| The Flow drill screw(FDS)riveting has been widely used in all-aluminum and steel-aluminum hybrid bodies due to its advantages of high mechanical strength,wide material adaptability,and unilateral connection.However,the current research in domestic and foreign about FDS process is still focused on the joint mechanical properties analysis and process parameters optimization,and there is little literature to systematically analyze and study the forming process of the FDS joint.In this thesis,the DP590 high strength steel/6061-T6 aluminum alloy FDS joining was taken as the research object,the simulation model of the joint forming based on the remeshing method was established and verified,and the temperature distribution,stress distribution and material flow law in forming process of the joint were analyzed.Further,the microscopic structure and mechanical properties of the joint s in FDS process were studied by combining the simulation model and profile test,hardness test,EBSD test and quasi-static tensile test.The research could provide theoretical basis and guidance for the using of FDS riveting in steel/aluminum dissimilar metals joining of the automotive industry.Firstly,models of the FDS joint forming based on the mesh deletion method,Euler method and the remeshing method were compared in this thesis,the results showed that the remeshing model could simulate forming process of the FDS joint most accurately.In the remeshing model,the temperature error is 2°C,the maximum temperature error is 12°C,the pressure error at the peak value is 19.3%,and the ratio of the bushing height and the petal height between the simulation and the experiment is 1.02 and 1.05,respectively.Subsequently,the temperature and stress distribution in the joint forming process were analyzed based on the process model in this thesis.The results showed that the temperature and stress distribution in the thread area of the sheet is uniform,which is beneficial to avoid the thread forming defects.And when the drilling speed at5000r/min,the peak temperature of the sheet is 387 °C,but duration and influence range of the above high temperature are limited to the internal thread area of the sheet.At the same time,the point tracking analysis of the upper,middle and lower materials of 6061-T6 aluminum alloy sheet was carried out.It is found that during the forming process of the bushing and the petal,the sheet material undergoes severe plastic deformation under the shear force of the screw,and finally the upper material forms the bushing and the middle and lower material forms the petal.Finally,the microscopic structure and mechanical properties of the joints under different drilling speeds were studied in this thesis.The results showed that good thread engagement could be formed between the screw and the sheet at the drilling speed of3000?8000r/min,but a small thread gap between the screw and the sheet were formed at low drilling speed.When the drilling speed at 3000?5000r/min,obvious recrystallization softening occur red at the tip of the internal thread of sheet,caused the hardness value decreases.And work hardening occurred at 500?m from the tip of the internal thread of sheet,caused the hardness value increases slightly.But when the drilling speed was 8000r/min,the recrystallization softening range extend ed to 500?m from the tip of the internal thread of sheet.Further,comparing the mechanical properties of the joints under different drilling speeds,it is found that the above differences in the morphology and hardness distribution have little impact on the joints mechanical properties. |
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