Study Of Laser In-situ Construction Of Surface Micro-texture On Ductile Cast Iron And Its Effect On Tribological Properties

Nowadays,ductile cast iron is widely used in the manufacture of gears and crankshafts due to its high mechanical properties and relatively low cost.However,the crankshaft journal surface is prone to wear due to the harsh working environment under long running conditions,which seriously effects the service life of the crankshaft.Based on this background,laser surface texturing technology provides a new direction for improving the tribological property of ductile cast iron crankshaft.Although the laser surface texturing technique can effectively improve the tribological property of the material surface,it still has some shortcomings.For example,micro-pits were usually formed by point-by-point ablation,the processing efficiency is too low for micron-scale texture to applied in mass production.In this paper,a process method of in-situ construction of surface micro-texture on ductile cast iron was proposed based on the problems of easy wear on the crankshaft journal surface of ductile iron and low processing efficiency of laser surface texturing technology.The effect of fluid dynamic pressure lubrication by micro-texture was verified by simulation,and the effect of micro-texture parameters on the fluid dynamic pressure effect was studied.The in-situ construction of surface micro-texture with good surface morphology were prepared on the surface of ductile cast iron using a picosecond laser by continuous scanning method.The effect of in-situ construction of surface microtexture on the tribological property of the ductile cast iron surface was investigated by friction and wear experiments.The main research content of the paper was as follows:(1)Two-dimensional and three-dimensional simulation models of micro-texture region were designed and established to verify the hydrodynamic pressure lubrication effect of micro-texture,the influence of the cross-section shape,the relative motion velocity of two friction pairs and the shape of micro-texture on the hydrodynamic pressure were analyzed,which provides a theoretical basis for the preparation of in-situ construction of surface micro-texture.The simulation results show that the existence of micro-texture can significantly produce hydrodynamic lubrication effect and improve the bearing capacity of lubricant film.The cross-sectional shape is triangular or semi-circular,and the surface shape is circular micro-texture has better fluid dynamic pressure lubrication effect.The relative motion velocity of the friction pair is positively correlated with the hydrodynamic pressure,the higher the relative velocity of the two friction pairs,the stronger the hydrodynamic pressure generated.(2)A picosecond fiber laser with wavelength of 1030 nm was used to in-situ construction of surface micro-texture on ductile cast iron.The effect of laser parameters on the surface morphology of in-situ construction of surface micro-texture was studied by varying the laser processing parameters,and obtained the good laser processing parameters.The surface morphology of in-situ construction of surface micro-texture was found to be superior when the laser power was 35.6 W,the laser scanning speed was 1000 ～ 2000 mm/s,the filling spacing was 0.01 mm,and the number of laser scans was 1time.The surface micro-hardness and crystallographic organization of the in situ induced microtexture were analyzed,and found that the surface micro-hardness of the in-situ construction of surface micro-texture was significantly increased.The structure inside the micro-pits was characterized by EDS and Raman spectroscopy,the graphene-like structure was found in micro-pits after laser irradiation,the mechanism of production may be related to the photothermal effect.(3)Friction and wear test were conducted using a ball-on-disk tribometer.The influence law of in-situ construction of surface micro-texture on the friction and wear properties of ductile cast iron was investigated by analyzing the friction coefficient,wear rate,surface oxygen content and surface wear morphology.It was found that all in-situ construction of surface micro-texture samples produced good friction and wear properties compare with the un-texture samples under the oil lubrication condition.This proved the reliability of the simulation results,and the hydrodynamic lubrication effect generated by the in-situ construction of surface micro-texture under oil lubrication can effectively improve the frictional wear performance of ductile cast iron.There is an optimal range for the area density of in-situ construction of surface micro-texture,it was found that the area density is about 20%,the in-situ construction of surface micro-texture has best wear reduction properties,the coefficient of friction and wear rate can reduced by up to 20% and 70%.In addition,the same area density samples were prepared by laser point-by-point ablation and in-situ construction micro-texture,and friction test were performed.The test results shows that the laser in-situ construction of surface micro-texture sample has better tribological properties than the laser point-by-point ablation sample,the coefficient of friction and wear rate are reduced by 10.7% and 3% respectively,which is attributed to the surface hardness improvement and graphene-like structures caused by the different laser irradiation methods,and it contributes to higher resistance ability of the generation of abrasive debris and thermal conductivity.