| With the rapid development of micro-electromechanical system and microsystem technology,higher performance requirements are put forward for sheet materials used in microforming.The blank for micro-forming should have good plasticity and a microstructure that is conducive to improving the mechanical properties,quality and precision of microparts.The commercial sheet usually exists large work hardening and anisotropy caused by severe rolling deformation,and the poor coordinated deformation ability among grains,which will affect the forming quality and mechanical properties of the microparts.Therefore.the performance improvement and microstructure control of sheet metal have become one of the key basic issues for the development of micro-forming technology.Severe plastic deformation process and the special energy field electropulsing treatment process have significant characteristics in terms of grain refinement and improvement of comprehensive mechanical properties,respectively,and are suitable for micro blanks to improve comprehensive properties and microstructure.However,most of the above-mentioned processes in the previous studies have focused on macro-scale materials,and there is a lack of research on related processing technology for micro-scale materials.Moreover,it lacks the in-depth research on process characteristics,size effect,dislocation configuration,microstructure evolution and mechanism.Therefore,put forward suitable for micro-forming blanks deformation method for grain refinement and electropulsing treatment,and studies the influence of process methods on the microstructure and properties and reveal the corresponding mechanism.This provides a new process reference for the pretreatment of micro-materials before micro-forming and promotes the application of micro-forming technology has important theoretical significance.In this paper,pure copper and copper alloy sheets were the research objects,with the goal of improving the microstructure and the mechanical properties,studied the process and its mechanism of deformation method for grain refinement and electropulsing treatment.The main research work of the thesis is as followsBy adopting the rolling method and design the tooth structure of the roll bending wheel,a cumulative plastic deformation method suitable for sheet metal strips—repetitive roll bending straightening method(RRBS)was proposed.The finite element method was used to analyzed the influence of rolling groove,tooth profile structure and teeth number of the rolling wheel on the blank forming quality,the effective strain accumulation and the uniformity of roll bending deformation.Numerical simulation shows that the deformation of the sheet metal can be accumulated continuously through multi-pass RRBS,but the deformation fluctuates periodically along the feeding direction of the blank with each tooth roll bending length as a unit.In order to effectively improve the uniformity of RRBS deformed sheets,reduce the strain concentration of samples and increase the number of deformation passes,a method of staggered roll bending for blanks was proposed.That is,when the blank is rolled between adjacent passes,the position is staggered by a certain distance with reference to the length of a tooth profile unit.Based on the analysis of the numerical simulation results of the RRBS process,the deformation device suitable for thin sheets of different thicknesses was designed and processed.Multi-pass RRBS experiments of pure copper was carried out,the influence rules of the pass number on mechanical properties variation of pure copper sheet was investigated.RRBS process can obviously refine the grains of copper sheet samples,and the composite treatment of RRBS process combined with recrystallization annealing was proposed,which eliminates work hardening and improves the plasticity of the sheet while making the grain further refined.Combined with the characteristics of RRBS process,the grain refinement mechanism of RRBS and recrystallization annealing was analyzed.Multi-parameter combinations such as current density,frequency,pulse width and duration were formulated to studied the influence of electropulsing on the mechanical properties and microstructure of RRBS copper sheets.Under proper pulse width conditions,current density and frequency as parameters that can change the pulse current energy instantaneously,have a significant effect on the mechanical properties of materials.Electropulsing significantly improves the plasticity of the RRBS deformed copper sheet,relieves the work hardening,and promotes the recrystallization of the material at medium and low temperatures to obtain a fine and uniform structure.The uneven distribution of sample temperature caused by Joule heating,thermal stress,and microstructure evolution caused by sudden temperature changes are important factors that affect the mechanical properties stability of samples.In order to improve the plasticity of cold-rolled copper sheets for micro-forming,the multi-scale copper sheets were treated with pulse current to improve plasticity Electropulsing can significantly improved the plasticity of cold-rolled sheet,and plasticity improvement effect was better than that of heat treatment.At the same time,it is found that cold-rolled sheet treated with electropulsing reflects the size effect phenomenon.With the decrease of the sheet thickness,the strength shows a "the smaller the stronger",but the electropulsing significantly weakens the size effect caused by grain sizes,and changed the Hall-Petch slope,making it lower than samples at room temperature without electropulsing treatment.Electropulsing has threshold effect on the mechanical properties of cold rolled copper sheet,and the effect is not only reflected in the current density,but also in the frequency and pulse width.The threshold is influenced by crystal defects such as grain boundary surface area and dislocation density.The contribution and action mode of the athermal effect of electropulsing to the plasticity improvement and structure change were confirmed and analyzed.Based on the impact of electropulsing on dislocations,the mechanism of electropulsing to improve the plasticity of copper sheet was obtained by analyzing the distribution of dislocations at grain boundaries,subgrain boundaries and in the grain interiors.Based on the high-efficiency electropulsing treatment method and its good mechanical properties improvement on pure copper sheet,it is proposed to directly use high current density electropulsing for aging treatment of Cu-Ni-Si alloy sheet for large-scale integrated circuit lead frame.Electropulsing promotes Cu-Ni-Si alloy to precipitated out dispersed nano-scale strengthening phases ?-Ni2Si and ?-Ni3Si,and the strength and electrical conductivity have been significantly improved in a short time while maintaining a lower temperature.Through the TEM and HRTEM observation and analysis of the precipitated phase,the mechanism of Cu-Ni-Si alloy precipitated phase promoted by electropulsing was revealed.Coupled physical effects caused by electropulsing promote the migration of vacancies,and the exchange of kinetic energy between drift electrons and solute atoms(Ni and Si)in copper matrix enhances the diffusion ability of solute atoms,and reduces-the energy barrier which should be overcome during the precipitation process,thereby accelerating the formation of the precipitate phase. |
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