Influence of Particle Shielding and Heat Accumulation Effects onto the Removal Rate for Laser Micromachining with Ultra-Short Pulses at High Repetition Rates.

High throughput is a key aspect in laser micro machining. In order to achieve excellent processing quality with high power, high repetition rate ultrafast lasers new and fast beam deflecting systems as e.g. polygon line scanners with marking speeds up to 100 m/s and more or – alternatively – fast rotating cylindrical work pieces are useful. These devices facilitate to work at high repetition rates in the MHz-regime and it has to be clarified if particle shielding and heat accumulation effects may become evident in this regime of repetition rates.
Experiments up to 43 W of average power at 6.8 MHz repetition rate and a pulse to pulse distance of one half of a spot radius have been performed on five different steel grades and on copper. Hints for a particle shielding effect were only found for steel 1.4301, but the effect is weak and, if it really exists, only in the range of 10%. For all other steel grades and copper no shielding or heat accumulation effects were observed up the maximum power of 43 W of the laser system.
Experiments with a galvo scanner and reduced pitches showed an oxidation effect which is not caused by the heat accumulation during the marking of a single line but eventually by the short time interval between marked lines which may lead to a long time heat accumulation.