Process And Mechanism Of Gradient Structural Pure Copper Sheets Prepared By Plastic Flow Machining

The ultra-fine grain/nanocrystalline materials have attracted much attention because of their excellent mechanical strength.However,with the increase of strength,the ductility of ultra-fine grain/nanocrystalline materials will decrease significantly.In this regard,many researchers have attempted to introduce gradient structure into ultra-fine grain/nanocrystalline materials to solve the problem of significantly reduced ductility.The plastic flow machining(PFM)process,as a new severe plastic deformation(SPD)process,can separate the surface layer of bulk workpiece and transform it into ultra-fine grain sheets or plates with gradient structure through controllable composite process of cutting and lateral extrusion.The sheets or plates have a high match between strength and plasticity.T2 pure copper,which has good plasticity and low strength after recrystallization annealing,is widely used,so it has a large space for property improvement.Therefore,it is of great significance to further study PFM process and the properties of pure copper sheets with gradient structure prepared by this process.The main contents of this study are as follows:(1)The PFM finite element model was established by DEFORM,and the effects of extrusion thickness and extrusion angle on the equivalent strain,equivalent strain rate,equivalent stress of pure copper sheet with gradient structure prepared by PFM process were studied.The results show that the equivalent strain gradient of pure copper sheet prepared by PFM process is obvious.With the increase of extrusion thickness and extrusion angle,the proportion of high equivalent strain layers decreases gradually,while that of low equivalent strain layers increases gradually.(2)The PFM experiment was carried out under the same parameters,and the pure copper sheet with gradient structure was successfully prepared.Then microstructure observation,hardness test and tensile property test of the sheet were carried out.The results show that the gradient structure of pure copper sheet prepared by PFM process has obvious grain size gradient and hardness gradient,and the gradient trend is similar to the equivalent strain gradient trend.With the increase of extrusion thickness and extrusion angle,the tensile strength of the sheet decreases gradually,and the elongation increases gradually.(3)Isochronous annealing treatment was carried out on the gradient structural pure copper sheet with extrusion thickness of 1.4 mm and extrusion angle of 120°.Then microstructure observation,hardness test and tensile property test were carried out on the annealed sheet.The results show that when the annealing temperature is 150 ?,the grain morphology of the annealed sheet is similar to that of the pre-annealed sheet,while when the annealing temperature is 350 ?,the annealed sheet has obviously gone through recovery,recrystallization and grain growth,so the grain morphology is similar to that of the original annealed sample.In addition,with the increase of annealing temperature,the hardness and tensile strength of pure copper sheet with gradient structure decrease gradually,and the elongation increases gradually.When the annealing temperature rises to 200 ? or above,the hardness decreases obviously.Therefore,in order to ensure the thermal stability of pure copper sheet with gradient structure prepared by PFM process,the recommended annealing temperature is 150 ? or below.