| At present magnesium and its alloys has become the research hotspot in the field of biomedical materials because of its good bio-compatibility and biodegradability.But the corrosion rate of magnesium and its alloys is too fast which is greater than the repair rate of the organization,thus how to control the degradation rate is the problem of magnesium as biomedical materials which is need to be solved currently.And the researches have show that the plastic deformation can improve the mechanical properties of materials,and grain refinement has an important effect on the corrosion resistance of materials which there has a close relationship between grain uniformity and corrosion resistance.Severe large plastic deformation is one of the ideal methods to prepare fine granular block materials,what's more mechanical properties and deformation uniformity of materials deformed by severe plastic deformation can be improved obviously,so doing research on the grain uniformity when the magnesium and its alloys are deformed by severe large plastic deformation has a great realistic significance on the magnesium and its alloys as biomedical materials.In this paper,the principle and method of the equal plastic angular extrusion process and the drawing process were used for the large plastic deformation process.A new multi-pass angle drawing(M-PAD)process is developed for pure magnesium wire with a diameter of lmm.By using the commercial software DEFORM to simulate the deformation process of M-PAD,the distribution of stress and strain,and the deformation uniformity are investigated.Based on the results of finite element numerical simulation the physical experiment was done to verify the feasibility of M-PAD process.Through the analysis of micro-structure and micro-hardness of M-PAD deformation,and the influence of the size of the back tension,passes of M-PAD and drawing speed on the grain refining were discussed.The results show that:(1)A new multi-pass corner drawing process for large plastic deformation process is developed.(2)The finite element analysis model of multi-pass corner drawing process is established,the influence of the technical parameters of the multi-pass angle drawing on the stress and strain during drawing process is discussed,and the change of drawing force which is caused by the process parameters in drawing process is analyzed;Besides the drawing force is decrease with the incensement of drawing radius,drawing angle and drawing number.And the best multi-pass angle drawing process is that when the radius of the rotation angle is 1.2mm,the drawing is 2 times and the drawing angle is 120 degrees.The effectiveness of the large plastic deformation process is verified by physical experiments.(3)The adjustable multi combination drawing die with multi-pass corners is designed,the structure of the drawing die is as follows:(1)two fixed plates and two forming rods with different radius of round angle;(2)there is a sliding groove on the fixed plate which can adjust the position of the deformation rod,and the different drawing angle can be obtained by changing the position of the forming rod in the sliding groove.In order to carry out a variety of different drawing experiments,the drawing angle is adjusted at 110 degree to 135 degree,and there are 4 choices between the drawing radius form 0.5mm to 1.6mm.(4)After M-PAD drawing the grains of pure magnesium wire can be effectively refined,with the increasement of drawing times the uniformity of deformation is enhanced,the grains are refined and uniform,but after 2 times of M-PAD deformation the grain refinement effect is no.longer apparent.And increasing the back pull the effect of grain refinement is gradually weakened,and the grain size of pure magnesium is the smallest and the refinement effect is the best when the drawing force is 5N.The optimal parameter of the drawing in this experiment is 5N and 2 times of M-PAD deformation. |
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