| Solid surface micro-texture has been widely applied in all walks of life,because it can effectively improve the surface friction and wear resistance,corrosion resistance,wetting property,etc.The laser surface micro-texture technology is one of the most popular methods among the current material micro-processing technologies due to high efficiency,high precision and clean to the environment.At present,most of the laser surface microstructures are machined by short pulses or ultrashort pulses,it has high processing quality,but its processing efficiency is low,its cost is higher and it has a higher demand for the control system and the working environment.The energy loss of the long pulse laser propagation process is small,and it has the advantages of long acting time and high energy coupling efficiency.At the same time,it avoids the influence of plasma shielding,shock wave and self-focusing phenomenon.Therefore,the research on processing of surface micro-texture based on long pulsed laser has a certain significance.In the research of laser surface microstructure,most of them are the research on the application of micro-texture.The discussion on micro-texture formation is relatively few,especially for micro-bump morphology.Most of micro-bumps are made by two steps or more,and there is still a lack of information related to creating micro-bumps using laser surface micro-texture technology in a single step.About study on wettability of surface micro-texture,it has been widely known that wettability is affected by surface free energy and surface roughness.However,there is relatively few studies on the effect of texture morphology and geometrical distribution on wettability.Therefore,in this thesis,the effects of laser processing parameters on the morphologies of different micro-textures by using the long-pulsed laser were studied,and different surface texture regions are obtained by different processing parameters.In addition,the wettability of different surface microstructure areas were examined,discussed and analyzed.The main research contents of this paper include:1)Construction of the long-pulse laser machining setup.In view of the advantages of long-pulse laser processing,the commercial fiber laser(YLR-200-AC,IPG)was operated in modulation mode to issue long pulse.The long pulse laser processing surface microstructure experiment equipment was set up independently.It consisted of a laser system,a beam delivery system,a motion system and a control system.By writing the corresponding program in the computer operation interface,the experiments of the surface micro-texture based on laser long pulse can be carried out according to the test design,which is easy to operate and control.It laid an important foundation for the processing and research of laser surface micro-texture based on long pulse.2)Experimental study on surface micro-texture of a single pulse laser.The 304L stainless steel with relatively wide application and good performance was selected as the workpiece.The dimple and bump microstructures on 304L stainless steel surface were processed by modulating the laser processing parameters(laser pulse width ?,laser output power P)on the workpiece.The effects of laser processing parameters(laser pulse width ?,laser output power P,pulse energy E)on the microstructure morphology of single pulse were studied,and the formation of different morphological texture was discussed and analyzed.The research results showed that dimples with better morphology can be obtained under the short pulse duration and high power.In contrast,bumps with better morphology can be obtained under the long pulse duration and low power.The pulse energy of the processing bumps was much larger than the pulse energy of the processing dimples.The influence of power on the shape of bump was relatively larger than that of pulse duration.In addition,it also included that a higher laser power combined with a shorter pulse duration could generate a higher temperature which is useful to create the micro-texture.But with the pulse energy increased,this effect became less obvious.When the pulse energy E unchanged,the depth and diameter of dimple became greater with the smaller pulse width.For the bump,a higher laser power was useful to create a bump with greater height and diameter when the laser energy was less than 5mJ.However,as the increase of the pulse energy,the difference in the size of bumps with different power was getting smaller and smaller under the same pulse energy.When the pulse energy reached about 7.2mJ,there were almost no differences in the shape of bumps with different power.In this experiment,the laser processing parameters of dimples and bumps with superior shape could be processed respectively,which provided an important basis for the experiment of processing micro-textured regions based on continuous pulse laser.3)Experimental study on surface micro-texture of continuous pulse laser.The grooves area and bumps area with different geometries were processed by modulating the processing parameters(laser output power P,laser scanning speed V,laser scanning horizontal spacing c)on the surface of 304L stainless steel by continuous pulse laser.It proved that the laser surface micro-texture technology was an effective method to fabricate the groove and bump texture region with high reproducibility and uniformity.Moreover,its control accuracy was high.Furthermore,it verified that the laser machining setup based on long-pulse could process the surface micro-texture with the better shape directly according to the designed size parameters.The influences of processing parameters on the microstructure area and the surface roughness Sa were analyzed in detail.The results showed that the scanning horizontal spacing c was the most important parameter which affected the surface roughness Sa of the textured area,the power P was the second,and the scanning speed Vhad the least effect on Sa.At the same time,the results were the basis for the change of wettability of the microstructure zone.4)Effect of laser micro-texture on wetting performance.By the detection of the contact angle between the water droplets and the microstructure zone obtained from different processing parameters,the changes of wettability of the untreated area and the surface micro-texture area before and after the treatment of the fluorosilane solution were discussed and compared.The effects of processing parameters(laser output power P,laser scanning speed V,laser scanning horizontal spacing c)on the surface wettability of 304L stainless steel workpiece before and after treatment with fluorosilane solution were carried out.The results showed that the laser surface microstructure zone had a stronger hydrophilicity than the untreated area.But after the treatment with fluorosilane,the surface was hydrophobic?and the laser surface microstructure zone had a stronger hydrophobic than the unprocessed surface.In general,the greater the surface roughness treated by the laser microstructure,the stronger the wettability.Wenzel wettability model can be used to explain:when the surface is hydrophilic,if the surface is rougher,the hydrophilicity will be stronger;when the surface is hydrophobic,if the surface is rougher,the hydrophobicity will be stronger.It was also found that the surface roughness,the surface microstructure morphology and the geometrical distribution of the microstructure on surface jointly determine the wetting properties of the laser microstructure surface. |
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