Real Time Monitoring of Laser Welding by Correlation Analysis the Case of AA5083 2007 Optics and Lasers in Engineering

7/28/2019 Real Time Monitoring of Laser Welding by Correlation Analysis the Case of AA5083 2007 Optics and Lasers in Engi

1/5

Optics and Lasers in Engineering 45 (2007) 10051009

Real-time monitoring of laser welding by correlation analysis:

The case of AA5083

T. Sibillanoa,b,, A. Anconaa, V. Berardia,b, P.M. Lugara` a,b

aCNR-INFM LIT3-Laboratorio Regionale di Ricerca, Formazione, Sviluppo e Trasferimento alle Imprese di Tecnologie Laser Innovative via Orabona 4,

I-70126 Bari, ItalybDipartimento Interuniversitario di Fisica, Universita di Bari e Politecnico di Bari, via Orabona 4, I-70126 Bari, Italy

Received 22 March 2007; received in revised form 17 April 2007; accepted 18 April 2007

Available online 3 July 2007

Abstract

In this study, we present an innovative real-time laser welding monitoring technique employing the correlation analysis of the plasma

plume optical spectra generated during the process. In order to look for a relationship between optical signals and welds quality, the

influence of the experimental conditions on the correlation plots are also investigated. The correlation analysis results allow to evaluate

the quality of the welds, through an on-line detection of common defects, such as oxidation or lack of penetration, with an excellent

spatial resolution.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Laser welding; Optical spectroscopy; Correlation analysis; Aluminum alloys

1. Introduction

Many systems for on-line monitoring of the laser

welding quality process have been developed in recent

years [1,2]. The spectroscopic analysis of the plasma is a

widely used technique, especially in an industrial environ-

ment [35]. Plasma emission spectrum generated during

laser welding process is characterized by the presence of

numerous emission lines, whose features suggest relevant

information about the process [6]. Several parameters can

be investigated from the spectroscopic characterization of

the plasma plume and in the last years a lot of studies have

demonstrated that there is a clear relationship between

those parameters and the quality of the welded joints. Inthis work, the detection of the weld defects on the weld

seam is achieved by real-time application of the covariance

mapping technique. By means of this technique we are able

to analyze the dynamics of the spectrum and to detect local

weld defects.

2. The covariance mapping technique (CMT)

The CMT is based on the calculus of the auto-

correlation of spectrum with the aim to find a relationship

between different regions of the spectrum itself. In our

previous works [7,8] we employed the CMT as a tool for

providing significant details about the composition of the

plasma, relating the dynamical evolution of the plasma to

the quality of the welded joints. In those works, we

calculated the correlation coefficients between all the

chemical species present in the plasma spectra generatedduring laser welding of aluminum alloy AA5083. In this

way, we have been able to build the covariance maps of the

process in optimal and defective conditions. A positive

correlation value between two chemical species present in

the spectrum indicated that they changed, as a function of

a known parameter, in the same way, i.e. that they were

formed by a process which had a similar characteristic. On

the other hand, a negative correlation indicated that the

two species were formed by competing processes. In this

work, we present the development of this new approach

ARTICLE IN PRESS

www.elsevier.com/locate/optlaseng

0143-8166/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.optlaseng.2007.04.002

Corresponding author. CNR-INFM LIT3-Laboratorio Regionale di

Ricerca, Formazione, Sviluppo e Trasferimento alle Imprese di Tecnologie

Laser Innovative via Orabona 4, I-70126 Bari, Italy. Tel.: +39 80 5443480;

fax: +39 80 5442219.

E-mail address: [email protected] (T. Sibillano).
http://www.elsevier.com/locate/optlasenghttp://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.optlaseng.2007.04.002mailto:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.optlaseng.2007.04.002http://www.elsevier.com/locate/optlaseng


2/5

consisting of monitoring the temporal evolution of the

correlation between significant emission lines in different

welding conditions, aiming to optimize the quality and the

reproducibility of the welded joints.

3. Experimental details

3.1. Set-up

A high power CO2 industrial laser was operated at its

maximum power of 2.5 kW. The beam diameter was 25 mm

and the divergence was 0.5 mrad. The laser head contains a

130 mm ZnSe focusing lens and a coaxial nozzle, supplying

an axial shielding gas, with a diameter of 3 mm. The nozzle

stand-off distance from the workpiece is adjustable, and it

is independent from the beam focal position. Helium was

used as shielding gas. The plasma optical emission was

collected by a quartz collimator of 6 mm focal length. The

collected light was transmitted to a PC interfaced miniaturespectrometer by an 50mm core-diameter optical fiber (see

Fig. 1a). The spectral range investigated was 550800 nm,

with an optical resolution of 0.3 nm. The detector

characteristics, such as the spectral range and resolution,

allowed us to catalogue the chemical species present in the

plasma. From our previous studies we can select the

plasma emission lines that give more useful information on

the performances of the process and when defects occurred.

The temporal resolution chosen was about 20 ms. Welding

tests were carried out on 2 mm thick plate of AA5083aluminum alloys [68].

4. Results and discussion

Plasma emission during laser welding of 2-mm-thick

plate of AA5083 aluminum alloy were studied under

different welding conditions: we selected three emission

lines for correlation analyses acquiring their temporal

evolution during the process. The selected emission lines

were the following: Al(II) at l 559:33 nm, Mg(II) at l 789:63 nm and O(II) at l 656:63 nm which are the most

reliable for the detection of the joint defects. We calculatedthe temporal evolution of the correlation coefficients mijbetween these three emission lines, along the workpiece

length. The spatial resolution of the computed plots

ARTICLE IN PRESS

Fig. 1. (a) Scheme of the experimental set-up; (b) example of an optical spectrum of plasma plume emission (from [8]).

T. Sibillano et al. / Optics and Lasers in Engineering 45 (2007) 100510091006


3/5

depended on the degree of freedom N [7,8] used to

calculate the correlation coefficient plots. The number N

also influenced the threshold value of mij corresponding to

the selected confidence level: in this study, it was chosen a

confidence level of 95%. Two series of welding tests were

carried out: in the first, we varied in each run the travel

speed v and the incident laser power P examining thecorrelation coefficient changes as a function of the thermal

input incident on the surface of the workpiece. In the

second sequence of welding tests the experimental para-

meters, e.g. laser power, welding speed, and gas flow rate,

were varied during the run. From the calculated curves it

was possible to find a relationship between the correlation

coefficients and the occurrence of some defects on the

welded joints: in particular, the correlation/anti-correlation

behaviour between Al and Mg, Al and O and Mg and O

emission lines was studied in this work. The first general

conclusions that can be drawn is that, in all the

experimental conditions investigated, the correlation be-

tween Mg and O was always well above the chosen

confidence level. We can thus assert that there was an

increase (decrease) in Mg emission which corresponded to

an increase (decrease) in the O emission. In fact, in a stable

helium environment, magnesium and oxygen easily reacted

to form complex magnesium oxides [7] always inducing a

strong correlation between these two species.

4.1. Results for constant operating parameters

The correlation AlMg and AlO was influenced by the

linear energy input, as showed in Fig. 2. It was evident that

correlation coefficient plots for both AlMg and AlOwere very similar. For low heat inputs HI 16:6 J=mm,when partial penetration depth of the workpiece was

achieved, an abrupt drop in the correlation coefficient plots

was observed well below the threshold limit of 95%

confidence level (Fig. 2a). As the thermal input increased,

some drops of the correlation AlMg and AlO below the

confidence level were observed only in few points along the

workpiece. These events corresponded to the occurrence of

some local defects such as craters or spatters on the welded

seams (Fig. 2b). As the welding speed decreased

(HI 41:6 J=mm corresponding to v 60mm=s), theprocess became more stable and we observed less defects

on the joint surface, the welded seams appeared more

regular. In fact for this experimental condition the

vaporization rate increased and the plasma was continu-

ously enriched by aluminum and magnesium so that

these species were always correlated, as well as Al and O

(Fig. 2c).

4.2. Results for variable operating parameters

In Figs. 35 we showed the evolution of the correlation

coefficients between the three species considered under

variable welding conditions: we varied the laser power, the

welding speed and the gas flow rate during the process and

we observed how the correlation between these chemical

species was influenced by the dynamics of the process. In

the first case (Fig. 3), we varied the incident laser power by

decreasing it from 2.5 to 1 kW: the welded seam appeared

stable and regular for high heat input (incident power

ranging from 2.5 to 2 kW), as expected by the results

reported in the previous paragraph. For such values of

incident power, the correlation between the species

considered was always well above the chosen confidence

level. It can be argued that part of Mg, as well as of Al,

enriches the plasma at the expenses of the molten region.

As the incident laser power decreased the shape of the

welded seams appears more irregular and affected by the

presence of some defects. We ascribed the occurrence of

local defects to the instability of the vaporization rate of

the alloy elements that caused a correlation drop between

Al and O and between Al and Mg (as shown in Fig. 3:

sketches 1 and 2). It was possible to relate the correlation

signal strength with the penetration depth of the weld: in

fact for lower heat inputs (incident power ranging from

1.25 to 1.0 kW) a partial penetration welding regime was

established, as showed in the particular of the welding

ARTICLE IN PRESS

Fig. 2. Correlation coefficient plots between Al and Mg for different heat

inputs (HI): operating conditions (a) P 2:0kW, v 120 mm=s; (b)P 2:5kW, v 80mm=s; (c) P 2:5kW, v 60mm=s. a factor indicatesthe chosen confidence level.

T. Sibillano et al. / Optics and Lasers in Engineering 45 (2007) 10051009 1007


4/5

joints corresponding to the low value of the incident power.

In these conditions the correlation plots appeared unstable

and showed many points under the chosen confidence level.

The same remarks can be applied to the results obtained at

variable welding speed: it was confirmed that the heat input

influenced the correlation between the emission lines

considered. The correlation coefficient plots calculated

between Al and O and between Al and Mg allowed us to

determine a threshold value for the welding speed

above which the welded joints quality is unacceptable.

In fact, for welding speeds above 70 mm/s (corresponding

to a H I 35:7 J=mm), we observed a discontinued full

penetration and a welded seam affected by some defects.

The calculated plots showed that, above 75 mm/s, the

correlation abruptly decreased under the confidence levelcorresponding to the transition from the full opened

keyhole to the partial penetration regime.

The keyhole shape was strongly influenced by the

welding speed, in fact the analysis of the welded joints

cross-section showed that, for high welding speed, the

keyhole was shallow and broad (welding speed ranging

from 40 to 70 mm/s) and therefore the beam absorption by

the workpiece was less efficient. For too low welding speeds

(welding speed below 50 mm/s), the energy deposited on

the workpiece surface was high enough to cause some

instabilities in the melt pool that affect the final profile of

the joint and the vaporization of aluminum and magne-

sium. For welding speed above 150 mm/s HI

16:6 J=mm the correlation coefficients increased abovethe confidence level: the analysis of the welded joints cross-

sections showed that at such high welding speed the mean

value of aspect ratio was always less than one, typical of

the conduction mode welding. In this regime, the energy

input was too low to sustain the keyhole and therefore

shallower penetrations were obtained. Nonetheless, there

was still some vaporization from the melt pool surface and

plasma formation but the overall optical plasma emission

was not affected by fluctuations and instabilities due to hot

vapors ejection from the keyhole. This resulted in a more

stable plasma and a high correlation among all the

ARTICLE IN PRESS

Fig. 3. (a) Correlation coefficient plots for variable incident power

(operating conditions: v 60mm=s, Q 60 l= min); (b) particular of thebottom view of the welded joint corresponding to low incident power.

Fig. 4. Correlation coefficient plots for variable welding speed (operating

conditions: P 2:5kW, Q 60 l= min).

Fig. 5. (a) Correlation coefficient plots for variable gas flow rate

(operating conditions: v 60mm=s, P 2 kW); (b) crater formation dueto too low gas flow rate.

T. Sibillano et al. / Optics and Lasers in Engineering 45 (2007) 100510091008


5/5

emission lines investigated. This welding regime was more

stable than the deep penetration welding and the overall

plasma emission intensity was not affected by fluctuations

and instabilities leading to a high correlation between the

emission lines considered.

Finally, we carried out a welding test in which the gas

flow rate was varied during the process, from 100 to 10 l/min so that after about 6 cm from the beginning of the run,

a completely inefficient gas shielding was established. The

correlation coefficient plots were shown in Fig. 5 together

with the picture of a evident crater observed along the

welded joint. It is worth noting that the abrupt drop of

the correlation, located at about 6 cm, corresponded to the

occurrence of the defect shown in the picture. The

correlation plot became unstable for the remaining part

of the weld, due to the lack of shielding gas.

5. Conclusions

Correlation spectroscopy can be used to detect local

defects during laser welding of metals. The results of the

welding tests performed showed that the CMT was

strongly influenced by the quality of the process due to

strong relation between the occurrence of the defects and

the optical emission coming from the plasma plume. The

presented results confirmed that this technique was efficient

for detecting the presence of local defect and for monitor-

ing the instabilities of the process caused by the variation of

the process parameters.

Acknowledgments

This work was supported by MIUR (Ministero dellIs-

truzione, dellUniversita` e della Ricerca, ITALY) under

project DD1105. The authors would like to acknowledge

Piero Calabrese for the technical support.

References

[1] Cobo A, Bardin F, Mirapeix J, Hand DP, Jones JDC, Lopez-Higuera

JM. Meas Sci Technol 2005;16:N16.

[2] Park YW, Park H, Rhee S, Kang M. Opt Laser Technol 2002;34:135.

[3] Bardin F, Cobo A, Lopez-Higuera JM, Collin O, Aubry P, Dubois T,

et al. Appl Opt 2005;44(19):3869.

[4] Ancona A, Spagnolo V, Lugara` PM, Ferrara M. Appl Opt2001;40(33):6019.

[5] Zhang Y, Lijun L, Zhang G. J Phys D Appl Phys 2005;38:703.

[6] Sibillano T, Ancona A, Berardi V, Schingaro E, Basile G, Lugara` PM.

Opt Laser Eng 2006;44(10):1039.

[7] Sibillano T, Ancona A, Berardi V, Lugara` PM. Opt Commun

2005;251:139.

[8] Sibillano T, Ancona A, Berardi V, Schingaro E, Parente P, Lugara`

PM. Opt Laser Eng 2006;44(12):1324.

ARTICLE IN PRESS

T. Sibillano et al. / Optics and Lasers in Engineering 45 (2007) 10051009 1009

Documents

Real Time Monitoring of Laser Welding by Correlation Analysis the Case of AA5083 2007 Optics and Lasers in Engineering