Effects of Short-Term Laser Beam Heating on the Absorptivity of Steel Sheets

The efficiency of laser beam processes basically depends on the efficiency of the laser beam source and the efficiency of the irradiated material's energy absorption. This absorptivity can be influenced by the surface condition. Besides coating or boundary layers, the surface topography is decisive. In this study, the effects of various time-temperature distributions on the absorptivity changes of steel sheets were investigated. For this purpose, three steels were chosen, namely, a stainless steel, a spring steel, and a hot work tool steel. Pre- and post-process characterizations of the absorptivity and surface topography were performed. Controlled heating with a laser beam was carried out using temperatures between 700 and 1200 degrees C and durations between 2 and 34 s. In order to compare the influences of these heating procedures on the absorptivity, a characteristic value, the temperature-compensated time, was introduced. It is shown that the surface roughness was influenced by laser irradiation but inadequately describes the increase of absorptivity. The changes in absorptivity are attributed to oxidation, which had an influence on the topography in a sub-micrometer range. Moreover, a saturation effect is observed for intense heatings. Furthermore, it is shown that the temperature-compensated time is a suitable value to describe absorptivity changes caused by short-term heating.