Influence of temporal and spatial laser power modulation on melt pool dynamics
The reproducibility of laser welded joints in highly reflective materials such as copper and aluminum is a difficult issue. Fluctuating energy deposition resulting from unsteady material and surface properties leads to unreliable processing conditions. The adaptation of the energy deposition to the material and workpiece properties achieved by a precise temporal and spatial control of the laser power offers new opportunities for such applications. Spatial power modulation influences the vertical and lateral melt flow velocity significantly and can be achieved for different welding applications with the use of highly dynamic galvoscanners. Together with fibre laser and Nd:YAG lasers capable of pulse durations of 100 ms, a modulation frequency of 2 kHz is possible. As for seam welding applications linear feed and oscillation shapes are superposed, the actual feed rate of the laser beam varies during the process. To ensure a consistent energy deposition, the temporal power modulation must be adapted to the spatial modulation. Furthermore the status of the keyhole can be influenced by fast power modulation with a frequency up to 10 kHz. In this paper the influence of power modulation on the melt flow is examined by means of high speed photography.