Impact of the thermally induced focus shift on the quality of a laser cutting edge

Abstract

The phenomenon of thermal focus shift in optical systems, mainly caused by thermal lensing, is well known. There are numerous publications dealing with this effect, but they do not answer the question of how relevant this problem is to the practical application of lasers in laser cutting machines. In this paper, we examine the effect of the thermally induced change of the focus position on the quality of the cut edge when laser cutting 3 mm thick stainless steel with an 8 kW disk laser. After measuring the magnitude of the focus shift, its impact on the quality of the resulting cutting edge was determined for three different scenarios: recommended process parameters, increased speed, and decreased gas pressure. Roughness and perpendicularity tolerance were used to quantify the quality of the cutting edges according to the ISO standard. It becomes clear that it is difficult to explain the effect of the change in the focus position on quality aspects in general, as this also depends on other process parameters. Nevertheless, it can be shown that the resulting cut edge quality decreases in all three scenarios as a result of the thermal focus shift. This problem cannot be avoided by simply changing the process parameter corresponding to the focal position in the opposed direction of the thermal behavior before starting. Even though the reduction of quality might not be severe today, the focus shift should be considered for all applications that require a very high cutting quality. Furthermore, this effect cannot be neglected anymore as laser power will continue to rise in the future.