| Silicon steel is the Fe-Si soft magnetic alloy containing low Si content and is the base material of electric power and telecommunication. It is well known that increasing the Si content improves the soft magnetic properties of silicon steel. However, as the Si content is increased, the material becomes extremely brittle and it is difficult to produce thin sheets by conventional rolling. Several methods have been developed to obtain high silicon steels including special thermomechanical rolling, rapid solidification, spay forming and chemical vapor deposition, etc. But each method has its limitations. Laser cladding technique aims at obtaining high performance alloy coatings on steel substrates, in which adhesion is obtained by surface melting of the substrate. In this process, a thin surface layer of the substrate is melted by the laser beam together with the additive or preplaced material to form the coating. The process has received a lot of attention over the years and is now applied commercially in a range of industries such as the automotive, mining and aerospace. The main advantages of laser cladding compared with the processes mentioned above are that the source of the energy can be localized and the size of laser beam can be controlled precisely. Moreover, the higher solidification rates induce a fine microstructure consisting of crystalline or amorphous phases and with an extended solid solubility of key alloying elements. The aim of the present work is to improve the Si content of the surface of silicon steel. The new method of laser assisted siliconizing is to laser clad a well-qualified high Si coating on the low silicon steel surface and then to prepare a high silicon steel or high si gradient silicon steel through the subsequent diffusion annealing treatment. It is a new potential technique to produce high silicon steel.The investigations of design of cladding material, exploration of the processing parameters and optimization of the parameters were carried out in the present work. Characterizations of surface morphology, microstructure, phase constituents, chemical composition and microhardness were carried out by using the optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and Mossbauer spectroscopy (MS). Based on the Miedema's formation heat model for binary alloys and the Toop's asymmetric model for ternary alloys, the formation heat, excess entropy and activity coefficients of Si from 1 900K to 4 100K in the Fe-Si-C melt formed during the laser cladding high Si coatings process were calculated. The exploration of the subsequent diffusion annealing treatment was carried and the magnetic properties of the laser cladding specimens after annealing were measured by vibrating sample magnetometer (VSM).During the single-track and multi-track laser cladding mixed powders of Fe and Si on low silicon steel surface, the laser energy and the laser scanning speed are found to the the key factors affecting the cladding process. When the laser energy is lower, the non-metallurigical bonding formed between the cladding coating and the low silicon steel substrate and there are a lot of cracks in the coatings. When the laser energy is higher, the surface oxidation of laser cladding coating is easy to occur and the dilution is high. The increase of laser scanning speed is helpful for the refinement and uniformity of cladding microstructure. At the same time, the thickness of laser cladding coating decreased and the dilution rate is also decreased. As for the preplaced powder bed with different thickness, the specific laser energy exists to produce a pore and crack-free coating with dense microstructure and high Si content. The higher specific laser energy causes a severe burning loss and is not helpful for obtaining the smooth cladding coating. The dilution at higher specific laser energy is also high. When the specific laser energy is lower, the energy for forming the metallurgical bonding between the coating and the substrate is not satisfied.By means of optimized single-track and multi-track laser cladding, a crack-and pore-free coating through excellent metallurgical bonding with the substrate was successfully prepared on the low silicon steel. After cladding, the surface of silicon steel could be divided into the cladding zone, the interface zone and the heat-affected zone (HAZ). The interface zone between the cladding zone and the substrate is a planar front solidified layer, epitaxially grown from the substrate. The cladding zone forms after rapid solidification of the molten cladding materials. In the cladding zone, the microstructures along the interface-to-top surface of the coating exhibit vertically aligned columnar and dendritic microstructures, which indicate that a vertical temperature gradient as heat is conducted away from the melt pool towards the substrate. And there are disoriented and finer dendrites near the top surface. In the cladding coating, the microstructure along the interface-to-top surface of the coating is not uniform and that of same highness has no obvious change and the microstructure is uniform. The HAZ near the substrate is characterized by a fine acicular martensite and the substrate still keeps ferrite microstructure. The secondary scanning occurs obviously during the multi-track laser cladding process and the microstructure of the overlapping zone is coarser and disoriented.The results of Mossbauer spectroscopy show that there are two hyperfine structures in the cladding coatings prepared under the optimized process parameters. One is the Fe-Si coating containing Si as high as 9.1wt.%. In this coating, the relative content of the a-Fe solution, the Fe-Si compounds and the y-Fe solution are 69.6%,13.0% and 17.4%, respectively. The main phase containing Si element of this coating is existing in A2 type (body-centered cubic structured) disorder solid solution ofα-Fe. The other is the Fe-Si coating containing Si as high as 15.20wt.% in which the relative content of the Fe3Si, the Fe-Si compounds and the y-Fe solution are 93.9%,5.8%and 0.3%, respectively. The main phase containing Si element of this coating is DO3 type (face-centered cubic structured superlattice) order solid solution of Fe3Si.The Si content of laser cladding zone is much higher than that of silicon steel substrate and the change of Si content is not obvious. And the Si content is decreasing in the interface zone and the HAZ. The change of microhardness is the same as that of Si content. The microhardness of the cladding zone is much higher that of the substrate as a result of the effect of solid solution hardened of Si atom and the refinement of the microstructure.During the laser cladding process, the Fe-Si-C melt formed. The iso-activity lines of Si distribute axisymmetrically to the incident laser beam in the melt pool vertically to the laser scanning direction. And the iso-activity lines of Si in the front of the melt pool along the laser scanning direction are more intensive than those in the back of the melt pool. The activity of Si on the bottom of the melt pool is lower than that in the effecting center of laser beam on the top surface of the melt pool and it may be the important reason of the formation of the silicides and excellent metallurgical bonding between the laser cladding coating and the substrate.The laser cladding specimens of Si content of 15.20wt.% were diffusion annealed at 1100℃for different times. The high Si gradient silicon steel with single phase ofα-Fe(Si) solution solid was obtained and the microhardness and Si content are distributed gradiently at the cross-sections of the specimens. The specimens as clad and as annealed are ferromagnetic at room temperature. The Si content of the surface of specimen as clad and annealed at 1100℃for 6 hours is 5.94wt.% and the Si content of the core of it is 3.68wt.%. And the DC magnetic property of specimens as clad and annealed at 1100℃for 6 hours is better with the specific saturation magnetization of 235.06emu/g and the coercivity of 1.5395G than the original low silicon steel. |
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