Surface Texturing And Tribological Properties Of Gray Cast Iron Prepared By Warm Micro-scale Laser Peening

Warm micro-scale laser peening(W?LP)combines the double modified technical advantages of strengthening and surface texturing.The dislocation pinning effect induced by dynamic strain aging under certain temperature was utilized to improve the strengthening effect and its high temperature stability of micro-scale laser peening.Combining with the heat dissipation gain of the surface texturing,the problem of heat fade on the surface of material during the friction process was effectively alleviated.In this paper,gray cast iron as typical material used for brake disc was selected as the research material.The process of W?LP was used to modify the surface of the material to improve the tribological properties of the gray cast iron surface,finally enhanced the ability of resistance to heat fade.Using the combined method theory,simulation and experiment,the pressure characteristics of the shock wave at microscale and the principle of dynamic strain aging under temperature assistant were explored.The effect of modified technology on the surface integrity of gray cast iron and its improving mechanism on tribological properties were studied.The main work of this paper is as follows:(1)The theory of warm micro-scale laser peening: According to the physical mechanism of the interaction between laser and matter,the generation mechanism of laser-induced plasma shock wave and the distribution characteristics of shock wave pressure at microscale were analyzed.Based on the propagation characteristics of shock wave in solid materials and the materials dynamic constitutive model,the deformation strengthening mechanism of materials was expounded.Combining with the microstructure characteristics on the material surface after modification,the influence mechanism of dynamic strain aging and dynamic precipitation on the effect of stability strengthened by micro-scale laser peening were analyzed from the perspective of dislocation pinning.According to the organization characteristic of grey cast iron and the generation mechanism of dynamic strain aging,the DSA and DP effect for the applicability of the gray cast iron surface modification were demonstrated,the temperature range of dynamic strain aging of the gray cast iron was determined.(2)Numerical simulation of temperature field of brake disc with surface texturing: Based on the distribution characteristics of the temperature field on the surface of smooth brake disc,as well as in view of the single emergency braking process of disc brake,the generation of frictional heat and the way of heat transfer during braking process were analyzed.The free energy function was introduced to derive the coupled thermoelastic basic equation.On this basis,comsol was used to establish the geometric model of brake disc/pad.The temperature field distribution on the surface of brake disc before and after surface texture was compared.And the influence of texturing depth on radial temperature distribution was also analyzed.(3)Surface modification of warm micro-scale laser peening: Based on the German Spitlight 2000 nanosecond laser pulse,the device of warm micro-scale laser peening was set up and the surface modification tests were carried out.The surface morphology,residual stress,elastic modulus and nanohardness of the gray cast iron subjected to warm micro-scale laser peening were measured by confocal microscope,XRD stress meter and nanoindentation instrument with different laser energy,different area density and different depth to diameter ratio of dimple,which was aimed to study the influence of process parameters on the surface integrity of the materials.A metallographic microscope was used to observe the microstructure on the area which was strengthened by warm micro-scale laser peening.The microstructure strengthening mechanism of gray cast iron was described from the perspective of residual stress strengthening,the precipitation of "bovine-eye ferrite" and grain refinement.(4)Tribological properties of texturing surface and its improving mechanism: Combining with the dry friction theory and wear mechanism,the friction coefficient and its stability,as well as the wear mass were used as the characterization parameters to carry out the dry friction and wear tests under different temperature conditions.The effects of different texturing parameters on the dry friction properties of gray cast iron were investigated.The wear morphologies were observed by Scanning Electron Microscope(SEM)and the wear mechanism was discussed.Based on EDAX and X-ray diffractometer,the evolution of the third body composition on the worn surface was analyzed.The mechanism of comprehensive effect with dynamic strain aging and dynamic precipitation on the tribological properties and its high temperature stability of gray cast iron surface were revealed by the comparison of phases.The results of theoretical analysis,numerical simulation and experimental study showed that warm micro-scale laser peening could introduce compressive residual stress and strengthening microstructure during the plastic deformation process.In combination with dynamic strain aging and dynamic precipitation,the proliferation and pinning of dislocations could effectively enhance the mechanical properties of gray cast iron surface.After the process of warm micro-scale laser peening,the tribological properties of the material surface under variable temperature were remarkably improved,and the heat fade of the frictional surface at high temperature was also alleviated.The technology could expected to be applied in the automobile braking system.