| As the key component of electrical machine,the magnetic properties of the stator core has drawn increasing attention from the industry.Electrical steel is the key material of stator,and the punched and laminated electrical steels are clamped and welded to form the stator.As laser welding has highly concentrated heat input and small welding deformation,it is one of the most potential method replacing the traditional TIG welding to join the electrical steel laminations.However,in laser welding of laminated electrical steels,interfaces between the steels hinder the heat transfer through the interface,and it is difficult to be estimate the weld bead characteristic and residual stress accurately.Meanwhile,the residual stress is induced into the electrical steel by the welding process,and the kinetic characteristic of the magnetic domain in the electrical steel is changed by the residual stress,it is difficult to estimate the magnetic hysteresis of the welded electrical steels.Besides,the insulation coating of the electrical steel in the weld seam zone is destroyed by welding,and eddy current distribution in the welded electrical steels is anomalous,it is difficult to estimate the eddy current losses.To overcome above issues,it is meaningful to conduct the research on heat transfer in laser welding of laminated electrical steels and magnetic properties degradation,obtaining the influence rules of interface on heat transfer and heat input on magnetic properties degradation,reducing the magnetic properties degradation caused by welding process.This dissertation focuses on laser welding of laminated electrical steels.The heat transfer in laser welding process and the magnetic properties degradation mechanism are researched with the methods of experiment,mathematical model and finiete element model(FEM).The detailed research procedure is as follows: at first,the laser welding equipment,torsion strength measurement equipment and magnetic properties measurement equipment are set up,and effects of welding parameters on the integrated performance of the stator are researched by experiments;secondly,the method to characterize the effect of interface on heat transfer in laser welding of laminated electrical steels is put forward,and the FEM is built to analyze the heat transfer in laser welding of laminated electrical steels;thirdly,the thermal-mechanical coupled FEM is built to estimate the residual stress in the welded laimnations,and the mathematical model to estimate the magnetic hysterisis property of the welded laminations is setup;fourthly,the mathematical model considering the characteristic dimension of the weld bead is built to estimate the eddy current losses of the welded laminations;at last,the mathematical model to estimate the torsion strength of the welded laminations at different welding parameters is built for industry application,and the critical weld bead area that met the torsion strength requirement is determined,providing a guide for choosing the proper welding parameters.The detailded content in this dissertation is as follows:1)Experimental investigation on the quality of laser welded electrical steel laminations at various parametersThe laser welding equipment,torsion strength measurement equipment and magnetic properties measurement equipment are set up.And the effects of laser welding parameters on the weld bead characteristic,microstructure,torsion strength and magnetic properties of the welded non-oriented B27ahv1500 electrical steel laminations are investigated by experiments.The experimental results show that with the increase of laser heat input the weld bead area and the torsion strength increase,while the magnetic properties deteriorate.The electrical steels welded at the speed of 10 mm/s have an increasement of 23.9 percent in iron loss compared with that of the loose laminations.2)Heat transfer model for laser welding of laminated electrical steelsIn laser welding of laminated electrical steels,the temperature distribution is discontinuous because of the hinderation of the interface.To characterize the effect of interface on heat transfer,the method to describe the thermal contact conductance of both the existing and vanished interface is put forward,and the FEM is built to analyze the heat transfer and temperature distribution in the welding process.Effects of sheet thickness and thermal contact conductance on the dimension of the weld bead are investigated.The result shows that the dimension of the weld bead decreases with the increase of sheet thickness.Compared to the weld bead area of 0.18 mm sheet,the weld bead area of 0.54 mm sheet has a decrase of 11 percent.3)Model to estimate the magnetic hysteresis property of the welded electrical steel laminationsThe thermal-mechanical coupled FEM is built to estimate the residual stress in the welded laimnations,and the accuracy of the coupled model is validated by measuring the residual stress in the welded specimen.The combined optimization algorithm is set up to determin the parameters of the magnetic hysterisis property model,which is used to describe the magnetic hysteresis property of the electrical steel without residual stress.And the mathematical model to estimate the hysterisis property of the welded laminations considering the residual stress distribution is setup,the accuracy of this model is validated by measuring the magnetic hysteresis property of the welded specimen.The result shows that the magnetic properties deteriorate with the increase of residual stress.4)Model to estimate the eddy current losses of the welded electrical steel laminationsThe section through the weld bead is divided into weld bead zone and base metal zone,and mathematical model considering the characteristic dimension of the weld bead is built to estimate the eddy current losses in the welded laminations.The FEM is also built to visualize the eddy current distribution in the welded laminations,validating the accuracy of the eddy current losses estimation model.Effects of sheet thickness,magnetic flux density,working frequency and dimension of the weld bead on the eddy current losses of the welded laminations are investigated.The research results show that the eddy current losses increasement caused by the welding process increases with the increase of the weld bead dimension,and the decease of the electrical steel thickness.5)Critical weld bead area meeting the torsion strength requirementWeld bead area has a significant effect on the integrated performance of the welded stator,too small weld bead area couldn't meet the torsion strength requirement,while too large weld bead area could lead to significant magnetic properties degradation.In this section,the mathematical models to estimate the weld bead area and the torsion strength of the welded laminations at different welding parameters are built for industry application,reducing the number of destructive experiments.And the critical weld bead areas for the stator with different specifications are determined from the torsion strength estimation model,providing a guide for choosing the proper welding parameters.In summary,the present dissertation has studied the heat transfer in laser welding of laminated electrical steels and the mechanism for magnetism degradation caused by welding process,and the critical weld bead area that met the torsion strength requirement is obtained.This dissertation provides a guide for designing the laser welding process for stator. |
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