By Eckhard Beyer and Achim Mahrle
The high power laser market has been remarkably influenced by the introduction of high-brightness lasers, i.e., laser sources offering a high optical output power in combination with a high beam quality or a low beam parameter product, respectively. These features are primarily exhibited by fiber and disk laser systems, but diode lasers are also increasingly available with improved beam quality and higher output levels. The development of new processes that make use of the advantages of high-brightness lasers was intensively pursued in recent years and some examples of innovative solutions were given in this paper.
On the other hand, the advent of high-brightness lasers also gave rise to some technical challenges, which still need a reliable solution. One point concerns the optical feedback due to back reflections during processing highly reflective materials. Another serious and often discussed topic, is the occurrence of unacceptable focus shifts when working with high-intensity laser beams.
OPTICAL FEEDBACK CONTROL
Laser material processing of highly reflective metals requires high laser intensities. This need is met by high brightness laser beam sources due to their good focusability. However, back reflections during the initial stage of the process in which the absorption of laser radiation is still rather low, are an important technical issue to contend with. The reflected laser light is capable of destroying fiber ends, coupler modules and splice connections. Research at the Fraunhofer IWS was aimed at preventing such back reflections to the optical components of the laser system. As a result, an optical diode was developed, allowing a blocking of back reflections to a minimum of 20 dB. Figure 1 shows how the optical diode does work. The emitted randomly polarized laser beam is initially split into two polarized laser beams with polarization planes perpendicular to each other. The Faraday rotators shift the orientation of these beams by 45 degrees. Finally, the two beams are united to the working beam focused onto the surface of the workpiece. In case of optical feedbacks, the reflected beam goes the same way backwards. The additional rotation of 45 degrees during the passing of the Faraday rotator enables the uncoupling of optical feedbacks.
FOCAL SHIFT DEVICE
The use of high-brightness laser sources can cause changes of the refraction index and the shape of lenses due to thermal interactions. In this way, deviations of the focal length as a function of the applied laser power occur. The resultant shift of the focal position can provoke a lower machining quality or, in extreme cases, a breakdown of the process. Corrections by moveable optical elements are possible but require a priori the instantaneous determination of the actual focal plane position. For this purpose, a measurement device was developed that uses the small amount of laser radiation reflected from the protection glass. This part is guided to a sensor which allows the calculation of the focal length on the basis of a ray transfer matrix calculation.
A very promising application area for high-brightness lasers is remote cutting of thin-section metals. In this cutting variant, the melt removal from the cut kerf is done by the action of the recoil pressure of evaporating material. An additional gas jet, like in conventional fusion cutting, is not necessary, and this feature allows the very fast movement of the laser beam along the cut contour by using scanner mirrors. Cutting speeds up to 800 m/min are realizable in this way. An outstanding application field of the innovative remote cutting process is cutting electrodes and separators of lithium-ion cells. The electrodes are made of two different composite materials with thicknesses of about 100 µm. The cathode is a coated aluminum foil whereas the anode consists of a coated copper foil. Typical electrode geometries can be cut in less than a half second with very acceptable cut edge qualities, as shown in Figure 2.
LASER WELDING WITH BEAM OSCILLATION
The excellent beam quality of high-brightness laser beam sources allows welding applications with spot sizes in the range of between 20 and 40 µm and intensity levels higher than those of electron beams. Issues related to these small spot sizes and high intensity levels are, however, very narrow weld seams with high aspect ratios of penetration depth and weld seam width, as well as severe weld spatter formation, in interaction with such high-intensity laser beams. One successful remedy is the possibility to apply beam oscillation techniques by use of highly dynamic scanning optics. The melt pool dynamics, the solidification behavior, and the resultant weld seam geometry can be strongly influenced by time- and power-controlled laser beam oscillations. Investigations on flux-coated aluminum components demonstrated that high-frequent beam oscillations considerably reduce the risk for splatters, pores and melt pool eruptions as shown in Figure 3. Similar improvements of the process characteristics were also achieved for welding steel.
LASER CLADDING WITH WIRE DEPOSITION
The use of metal wires instead of powder materials is considered as a promising solution for prospective laser cladding applications in prototype or tool and mold construction. The process enables a material utilization of 100 percent without the risk of contaminations with impurities during the deposition process. Exemplarily, a multi-beam laser head with centric wire feed and a generated turbine blade are shown in Figure 4. The head developed at Fraunhofer IWS allows omni-directional 3D processing with higher processing speeds, higher deposition rates and much better material utilization than a comparable powder laser cladding process and is available for fiber and disk laser beam sources.
Issues and applications in the field of high-power laser material processing with recent laser sources and latest system components were presented. Most of these developments would have hardly been possible to realize without use of the advantages of recent high-power and high-brightness laser systems. The advent of these laser beam sources has had a noticeable impact on research and development activities in laser material processing.
Dr. Eckhard Beyer is the Executive Director of Fraunhofer IWS, Dresden and Dr. Achim Mahrle is a Research Fellow at Dresden University of Technology.