Effect of Surface Roughness on CO₂ Laser Absorption by 316L Stainless Steel and Aluminum

The roughness of a material’s surface is a significant factor affecting the absorption of energy when scanned with laser beam radiation. It is a robust variable affecting the entire laser process cost and the appropriate levels for the laser processing parameters, such as the power applied. It is thus an important factor for many processes, such as laser cutting, welding, roughening, and polishing. As such, an examination of the effect of roughness on laser absorption is desirable to determine how input parameters, such as power, must be adjusted when working with different roughness and the potential power savings of roughening samples, or eliminating polishing or finishing steps, before laser processing. In this study, a comparison of the effect of the surface roughness of flat 316L stainless steel and aluminum samples on the thermal energy absorbed in laser processing was investigated. The aim was to characterize the effect and identify the potential energy savings. Samples were processed as received and either polished or roughened, such that there were five different roughness levels for each material. Individual carbon dioxide laser shots were applied with large unprocessed gaps in order to allow clear investigation of the crater depth dimensions. The width, length, and depth of the craters were measured by both scanning electron microscope and three-dimensional microscope, with larger dimensions indicating a greater absorption of the laser energy. The expected outcome of the study was confirmed, finding a noticeable improvement in the absorbed energy on the rougher surfaces for both materials, with a saving of ∼62 % in the laser power applied when the aluminum surface roughness increased from 2.4 μm to 7.03 μm.