By Andreas Heider, Axel Hess, Rudolf Weber, Thomas Graf
Institut fuer Strahlwerkzeuge (IFSW), Pfaffenwaldring 43, 70569 Stuttgart, Germany
Due to their material properties such as high electrical and thermal conductivity, copper is more and more demanded for industrial applications. The same material properties make laser welding of copper a challenging task. To achieve welds with penetration depths of several millimeters in copper using commercially available laser sources relatively low feed rates of less than 10 m/min are needed. Welds in copper at such low feed rates with 1 µm wavelength lasers often suffer from defective weld seams with melt ejections and pores. Furthermore the absorptivity of pure copper is very low at this wavelength.
The outcome of melt ejections are reflected in holes in the solidified seam surface. These holes are the result of the lack of mass resulting from the ejection of molten material out of the melt pool during the welding process. In most of the cases such holes extend over the complete penetration depth. For industrial applications primarily the cross sectional area of the welds is of interest in order to achieve the highest possible electrical and thermal conductivity. Hence it is of great importance to minimize or even avoid such weld imperfections.
On the one hand the absorptivity of copper strongly increases in the visible wavelength range. On the other hand modulated laser power can reduce melt ejections. A combination of these two approaches is presented in this article. A frequency doubled “green” thin-disk laser at a wavelength of 515 nm with a maximum fundamental mode cw output power of 200 W and a 5 kW thin-disk “IR” laser at the wavelength of 1030 nm were geometrically combined. The focal spot diameters were 25 µm for the green and 100 µm for the IR laser, respectively. The experimental setup is shown in Figure 1.

Figure 1: Experimental Setup.
In order to analyze the influence of the thermal conductivity, a copper alloy CuSn6 (referred to as “bronze”) with a moderate and a pure copper (Cu‑ETP) with very large heat conductivity were examined. The welded samples were analyzed with respect to the number of melt ejections, the penetration depth and quality of the solidified seam surface. The number of melt ejections was counted from the weld seam surface: Each hole located in the melt seam after the solidification was counted as melt ejections. All welds were 80 mm longs bead on plate welds.
Process Stabilization by Laser Power Modulation
In order to investigate the effect of the formation of melt ejections the laser power was sinusoidally modulated. The laser power was modulated around the deep penetration threshold with an amplitude of 50 % of the average power. The results of these investigations are shown in Figure 2.


Figure 2: Seam Surface (top) and longitudinal cross-section (bottom). Cu-ETP, comparison between cw weld (upper) and modulated weld (lower) at 500 Hz and an average power of 1.7 kW.
Presuming the correct parameters modulation of the power has very distinct advantages: On the one hand there is a reduction of 70 % to 90 % in number of melt ejections. On the other hand a significantly more regular and homogeneous solidified weld seam surface for the modulated welds was achieved as shown in Figure 2.
Increased Process Stability by applying an Additional Green Laser
The output power of 200 W of the green laser is not sufficient to achieve penetration depths of a few millimeters in copper despite the much higher absorptivity at this wavelength. Therefore it is quite straight forward to combine two laser sources for welding copper: A “green” low power laser and a kilowatt infrared laser. On this background the two lasers described above were combined geometrically for the experiments (see Figure 1). For the experiments the green focal spot was set 130 µm in front of the IR with respect to the direction of movement.
Figure 3 shows a comparison of the “IR-only” welding process and the IR-green combined process with the green laser for pure copper. In order to keep the total power constant for the combined process with the 200 W of green laser, the IR laser power was reduced by the power of the green laser. The modulation frequency of the IR laser was set to 500 Hz with an amplitude of ± 750 W.

Figure 3: Melt ejections per weld and penetration depth for Cu-ETP at 6 m/min and a power of 1.7 kW; Left: IR-only; Right: combined process (green and IR).
Due to the combined process the melt ejections could be reduced to not more than one ejection per weld. In addition the penetration depth could also be increased (see blue line Figure 3).
In Figure 4 the solidified seam surface for bronze for the IR-only (top) and the combined process (bottom) is shown. Very homogeneous and regular seam surfaces without any melt ejection could be obtained with the combined process.

Figure 4: Weld seam surfaces for CuSn6 with surface defects; Top: IR cw, bottom: Combined process: Green and IR.
Due to the power modulation the melt flow is influenced in a way that less melt ejections occur. One possible explanation for the improved weld seam quality achieved with the combined process is believed to be a preheating effect. The green laser preheats the material and improves the incoupling of the IR bream.
Conclusion
Weld seams in deep copper welding at low feed rates suffer from many melt ejections and surface inhomogeneities. The presented experimental results clearly show that modulation of the laser power significantly improves the weld quality by considerably reducing the number of melt ejections. Combining the modulated IR laser with an additional green laser allows producing almost ejection-free deep welds and a perfectly homogeneous seam surface.
Acknowledgment
The presented work was funded 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.
Contact
Andreas Heider, Institut fuer Strahlwerkzeuge (IFSW), University of Stuttgart, Germany
Pfaffenwaldring 43, D-70569 Stuttgart, Germany
Phone ++49-(0)711-685-69730
Fax ++49-(0)711-685-59730
e-mail: andreas.heider@ifsw.uni-stuttgart.de
http://www.ifsw.uni-stuttgart.de