Energy Coupling of Laser Radiation on AISI 304 Stainless Steel: Effect of High Temperatures and Surface Oxidation

Abstract

The industrial application of laser materials processing methods is still far ahead of research into the physical phenomena occurring during these processes. In particular, the effect of high temperatures on the energy coupling of laser irradiation of metals is poorly understood. However, most processes in laser materials treatment involve temperatures above the melting point or even cause evaporation. This study therefore evaluates the effect of high temperatures on the energy coupling efficiency of stainless steel experimentally for three typical laser wavelengths (515 nm, 1.07 µm, 10.6 µm). As a result, it is shown that the effect of temperature on the energy coupling efficiency depends on the wavelength. In this context the relevance of the X-point phenomenon known from the emissivity theory could be demonstrated for laser material processing. Further, the effect of a process-induced surface oxidation is analyzed. At temperatures above 650 °C the energy coupling efficiency dramatically increases to around 65% at melting point and stays at this high level even in the liquid phase.

Change of dominant material properties in laserperforation process with high-energy lasers up to 120kilowatt

“…For steel samples a similar behaviour is observed but with a much stronger influence of the oxidizing atmosphere 25,26,[31][32][33][34] . At room temperature, depending on the surface state, an absorptivity from 26 % to 50 % is reported.…”

“…Finally, the influence of the fit point at 800 • C in the simulations is also unclear. Reported values of around 25 % of effective energy coupling for experiments in inert gas atmosphere and increasing values up to 65 % in oxidising atmosphere 31,34 do not reflect the experimental conditions of this paper. Here, almost pure iron was used which already had an oxidised surface compared to the polished stainless steel samples used by Hip et al 34 .…”

“…Reported values of around 25 % of effective energy coupling for experiments in inert gas atmosphere and increasing values up to 65 % in oxidising atmosphere 31,34 do not reflect the experimental conditions of this paper. Here, almost pure iron was used which already had an oxidised surface compared to the polished stainless steel samples used by Hip et al 34 . For already oxidised steel samples, the absorptivity was reported to be around 80 % 33 with only small changes with temperature or irradiance (note, overall much lower intensities compared to those used here).…”

“…At room temperature, depending on the surface state, an absorptivity from 26 % to 50 % is reported. With increasing temperature the absorptivity increases due to oxidation 33,34 . Therefore, a strong increase in absorption is observed at about 600 • C. If a mean value for the absorptivity is determined for the entire laser processing time, a dependence on the laser spot size is observed 25,26 .…”