Abstract
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 °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.
Highlights
Laser beam sources are widely used in current production applications
Surface modifications induced by short-term laser beam heating are insufficiently described by conventional roughness values
Conventional roughness values are inappropriate for indicating absorptivity changes due to a laser beam heating process
Summary
Laser beam sources are widely used in current production applications. For material processing, the efficiency of the laser beam process is determined by the absorption of the radiation by the irradiated material. The absorptivity of a material depends amongst other things on the laser beam wavelength, the material, and its surface topography Measuring this absorptivity was first carried out using a calorimetric approach with thermocouples [1]. For the stainless steel 1.4301, investigations of the oxidation process showed that the annealing color represents the oxide layer thickness [8]. Investigations of diffusion processes in vacuum indicate the formation of a healing layer at the grain boundaries which modifies the rate law [16] This is more pronounced the higher the chromium content and the annealing times; no deviation from the parabolic law was measured for up to 30 min at 700 ◦C [16]. That characteristic value had not yet been described for the absorptivity changes caused by thermal influences on steel sheet surfaces
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