AbstractDue to their material properties such as high electrical and thermal conductivity, copper materials are more and more demanded for industrial applications. The same material properties make laser welding of copper a challenging task. Laser welds often suffer from defective weld seams with ejections, pores and a large fluctuation in the penetration depth. In this paper the influence of laser power modulation during copper welding on weld imperfections is discussed. It is shown that a sinusoidal power modulation leads to a strong reduction of melt ejections and also to an increase in penetration depth

Graf, Thomas

Heider, Andreas

Hess, Axel

Stritt, Peter

Weber, Rudolf

Elsevier - Publisher Connector 

Process Stabilization at welding Copper by Laser Power Modulation 

Available online at www.sciencedirect.com
Physics Procedia 12 (2011) 81–87
1875-3892 © 2011 Published by Elsevier Ltd.
doi:10.1016/j.phpro.2011.03.011
LiM 2011 
Process Stabilization at welding Copper by Laser Power Modulation   
Andreas Heider1*, Peter Stritt2, Axel Hess1, Rudolf Weber1, Thomas Graf1 
1Institut fuer Strahlwerkzeuge (IFSW),Pfaffenwaldring 43, 70569 Stuttgart, Germany 
2GSaME Graduate School of Excellence advanced Manufacturing and Engineering, Nobelstr. 12, 70569 Stuttgart, Germany 
 
 
Abstract 
Due to their material properties such as high electrical and thermal conductivity, copper materials are more and more demanded 
for industrial applications. The same material properties make laser welding of copper a challenging task. Laser welds often 
suffer from defective weld seams with ejections, pores and a large fluctuation in the penetration depth.    
In this paper the influence of laser power modulation during copper welding on weld imperfections is discussed. It is shown that 
a sinusoidal power modulation leads to a strong reduction of melt ejections and also to an increase in penetration depth. 
 
Keywords: Laser Welding; Copper Welding; Power Modulation; Process Stabilization; CW; Spatter; Ejection 
1. Introduction 
The current efforts in the electrification of automobiles significantly increase the need for a reliable and effective 
copper welding process. Due to the low absorptivity at 1 μm wavelength and the high heat conductivity, laser 
welding of copper materials often results in defective welds. Such weld seams suffer from many ejections and pores. 
Furthermore a strong fluctuation of the penetration depth along the weld seam can often be seen. For industrial 
applications primarily the cross sectional area of the welds are of interest to achieve the highest efficiency of 
electrical and thermal conductivity. Hence it is of great importance to minimize or even avoid such weld 
imperfections. Figure 1(a) shows the appearance of a melt ejection from a high-speed video observation of a 
welding process. In Figure 1 (b) and (c) the solidified weld seam of a welded sample with a lot of holes can be seen. 
These holes result, when molten material is ejected out of the melt pool during a weld.   
 
 
 
 
 
 
 
Figure 1: Welded copper sample, Cu-ETP, v = 6 m/min, P = 1500 W. (a) Formation of a melt ejection out of high-speed-video; (b) Weld seam 
with melt ejections; (c) Enlarged image of a melt ejection.  
 
* Corresponding author. Tel.: +49-711-685-69730; Fax: +49-711-685-59730. 
E-mail address: Andreas.Heider@ifsw.uni-stuttgart.de. 
Open access under CC BY-NC-ND license.
82  Andreas Heider et al. / Physics Procedia 12 (2011) 81–87
A weld for industrial applications has to fulfil several specifications. One of the most critical requirements is the 
strength of a weld depending on the connected area. Furthermore the electrical and thermal resistance defined by the 
uniformity of material is especially important for the use of copper materials. Melt ejections lead to a hole in the 
weld seam and degrade the mechanical and electrical properties of the weld.      
In general the laser welding process has two distinct regimes, heat-conduction welding and deep-penetration 
welding. The latter is characterized by the formation of a capillary which forms a deep and narrow weld. It is known 
that ejections out of the melt pool and the building of pores may result from the oscillation of the keyhole as a result 
from the collapse of the keyhole [1]. The formation of a capillary starts at the threshold from heat-conduction to 
deep-penetration welding. In the vicinity of the threshold it is possible to influence the formation of the capillary by 
changing the laser power. Thereby the keyhole dynamics can also be influenced in order to stabilize the process. In 
this paper the effect of laser power modulation around the deep-penetration threshold will be discussed regarding 
ejections and the formation of pores. Furthermore the change in penetration depth due to the modulation of the laser 
power will be discussed.  
2. Experimental Setup 
All experiments were made with a TruDisk 5001 laser with a fibre delivery with 100 μm core diameter. The laser 
has a maximum output power of 5000 W at a wavelength of 1030 nm. All welds were made with a focus diameter of 
100 μm. The laser optics was set to an angle of incidence of 18° to prevent it from damage by the back reflected 
light. To analyze the influence of heat conduction, a copper alloy (bronze, CuSn6) and a pure copper (Cu-ETP) with 
higher heat conductivity were used for the experiments. The relevant thermo-physical properties are summarized in 
Table 1. All welds were 80 mm long bead on plate welds in 2.5 mm thick samples. No shielding gas was used.  
Table 1: Material properties of the copper materials under investigation at ambient temperature [2]. 
Properties CuSn6 (bronze) E-Cu-58 (ETP) 
Heat conductivity k [W/(mK)] 75 390 
Heat capacity cp [J/(gK)] 0.377 0.386 
Density r [g/cm³] 8.8 8.93 
Thermal diffusivity ț=k/(rcp) [cm²/s] 0.226 1.131 
 
Figure 2 shows the experimental setup. Power modulation is done by a function generator (1) which gives a fast 
analogue input signal to the laser (2), setting the actual laser power. With the fibre delivery (3) the laserlight is 
transported from the laser to the optics (4) which focuses it onto the workpiece (8).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2: Experimental setup of the processing system.  
(1) Control and Measurement PC and Function Generator, 
(2) Laser TruDisk 5001,  
(3) Light Conducting Cable,  
(4) Beam-Splitter,  
(5) Photodiode, for the Back-Reflection Signal, 
(6) Interference Filter (1030 nm), (7) Neutral Gray Filter,  
(8) Workpiece,  
(9) Moved Axis,  
(10) High-Speed-Camera,  
(11) PC for High-Speed-Images. 
Andreas Heider et al. / Physics Procedia 12 (2011) 81–87 83
In order to analyze the process dynamics all welds were observed with a high-speed camera at a frame rate up to 
10 kHz. Furthermore the back-reflected laser light form the workpiece was observed using a photodiode. 
The welded samples were analyzed with respect to the number of melt ejections, the number of pores, the 
penetration depth and the fluctuation of the penetration depth. The melt ejections were counted from the weld seam 
surface. The number of pores and the penetration depth were measured from the longitudinal and cross-sections.  
3. Results and Discussion 
3.1.  Melt Ejections 
Figure 3 shows a longitudinal cross-section of a welded sample (Cu-ETP, v = 6 m/min, P = 1500 W). In the 
majority of cases melt ejections extend over the complete penetration depth as seen in Figure 3 and therefore they 
turn out to be the most critical weld imperfection.  
 
 
 
 
 
Figure 3: Longitudinal cross-section of a weld with many melt ejections. 
The susceptibility to weld imperfections strongly depends on the feed rate. At low feed rates (v < 8 m/min) the 
amount of ejections increases with decreasing velocity as shown in Figure 4. To investigate this effect the laser 
power was kept constant while increasing the feed rate and the number of ejections was counted. The diagram 
shows, that pure copper (Cu-ETP) suffers from about a factor of two more ejections than bronze (CuSn6). This 
behaviour is confirmed by the high-speed images that show stronger fluctuations of the melt pool.   
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4: Correlation between melt ejections and feed rate at a constant laser power for two different copper materials.  
The following analysis was performed at a feed rate of 6 m/min due to the welding depth of several mm which 
can be reached with this parameter. The laser power was sinusoidally modulated with an average power of 1000 W 
for CuSn6 and 1500 W for Cu-ETP. The average power chosen corresponds to the threshold power for deep 
penetration welding [4]. Welds were performed with different modulation frequencies at constant modulation 
amplitude of േ 750 W.  
Figure 5 (left) shows the penetration depth and the number of ejections depending as a function of the modulation 
frequency. The penetration depth increases at all modulated welds in comparison to the welds with constant laser 
power (modulation frequency = 0 Hz). The best results regarding the melt ejections are achieved at a frequency 
84  Andreas Heider et al. / Physics Procedia 12 (2011) 81–87
between 400 - 600 Hz at Cu-ETP. The corresponding longitudinal cross-sections and weld seam surface can be seen 
in the Figure 5 (right). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5: Left: Penetration depth and melt ejections as a function of the modulation frequency. Right: Longitudinal cross-sections and weld 
surface of pure copper (Cu-ETP); top: modulated welding at 500 Hz; bottom: unmodulated. 
For the bronze material it is possible to produce a weld with few or even without any ejections at a frequency of 
about 350 Hz. The higher heat conductivity of Cu-ETP leads to a smaller penetration depth and a larger number of 
ejections caused by a stronger oscillation of the keyhole as can be seen in the high-speed videos. 
An explanation for the instability of the welding process at low feed rates has been illustrated by a model of 
R. Fabbro [3] which can be applied and extended for the present case: The welding process becomes instable at 
lower feed rates as a consequence of a small angle of the keyhole front wall. The evaporation pressure decreases 
with decreasing angle of the keyhole front wall relative to the incident beam and also decreases with decreasing 
absorbed intensity. As soon as the closing pressure of the keyhole - resulting from the local surface tension -  
overbalances the evaporation pressure the keyhole collapses. As a result of the collapse of the keyhole melt ejections 
occur.  
Following this model the keyhole stability is influenced by a varying incident intensity. This effect is considered 
to be very pronounced if the intensity is periodically modulated across the deep penetration threshold. The 
evaporation pressure in this case strongly changes with the intensity directly influencing the balance between 
evaporation pressure and surface tension. In addition, as the evaporation pressure is dependent to material properties 
different behavior is expected for the used materials under consideration as seen in Figure 5.  
3.2. Stability of welding depth 
For almost all welds the molten area is one of the most important criteria. The quality of a weld seam is 
significantly affected by the solidified including the thermal and electrical resistance. Therefore a critical weld 
characteristic is the fluctuation in penetration depth. Figure 6 left shows two longitudinal cross-sections with the 
fluctuation in welding depth for the bronze material (CuSn6). The fluctuation is the peak-to-peak variation of the 
welding depth represented by the dashed lines. The largest penetration depth measured in the longitudinal cross-
section is defined as 100 % penetration depth. The difference between the highest and lowest penetration depth 
neglecting the melt ejections is defined as the fluctuation in penetration depth. 
 The diagram in Figure 6 right shows the measured fluctuation in penetration depth for the two investigated 
copper alloys. The fluctuation at the bronze material is larger than at pure copper as can be seen from Figure 6 
(right). With a modulated laser power it is possible to reduce the fluctuation up to 60 % (Figure 6 right). These 
results were confirmed by repeating the experiments several times. 
Andreas Heider et al. / Physics Procedia 12 (2011) 81–87 85
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6: Left: Longitudinal cross-sections of modulated (top) and not modulated (bottom) weld. Right: Fluctuation in penetration depth for 
CuSn6 and Cu-ETP with modulated and not modulated laser power.  
3.3. Formation of pores 
The formation of pores was investigated at the sinusoidal modulation of the laser power. Figure 7 shows two 
longitudinal cross-sections. The upper image shows a modulated weld with a lot of pores. The pores are mostly 
situated at the lower part of the solidified weld seam. In contrast the unmodulated weld (lower image of Figure 7) on 
the one hand shows less pores but on the other hand the complete weld is much more inhomogeneous regarding the 
welding surface and the penetration depth.    
 
 
 
 
 
 
 
 
 
Figure 7: Top: Modulated weld with many pores; CuSn6, f = 375 Hz, Paver = 1000 W, Pampl = +/- 750 W, v = 6 m/min. Bottom: Not modulated 
weld; CuSn6, f = 0 Hz, Paver = 1000 W, v = 6 m/min. 
An analysis of the location of the pores shows that the pores arise frequently at the trailing edge of the laser 
power. One can clearly see this behaviour at a modulation frequency of 40 Hz (Figure 8 left). The green curve 
shows the laser power and the red signal shows the normalized back-reflection. The longitudinal cross-section is 
aligned to the laser power. If the capillary is closed most of the laser power is reflected back to the optics and can be 
detected as a rise in the observed back-reflection signal. This correlation is usually seen when passing the threshold 
from heat conduction welding to deep penetration welding [5]. The rise in the back-reflection signal at decreasing 
laser power as seen in Figure 8 is a clear indication for a closing capillary. At this point the capillary collapses and a 
pore can form. 
A closer look to the geometry of the solidified weld seam shows two different angles (Į and ȕ in Figure 8 right) 
that can be observed at the trailing and rising slope. As the opening and closing processes of the capillary are 
dominated by different physical phenomena [6] the angles of the two slopes indicated by the red and the blue dashed 
line in Figure 8 (right) are unequal as well. A specific analysis of the opening and closing behaviour of the capillary 
is reported in [6]. 
 
 
a
b
86  Andreas Heider et al. / Physics Procedia 12 (2011) 81–87
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 8: Left: Top: Longitudinal cross-section of a weld, f = 40 Hz, CuSn6, P = 1000 W, v = 6 m/min; bottom: Normalized back-reflection and 
measured power. Right: Longitudinal cross-section with different angles of the trailing and rising slope of solidified weld seam.  
3.4 Surface quality 
Another criterion to evaluate a weld is the subjective surface quality of the weld. Melt ejections lead to the loss of 
heat and the collapse of the keyhole and not only cause a fluctuation in penetration depth but also a variation in the 
weld seam width. Due to the effect of stabilization of the keyhole by modulating the laser power the weld seam 
width is almost constant and the weld surface shows good uniformity. Figure 9 shows the improvement of the weld 
seam surface by modulating the laser power. In comparison to the welds without modulation a significantly more 
regular surface can clearly be seen at the modulated welds.  
 
 
 
 
 
 
 
 
 
 
Figure 9: Top: Weld seam surface of Cu-ETP, modulated and not modulated. Bottom: Weld seam surface of CuSn6, modulated and not 
modulated. 
4. Conclusion 
The presented experimental results clearly show that the modulation of laser power has a mostly positive 
influence on the weld quality in copper. On the one hand, an increased formation of pores was observed using this 
method. Pore formation was observed mostly at the trailing edge of the modulated laser power during the collapse of 
the keyhole. This might be overcome by using optimized shape of the modulation signal as was reported [7]. 
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Andreas Heider et al. / Physics Procedia 12 (2011) 81–87 87
On the other hand it is shown that it is possible to significantly reduce the melt ejections with a sinusoidal 
modulation of the laser power across the deep penetration threshold. The power modulation causes a stabilization of 
the keyhole and reduces the fluctuation of penetration depth. Furthermore the penetration depth could be increased 
by the modulation. In addition, smooth weld seam surfaces and a constant weld seam width could be obtained.   
5. Acknowledgment 
All the investigations are accomplished in the context of the CuBriLas-project (FKZ: 13N10577) which is 
sponsored by the Federal Ministry of Education and Research (BMBF). The CuBriLas-project belongs to the 
MABRILAS-network. The responsibility for this paper is taken by the authors. 
References 
[1]   Hohenberger, B: „Laserstrahlschweißen mit Nd:YAG-Doppelfokustechnik“, Dissertation, Universität Stuttgart (2003). 
[2]   Wieland Werke: Copper data sheet of B16 (CuSn6), K32 (E-Cu-58); www.wielandwerke.de (2011). 
[3]   Fabbro, R.: “Melt pool and keyhole behaviour analysis for deep penetration laser welding”, Proceedings of J. Phys. D: Appl. Phys. 43, 2010 
[4]   Hess, A.; Heider, A.; Weber, R.: ”Benefits from Combining Laser Beams with different Wavelength (Green and IR) for Copper Welding”, 
Proceedings of ICALEO 2010.  
[5]   Stritt, P.:  “Laser Power Modulation at the Threshold from Heat-Conduction to Deep-Penetration Welding”, Proceedings of ICALEO 2010. 
[6]   Stritt, P.: “Utilizing Laser Power Modulation to Investigate the Transition from Heat-Conduction to Deep-Penetration Welding”, 
Proceedings of LIM 2011 
[7]   Tsukamoto, S.; Kawaguchi, I.; Honda, H.; Arakane, G.: “Suppression of Porosity using Pulse Modulation of Laser Power in 20 kW CO2 
Laser Welding”, Proceedings of ICALEO 2001. 


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Process Stabilization at welding Copper by Laser Power Modulation

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