| With the development of production and life,it is imperative to further increase the speed of railway trains,and the increase of running speed puts forward higher requirements for train braking performance.The traditional braking system has a relatively short service life due to failure behaviors such as wear and thermal fatigue,and it is difficult to meet the operating requirements of existing high-speed trains.At present,the related research of the braking system mainly focuses on the enhancement of the surface of the brake disc to improve the performance of the brake disc and prolong its service life.In this paper,the composite coating was prepared on the surface of Invar alloy by laser cladding technology,and the effect of the addition of reinforced particles with different ratios of B and Ti on the microstructure and properties of the composite coating was studied.Optical microscope,scanning electron microscope,EBSD,X-ray diffractometer,DSC,etc.were used to study the influence of in-situ precipitation on the microstructure,morphology and solidification process of the Invar alloy-based composite coating;Vickers microhardness tester,friction The wear testing machine and static thermomechanical analyzer were used to characterize the performance of the Invar-based composite coating,and draw the following conclusions:（1）The composite coatings prepared under different line energy conditions are in good bonding with the substrate,with metallic luster and no obvious defects such as holes and cracks in appearance.As the line energy decreases,the number of Ti B2 particles increases and the size decreases,mainly because the scanning speed is fast and the laser stays in the molten pool for a short time,which reduces the energy input and causes the viscosity of the liquid phase to increase,and the temperature of the molten pool decreases.,the range is reduced,resulting in restricted kinetic growth conditions,so the size shows a certain degree of reduction.With the increase of line energy,the overall Vickers hardness does not change much,and the maximum Vickers hardness is 290 HV0.2.At this time,the strengthening mechanism of the coating is relatively simple,which is in-situ precipitation strengthening.The main wear mechanism of the coating is abrasive wear.Compared with the Invar alloy,the composite coating prepared under different line energy conditions is more than twice that of the Invar alloy in the low temperature range,indicating that the number of reinforcing particles is inversely proportional to the thermal expansion coefficient.（2）The composite coatings prepared with different B/Ti additions are in good bonding with the substrate,but with the increase of the addition,the Ti content decreases,and penetrating cracks appear,which is preliminarily inferred to be due to the formation of borides.With the increase of B/Ti addition,the Ti B2 particles gradually decreased,and the Ni3B content increased gradually.When B/Ti>1.66,the coating is mainly Ni3B/Fe64Ni36eutectic structure.Through the theoretical calculation of mismatch degree,it is determined that Ni3B can be heterogeneously nucleated with Ti B2 as the core,while the mismatch between the Invar matrix and Ti B2 particles does not satisfy the heterogeneous nucleation conditions.With the increase of B/Ti addition,the overall Vickers hardness of the coating showed an upward trend,and reached the maximum value when B/Ti=1.82,and the average Vickers hardness was 315 HV0.2.At this time,the strengthening mechanism of the coating It is an in-situ-eutectic composite strengthening mechanism.The average friction coefficient reaches the minimum value of 0.557when B/Ti=1.53,and the main wear mechanism of the coating changes from abrasive wear to fatigue wear.The thermal expansion coefficient is close to that of Invar alloy at high temperature,maintaining a relatively excellent Invar effect. |
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