| With the continuous promotion of strategies including Circular Economy.Low-Carbon Economy and Resource-Conserving Society of our country.surface engineering technology.which can produce new surfaces with wider application fields.better performance and more functions than bulk materials by using less energy and low cost,has become the leading technology of advanced manufacturing technology and the key technology of modern industry,and it is also of great significance to the sustainable development and the socialist modernization of our country.In order to meet the requirements of modern industrial and agricultural production and realize the quick development of special functional surfaces and the rapid manufacturing of special strutures on material surfaces,in this research,with the advantages of long pulse laser,the mechanism of laser micromachining of 304L stainless steel is studied theoretically,numerically and experimentally,and various performances of 304L stainless steel surface textured by long pulse laser are studied and analyzed,providing theoretical and experimental bases for the morphology design,parameter optimization and technique improvement of laser surface texturing.Main works and conclusions in this research are as follows:1.Numerical simulation and experimental study on the micromachining of 304L stainless steel using single laser pulse.For the order of microsecond laser power density in the experiments,a mathematical model which incorporates heat and mass transfer as well as fluid flow is developed on the basis of continuity equation,momentum conservation equation and energy conservation equation.An oxidation kinetics model is established to describe the mass inflow caused by high temperature oxidation of stainless steel in both solid and liquid phases,and the phase transitions of both melting and vaporization are considered.A modified level-set method is developed to trace the free surface,and possible effects on the free surface boundary containing surface tension,recoil pressure,Marangoni force,shear stress caused by metallic vapor,gravity,buoyancy force and stagnation pressure firstly proposed in this study as well as mass loss are incorporated.In this study,the commercial software COMSOL Multiphysics is employed to perform the numerical simulation,and experiments are carried out based on the experimental device for laser surface texturing.Comparing the numerical results and the experimental data of 304L stainless steel surface microstructures induced by single pulse laser,the vadility of the model is verified,and the evolution of surface topography of irradiated zone from the bump to the crater and an ejection threshold are obtained.The results show that with the increase of laser fluence the irradiated zone on the surface of 304L stainless steel presents the"flat-topped" bump,the "M-shaped" bump and the "Gussian-like" crater with a circular rim successively,which correspond three different laser fluence ranges revealing different physical and chemical processes.2.Morphology study and mechanism analysis on the micromachining of 304L stainless steel using single laser pulse.On the basis of heat transfer,mass transfer and hydrodynamics,the process and results of Isaser micromachining of 304L stainless steel with single pulse and a pulse width of 10 μs are studied systematically by numerical simulation.Firstly,the influences of laser fluence and ambient air on machining process and surface topography are analyzed,and the conclusions are as follow:laser fluence determines the final surface morphologies by influencing the temperature fields,oxidation reactions,phase changes,drving forces and flow fields;the influence of ambient air is mainly reflected in two aspects which include the effect of oxidation on mass flow and the influence of dissolved oxygen on surface tension.Secondly,the dynamics of melt ejection and the influence of various driving forces on melt ejection are analyzed,and the results show that the stagnation pressure and the shear stress caused by metallic vapor play significant roles in the formation of melt ejection,and the influence of Marangoni force can not be ignored,while the main obstacle to melt ejection is surface tension.Finally,the influences of laser pulse duration on surface morphologies of microstructures at different laser fluences are predicted by numerical simulation method.3.Experimental study on influence of process parameters for micromachining with multiple laser pulses on surface topography of 304L stainless steel.By using the experimental device for laser surface texturing,the influences of number of laser pulses,laser fluence and defocusing distance on surface topography of irradiated zone on the surface of 304L stainless steel are studied,and the evolution of surface topography from the bump to the crater and the formation mechanisms of various morphologies with multiple laser pulses are analyzed.Furthermore,the results and the mechanisms of micromachining of 304L stainless steel with single laser pulse and multiple laser pulses are compared.The main conclusions are as follows:(1)Increase of number of pulses can slightly adjust the surface topography of irradiated zone,while the rise of single pulse fluence has a much greater impact on surface topography and may limit the increase of the height difference of microstructures.(2)With the increase of single pulse fluence,the evolution of surface topography of irradiated area on 304L stainless steel surface can be devided into five phases including the phase of "sombrero-shaped" bump,the transitional phase,the phase of"Gaussian-like" crater,the phase where plasma plays an increasingly significant role and the phase characterized by a saturation presented in plasma shielding mechanism,and each phase reveals different physical and chemical processes.(3)Under certain processing conditions,the evolutions of surface topography of microstructures in the range of positive defocus and negative defocus are similar.However,there is still a difference that the inner diameter,the outer diameter and the rim height of microstructures in the range of positive defocus are obviously smaller when the absolute value of defocusing distance is fixed.4.Fabrication and performance study of laser micro-textured surfaces.18 types of specimens with typical texture morphologies and texture area densities are processed by long pulse laser,and the hardness and roughness of laser-textured and untextured(control)surfaces are tested.Moreover,the tribological performances of laser-textured surfaces and untextured surfaces under dry and lubricant sliding are studied,and the wetting performances of both types of surfaces are investigated.The main conclusions are as follows:(1)With different texture morphologies,the hardness inside laser micro-textures is greater than that outside laser micro-textures,and the hardness outside laser micro-textures is greater than that of untextured surfaces.(2)The influences of different texture morphologies on surface roughness are different,and with the increase of texture area density the surface roughness of specimens rises continuously.(3)Under dry and lubricant sliding,with certain processing parameters,only the laser micro-textures with appropriate texture morphologies and appropriate texture area densities can reduce the friction coefficient and wear amount as well as improve the tribological performance of a surface.(4)With the increase of texture area density,all the contact angles of laser mico-textured surfaces with different texture morphologies witness an upward trend.Furthermore,the wettablility of a surface is determined by both surface topography(texture topography,texture area density and roughness)and surface chemical composition. |
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