Research On Melting Forming Mechanism And Properties Of Laser Texturing On Roll Surface

Cold rolled sheet is an important material widely used in automobile,home appliance and other industries.Its surface morphology directly affects the properties of cold rolled sheet,such as stamping forming,painting and wear resistance.In the process of cold rolling,the microscopic morphology of the roll surface is transferred to the surface of cold rolled sheet with a certain attenuation ratio.Therefore,the surface roughness pretreatment of cold roll is very important before rolling forming.At present,the roller surface is mainly treated by electrical discharge texturing(EDT).However,the EDT equipment is expensive and depends on import,and the EDT still has some problems,such as high energy consumption,serious oil pollution and poor morphology retention.Laser texturing technology has the advantages of high morphology quality,environmental friendliness,moderate operation cost.By using high energy laser beam to irradiate the roller surface,the morphology and distribution of the texturing on the roller surface can be precisely controlled by adjusting the processing parameters,and the quality of cold rolled sheet can be effectively improved.However,the forming mechanism of laser texturing is not clear at present.The conventional laser texturing distribution has the characteristics of regularity and consistency,which will lead to the appearance of stripe color difference,uneven coating,local cracking and other defects.The effect of laser texturing morphology and distribution on rolling transfer and wear resistance is not clear.Therefore,this paper focuses on the basic theoretical issues related to controllable laser texturing on roll surface,and carries out in-depth and systematic research.(1)Based on the melting flow and evaporation theory of metal surface radiated by laser heat,a laser texturing model was established and the melting forming mechanism of laser texturing was studied.The effects of laser power and pulse width on the laser texturing and morphology evolution of the two morphological features were described.It is found that when the laser power is low or the pulse width is small,the metal in the radiation zone mainly melts and flows,and the spherical coronal texturing is formed in the molten pool driven by capillary force and thermal capillary force.When the laser power or pulse width increases to a certain extent,there is both molten flow and massive evaporation of metals in the radiant zone,the recoil pressure caused by evaporation makes the center of the molten pool obviously concave.In the process of condensation,the molten pool reflow under the action of capillarity force and thermal capillarity,and the depth of the central pit is reduced,and finally the cratering texturing is formed.With the increase of laser power or pulse width,a large amount of evaporation is the main reason for the texturing to change from spherical coronal texturing to crater texturing.This research has important reference value for the selection and precision machining of laser texturing.IV(2)Based on the requirement of regular distribution and random distribution of laser texturing processing on roll surface,the laser equipment which can meet the function requirement was developed,and the process experiment of Cr12 steel surface texturing was studied.The research shows that the texturing surface machining with different distribution characteristics can be realized by adding an acoutooptic deflector in the optical path system which can control the workpiece movement and beam deflection.The research of laser texturing process found that when the laser focusing radius is about 150 μm and the laser power is 60 W,and the pulse width increases from 0.4 ms to 1 ms,the spherical coronal texturing is obtained.When the pulse width increases from 1.5 ms to 4.5 ms,the crater texturing is obtained.When the pulse width is set to 1 ms and the power is set to 50 W,the spherical coronal texturing is obtained.When the power increases from 100 W to 300 W,the crater texturing is obtained.The results show that spherical coronal texturing is formed at lower power or smaller pulse width,and crater texturing is formed gradually with the increase of power or pulse width.By comparing the experimental results with the simulation results,the rationality and feasibility of the theoretical model are verified.(3)The laser texturing surface was modeled by Matlab,and the spatial distribution of laser texturing on roll surface was studied based on regional autocorrelation function and regional power spectral density function.The results show that the texturing morphology and spacing have little effect on the degree of isotropy of texturing surface.While the texturing distribution has a great effect on the degree of surface isotropy,and the degree of isotropy of the randomly distributed texturing surface is significantly higher than that of the regular distributed texturing surface.Therefore,texturing surfaces with different roughness indexes can be obtained by controlling texturing characteristics such as topography height and depth and texturing spacing,and texturing surfaces with higher degree of isotropy can be obtained by processing randomly distributed texturing.(4)The effects of the texturing morphology,spacing and distribution characteristics on the surface roughness and the transfer properties of the morphology before and after rolling were studied by laser texturing processing on the roll surface and characterization.The results show that when the texturing morphology and distribution form are the same,with the increase of texturing spacing,the surface roughness and peak number of texturing roller decrease.The rolling experiment shows that the surface of randomly distributed texturing aluminum plate has no obvious striations,which is of good quality,while the surface of regular distributed texturing aluminum plate has obvious striations,which affects the appearance quality.The spherical coronoid texturing has a higher morphology transfer rate than the crater texturing.Therefore,suitable texturing morphology,spacing and distribution characteristics can be selected according to the quality requirements of cold rolled sheet.(5)Based on experiments in mechanics and tribology,the mechanical properties and wear resistance of laser texturing were studied.The results show that the surface microstructure of Cr12 steel with laser texturing is more dense and its microhardness is about two times that of Cr12 steel.Under the same conditions,the wear properties of the texturing surface with different distribution characteristics are improved compared with the smooth surface without texturing treatment.Attributed to,on the one hand,laser texturing processing has a hardening effect on the surface material.On the other hand,the laser texturing with concave-convex composite morphology can store lubricating oil and wear debris,thus improving lubrication and wear.In addition,the friction coefficients of randomly distributed texturing surfaces are slightly lower than those of regular texturing surfaces when the texturing morphology and spacing are the same.