Control Process And Mechanism Of Magnetic Treatment On Microstructures,Properties And Residual Stress Of Auto Tailor Welded Blanks

Automotive lightweight is the development trend of the vehicle,and also the national major need in the science and technology.Laser tailor welded blanks(TWBs)is one of the most important ways to achieve the automotive lightweight,which has been widely used in the vehicle industry.However,the microstructures and properties of weld seam are different to those of the base material,which seriously affect the mechanical properties and formability of TWBs.In addition,the maximum value of residual stresses in the weld seam and the surrounding region is close to the yield strength of the base material,which can reduce the size stability and the fatigue strength of body structures.In view of the above problems,the alternating magnetic treatment was proposed to regulate and control the microstructures,properties and residual stress of the auto TWBs.The main research contents are listed as follows.Based on the ANSYS electromagnetic field simulation module,the magnetic treatment model of the auto TWBs was developed,and the distribution of magnetic field intensity in the TWBs was then simulated.According to the results of finite element simulation,the experiment platform of magnetic treatment was established.This platform had the adjusting function of magnetic field intensity,frequency and time,which provided the test condition for the following researches.The process parameters of the magnetic treatment for auto TWBs were analyzed theoretically.The orthogonal experiment was carried out to verify the process parameters,which were finally determined as: 10000A/m of the magnetic field intensity,50 Hz of the frequency and 90 s of the time.The effects of magnetic treatment on microstructures and mechanical properties of auto TWBs were then studied by transmission electron microscopy(TEM)and uniaxial tension,etc.The results of microstructures observation showed that the martensite and ferrite in the TWBs were almost unchanged,while the dislocation density increased and the dislocation distribution was more uniform after the magnetic treatment.The results of mechanical tests showed that the strength of TWBs remained unchanged and the elongation was increased by about 11.13% after the magnetic treatment.Furthermore,after the magnetic treatment the hardness of weld seam was increased by about 7.21%,while the hardness of base material had little changes.The effect of magnetic treatment on the formability of auto TWBs was studied.Based on the Erichsen test,the formability of TWBs before and after the magnetic treatment was researched.The results showed that the formability of TWBs after the magnetic treatment was improved,that is,the IE value increased by about 6.94%.The reasons for the improvement of the formability of TWBs were then revealed from the view of the changes of dislocation morphology and residual stress.The effect of magnetic treatment on the residual stress of auto TWBs was studied.Based on the multiphysics simulation module of ANSYS,the distribution of welding residual stress of TWBs was simulated.The residual stresses in the “key points” before and after the magnetic field were measured by X-ray diffraction(XRD),and then the influence of the magnetic treatment on the residual stress of the TWBs was analyzed.The results showed that the welding residual stress concentration reduced after the magnetic treatment,the welding residual tensile stress along the welding direction in the high stress areas(weld seam and the surrounding region)had significantly decreased(about 23.5%~32.9%),while that in the low stress areas had little change.The absolute value of welding residual stress perpendicular to the welding direction changed little in general.The mechanisms of magnetic treatment on auto TWBs based on the dislocation dynamics was studied.According to the theories of dislocation,the long-distance stress field in continuous medium was analyzed and the stress field of dislocation core based on P-N partial discrete model was also obtained.The equation of total free energy of ferromagnetic materials under the action of magnetic field was established.Based on the thermodynamic law of the minimum total free energy of the material,the residual stress variation caused by the displacement of the domain wall and the rotation of the magnetization vector were analyzed.According to the theories of dislocation dynamics,the long-distance motion equation of dislocation,the critical condition equations of dislocation movement and multiplication under the magnetic field were established.The mechanism of magnetic treatment technology was then revealed.By combining the theories of ferromagnetism and dislocation dynamics,the regularity and mechanism of magnetic treatment technology were then revealed.This has important practical significance and theoretical value in improving the performance of auto TWBs and developing the technology of magnetic treatment.