| In the whole life cycle of metal materials from preparation,forming to processing into components and assembly and service,there will inevitably always be residual stress inside.However,residual stress is a potential,negative and destructive factor in most cases.It is also a hidden defect that becomes more and more serious with the increase of material strength.At present,there is still a lack of fast,effective and green residual stress reduction methods.Therefore,it is of great significance to carry out the research on new methods,new technologies and new devices for stress reduction of ferromagnetic materials.Aiming at a new method magnetic-vibration stress relief to reduce residual stress proposed by the research group,this dissertation develops a magnetic-vibration stress relief experimental system platform,studies the evolution of residual stress,material properties and microstructure in the process of reduction,reveals the coupling mechanism and enhancing effect mechanism of magnetic-vibration stress relief from different scales,and studies the use and effect of magnetic-vibration stress relief method,Systematically and deeply study the behavior and law of enhancing effect of this new method.The main research work and results of this dissertation are as follows.(1)An experimental study was carried out on the feasibility and effectiveness of the new method of magnetic-vibration stress relief to reduce residual stress.The results show that the reduction effect of magnetic-vibration stress relief is better than that of single magnetic treatment or vibration treatment,and has a"1+1>2" enhanced reduction effect;The tensile properties,hardness,fatigue properties,electrical conductivity and magnetic properties of silicon steel after magnetic treatment,vibration treatment and magnetic-vibration stress relief are systematically measured to analyze their influence laws.The research results show that different methods have little negative impact on various properties of materials,and even the properties of individual materials have improved after magnetic-vibration stress relief;At the same time,it is found that high-frequency pulse stretching has a similar reduce effect on the material;The measured analysis results and mechanical analysis of the material show that the magnetic-vibration stress relief process mainly depends on the pulsating tensile action to make the material produce local micro plastic deformation,so as to reduce the stress.Based on the fatigue yield and failure theory of metal materials,the hypothesis of residual stress reduction mechanism and homogenization mechanism of "local micro plastic deformation" caused by "material softening" and "deformation superposition" of magnetic-vibration stress relief is put forward.(2)The microstructure of the materials treated by various methods are measured and analyzed,and the mechanism of reducing residual stress by magnetic-vibration stress relief and the reasons why it affects the properties of the materials are revealed.The results show that after different treatment,the grain size of silicon steel increases slightly,the energy of Goss texture increases,the brittle phase decreases,the distribution of Si elements is uniform,the grain boundary narrows,and the dislocation density increases in different degrees,which may be the reason for the change of mechanical properties of the material;Among the different methods,the magnetic-vibration stress relief has the most significant effect on the dislocation,but the energy provided by the treatment is weaker than that of thermal treatment;The magnetic domain structure of the materials treated by different methods has changed.The magnetic-vibration stress relief can significantly increase the parallel domain and reduce the labyrinth domain,which may lead to the change of the electromagnetic properties of the materials.The pulsating tensile stress synchronized with the vibration can also change the magnetic domain structure;Although the applied load level is low compared with the material strength,the material still has local micro and even mesoscopic plastic deformation during magnetic-vibration stress relief process,which is consistent with the macro size measurement results.The correctness of the stress reduction mechanism caused by local micro plastic deformation of magnetic-vibration stress relief is verified.(3)The enhancement mechanism of the residual stress reduction effect of magnetic-vibration stress relief is revealed through the experimental study and the first-principles calculation and analysis.The experimental study found that the ferromagnetic materials with different structures all have the stress-magnetic coupling effect in which the magnetic induction intensity increases with the increase of tensile stress and decreases with the increase of compressive stress;Based on the first principle calculation and analysis of the relationship between the stress and atomic magnetic moment of Fe crystal and DO3 crystal,it is concluded that the atomic magnetic moment of the two crystal systems increases with the increase of tensile stress and decreases with the increase of compressive stress,and the atomic magnetic moment responds more strongly to the tensile stress.The results of "force enhanced magnetism" are qualitatively consistent with the results of experiments;At the same time,it was found by calculation that the changes of energy band and electronic density of states both showed that tensile stress can make electrons approach the Fermi level and enhance the activity of the atomic system.Under the action of electromagnetic field,it can enhance the possibility of micro atomic migration and dislocation evolution,showing the stress-magnetic coupling effect of "magnetic strengthening force";The two-way strengthening stress-magnetic coupling effect of the deformation force and the magnetic field determines that the magnetic-vibration stress relief has a "1+1>2"residual stress reduction and enhancing effect compared with vibration or magnetic treatment.(4)Based on COMSOL finite element software,the multi physical field coupling simulation models of macroscopic and mesoscopic scale magnetic-vibration stress relief process were established respectively.The effects of treatment parameters,part material properties,size and grain characteristics on the residual stress reduction effect are systematically studied,the reduction and enhancement effect of magnetic-vibration stress relief is quantitatively revealed,and the hypothesis of residual stress reduction mechanism is indirectly proved.The simulation results show that when the electromagnetic field acts vertically on the plate and applies tensile stress,reducing the magnetic frequency and increasing the magnetic voltage or material magnetostrictive coefficient or exciting force or exciting frequency will improve the effect of reduction;The larger the grain size and the narrower the grain boundary of ferromagnetic materials,or the more the grains with high magnetic properties,the greater the reduction and enhancement effect can be obtained by magnetic-vibration stress relief.It shows that the magnetic-vibration stress relief method and its enhancement effect are closely related to the magnetic properties of parts;The simulation reproduces the enhancing effect of "1+1>2" of magnetic-vibration stress relief.It is found that when the magnetic frequency is the same as or integral multiple of the vibration frequency,there is a greater enhancing effect,and the simulation results show that the magnetic-vibration stress relief has the enhancing effect of "1+1≈2.78";Through strain measurement and simulation analysis of cycle period,magnetic frequency and excitation frequency,it is proved that there is material softening in the process of magnetic-vibration stress relief,which supports the mechanism hypothesis of residual stress reduction in this dissertation. |
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