Research On TiB₂/Fe₆₄Ni₃₆ Invar Based In-situ Composite Coating By Laser Deposition

At present,China's railway transportation is developing in the direction of high speed and heavy load,which puts forward higher requirements for the safe operation of trains.Among them,the requirements on the braking system are also more stringent.The existing traditional brake discs are difficult to meet the practical needs of modern high-speed trains.Using laser deposition technology to strengthen the surface of the existing brake discs can greatly improve the braking performance of the brake system.Laser deposition can also repair the brake discs that have failed,extend the service life of the brake discs,and save The operating cost of the train.In this paper,laser deposition technology is used to prepare Invar based in-situ composite coating,and the effect of adding different proportions of Ti and B on the microstructure and properties of the coating is studied.Use optical microscopes,scanning electron microscopes,X-ray diffractometers and other instruments to study the effect of enhanced relative Invar coating on the macro morphology,microstructure and solidification process;use Vickers microhardness tester,friction and wear testing machine and electrochemistry The workstation and other equipment characterized the composite coating,studied the Vickers hardness,friction coefficient,surface roughness and corrosion resistance of the composite coating,and reached the following conclusions:(1)The thermodynamics of the reaction system was calculated.Through the calculation of the change of enthalpy and Gibbs free energy of each reaction,the change function of the related reaction was obtained.It was determined that Ti B₂ met the conditions of the most preferential formation in the system.The feasibility of preparing Ti B₂/Fe₆₄Ni₃₆ composite coating by laser deposition was proved.It provides a basis for subsequent experiments.(2)The coating with Ti and B additions has good bonding with the substrate and smooth surface.With the increase of Ti/B ratio,the content of Ti B₂ has increased but the content of Ni B has decreased.When Ti/B<0.5,the microstructure of coating is mainly hypoeutectic structure of Invar alloy and Ni B;when Ti/B>0.5,the microstructure of coating is mainly composite structure of Ti B₂ particles dispersed in Invar alloy.Through the theoretical calculation of mismatch degree,it is confirmed that Ni B can form heterogeneous nucleation with Ti B₂ as the core and form core-shell reinforcement,while the heterogeneous nucleation condition is not satisfied between Invar alloy matrix and Ti B₂.With the increase of Ti/B ratio,the Vickers hardness first increases and then decreases,and reaches the maximum value when Ti/B=0.5,and the average Vickers hardness is 344 HV_0.2.The friction coefficient decreases first and then increases,and the minimum value is 0.563.The main wear mechanism of the coating is from fatigue wear to abrasive wear.With the increase of Ti content,the electrochemical impedance value increased continuously,indicating that the corrosion resistance improved continuously.(3)When the ratio of Ti/B is 0.5,impurities and holes appear on the surface of pure Invar alloy coating;The macro morphology of the composite coating is good when the content of Ti-B is 3.21 wt.%?9.63 wt.%.However,when the content of Ti-B is 12.84 wt.%,cracks appear in the vertical deposition direction due to the formation of excessive borides.With the continuing increase of B and Ti content,the macro morphology of coating changed from divorced eutectic to dendrite,and finally the eutectic structure disappeared,meanwhile,a large number of core-shell structures with Ti B₂ as the core appeared.With the increase of B and Ti content,the Vickers hardness of coating increases continuously,the maximum value is 417 HV_0.2 when the content of Ti-B is 12.84 wt.%.The friction coefficient of the coating is decreasing first and then stable,but the coating with 12.84 wt.%Ti-B additions fluctuates greatly from friction coefficient.