Beyond Fresnel: absorption of fibre laser radiation on rough stainless steel surfaces

Abstract

The standard procedure for the calculation of the angle-dependent absorptivity consists of applying the well-known Fresnel formulas to the material property refractive index n and extinction coefficient k. However, the Fresnel formulas are only applicable for perfect smooth surfaces. Hitherto the effect of rough surfaces on absorptivity values and profiles are not thoroughly explored. Yet, this knowledge is vital for many kinds of theoretical consideration in computational models as they are applied in laser material processing, imaging and metrology. Therefore, we present experimental results on the angle-dependent absorptivity of stainless steel with five different surface finishes for fibre laser radiation with a 1.07 µm wavelength and for s- and p-polarisation separately. We can show that for probes which fulfil the modified Fraunhofer smoothness criterion at normal incidence (R z < λ/32), the Fresnel formalism is applicable. For the tested probes with higher surface roughness, the real absorptivity differs considerably from this solution: the maximum of absorptivity at the pseudo Brewster angle is decreased and shifted towards lower inclinations. Furthermore, other effects are observed which are not explained by theoretical approaches like Fresnel or ray-tracing models.

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

“…The measured trend in our experiments is identical compared to results in the literature [24], however, our values are lower. This can be a consequence of the much smaller surface roughness [7] of our used probes.…”

“…Some studies tried to examine the effect of non-laboratory conditions on the energy coupling of laser radiation. While for the effect of surface roughness, both theoretical calculation models [4,5,6] and experimental results [7,8] were published, but the results for the temperature dependence are only sparse. Using Kirchhoff’s law of thermal radiation, which states that under the assumption of a thermodynamic equilibrium the direct spectral emissivity corresponds to the direct spectral absorptivity, results on the temperature dependence of the emissivity can be considered instead.…”

Multi-Objective Optimization of Laser Cleaning Quality of Q390 Steel Rust Layer Based on Response Surface Methodology and NSGA-II Algorithm

“…In the pre-experiment stage, the surface rust layer of the sample was unevenly distributed, which made it difficult to accurately control the process parameters of singlelaser cleaning and could have easily lead to ablation of oxide residues in some areas. Therefore, the high-power and low-power double-laser cleaning method was adopted during the secondary cleaning, and differences in the laser absorption rate of the material were utilized for process refinement [20][21][22]. The parameters of the second laser cleaning process significantly affect the quality of the cleaned surface.…”