Cyclic Mechanical Behavior Of Pure Copper With Different Passes Under Strain Control

In recent decades,copper has been widely used in the electronics industry,transportation,construction industry and aerospace.However,with the complexity and harshness of the actual working conditions,the performance requirements for copper are also getting higher and higher,especially regarding its strength and hardness.The technology of equal channel angular pressing can make large plastic deformation of the material without changing the shape and size of the material,refine the grain,improve the distribution of the structure,and significantly improve the comprehensive mechanical properties of the material.As a part or component,copper will inevitably bear the cyclic stress/strain effect.In order to ensure the safety performance of the material,the mechanical behavior of copper under cyclic loading must be studied accordingly.Therefore,this paper takes industrial pure copper T2 material as the research object,and different passes of pure copper materials are prepared by equal channel angular pressing test.The comparative analysis of different passes of pure copper under symmetric strain control and asymmetric strain control cyclic loading conditions cyclic mechanical behavior,and the EBSD microscopic characterization technique was used to analyze and discuss the microstructure changes before and after the 8 pass of pure copper cyclic loading.The main research contents and innovations of the thesis are as follows:1.The cyclic loading experiments of different passes of pure copper under symmetrical strain control were carried out,and the cyclic characteristics under different strain amplitudes were compared and analyzed.Including cycle stability hysteresis loop,shape factor and Bauschinger effect,plastic strain amplitude change,Massing characteristics and cyclic soft hardening phenomenon.It is found that the uncompressed pure copper sample has the best stability hysteresis curve under different strain amplitudes,and plastic deformation can be produced under the control of small strain amplitude(0.1%).And as the strain amplitude increases,the corresponding shape factor increases gradually,and the Bauschinger effect becomes weaker.However,the hysteresis curves of other pure copper at different strain amplitudes show different degrees of intersection,and as the strain amplitude increases,the corresponding shape factor decreases first and then increases,and the Bauhinth effect first increases and then weakens.2.EBSD characterization technique was used to analyze and discuss the microstructures of 8-pass pure copper before and after symmetrical cyclic loading.The effects of symmetrical cyclic loading on grain morphology,grain size,grain boundary orientation difference,Schmidt factor,recrystallization,dislocation density and texture are discussed.It is found that after cyclic loading,the grain size of 8 passes pure copper coarsens,the average orientation angle increases,the dislocation density decreases and the texture weakens obviously.3.The cyclic loading experiments of different passes of pure copper under asymmetric strain control were carried out,and the cycle characteristics under different strain amplitudes were compared and analyzed.These include cyclic stress response,cyclic softening factor,average stress relaxation law,and the relationship between mean strain and fatigue life.The study found that the first cyclic average stress of different grades of pure copper material is determined by the strain amplitude and strain ratio.The smaller the strain amplitude,the larger the strain ratio,and the greater the average stress for the first cycle.For unextruded pure copper samples,the average stress relaxation rate increases with different strain ratios as the strain amplitude increases.For the pure copper sample after extrusion,at the lower strain amplitude(0.4%),the cycle period required for the average stress relaxation at different strain ratios is longer,and the average stress relaxation is slower.Under different strain ratios,the average strain ?m of pure copper materials with different passes has a linear relationship with the logarithmic coordinates of alternating life 2Nf,and can be linearly fitted by the formula ?m=C(2Nf)m.