Investigation On Optimal Extrusion Route Of Pure Copper During ECAP Process

ECAP is now widely used in the processing of ultra-fine grained materials.Its obvious feature is that the shape and composition of material does not change significantly after the extrusion,so that the billet can be extruded multi-passes.With the strain accumulation and the application of simply shearing deformation,the purpose of grain refinement can be achieved.There are many factors that affect the grain refinement efficiency during the ECAP processing,such as mold structure,extrusion passes,extrusion temperature,extrusion route,backpressure,etc.This research mainly focused on the effect of different extrusion routes on the microstructure and properties of pure copper.Among the traditional extrusion process,the most efficiency of grain refinement is route A when using a die with an internal angle Φ equal to 120°.In this experiment,the rotation angle of each pass was calculated by the angle between the grain elongation plane and the shear plane of next pass,when the material in the channel,the slip system is in the easiest position to move,and the route X was designed to be extruded together with the route A and route BC.The properties of the extruded material are compared in order to find the best ECAP processing route.After processing,the metallographic,EBSD,TEM and mechanical property tests were carried out to reveal the difference in the effect of refinement,texture evolution,hardness,tensile strength and elongation between different routes.The following results are obtained:(1)Route X extruded material grains are mostly equiaxed crystal,and the grain size is smaller than the other two routes.Grain size of route A is small but the phenomenon of grain elongation is serious.Grain size of route BC is larger.So the route X is better than route A and route BC about the refinement effect.(2)Due to the deformation characteristics of the ECAP process,a large number of dislocations are formed with the severe shear deformation.With the proliferation and movement of dislocations,a large amount of subgrains are formed in the grains.As the extrusion pass increases and the strain accumulates,The misorientation between subgrains increases gradually,and the low angle grain boundaries gradually change to high angle boundaries.However,due to incomplete deformation,the subgrain boundaries do not all transform into high angle grain boundaries,and many low angle grain boundaries exist.(3)After the extrusion,the hardness and tensile strength enhancement is obvious in the first pass in three routes.With the increase of passes the hardness enhancement effect gradually reduced.After 8 passes,the hardness and tensile strength of the route X is higher than route A and route BC.(4)The materials was annealed to research the recrystallization behavior at 150℃,200℃ and 250℃.Through the metallographic examination and EBSD,it is found that the route X extruded material exhibited better thermal stability.