Impact of Ultra-Short Pulsed Laser (USPL) Ablation Process on Separated Loss Coefficients of Grain Oriented Electrical Steels

The purpose of this article is to study the impact of surface laser treatments with ultra-short pulses (USPs) (femtosecond laser) on the magnetic properties of grain-oriented electrical steels (GOESs) using the two-temperature model for the ablation process and the magnetic loss separation model of Bertotti. We demonstrated that the hysteresis and excess loss coefficients behave differently depending on the type of laser treatment and its pulse duration [long pulse (LP), short pulses (SPs), and USP]. We also presented the adjusted models to estimate the impact of the USP on the sheet surface in terms of laser energetic quantities; more precisely, the groove depth, the plasma maximum temperature, and the peak surface wave pressure were estimated, relative to its nominal value. The latter physical impacts of laser pulses were then correlated with Bertotti's loss coefficients: the static hysteresis loss coefficient and the excess loss coefficient. The laser process is not always able to reduce simultaneously both loss contributions. Thus, a compromise must be found to optimize the process. The variation of the flux density level as a function of the applied magnetic field was measured with a single sheet tester (SST) under a one-directional field parallel to the rolling direction. From these measurements, we deduced the whole power loss contributions. Results showed that an optimization of the laser's parameters ensured an iron loss reduction at 50 Hz up to -30% for an induction below 0.5 T and a percentage close to -15% for an induction above 1.5 T.