Vorteile und Herausforderungen beim Laserstrahlschweißen mit Strahlquellen höchster Fokussierbarkeit

Abstract

Die Vorteile einer sehr guten Fokussierbarkeit von Scheiben- und Faserlaser lassen sich auf verschiedene Art und Weise nutzen. Kleinere Fokusdurchmesser und die daraus resultierenden hohen Intensitäten ermöglichen einen Tiefschweißprozess bei sehr hohen Vorschubgeschwindigkeiten. Für den industriellen Einsatz ist vor allem eine reproduzierbare hohe Qualität des bearbeiteten Bauteils entscheidend. Schädigungen, wie insbesondere der Verzug, sollen so weit wie möglich reduziert werden. Untersuchungen zeigen, dass bei einer konstanten Einschweißtiefe mit einem kleineren Fokusdurchmesser, einer höheren Vorschubgeschwindigkeit und einem kleineren Divergenzwinkel der Verzug stark reduziert werden kann. Der Vorteil einer sehr guten Strahlqualität kann auf der anderen Seite sehr hohe Intensitäten in den Schweißoptiken mit sich bringen. Durch Absorptionen im Bulkmaterial und den Beschichtungen kommt es zu thermischen Effekten und somit zu einer Verschlechterung der Strahlqualität und zu einem Fokus-Shift. Diese können so gravierend sein, dass Auswirkungen auf den Schweißprozess einhergehen. In der vorliegenden Arbeit wurden mit hochwertiger und komplexer Messausrüstung transiente und stationäre Strahlqualitätsmessungen an Schweißoptiken durchgeführt. Ein neuentwickeltes und neuartiges Verfahren ermöglicht eine einfache Qualifizierung von Optiken. Der sogenannte „Referenzprozess“ macht dazu Veränderungen der Intensität bzw. der Intensitätsverteilung auf dem Werkstück sichtbar, die zu unterschiedlichen Schweißergebnissen führen und sich mittels Messschieber einfach auswerten lassen. Ein weiterer Vorteil der hohen Intensitäten von im infraroten emittierenden Laser-strahlquellen ist die Bearbeitung von Kupferwerkstoffen im Dauerstrichbetrieb. Dabei lassen sich mehrere Millimeter Einschweißtiefe erreichen. Eine Herausforderung stellt die geringe Absorption von gerade einmal 5 % zu Prozessbeginn dar. Starke Rückreflexe können dabei zu Schädigungen in den optischen Komponenten führen. Durch die Verwendung von frequenzverdoppelten Lasersystemen kann die Absorption etwa versiebenfacht werden, so dass Rückreflexe reduziert werden. Diese neuartigen im „grünen“ emittierenden cw-Laser sind in ihrer Leistung noch stark begrenzt, weshalb in dieser Arbeit ein Kombinationsprozess realisiert wurde, mit dem höhere Einschweißtiefen möglich werden und der Vorteil der kürzeren Wellenlänge deutlich wird.

New developments in diode-pumped fibre lasers and disc lasers open up opportunities for completely new process strategies as a result of their incomparable focusability at high laser power. This yields unprecedented beam parameter quotients (power divided by focus diameter) which are needed for high welding velocities. In addition this allows deep penetration welding in highly reflective materials such as copper. The laser sources used for this work were fibre coupled allowing high accessibility and flexibility as well as lower costs for the handling devices. Another important property of shorter wavelengths is the generally increased absorptivity in metals. With the use of 1 µm lasers instead of CO2 lasers, an increase of absorptivity has already been attained. A further improvement of the absorptivity, especially in copper material, can be achieved by using laser sources in the visible range. This kind of novel laser sources, which have not been available so far, goes along with new possibilities in material processing. For industrial application the above mentioned advantages can be exploited in high efficiency processes. The small interaction zones enable deep penetration welding with less distortion of the workpiece. The joint geometries of thin samples of less than one millimeter have never been achievable before and were therefore never examined in the past. The minimization of distortion through minimizing the heat input is clearly shown in this work. For this purpose samples of stainless steel with a thickness of 500 µm were used. The distortion angle (averaged angle of the distorted workpiece adjacent the weld seam) was measured with a surface topography system. Therewith a comparison of different welding parameters could be performed. Three laser sources with different beam qualities were used for the investigations. Different focal diameters in the range from 14 µm to 200 µm were achieved with different focusing arrangements. A reduction of the distortion angle was demonstrated for: Smaller focus diameter; Higher processing speeds (up to 50 m/min); Smaller divergence angle of the laser beam. In doing so the penetration depth of welds was kept constant. The above mentioned three parameters directly reduced the cross-sectional area of the weld seam and therefore resulted in a smaller distortion angle. Due to the decreasing focal diameters new clamping devices with a high position accuracy had to be developed. A magnetic clamping device fulfilling this demand was constructed and manufactured. However, high average power very often causes thermal beam distortion inside trans-mitting optical elements. This effect is especially pronounced in contaminated optics. It results in a significant decrease of the beam quality and a shortening of the effective focal length, also referred to as focus shift. Both effects directly modify the laser spot size and thus the intensity on the workpiece. This is very critical for most laser material processing applications. Especially welding of highly reflective materials such as copper is very sensitive to changes in the laser intensity. To prevent process failures, the focus shift and also the beam quality have to be measured behind the optical components. Static as well as transient measurements showed that the focus shift and the degraded beam quality directly reduce the intensity on the workpiece. In addition the power which is absorbed, reflected, and scattered in the optical components also reduces the intensity on the workpiece. Reductions of the intensity on the workpiece of up to 88 % were measured. The remaining 12 % obviously are not sufficient to allow a stable welding process. These results imply that the use of appropriate and very clean processing optics is mandatory for a reproducible and stable welding process. The above mentioned measurements require very expensive and complex measure-ment equipment. In this work, a novel and simple method is presented, which uses the welding process itself to quantitatively determine the focus shift. The so-called “reference process” was developed, which takes benefit of the dependence of the deep penetration welding threshold on the beam diameter. After a defined preheating time in order to control the thermal load of the optics, a weld trace of a few centimeters is generated in a sample by applying a laser power ramp. The position of the transition from heat conduction welding to deep penetration welding is easily noticeable by visual inspection of the seam width on the workpiece surface. This allows measuring it with a simple caliper, yielding the threshold transition power. By the outcome of four different welds with suitably varying laser parameters, the focus shift can be calculated with appropriate accuracy. With this method, a new and easily applicable tool is available. This is an important requirement for a reproducible and stable welding process and even holds for changing conditions, as e. g. slowly contaminated safety glasses. Presuming correct conditions of the laser beam on the workpiece, also highly reflec-tive materials can be welded by the means of highly brilliant 1 µm laser beams. With the use of 1 µm lasers the absorptivity can be increased from about 1 % to about 5 % in copper material at room temperature as compared to CO2-lasers. Using high-power 1 µm lasers this absorptivity increase guarantees that enough energy absorbed for starting a deep penetration welding process. In addition the absorptivity of copper increases with temperature and even shows a large step in absorptivity at the phase transition from solid to liquid from about 8 % to about 13 %. When the processing optics are not aligned and shielded correctly, the large amount of back reflected laser light causes severe damage somewhere in the optics chain. This happens especially at the process start. To control the process start condition, the absorptivity was increased by coupling more laser power in the copper material. This was done by using a beam source with a once more shorter wavelength. A frequency-doubled thin-disk laser with a wavelength of 515 nm and a maximum average output power of 100 W was available for the experiments presented here. At this wavelength, the absorptivity for the “cold” copper material is more than seven times higher as compared to the infrared (IR) laser beam. However, due to the limited power the penetration depth of pure copper welds was only about 100 µm, which is not sufficient for most industrial applications. Therefore, a commercial thin-disk laser (λ = 1030 nm) and the frequency doubled thin-disk laser were combined allowing a so-called hybrid process. The preceding green laser beam with a focal diameter of 25 µm was used to heat up, melting and forming a small keyhole on the surface of the copper material. The IR laser beam was focused onto the workpiece to a diameter of 100 µm and the distance between the two laser beams was adjusted to 100 µm. The large distance was chosen so that there was no interaction between the keyhole created by the green laser beam and the interaction region of the IR beam. Numerical simulation of these conditions showed an increase of absorptivity in the interaction zone of the IR beam from 5 % to about 11 %. Under these conditions the IR beam interacted with the preheated material which leads to an enhanced absorptivity and a lower threshold for deep penetration welding. Welding experiments confirmed the predictions. For the described hybrid process two laser sources as well as two very accurately aligned processing heads were necessary. The high complexity of the optical arrange-ment and the resulting high cost make the hybrid process only reasonable for specific industrial applications. Nevertheless, the encouraging results using the hybrid process suggest further research on laser sources providing - at least at the process start - enough “green” laser power to force well-controlled deep penetration welding in copper materials.