setup

A diagram of the instrument setup is shown in

Fig. 1. Setup of the experiment showing the different components. 1, welding head; 2, sample and welding direction; 3, laser beam;4, plasma; 5, protection-gas nozzle; 6, collection lens; 7, two-dimensional positioner equipped with the fiber end; 8, fiber optic; 9,spectrograph; 10, CCD; 11, PC; 12, transient recorder; 13, fast laser-power monitor; 14, fast motion analyzer.

( )S. Palanco et al. � Spectrochimica Acta Part B: Atomic Spectroscopy 56 2001 651�659 653

ŽFig. 1. A high power CO industrial laser Rofin2.Sinar, RS-10000 capable of up to 10 000 W in

CW mode was operated at 5000 W and focusedŽwith a water-cooled parabolic folding mirror f�

.300 mm to produce continuous welds onaluminum alloy samples at a speed of 50 mm s�1.Plasma emission was spatially integrated orresolved depending upon the experiment. A fused

Ž .silica biconvex lens f�50 mm, d�25.4 mmseparated 550 mm from the plasma was used tocouple the whole emission to a 2-m fiber opticŽ .d�200 �m, 0.27 N.A. . For spatial resolution ofthe emission, the lens was replaced by another

Žfused silica biconvex lens f�100 mm, d�25.4.mm in a 1:1 magnification scheme and the fiberŽ .optic d�200 �m, 0.27 N.A. was shifted along

the axis of the inverted image with a micrometriclinear stage. With this arrangement, the plasmaemission could be spatially resolved in 200 �mslices. Lens protection was provided with a crossedgas jet in both configurations. Light exiting thefiber optic was taken to the entrance slit of a

Ž .F�3.9 imaging spectrograph Oriel, MS-257equipped with a 1024�256 pixels CCD detectorŽ .Andor, DU-420 . Video recording of the plasmaand the molten pool fluctuations was carried out

Žwith a fast motion analyzer Eastman Kodak.Company capable of up to 18 000 fps. Necessary

illumination was granted by a fiber-coupled in-frared laser diode whose output was focused tothe vicinity of the welding zone. A fast-readoutlaser power monitor was used for monitoringpossible transient instabilities in the CO laser2output which may affect further spectroscopicmeasurements. A multichannel transient recorderŽ .Nicolet 2580 was used to provide the necessarysynchronization among the described instrumentsand a wave function generator was used in someexperiments to modulate the laser output in am-plitude and produce pulse bursts.

2.1. Samples

All samples were 2.5-mm-thick plates made ofŽthe 6013 aluminum alloy composition, in mass

percent: Si 0.71, Fe 0.25, Cu 0.88, Mn 0.33, Mg.0.92, Cr 0.03, Zn 0.03, Ti 0.01 . In order to avoid

certain welding problems associated to surfacecontamination and moisture, all samples werecarefully cleaned with high-purity acetone previ-ous to the weld.

3. Results and discussion

3.1. Spectral analysis

As a first step, the whole plasma emission wasspatially integrated. Acquisition time was set to11 ms and spectra were acquired at the maximumrate allowed by the CCD electronics, approxi-mately 58.8 spectra per second. Fig. 2a illustratesthe emission spectrum obtained under these con-

Ž .ditions. As shown, the 285.21 nm Mg I line isnoticeably self-reversed. This circumstance is fullyrepresentative of these low temperature weldingplasmas in which the number density of non-excited atoms is high, causing self-reversal ofemission lines of more volatile elements. A dif-ferent situation is presented in Fig. 2b whichcorresponds to the end point of a weld, where thesample speed is considerably reduced and hence,the energy input is higher at a given laser power,inducing a plasma temperature increase. Underthese circumstances, self-reversal in the 285.21

Ž .nm Mg I emission line is not apparent. Theelectron number density was estimated by thewell-known Stark-broadening method. The spec-trograph was centered at 559.6 nm and the 559.32

Ž .nm Al II line FWHM was measured and cor-rected from instrumental line broadening, yield-ing an electron number density of 4.3�1016 cm�3.Using this value and assuming local thermody-namic equilibrium, the Saha equation allowed toobtain the electron temperature value from therelative line intensities of two consecutive ioniza-tion stages of aluminum. The former emissionline, corresponding to single-ionized aluminum, in

Ž .conjunction with the 569.66 nm Al III and 572.27Ž .nm Al III emission lines were used to estimate

an electron temperature of 7100 K. For suchelectron temperature and density values, a mini-mum electron density of 1.0�1015 cm�3 was

� �calculated which ensured LTE fulfillment 19,20 .


Ž . Ž . Ž .Fig. 2. a Typical emission spectrum of a CW-laser welding plasma showing self-reversal of the 285.21 nm Mg I emission line. bEmission spectrum of a CW-laser welding plasma obtained at the end of a weld.

The behavior of these CO welding plasmas have2� �been described elsewhere 21 and therefore, no

further discussion is provided here.Correlation between weld defects and changes

in spectral features was attempted during normalweld operation without conclusive results. Severalseries of 100 spectra each were acquired at dif-ferent spectral regions without performing signal


accumulation. A noticeable spectrum-to-spectrumsignal fluctuation was found. As an example, rela-

Ž .tive standard deviation RSD values rangedŽ .between 39% for the 266.04 nm Al I line and

Ž .47% RSD for the 260.57 nm Mn I line in thespectral window shown at Fig. 2. Even thoughsuch values were better than a priori expectedfrom the visual inspection during the weldingprocess, probably due to the particular configura-tion of the light collection arrangement, designedto minimize the fluctuation induced by plasmamotion in the light coupled to the fiber.

Further experiments were devoted to spatiallyresolve the plasma emission in height using adifferent collection arrangement, as previouslydescribed. A set of 100 spectra were acquired andaveraged for each sampling height in the plasma.The results show the same tendency for the vari-ous lines studied with the sampling position, asillustrated in Fig. 3. Besides the overall signaldecay with sampling height, no further correlationcould be found, indicating the loss of spatialinformation probably due to the violent plasma

motion. Fig. 4 shows three photograph sequencesextracted from a fast video recording performedwith the fast motion analyzer. The first sequenceŽ .Fig. 4A has been recorded under normal weld-ing operation and provides clear evidence of theamplitude of plasma fluctuations, which are ofthe same size as its average dimensions. Relatedto this, the speed of such fluctuations is someorders of magnitude faster than typical acquisi-tion rates used in LIBS. This may lead to a wrongperception of the phenomena occurring in theplasma.

3.2. Fast kinetics experiments

The absence of clear correlation between theoccurrence of weld defects and plasma emissionand the incapability of obtaining spectral infor-mation at different heights in the plasma underthe given acquisition conditions required the useof an acquisition rate comparable to that of thefast motion analyzer. For that purpose, a specialreadout mode of the CCD camera was used which

Fig. 3. Response of several emission lines at different heights in the plasma. Each point is the average of 100 spectra. AcquisitionŽ . Ž . Ž . Ž . � Ž .time was 11 ms. � 552.84 nm Mg I , � 559.32 nm Al II , � 569.66 nm Al III , � 571.11 nm Mg I , 572.27 nm Al III . The scale of

each plot has been changed to fit all together in the same chart.


Ž . Ž . Ž .Fig. 4. Photograph sequences extracted from a fast video recording A before, B during and C after the occurrence of aŽ . Ž . Ž .blowhole. Three main welding zones have been identified and marked in image A-1: a solidified material, b molten pool and c

Ž .plasma. Sequence A illustrates the rapid changes in position and shape of the welding plasma during normal welding operation.Ž .Sequence B shows the evolution of the plasma and the molten during the development of a blowhole. From image B-1 to B-7, the

plasma gradually detaches from the sample and disappears from the camera field-of-view. Almost simultaneously, the detonationwave induces an ejection of molten material as shown in images B-7 to B-12. As a consequence, a hole is produced as illustrated in

Ž .sequence C , where the plasma emerges as new material enters the laser path.

allowed to divide the chip into 16 horizontalŽ .regions of 16 rows each 1024�16 pixel . Only

the upper region was illuminated at each spec-trum acquisition, the 15 rows below remaining inthe dark. Therefore, by vertically shifting thecharge inside the chip by 16 rows at a time, aseries of 15 spectra could be acquired at 4�103

spectra per second. Adequate illumination of the

CCD was done by positioning the fiber optic tipon the top of the entrance slit of the imagingspectrograph with a micrometric translation stage.

3.3. Presence of oxides

Using this fast acquisition approach and byspatially resolving the plasma emission the con-


tribution of different species could be successfullyassociated to its position in the welding plasma.In particular, aluminum and magnesium oxidescould be detected. The formation of such speciesduring the welding process can severely affect thefinal mechanical properties of the joint. The emis-sion spectrum in Fig. 5 shows the spectra ob-

Žtained at the plasma base close to the sample. Žsurface and at the plasma outer edge 2.1 mm

.above the sample surface . Whereas only atomicmagnesium emission is detected at the base,

� �emission related to molecular species 19 alsoappears in the spectrum corresponding to theplasma edge. In terms of weld quality, this is aless critical situation, as the plasma base in thevicinity of the keyhole is virtually free of oxidecontamination. A defective protection-gas streamcould be considered the possible cause for thiseffect.

3.4. Detection of blowholes

In order to synchronize the acquisition timingwith the occurrence of a weld defect, a method

� �presented in a previous work 8 was used toartificially introduce instabilities in the weld. Bymodulating the original CW 5000 W laser outputat 1 kHz and 30% amplitude, weld defects wereefficiently developed and, although they are syn-thetic, the present method is the only way toreproduce the phenomenon on a regular basis. Awave function generator was in charge to gener-ate the laser burst and to trigger the CCD acqui-sition sequence. Under such conditions, the rela-tionship between the blowholes and minor volatilecomponents of the alloy was investigated. Zincand manganese emission was studied at differentconditions without finding clear evidence pointingto these elements to be the cause of blowholes. In

Ž . Ž .Fig. 5. Spatially resolved spectra obtained at high acquisition rate from the base z�0 mm and the outer edge z�2.1 mm of theplasma illustrating molecular emission of the B-X system of AlO locating the presence of aluminum oxide. For ease of viewing thespectra are not to scale.


general, their emission behavior was found to besimilar to that of major constituents of the alloysuch as aluminum or magnesium.

According to spectrometric measurements andto recordings with the fast motion analyzer, CW-laser sustained plasma can induce shock wavessimilar to those produced in pulsed laser experi-ments. Such plasma expansions seem to be par-tially absorbed in the molten pool, giving rise topool waving which is favored by the low viscosityof liquid aluminum. Fig. 6 shows a representativetime profile of the events occurring during a welddefect. The intensity of relevant aluminum andmagnesium lines is plotted for each spectrumcovering a 256 �s time window. As illustrated,rapid alterations in plasma spectral features occurbetween spectra �9 and �12. These alterationsinclude the growth of aluminum ionic lines at559.32 nm, 569.66 nm and 572.27 nm, and thebroadening of the magnesium atomic line at552.84 nm. Such behavior, associated to welddefects, may suggest a significant raising in plasmatemperature right before the occurrence of adetonation wave. Fig. 4B shows the photograph

sequence corresponding to the development ofthe weld defect. Such an event was observedbetween spectrum �10 and �11 and gave anunequivocal place to the formation of a blowholein this particular case. Following the detonation,the spectra show two alternative patterns depend-

Ž .ing on the type of defect that occurred: i acomplete signal depletion in the case of a blow-hole as shown in Fig. 6. This effect is attributed tothe lack of material at the beam focus after thedetonation. Such an assumption is in agreementwith the image sequence shown in Fig. 4C wherethe plasma gradually emerges as fresh sample

Ž .enters into the laser path. ii In the secondinstance, the signal rapidly recovered its originallevel prior to the detonation. This fact is compati-ble with the formation of a notch, a defect char-acterized by a shape irregularity at that particularpoint in the final welded joint.

4. Conclusions

Real-time spectroscopic monitoring of the oc-

Ž .Fig. 6. Net intensity of several aluminum and magnesium emission lines during the occurrence of a blowhole. � 552.84 nm Mg I , �

Ž . Ž . � Ž .559.32 nm Al II , � 569.66 nm Al III , 572.27 nm Al III . Each spectrum covers a time window of 256 �s. Background has beensubtracted for viewing convenience.


currence of weld defects in CW-laser welding ofaluminum alloys has been demonstrated. Blow-holes and notch defects are preceded by a suddenalteration of plasma emission characteristicswhich have been confirmed by spectral measure-ments and fast video recordings. It is feasible thatthis situation leads to the formation of a strongplasma shock wave that can no longer be absor-bed by the molten pool. The momentum excess isreleased through an ejection of the material sur-rounding the plasma. Although the experimentalsetup allowed to spatially resolve the plasmaemission when using high acquisition rates, workis in progress to include a light-collection ar-rangement perpendicular to the part, i.e. alongthe plasma z-axis. This configuration might allowboth to minimize fluctuations resulting fromplasma motion and to obtain information insidethe keyhole, which could provide additional in-sight of the processes leading to the developmentof weld defects.

Acknowledgements

The authors would like to thank L.O.T.-OrielGmbH & Co KG for the loan of the equipmentused for all spectrometric measurements in thiswork and especially to Dr Jurgen Schlutter for his¨ ¨efforts and interesting suggestions. This work hasbeen partially supported by project PB97-1107 ofthe Direccion General de Investigacion Cientıfica´ ´ ´

Žy Tecnica Ministerio de Educacion y Cultura,´ ´.Madrid, Spain .

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spectroscopic diagnostics on cw-laser welding plasmas of aluminum alloys

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