Laser Micromachining of Metals with Ultra-Short Pulses: Factors Limiting the Scale-Up Process

Especially for metals it was shown that the material removal process with ultra-short laser pulses shows an optimum point with highest efficiency i.e. with maximum specific removal rate. A scale-up process should therefore be realized by keeping the applied fluence near this optimum value i.e. by linearly increasing the repetition rate with the average power. In this paper we show that this scale-up is affected by two factors, the heat accumulation and by shielding effects. The latter leads to a significant reduction of the specific removal rate which becomes significant in the case of copper or brass when the repetition rate is increased into the multi-MHz range. In contrast shielding effects are much less pronounced for stainless steel (1.4301) but the surface quality is strongly affected by heat accumulation. However, we show that removal rates in the range of 40 [mm.sup.3]/min are achieved for all studied metals with average laser powers above 300 W. Keywords: ultra-short laser pulses, micromachining, scale-up and throughput, heat accumulation, plasma-shielding, particle shielding