Research On The Surface Roughness Evolution And Surface Finish Controlling In Cold Plastic Deformation

The surface topography of the workpiece varies greatly and can hardly be controlled during plastic deformation,hence the plastic deformation process is generally considered unsuitable for the production of final product.However,due to the shape and special usage conditions of the parts,ultra-high-precision surfaces must be obtained by plastic deformation in some specific engineering applications,such as the aluminum wave-guide tube.To meet the requirements of those specific engineering applications,the surface roughness evolution during plastic deformation was studied in this thesis thoroughly,and the roughness controlling maps of the free surface and the constrained surface were established.The main research contents of this thesis are as follows:?1?The three dimensional crystal plasticity finite element model was built to analyze the surface roughness evolution during plastic deformation.And a more general method is developed to apply the periodic boundary conditions to both periodic mesh and aperiodic mesh.Then the methods of “macro-meso coupling” and “symmetry boundary condition + periodic boundary condition” were proposed to simulate the surface roughness evolution during complex plastic deformation and simple plastic deformation,separately.?2?Based on the 3D crystal plasticity finite element model,the effects of strain path,initial surface roughness,grain size and crystal orientation distribution on the roughness evolution of free surface were analyzed.The results indicated that the surface roughness of the model with flat surface depended approximately linearly on strain for five typical strain paths,and the final surface roughness under in-plane pure shear condition was smallest.The initial surface roughness had less influence on the stress and the strain distributions after deformation,which was more similar to have a superimposed effect on the final surface roughness.The concepts of equivalent grain size,which represents the comprehensive effect of grain sizes in different directions,and the SD_c HR,which represents the effect of orientation distribution,were proposed in this thesis to quantitatively describe the surface roughness evolution.Then the prediction model of surface roughness was established,and the predicted values of this model showed a favorable agreement with the uniaxial tensile results.?3?The effect of pretreatment process parameters on the microstructure and orientation texture were studied by the pre-deformation and heat treatment experiments and the artificial neural network method.The roughness controlling map of free surface under uniaxial tension was established based on the results of pretreatment experiments and the roughness prediction model.According to the roughness controlling map,it could be found that larger deformation and lower heat treatment temperature leaded to smaller surface roughness after uniaxial tension,and the final surface roughness showed a tendency of decreasing first and then increasing over the pre-heat treatment time.The roughness of free surface after plastic deformation could be indirectly adjusted by adjusting the material parameters based on the specified deformation process and the roughness controlling map.?4?The effects of ultrasonic vibration time,dispersant content and p H value on the dispersion of nano-TiO₂ were analyzed to obtain the optimum dispersion conditions of nano-TiO₂.Based on ring-on-disc test,the influences of nano-TiO₂ content,load and rotation speed on the tribology performance of nano-TiO₂ lubricant were in-depth researched.The nano-TiO₂ particles,which were adsorbed on the rubbing surface,could play a role in stabilizing the friction process and reducing the fluctuation of friction coefficient.Moreover,the lubricant with 0.3 % 0.8 % nano-TiO₂ had best tribology performance.?5?The roughness evolution of constrained surface under different lubrication conditions were analyzed by single-pass rolling experiment.A new parameter named valley volume of surface topography was proposed to represent the asperity flattening during plastic deformation,and the experiment results indicated that the surface roughness after rolling was approximately linear with the valley volume.Based on the single-pass rolling experiment,the roughness controlling maps of constrained surfaces under different lubrication conditions were established.It was found that both thickness reduction and rolling speed had great influences on the final surface roughness under the condition of no lubrication.However,under lubrication conditions,the effect of rolling speed on the final surface roughness could almost be ignored after the thickness reduction exceeds 34 %.In general,a little larger thickness reduction and smaller rolling speed could result in a smoother surface after rolling.