Magnetic losses reduction in grain oriented silicon steel by pulse and continuous fiber laser processing

As an aspect of the worldwide trend towards energy consumption and preservation of the natural environment, the reduction of electricity consumption has become an extremely crucial matter in recent years. Grain-oriented (GO) silicon steel represents conventional soft magnetic material having high permeability and low core losses along the rolling direction and is commonly used for cores of electric transformers. Its magnetic properties are closely related to the sharpness of {110}<001> crystallographic texture, i.e. the Goss texture, which is evolved by secondary recrystallization during the abnormal grain growth.^1,2

The Goss texture forms as a result of technological route proposed by Goss³ in 1934. The procedure has been continuously improved and developed in order to achieve the best final properties of the material. Nowadays, the driving forces for research and development are the increase of quality and reduction of manufacturing costs. In the last few years, these aspects have become the mainspring for most of the industrial research and development activities.⁴ Improvements in the magnetic properties of electrical steels over the past 80 years have resulted from the control of impurities, thinner strips, improved coatings, improved texture and optimization of grain size.⁵ It is likely that trend will continue and will lead to further incremental improvements. Magnetic domain structures of secondary recrystallized grains and especially their movement have a strong effect on the core losses of GO silicon steel. Pinning of domain walls movement and surface closure domains must be considered in order to reduce the magnetic losses of GO steel.⁵ 

Core magnetic losses are usually separated on the one hand into structure dependent, frequency independent static hysteresis losses and on the other hand frequency dependent dynamic losses. The dynamic losses are divided into the classical eddy current losses which can be calculated with the help of Maxwell’s equations and anomalous eddy current losses which are principally dependent on the domain wall motion.^6–8 The magnetic domains of GO electrical steel consist of two components: 180° main domains and 90° surface closure domains.⁹ Eddy-current losses monotonically decrease with decreasing 180° wall spacing and increase with increasing amount of 90° domains. To reduce the eddy-current losses, domain refinement techniques are being investigated by eliminating unfavorable internal stresses and providing favorable stresses that refine the magnetic domains.¹⁰ 

Several different domain refinement techniques, such as mechanical scratching, plasma irradiation, spark ablation and laser scribing have been studied for many years. All these methods can induce favorable stress that refines the magnetic domains due to mechanical or thermal strains introduction. In these cases, generally static hysteresis losses are increased due to internal stresses generated by dislocations caused by mentioned techniques while dynamic losses are reduced by domain refinement. If the reduction of dynamic losses is larger than the increase of static losses for the specific frequency of AC magnetic field total core losses can be reduced.¹¹ For the industrial utilization of refinement process, the so-called “laser scribing” technique is usually applied due to its no-contact nature, high flexibility and little damage of the surface coating.¹² 

Typical core loss reduction of about 10% could be achieved with several kinds of laser beam sources. The theoretically predicted core loss reduction of 70% encourages unceasing motivation for further developments of the refinement processes.¹³ 

Our previous work¹⁴ was focused on optimizing the laser scribing conditions of Fe-3%Si electrical steel sheets for single pulse laser regimes. The present work represents an extended and continuing research study to our preliminary investigation¹⁵ of the effects of continues fibre laser processing on magnetic domains structure and surface changes of the same material in correlation with its resulting soft magnetic properties measured in AC magnetic field.

The objective of this work is to optimize the scribing process by means of fiber laser in continuous and pulse regime in order to reduce core losses of investigated GO steel by introducing favorable stresses leading to the magnetic domains refinement.

Sample material was grain oriented 3% silicon electrotechnical steel which was taken after final box annealing and coating from an industrial line. The domain modification was carried out on the strips of 30mm width, 80mm length and 0,28mm thickness. These samples were prepared by electric spark cutting and the longest side of samples was parallel to the rolling direction. Then these strips were subjected to stress relief annealing at 800°C for 30 min. in dry hydrogen atmosphere.

The laser scribing process was carried out on the surface of experimental samples by means of fiber laser using the pulse and continuous regimes with wavelength 1064 nm from “TRUMPF Company”. A scheme of the experimental setup for laser scribing as well as diagram of laser scribing process on the surface of experimental strips is shown in figure 1(a) and 1(b), respectively. The samples were individually treated in air atmosphere by laser beam in either continuous or pulse mode. The power density of the laser beam in case of continuous regime was in the range from 12W up to 30W with step of 6W. During the application of this scribing process, the laser beam spot on the samples surface with 30μm in size moved with the velocity of 100mm/s. In the case of pulse mode, the input power of laser beam varied from 30 W to 240 W with a pulse duration of 100μs and 100Hz frequency. The distance between the neighboring pulses was ΔL=0,3mm. The laser scribing was performed in the form of lines on the steels surface perpendicularly to the rolling direction. The distance between the lines was selected as following: L₁=4mm, L₂=6mm, L₃=8mm and L₄=10mm.

The magnetic properties of experimental samples before and after laser scribing were represented by core losses which were measured in AC magnetic field. The open samples in the form of strips were measured by the closed magnetic circuit with a symmetrical yoke. The hysteresis loops at 50Hz frequency were measured by fluxmeter based hysteresisgraph at the maximum magnetic induction of 1.5T. The total core losses in J/m³ were calculated as an area enclosed by the hysteresis loop.

The magnetic domains were observed by means of Bitter method which is based on the use magnetic colloid suspension Ferrofluid for magnetic domains visualization in the light optical microscope “Olympus Inverted System Metallurgical Microscopy GX-71”.

The most representative samples were chosen for the texture analysis which was carried out by an electron back scattered diffraction (EBSD) method. The data obtained were processed by the CHANNEL-5 software package.

The common microstructure and texture of the experimental GO steel which was taken from industrial line after final treatment is presented by Inverse Pole Figure (IPF) map as well as Orientation Distribution Function (ODF), see figure 2(a) and 2(b), respectively. Here, the EBSD analysis was carried out on sample surface perpendicular to its transverse direction (TD) and rolling direction (RD). As one can see, the material is characterized by primary recrystallized grain matrix with complete abnormal growing grains. The average grain size is approximately some millimeters in size. The IPF map as well as the ODF section taken at φ₂=0° show that the presented grain matrix is formed by the grains with preferable {110}<001> Goss crystallographic orientation. It means that the <001> direction of easy magnetization is mostly parallel to the sample RD.

The refined magnetic domains structures in the vicinity of laser affected zones by pulse and continuous processing are shown in figure 3(a) and 3(b), respectively. These images have been obtained by using the Bitter technique. The method based on the use of the colloidal suspension of iron oxide particles which are attracted by the external magnetic field and outlines the domain walls on the sample surface. Here, the width large 180° magnetic domain walls are in the range 10-50 μm and their oriented along the rolling direction of the sheet plane. The outgoing domains structures are refined by the irradiation spot in cases of pulse mode and irradiation line in case of continuous mode, see figure 3(a) and 3(b), respectively. New subdomains which are different from the magnetization direction of 180° main domains occur along laser irradiation spot or line. As one can see, complex domain structures are observed in the vicinity of laser scribed parts, wherein it can be assumed that degradation of domain walls takes place around the fast heated and cooled zones. The refinement of the magnetic domain structure after laser scribing is clearly visible, so it can be assumed that the eddy current component of the total losses will be lower after laser treatment.

The IPF and local misorientation maps as well as ODF section of sample cross-section microstructure with the thermal affected zone which was induced by laser beam in continuous regime with power of 24W is presented in figure 4. Here it is evident from IPF map and ODF section that laser affected zone is characterized by Goss crystallographic texture, see figure 4(a) and 4(b). The high intensity of η-fiber which is the most relevant fiber for GO steel show the excellent sharpness of {110}<001> crystallographic orientation around as well as inside irradiation spot without significant deviation, see figure 4(c). The comparison of Goss crystallographic texture sharpness measured in the samples before and after the laser beam treatment, clearly demonstrates that the laser scribing does not have a strong influence on the sharpness of the Goss texture. On the other hand, the local misorientation maps obtained during the EBSD analysis shows significant strains on the surface of irradiation zone, see figure 4(c). Direct mapping of the intragrain misorientation clearly shows the small thermal stresses associated with the laser scribing. Within the analyzed grain, the misorientation between the reference pixel and every other pixel is plotted using a color map from blue (0°) to red (4°). Small misorientations (0.1°-1° blue) represent small amounts of intragrain misorientation/lattice rotation and therefore little deformation. Large misorientations (2°-4° green and red) represent large amounts of intragrain misorientation/lattice rotation and thus large thermal stresses region. This indicates that laser beam with power of 24 W interacting with the steel surface induces only slight changes of substructure parameters in the place of contact and the texture of the sample before and after laser scribing is similar and characterized by high intensity of {110}<001> Goss component.

Precise estimation of the relation between the parameters of laser beam, number of laser scribing lines on the surface of samples and domain wall motion can be obtained through the measurments of magnetic losses in the AC magnetic field. The core losses change of the samples after application of laser scribing process at continuous and pulse regime is presented in figure 5. Here, figure 5(a) present the data on the effects of laser beam power in continuous mode and distance between the scribe lines perpendicular to the rolling direction. As one can see the samples which were scribed by laser beam with the distance between lines of 10mm are characterized by the lowest reduction of core losses, compared to the samples with shorter distances between laser scribe lines. The most significant reduction of core losses was achieved for the GO steel samples which were laser treated with distance between lines of 4mm. The data show that the best achieved core losses reduction of 38% (from 368 J/m³ to 226 J/m³) was obtained for the samples treated by 24W power of laser beam.

Figure 5(b) show the core loss data of the samples scribed by the laser beam in the pulse regime. In this case the most apropriate changes of core losses were achieved for the samples treated by laser beam with power values of 120W and 240W. The maximal core loss reduction of about 15% (from 368 J/m³ to 312 J/m³) was obtained for the samples treated by the laser beam with power of 240W and distance between the scribe lines of 4mm. As one can see, the results presented on this graph (see figure 5(b)) show the linear dependence of core lose reduction on the power of laser beam.

Magnetic losses measurements of the experimental samples after domains refinement demonstrate clear improvement of their magnetic characteristics for the both laser scribing techniques, compared with laser unaffected samples. The data show that the best achieved core losses reduction of 38% was obtained for the strips treated by laser processing using continuous regime. These results indicate significant improvements in comparison with the samples treated by discrete pulse regime and can be attributed to uniform refinement of 180° main domains by introduction of laser scribe lines.

The improvement of magnetic properties of GO silicon steels subjected to laser scribing is mainly attributed to magnetic domain refinement. These kinds of steels are characterized by microstructure with huge grain size resulting in the enlargement of magnetic domains. These large domains structures increase magnetic losses as the domain walls move back and forth under the action of alternating field. If the domain size is kept small, the walls do not move as far and less energy is lost in moving them. The laser scribing enables to reduce the average size of magnetic domain walls because during this process the laser beam introduces thermal stresses on the surface of the material. This thermal input is localized to a small spot or line on the surface.

Because the surrounding material is not affected, the heated area is constrained to react only by expanding out of the plane of the material surface. This rapid surface displacement launches a dilatational wave that propagates the stress distribution throughout the depth of the material. The resultant strain is believed to cause a slip in the plane that provides the new wall boundaries.¹² 

The magnetic domains refinement was performed by laser irradiation depending on its power density and scribe lines distances on the samples surface. The experimental results have clearly shown beneficial effect of laser scribing on the core losses reduction of GO steels. Both continuous and pulse laser scribing regimes were applied on the grain oriented steel strips. The optimal conditions of pulse and continuous laser scribing treatment resulted in core loss reduction from original 368 J/m³ to newly obtained 312 J/m³ and 226 J/m³, respectively. The energy of laser beam induces thermal shock which in combination with correct density of scribe lines may decrease the samples core loss. The texture analysis demonstrated that small spots of thermal stresses do not influence the outgoing crystallographic orientation. Summarily, performed investigations revealed that the best soft magnetic properties improvements for the pulse and continuous regimes with the same scribe lines distances were related to 15% and 38% core loss reduction, respectively.

This work was carried out within the project, which is supported by the Slovak Research and Development Agency under the contract No. APVV-15-0259. This work was also partially supported by the Slovak Grant Agency VEGA, project No. 2/0081/16, No. 2/0120/15 as well as COST Action MP1401. Also, the work was realized within the frame of the project ITMS 26220220037, ITMS 26220220061 and ITMS 26220220064.