Time-resolved temperature measurement during laser marking of stainless steel

Abstract

A new measurement system was developed for time-resolved surface temperature measurement in nanosecond time scale. A study of surface temperatures reached by different parameters of laser marking and their correlation with resulting microstructure, phase composition and corrosion tests performed on marked samples is presented. The marking was done using a nanosecond pulsed fibre laser with variable pulse duration (from 9 to 200 ns), repetition frequency and pulse energy. Different phase composition and corrosion resistance were observed for visually similar marking results obtained by different laser parameters. This correlates well with maximum temperatures reached in the laser spot, which varied from less than 1100 °C for longer pulses to more than 1800 °C for shorter pulses. Melting of the surface with up to 4 µm thickness was observed for marking processes inducing high temperatures. The maximum temperatures in the pulse depend not only on pulse duration but also on previous pulses due to the heat accumulation effect. A temperature difference of up to 500 K was observed due to heat accumulation. From the results it can be concluded that combinations of longer pulse duration and higher repetition rate are the most suitable parameters for preserving corrosion resistance of stainless steel after laser marking.