| Laser shock micro-forming technology is a new micro-forming technology.Due to the limitations of experimental methods,it is difficult to observe the evolution of micro-structure during forming process.Therefore,molecular dynamics method is used to study this subject: Atomsk software is used to build the quasi three-dimensional models of copper;LAMMPS is used to relax the models and to load the deformation,then output the simulation results;and the simulation results are visualized by OVITO.The evolution process of micro-structure and the formation mechanism of nano-structures of copper under high strain rate loading are systematically studied.The main contents and achievements of this dissertation are as follows:(1)The method of molecular dynamics simulation is studied,and the basic process of simulation is determined.The basic parameters of system initialization,setting of geometric and numerical conditions are discussed,and the selection of appropriate algorithm to solve Newton equation,obtaining parameters such as particle position and velocity are studied.Then,the post-processing of simulation results,obtaining macro physics and visual analysis are discussed.(2)The microstructure evolution of nano-polycrystalline copper under high strain rate loading was studied by molecular dynamics simulation and compared with the previous experimental results,the results show that:1)In the early stage of loading,the plastic deformation is dominated by dislocation.With deformation going on,the slip systems in the grains are activated in turn,and dislocations move along the slip planes.When the stress is concentrated,there will be secondary slip and cross slip.There is also alternation of slip systems in grains;2)Dislocations can form sub-grain boundaries,and sub-grains can be transformed into fined grains by dynamic recrystallization.In previous studies,the effect of dislocation was ignored because the deformation mechanism was inversely deduced from the samples after deformation.According to the experimental results and simulation results,dislocation movement is also an important reason for the formation of nano-grains;3)The formation mechanism of nano-twin bundle is revealed.This special structure is transformed from parallel intrinsic stacking faults,and the parallel stacking faults are expanded into nano-twin bundle by dislocation movement;formation and extinction of twins are simultaneous.(3)The micro-structure evolution of twinning-polycrystalline copper under high strain rate loading is studied by molecular dynamics simulation,the results show that:1)In the quasi three-dimensional model,the dislocations mainly slip in two ways: soft mode and hard mode I.Frequent soft mode slip can cause twin fusion;there are three modes of dislocation formation in nano-twin copper;2)Nano-twin can provide work hardening effect and maintain stable plastic deformation ability at the same time.(4)The micro-structure evolution of nano-twin copper under tension and compression loading with different angles(0 °,15 °,75 °,90 °)was studied by molecular dynamics simulation,the results show that:1)Under the same strain rate and strain,the change of micro-structure under compression loading is more severe than that under tension loading.Dislocations are easier to emit and secondary twins are easier to form;2)The larger the angle between the loading direction and the twin,the easier it is to form stable secondary twin structure;3)The soft and hard mode of dislocation slip can be transformed into each other by cross slip;secondary twinning can cause the phenomenon of twinning intersection.In this dissertation,the evolution process of micro-structure and the formation mechanism of nano-structures of copper foil under laser shock micro-forming are studied at atomic level,and the previous research conclusions based on experimental research are supplemented and improved,which provides theoretical support for the effective control of micro-formity and the realization of high quality micro-forming. |
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