characterisation of cw nd : yag laser keyhole dynamics

Optics & Laser Technology 33 (2001) 201–207www.elsevier.com/locate/optlastec

Characterisation of cwNd : YAG laser keyhole dynamicsBruno Martin ∗, Alexandre Loredo, Michel Pilloz, Dominique Grevey

Universite de Bourgogne, Laboratoire Laser et Traitements des materiaux (EA 2976), 12, Rue de la Fonderie, F71200 Le Creusot, France

Received 18 January 2001; accepted 30 January 2001

Abstract

The paper concerns laser–matter interaction characterisation. In this work, we use a rapid CCD camera located coaxially to the laserbeam and we compare recorded images with those obtained by numerical modelling. Because images are di8cult to understand, wecompute thermal radiation emitted by a keyhole of 9xed geometry and we adjust it trying to approach the camera record. The modellingtreats radiative heat transfer within the keyhole and determines the sensor illumination map. By adjusting the geometrical characteristicsof the hole, we seek to obtain the image that corresponds as well as possible to the realised experiment. Results are compared with otherexperimental methods simultaneously performed plume characterisation with an electric probe and spectrometric analysis. They show theexistence of two distinct behaviours of the keyhole: a pseudo-steady state associated with regular and pseudo-constant keyhole shapes,low frequencies of electric current in the plume, and generally good welding results, and a highly dynamic mode associated with irregularand rapidly varying keyhole shapes, high frequencies in the plume current and generally poor welding results. c© 2001 Elsevier ScienceLtd. All rights reserved.

Keywords: Laser; Keyhole; Dynamics; CCD; Radiation; Measurement; Modelling

1. Introduction

Today laser welding presents an interesting alternative toprocesses classically used in industry. However, di8cultiesremain in certain cases such as aluminium alloys welding.They come from complex interaction phenomena occurringbetween the beam and the matter. Therefore, their compre-hension is necessary for the development of this technol-ogy. During a welding operation, the intense evaporation(induced by the laser beam) pushes away by reaction themolten metal: a keyhole full of vapour and micro-droplets iscreated, which traps the incident radiation. The knowledgeof its geometry and its temporal evolution is thus necessaryto understand the energy transfer from the incident beamto the metal. In this work, we chose to observe the irradi-ated surface using a rapid CCD camera. These 2D imagesdo not make it possible to 9nd directly the geometry of thekeyhole. This is why we developed a numerical model. Itallows, starting from 3D geometry, to calculate the emittedenergy density of each point of the keyhole in the directionof the sensor. The walls of the keyhole are supposed to be

∗ Corresponding author. 3, rue Anthony DuVivier, 58000 Nevers,France.

E-mail address: [email protected] (B. Martin).

isothermal at the substrate boiling temperature and startingfrom the radiative equilibrium conditions between the var-ious points of the keyhole surface, the radiated energy ofeach of these points towards the sensor is calculated.The interaction plume has been characterised and its inDu-

ence on CCD camera results is considered. Further, record-ings are correlated with electric probe results.

2. Welding and keyhole

The very strong density of power delivered by the laserbeam causes the metal to boil a few milliseconds after thebeginning of irradiation [1]. The pressure created by theintense vaporisation tends to dig the molten metal zone.Then, the laser beam penetrates deeper and deeper in thematter, and the supplied zone becomes a thin hole, namedkeyhole. This phenomenon allows deep welding.During Nd : YAG continuous wave welding process and

when no major problems occur, the keyhole shape is closeto an inclined conical form (Fig. 1). Its depth varies fromone to few millimetres and its diameter is a few hundredmicrons, approximately the beam diameter. In this case, onecan suppose that the shape Ductuates round an equilibriumcon9guration. In a 9rst step these motions are small enough

0030-3992/01/$ - see front matter c© 2001 Elsevier Science Ltd. All rights reserved.PII: S 0030 -3992(01)00014 -7

202 B. Martin et al. / Optics & Laser Technology 33 (2001) 201–207

Fig. 1. Flash radiography of a keyhole.

Fig. 2. Example of keyhole geometry. Virtual view before collapse.

to be neglected. So one can take a mean shape for geomet-rical purposes.Serious problems appear when the shape Ductuations be-

come greater, giving to the keyhole a strongly unstable char-acter. During aluminium alloys welding, such behaviour iscustomary. The reasons for these instabilities are not exactlyknown, even if explanations were proposed in some recentworks. They can be due to the unstable behaviour of theRear Keyhole Wall (RKW) [2] due to a variable irradiationby the reDected beam and=or humps created on the FrontKeyholeWall (FKW) [3]. Some authors showed experimen-tally that the keyhole could adopt some very tortuous shapes[4,5] and could even collapse. Many other phenomena caninduce an unstable behaviour of the keyhole. For example,when the keyhole comes out of the welded sheet the key-hole wall equilibrium conditions are suddenly changing andinstabilities may appear.The shape of the keyhole plays an important role in the

laser–matter interaction, because the multiple reDections oc-curring leads to a more or less trapped beam. These reDec-tions of the incident radiation govern the energy deposit law,which can be calculated if the shape of the keyhole and thecharacteristics of the beam are known.The evolution of the keyhole, especially when closure

appears on its top (see Fig. 2 for example), can induce an

augmentation of eKective power, due to the trapping eKect.The increased amount of ejected matter must pass througha smallest exit section. We think that it could explain someliquid jets, often associated with welding problems.

3. Investigation means

The experimental set-up is composed of an Nd :YAGlaser for the welding treatment and an analysis set (electricprobe, rapid CCD camera, spectrometer, etc.) for the laser–matter characterisation.We used two types of Nd :YAG lasers for the welding

process. The 9rst one is a pulsed Haas laser (maximum 50J=pulse). The beam is carried to the target via a 400 �m opti-cal 9bre core and via a focusing head (magni9cation: 0.75).The second one is a cw Haas laser able to deliver 3 kW.The beam is dispatched to the work piece after crossing a600 �m optical cable core and a focusing system (magni9-cation: 0.75). The reference material for the target is stain-less steel (304L).

3.1. The beam–plume interaction. Spectrometric analysis

From previous works [6,7] it has been possible to de9neproperties of the interaction plume. Obtained results are ingood agreement with those obtained by Matsunawa et al.[8]. The main characteristics of the plume are summarisedbelow:

• the plume temperature is in the range 3000–5000 K. Thehigher temperatures are obtained when a pulsed laser isused;

• the electronic density, Ne, is 1020–1022 m−3, and theatomic density is about 1024 m−3. Therefore, the ioni-sation degree is in the range 10−4–10−2. Because the(Ne=T3) factor is smaller than 2 1014 m−3 K−3 the plumemedium acts as a gas and because of the ionisation de-gree, it acts as a highly ionized gas [9];

• for the laser wavelength (1:06 �m) and the measured tem-perature of the plume, the calculated optical index is closeto 1 and the absorption coe8cient is close to zero (seeFig. 3). Therefore, modi9cation of the beam trajectory isnot signi9cant. The same conclusion is valid for the vis-ible wavelength we choose for camera observations.

However, the beam is diKused because of the presence ofmicro-droplets in the plume. An approach of this diKusion(particularly the inDuence of both volumetric fraction andsize of the droplets) has been calculated by Lacroix [7](Fig. 3b). A part of about 50% of the incident beam isconsidered. These results are in good agreement with otherworks [6,10] which show that the upper part of the meltingzone (nail head) is due to the diKusion of the beam. We cansuppose that when the amount of ejected matter increases,the micro-droplets volumetric fraction also increases. ThediKusion of the beam causes a decrease in the incident power

B. Martin et al. / Optics & Laser Technology 33 (2001) 201–207 203

Fig. 3. Values of optical factor for stainless steel plume created by a Nd :YAG radiation [7]. (a) Optical index, n. (b) Isothermal lines due to the beamdiKusion by micro-droplets (Y axis: height; X axis: surface position). (c) Absorption coe8cient.

Fig. 4. Sketch of the measured potential diKerence between the workpieceand the sounding.

reaching the keyhole. This leads successively to a reducedkeyhole with less ejected matter, a small beam diKusion, ahigher power entering into the keyhole, and so on.This irregular power supply could explain the unstable

keyhole behaviour. In order to con9rm this hypothesis, weneed further investigations about the creation process ofdroplets.Because of the relatively high distance between the key-

hole and the collimating lens, (more than 200 mm) a fewdiKused rays reach the CCD camera. Therefore, diKusionleads to a global attenuation but the fuzziness is limited.

3.2. Electric probe characterisation

The electric probe is made with a tungsten rod of 1:6 mmdiameter (Fig. 4). The idea, here, is to consider the interac-tion plume as a current supply and to measure the potentialdiKerence between its bounds.

U (t) = Ri(t) with i(t) = J (t)A(t)

and

J (t) = JD(t) + JE(t) + JI(t);

where A(t) illustrates the sounding area inside the plume.This surface can be variable if the plume size Ductuates,J (t) is the current density, JD(t) represents the displace-ment current density: JD(t) = O(dE=dt) + (dP=dt), JE(t)is the electronic current density: JE(t) = NE(t)V (t)q, NE(t)is the electronic density, V (t) the electron speed and q theelectron charge, JI(t) illustrates the ionic current density:JI = (mE=mI)JE, mE is the electron mass and mI is the ionmass. JI is negligible.Thus the variations in the measured potential diKerence

will come from the variations of current density displace-ment linked to spatio-temporal variations of plume compo-sition for the 9rst part, and to the other part, from variationsof electronic current density associated to electronic densitymodi9cations and eventually electrons ejection speed.Because of the dynamic behaviour of the keyhole, the

matter Dow is not constant and does not present the sametemporal variation for two diKerent pulses. But, to obtaina given result (penetration for example), a certain quantityof matter has to be ejected. Because of the proportionalityof measured tension with matter Dow density, we have toconsider the integral

∫U dt, which is proportional to the

ejected matter density. A series of trials were made with aview to verify that the integral value is constant when thelaser parameters (pulsed laser) are constant. It shows thatthe standard deviation is about 10% of the mean value. Atthis time, we consider that we can use this parameter for theinteraction characterisation.Fig. 5 illustrates obtained results when a target is irradi-

ated with a pulsed laser. Part (a) is devoted to a stainlesssteel substrate. According to the previous indications, theintegral value (named “Area”) is increasing with the pene-tration. In return, in case (b), the “Area” is decreasing whenthe penetration grows. This fact has to be compared withplasma analysis of Al–Mg alloy welding shown by Kimet al. [8]. The plasma intensity is lower when laser powerdensity increases because of self-absorption phenomenaoccurring in the plume.Now, the temporal evolution signal study makes it pos-

sible to distinguish two behaviour classes: the 9rst one cor-responds to signals with low frequencies (between 2 and8 kHz), and the second class includes signals with “high”


Fig. 5. Evolution of the integral value according to the penetration—(a) stainless steel, (b) Aluminium alloy.

Fig. 6. Electric probe sounding—FFT of recorded signal—(a) 9rst class and (b) second class.

frequencies (2–15 kHz). Fig. 6 illustrates this point. If weassume that these variations are due to matter Dow variationduring laser interaction, it shows that the keyhole behaviourpresents two diKerent possibilities.We generally observe that correct welding is associated

with the 9rst class signal, and poor welding results areusually associated with the second-class signal. Observa-tions made with the camera will add some interesting resultsconcerning the time-dependent evolution of the keyhole, dif-ferent from each case.

3.3. Observation with a camera

In the set of existing methods for experimental study ofphenomena prevailing in the process, the imagery plays aninteresting role. Indeed, the keyhole and the molten poolthat surrounds it are the seat of very fast movements whosedynamics is complex. Straightforward models are very sen-sitive to physical data and lead to poor results in the generalcase and complete modelling is too complex. Images ob-tained with the camera help us to understand the shape ofthe keyhole and its evolution in time.Because of its internal surface temperature (around the

metal boiling point) the keyhole emits a signi9cant ther-mal radiation. The spectrum of this diKuse radiation variesaccording to the brightness temperature. A part of this ra-diation is shaped using the laser focusing optics and the550 nm component of the radiation is selected before send-ing it to the CCD camera (Fig. 7). The fast CCD cameratakes 128 × 128 pixels images at a rate of 1000 s−1, andthen images are digitised and stored. They can reveal cer-

Fig. 7. Experimental device.

tain aspects of the keyhole, in particular its diameter and itsemerging character.However, trying to deduce the geometry completely from

these images is di8cult for two reasons. First, only one im-age in two dimensions does not contain enough informationto represent a 3D shape. It is theoretically possible to im-prove measurement using two cameras, but it is technicallydi8cult. Moreover, the image obtained corresponds to thetotal power (intensity) sent by each point of the surface inthe direction of the camera. This power is the addition ofemitted and reDected thermal radiation, and then stronglydepends on the keyhole geometry because the bottom ofthe keyhole receives more radiation from itself than the top.On the other hand, it is possible to calculate the energy re-ceived by the sensor if the geometry is known a priori, by


Fig. 8. a: Calculated picture—stable class, b: recorded picture—stable class.

means of an established method, which is brieDy describedbelow.

4. Keyhole radiation study

The radiosity method is used here to compute radiationexchanges within the keyhole and to determine incidentradiation Dux striking the sensor.

4.1. View factor between two elemental surfaces

The view factor between two elemental surfaces i and j,whose area are dSi and dSj, is de9ned as the fraction ofradiative energy leaving j that directly strikes i.Let r be the distance between the two surfaces, �i, �j be

the polar angles between the normal to the surfaces and theline joining i and j, the view factor is commonly de9ned as

dFji =cos �i cos �j dSi

�r2: (1)

It is easy to remark that dFji dSj=dFij dSi is known as thereciprocity relation. If we have to work with non-elementalsurfaces, we must integrate over the respective surfaces.However, in our case, we use small Dat surfaces and do notperform integration.

4.2. Radiosity set of equations

The keyhole consists of n − 1 elementary surfaces i ofarea Si. The lens of the focusing system corresponds to sur-face 1. For each surface, we de9ne, respectively, radiosity,hemispherical emitted power, irradiation.

Ri; Mi = i�T 4i ; Hi: (2)

Irradiation at the surface i is the sum of all surface con-tributions, and could be expressed as

Hi = 1=Sin∑

j=1SjFjiRj =

n∑

j=1FijRj: (3)

The reDected power by unit of area of surface i is

�in∑

j=1FijRj = (1− i)

n∑

j=1FijRj: (4)

Radiosity at surface i is the sum of emitted power andreDected power

Ri = i�T 4i + (1− i)n∑

j=1FijRj: (5)

This set of n equations is a linear system easy to solven∑

j=1AijRj = Bi or [A]{R}= {B};

where Aij = ij − (1− i)Fij and Bi = i�T 4i : (6)

4.3. Modelling

The supposed surface of the keyhole is divided intoquadrilateral elements of su8ciently low size to give mean-ing to the view factors [Eq. (2)]. The main geometricalcharacteristics of the keyhole are: its diameter, its depthand its slope. We can generate the shape of the keyhole bya mathematical choice of functions giving its surface. Withthe program, the user can also de9ne an arbitrary form. The9rst lens of the focusing system is an additional facet, whoseposition compared to the keyhole can be adjusted, and itstemperature is set to the ambient temperature. Consideringthe importance of temperatures taking place in the keyhole,the environment is passive with respect to the exchanges.First, the view factors are computed, then the linear sys-

tem to be solved is built [Eq. (7)]. The resolution gives thehemispherical emitted power of each facet. Then, the inci-dent radiation Dux coming from surface i and striking thesensor is determined as

’i = "RiFijS1Si cos �i

= "Ri

cos �iFi1; (7)

where " is a constant depending on optical characteristicslike focal distance, 9lter and mirror attenuations.


Fig. 9. a: Calculated picture—unstable class, b: recorded picture—unstable class.

This gives a map, which can be compared with the imagesof the camera.

4.4. Model exploitation

By successive approaches, we choose a keyhole geom-etry for which the illumination of the sensor correspondsto obtained images with the camera. However, remarks ofSection 3.3 leads to the fact that various shapes could givesimilar images. Nevertheless, the method permits to excludesome pro9les whose existence was supposed a priori.We have chosen pictures corresponding to two typical

behaviours, one for the stable case and the other one for theunstable keyhole.In the stable case, all the images are similar. The choice of

one of them (Fig. 8b) is then very easy. Using the proceduredescribed above, we obtain a shape resembling a slowly bentand curved cone (Fig. 8a).In the case of the unstable behaviour, successive images

are very diKerent. So, we have chosen one that appears fre-quently. It seems to correspond to a throttled shape (Fig. 9b).The keyhole aperture tends to close, increasing the trappinge8ciency of the beam. Then we can suppose that the pres-sure augmentation induced will promptly reopen the key-hole, and perhaps eject some molten metal inducing chaoticbehaviour.

5. Conclusion

The laser–matter interaction modelling is of great impor-tance for the user. It will allow to use the laser as an interest-ing tool for industry especially for the welding process. Inthe meantime, this theoretical approach needs some exper-imental works for interaction characterisation. Because ofthe high complexity of occurring phenomena, many meth-ods have to be implemented together. One of them is the“visual” observation of the irradiation area. If we observe

this area coaxially to the laser beam it is possible to havean idea of the keyhole behaviour. This point is particularlytrue when we use a cw Nd :YAG laser and stainless steelfor target. Indeed spectroscopic analysis shows that infor-mation is not highly disturbed when it passes through theplume.With a view to understand recorded pictures a keyhole

radiation model was developed in which an a priori keyholegeometry is supposed. The calculation gives the radiationpattern of this theoretical keyhole. After comparison of thispattern with the recorded picture and after adjustment of thekeyhole geometry it is possible to see that there exist twoclasses of welding keyhole: a pseudo-steady state associ-ated with regular and pseudo-constant keyhole shapes, lowfrequencies of electric current in the plume, and generallygood welding results, and a highly dynamic mode associ-ated with irregular and rapidly varying keyhole shapes, highfrequencies in the plume current and generally poor weldingresults.Due to the low rate of the CCD camera (1000 frame s−1)

we are only able to say that there exist two classes of weldingbehaviour. In the future we will use a more rapid camera inorder to havemore details concerning the keyhole behaviour.More substrates such as aluminium or magnesium alloyswill be studied because of great industrial interest.

Acknowledgements

The camera pictures of the welding process wererecorded with the coaxial process control system of theFraunhofer Institute for Laser Technology ILT (Stein-bachstraTe 15, 52074 Aachen, Germany). We thank MrAbels (E-mail: [email protected]) and Mr Kratzsch (E-mail:[email protected]) from the Fraunhofer ILT for their sup-port during the experiments with the camera system. Theyare responsible for the development of the coaxial processcontrol system at the ILT.


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characterisation of cw nd : yag laser keyhole dynamics

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