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