| High-quality cut-to-length machining is essential to realize the wide range of applications of magnesium alloy sheets in the aerospace and rail transportation industries.However,the existing magnesium alloy sheet sizing and forming processing method processing efficiency is low,sawdust magnesium chip processing cost is high,and there is a hidden danger of combustion and explosion.The problem is becoming more and more prominent,and it is difficult to meet the quality of shear to improve and reduce the cost of controlling the effective synergy of the actual production needs.Given the many limitations of the traditional sizing process,ultrasonic vibration-assisted machining technology has the potential to solve the problems of poor sizing quality and high machining production costs.To this end,this thesis proposes an ultrasonic-assisted shearing processing method to improve the forming quality of magnesium alloy sheets.Carried out research on the shearing deformation characteristics of magnesium alloy sheets under different ultrasonic vibration-assisted shearing conditions and the macro-and micro-morphological characteristics of the sheared section.There is a quantitative correlation between the effect of ultrasonic vibration on the shear intrinsic characteristics of magnesium alloy plates and the corresponding material fracture behavior,microscopic deformation mechanism,and property evolution characteristics.The main research work and contents are as follows:(1)The shear deformation characteristic of magnesium alloy sheets under conventional shearing process was analyzed.By carrying out the conventional warm shearing experiment of AZ31 magnesium alloy sheet after rolling under different conditions,analyzed the influence law of different process parameters on the burnish zone height and burnish zone area percentage of the sheared section,explored the evolution law of the quality of the sheared section under the interactive influence of the sheet temperature and the shear edge clearance,and used simulation to clarify the key factors affecting the quality of the sheared sheet section by adopting the orthogonal design method,in order to provide a reference basis for the further optimization of the magnesium alloy sheet cut-to-length processing technology to provide reference basis.The study shows that the sheet temperature has the most significant effect on the section quality of magnesium sheet after shearing,followed by the shear edge clearance.Under the condition of warm shearing,150℃ sheet temperature and 12% shear edge clearance,the section of magnesium sheet after shearing has the largest burnish zone area,and the section quality is better.(2)The ultrasonic vibration-assisted shearing process method was proposed,and an experimental platform was constructed,consisting of an ultrasonic vibration-assisted shearing system,shear edge fixing and adjusting system,shear force measuring system,fixed bracket molds,and other parts.The experimental platform could adjust the ultrasonic amplitude and shear edge clearance.By applying ultrasonic vibration to change the contact pattern between the shear blade and the sheet,so as to realize the purpose of improving the quality of shear forming and reducing the processing load.The trajectory of the shear blade under the action of ultrasonic vibration was deeply analyzed,and the equipment for the ultrasonic vibration-assisted shearing process was selected and designed.The study shows that the critical shear speed threshold can be satisfied to realize periodic,intermittent contact and separation between the shear blade and the sheet.Based on the required shear resistance of a ranging from 1 mm to 10 mm magnesium alloy sheet,a 2600W-20 k Hz ultrasonic generator and corresponding type of ultrasonic transducer are selected.Based on the half-wavelength principle,the lengths of the ultrasonic horn and the upper shear blade were calculated to be129 mm and 120 mm,respectively.In the meantime,the results of the modal analysis showed that the designed ultrasonic vibration-assisted shear machining system efficiently transmits and amplifies the ultrasonic vibration.(3)The influence of process parameters on the distribution and transformation of equivalent stresses of magnesium alloy sheets under an ultrasonic vibration-assisted shearing process was explored,and the shear deformation characteristics of magnesium alloy sheets were analyzed.Based on the vibration-thermal coupling finite element method,a converter connecting the reference point of shear displacement and the reference point of ultrasonic vibration is constructed,and a three-dimensional analog simulation model of ultrasonic vibration-assisted shear processing is established by coupling the plasticity intrinsic model of magnesium alloy sheet and the damage failure model and by adopting the double-precision control method to delineate the mesh of the area affected by shear deformation.The distribution law of equivalent stresses at the same moment and the evolution characteristics of equivalent stresses at the same position of magnesium alloy sheets under conventional shearing processes and ultrasonic vibration-assisted shearing processes are analyzed.The study shows that the stress status of magnesium alloy sheet under ultrasonic vibration in shear deformation near the inhomogeneous deformation zone was significantly improved,the maximum equivalent stress and the average equivalent stress were reduced compared with conventional shearing,and the trend of the equivalent stress change appears to be the superposition of stress and stress fluctuation.In addition,the delay in the time to reach the maximum equivalent stress means that the elastic-plastic deformation stage of the sheet is prolonged,and the time of the crack extension is increased.Based on the experimental results,the optimized behaviors of ultrasonic vibration on the maximum equivalent force and average equivalent force of magnesium alloy sheets under different conditions were further quantified,providing quantitative theoretical guidance for the optimization of a high-quality and low-cost ultrasonic vibration-assisted shearing process for magnesium alloy sheets.(4)The synergistic effect of process parameters on the quality of ultrasonic vibration-assisted shear processing was investigated,and the mechanism of ultrasonic vibration on the organization and properties of magnesium alloy sheets was elucidated.Based on the above experimental platform,ultrasonic vibration-assisted shear processing experiments of AZ31 magnesium alloy sheet were carried out.With the help of OM,SEM,microhardness tester,pressure sensor and other equipments and technologies,the geometric regional distribution characteristics of magnesium plate section after shearing under different process parameters,macro fracture morphology characteristics,as well as microstructural characteristics near the fracture,microhardness distribution characteristics along different directions and the characteristics of the processing of the whole process of the change of the shear force were analyzed.The main control methods(crack initiation and extension,fracture mechanism,work-hardening behavior of the fracture,dynamic recrystallization nucleation,and growth)and their laws of action for the effective improvement of the shear quality of magnesium alloy sheets were investigated.The study shows that the processing quality of magnesium sheet after shearing under high amplitude ultrasonic vibration is the best.In addition,after applying ultrasonic vibration,the fracture behavior of the magnesium sheet underwent a tough-brittle transition,the microstructure grain near the fracture is obviously refined,and the maximum microhardness position is shifted downward,so the ultrasonic vibration effectively inhibits the internal cavity sprouting and crack expansion of the sheet,and the elasticity-plasticity deformation stage is prolonged,which is conducive to the improvement of the quality of the sheared section.The mathematical response model and shear quality prediction model between multivariate process parameters and quality evaluation indexes(rollover width,burnish height,and maximum shearing force)were established based on the mechanism of the influence of process parameters on the quality of the shearing process,and the optimal process parameter solutions that can satisfy the optimization conditions were finally obtained.In summary,this paper adopted ultrasonic vibration-assisted shearing method to obtain magnesium alloy sheet with excellent processing quality,clarified the influence mechanism of ultrasonic vibration on the macroscopic fracture behavior and microscopic deformation mechanism of magnesium sheet shear,analyzed the synergistic effect law of the process parameters on the shear quality indexes(dimensional accuracy,section quality,and processing load),provided technical guidance for the optimization of the cut-to-length shaping and processing process with the purpose of improving the shear quality of magnesium alloy sheet,and offered an important theoretical basis and practical significance of the project for the expansion of magnesium alloy sheets for the application scope of the lightweight molding components in the fields of automobile manufacturing and aerospace in a large market scale. |
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