| Carbon Fiber Reinforced Polymer(CFRP)has been widely used in advanced structures in aerospace,rail transit and national defense industry due to its high strength and stiffness weight ratio.CFRP structural components are usually manufactured by near net forming technology.In order to meet the expected accuracy,surface quality and assembly requirements,subsequent secondary processing is still needed.However,CFRP composite is a typical difficult-to-process material which is difficult to machining due to the significant difference in mechanical properties between carbon fibers and resin matrix.Moreover,the hygroscopicity of resin matrix will lead to water absorption and expansion of CFRP composites,seriously affecting its mechanical properties,which limits the application of flood cooling lubrication in the grinding of CFRP composites.To date,dry grinding is mostly used.However,dry grinding of CFRP composites usually results in wheel blockage and various damages,such as fiber pullout,fiber fracture,resin coating,interface cracking,fiber matrix debonding and delamination,which severely limits the application of CFRP composites in high-end equipment.To address the above technical bottlenecks,this paper proposes a green processing strategy based on Nanofluids Minimum Quantity Lubrication(NMQL)for CFRP composites with high quality,high efficiency and low damage.Firstly,CNTs nanofluids are determined for the temperature sensitive problem of resin matrix.Through dispersion mechanism analysis and experimental study,surfactants were optimized to solve the bottleneck of CNTs agglomeration.The material removal mechanism of single grain cutting CFRP composites was studied,and the mechanical models under different lubrication conditions were established and verified by experiments.The surface quality,morphology and processing damage of CFRP composites grinded under different lubrication conditions were further studied and evaluated by experiments.The main research work of this paper includes the following contents:(1)The dispersion properties of different surfactants were researched,and the effects of molecular structure and physicochemical properties of different surfactants on their dispersion mechanism were analyzed.The dispersion mechanism of CNTS nanofluids with different surfactants was studied from the aspects of space barrier effect,reverse micelle theory,hydrocarbon chain lipophilicity,London force and synergistic effect.The dispersion stability and viscosity of CNTS nanofluids with different surfactants were evaluated experimentally.(2)From the mesoscopic point of view,cutting CFRP with a single grain reveals the complex material removal mechanism.Based on the difference of contact state between grain and carbon fibers in different stages of cutting arc length,four kinds of contact models and cutting models(Hertz contact model between grain tip blunt cone and carbon fibers,extrusion model of grain on carbon fibers,elliptical local contact model of grain and carbon fiber,single carbon fiber cutting model)are established respectively.Geometric analysis of the influence of different processing parameters on the cutting zone was carried out,and the variation of the length and width of the cutting zone with the depth of cut,the circular speed of grinding wheel and the feed speed of workpiece were revealed.(3)On the basis of the removal mechanism of single grain cutting CFRP composites,considering the uniformity and randomness of carbon fiber arrangement,the corresponding mechanical model was established according to the different contact state and material removal behavior of grain and carbon fiber at different stages of cutting arc length.And the cutting forces at different points of cutting arc length were calculated.Considering the influence of lubrication conditions on friction,the friction and wear experiments of diamond material and CFRP composites as friction pairs were carried out.The friction coefficients of different lubrication conditions needed in the mechanical model were obtained,and the friction between single grain and CFRP at different stages was calculated.Finally,the tangential force and normal force are obtained by decomposing the cutting force and friction force of a single grain.Then the cutting force model is validated by the single grain grinding experiment under the corresponding processing conditions.(4)By comparing with dry and MQL grinding,the grinding performance of CFRP composites using NMQL technology was studied,with emphasis on the grinding surface quality and machining defects.Firstly,Ra,Rz and RSm value are compared,which characterizes the general surface characteristics and service properties of surface roughness,the fiber pull-out phenomenon by the peak height of the reaction profile,the roughness characteristics in the horizontal direction of the reaction,respectively.Secondly,the micromorphology of grinding CFRP surface under different working conditions and the damages in detail were qualitatively analyzed by SEM.Finally,the high-frequency texture features of surface roughness signals and SEM images in different directions are quantitatively analyzed by using wavelet analysis and energy distribution ratio of wavelet decomposition.The defects such as fiber pull-out,fiber breakage,resin coating and debonding between fiber and matrix are characterized.(5)The surface morphology fractal characteristics and wheel blockage of grinding CFRP composites under different lubrication conditions were compared and evaluated.Firstly,the fractal dimension is used to quantitatively characterize the holistic fractal characteristics of the machined surface in different dimensions.The two-dimensional fractal dimension of grinding feed direction and fiber direction surface roughness profile signal was calculated and analyzed,and the three-dimensional fractal dimension of SEM surface micromorphology signal at different magnification was calculated and analyzed.Secondly,the multi-fractal spectrum of the surface roughness profile curve is established,and the local morphology characteristics of the grinding surface of CFRP under different working conditions are qualitatively analyzed.On the basis of the multi-fractal spectrum,the spectral width and spectral difference of the corresponding working conditions and different directions of the workpiece surface are calculated,and the local characteristics of the grinding surface morphology are quantitatively described.Finally,the grinding wheel surface images of CFRP composites processed under different working conditions were captured by microscopic imaging.The grinding wheel blockage and oil film infiltration and spreading on the grinding wheel surface under different lubrication conditions were characterized and analyzed. |
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