Study On Micro ECAP Forming Technology And Process Of Pure Magnesium

With the rapid development of science and technology,the demand of micro products is increas;ing,especially in the micro electro-mechanical system which the shape size and operation scale is tiny.Under the huge development prospect and market potential,microforming technology has become the hot research by international experts and scholars.The micro plastic deformation is not reducing the traditional macro plastic deformation process proportionally.With the decrease of the billet size,the friction force of deformation increases dramatically,and the materials present anisotropic and flow non-uniformly,and then present the characteristics which is different with macroforimg,and that is the size effect of microforming.Equal Channel Angular Pressing(ECAP)can prepare the fine grained materials which have excellent mechanical properties by realizing severe plastic shearing deformation,and effectively weaken the size effect,also improve filling property of the material So combining ECAP and micro forming technology is a important research direction in the field of micro manufacturing.In this paper,high purity magnesium(purity:99.95%)was chosen as research object,and the micro ECAP(M-ECAP)process was designed for the study.By using the commercial software DEFORM to simulate the deformation process of M-ECAP,the flow of material,forming load,and the distribution of velocity field,equivalent stress and strain was investigated.The convexity of extruding was taken as evaluation standard of M-ECAP parts'forming quality,and the best on the bast extrusion process was obtained by using orthogonal test,and then the grain-refining pure magnesium billets were prepared by ECAP.Based on these results the physical experiment were done to verify the feasibility of M-ECAP process.Through the analysis of the variation of surface morphology,microstructure and micro-hardness of M-ECAP parts,the effects of different grain size,grain distribution uniformity and M-ECAP passes on the quality of micro parts were discussed.The results show that:(1)The grain-refinining and shape normalization micro parts can be prepared by M-ECAP.So this process which provides the methods to prepare the medical micro parts of magnesium alloys that have high strength,high toughness and high corrosion resistance is feasible.(2)Based on finite element analysis of M-ECAP deformation,here comes to the conclusion that the materials deforming is non-homogeneous.When passing the Conner area where is the main deformation area,the material nearing outer corner can obtain lager equivalent strain and faster velocity than that at inner under the effects of shear force.This phenomenon is similar to traditional ECAP process.With the increase of extrusion passes,the effective strain of sample can be effectively accumulated.Because of the friction size effect in M-ECAP experiments,the effects of friction force on deformation is greater than marcoforming.The flow speed of outer metal is lower than the inner metal due to its movement is blocked.The more the extrusion passes,the greater the flow velocity difference of inner and outer metal.At the same time,the extrusion load increases step by step.(3)Through the analysis of orthogonal test,the result shows that the optimal plan is that pretreating the billet by 4 passes ECAP,and letting the friction coefficient be 0.05,then extruding the billet with speed of 0.1 mm/s at 275.Under this plan,the forming parts can obtain the smallest convexity of extruding,and also the best flow uniformity.(4)The grain-refining material can effectively weaken the size effect.The fine grains and homogeneous distribution of initial microstructure can make the material deformation more homogeneity,and the shape more normalization.So the size effect is weakened.On the contraty,the surface cracks occur,and even fractures at corner area.(5)To increase the extrusion passes is conducive to grain refinement as well as homogeneous distribution.However,due to the combined effects of dislocation increasing and annihilation,the refinement of grain size by increasing extrusion passes will gradually weaken,and this rule is same to the traditional ECAP.With the effect of friction size effect,the microstructure of part will present a hierarchical state that the fine grain at outer metal and coarse grain at inter area.This phenomenon is not obvious in the macroforming.The grain can be significant refinement by M-ECAP.The grain size of billet has little impact on the micro parts,but the microstructure uniformity of the billet has significant effect on the micro parts.(6)Under the effect of fine grain strengthening,the micro-hardness value of micro parts have varying degrees of increase,at the same time the micro-hardness variation coefficient is greatly reduced comparing with the billet,so the deformation is more uniform.