Research On Laser Cladding Process And Performance Of Brake Disc Material For High-speed Train

High-speed train brake disc is one of the important parts to ensure the safety and reliability in the long run process,the brake failure occurs due to the friction and wear,and thermal failure of brake disc.Regardless of the wear or scratches of the brake discs,thermal fatigue occurs only on the friction surface of the brake disc or somewhere near the friction surface.The laser cladding coating is metallurgically bonded to the matrix,with a narrow heat affected zone and small thermal deformation.This paper applied laser cladding technology to the development of a new brake disc with metal matrix composite.By improving the braking performance and extending the life of the brake disc,the safety and reliability of brake components can be improved and operating and maintenance costs can be reduced.In this paper,the relationship between the laser cladding material and the process parameters and the cladding of the brake disc material for high-speed train is determined through the single factor and orthogonal test of the cladding process parameters,and the selection of cladding material.The microstructure,hardness,element distribution and strengthening of the laser cladding layer were analyzed by microscopic metallography,microhardness,scanning electron microscopy(SEM),X-ray energy dispersive spectroscopy(EDS)and X-ray diffraction(XRD);The mechanical properties of the cladding layer,the bonding strength between the cladding layer and the matrix,and the high temperature wear properties were evaluated by tensile test,micro shear and high temperature friction and wear test;Finally,the inertial braking test is used to evaluate the braking performance of the cladding layer,the matching performancer and wear mechanism between the cladding layer and the brake pads.(1)The Co and Ni-based clad coatings were prepared on the base material of the brake disc by laser cladding by using Ni45,Ni60,Co06 and Ni40 powders.Ni40 and Co06 powder,with no pores and cracks in their coatings,were selected to carry out the single factor and orthogonal test of the process parameters.The regular relationship between the process parameters and the cladding was obtained,according to which guide the superior cladding process parameters of the brake disc were determined.(2)The microstructure of the Ni40 and Co06 cladding layers,from the near-fused line,the bottom,the middle,to the top of the cladding layer,is planar crystal,columnar crystal/cell crystal,dendritic crystal and equiaxed crystal.Co06 and Ni40 cladding monolayer and hardness are about 370HV0.3.The hardness of the heat affected zone is higher than that of the substrate and the cladding layer due to the laser quenching effect.With the increase of the laser power,the hardness of the heat affected zone increases and the hardness of the cladding layer decreases.(3)Co06 has a micro shear strength of 771 MPa in the largest of all microstructures,while the Ni40 cladding layer has a shear strength of 551 MPa and is the smallest in all microareads.Co06 and Ni40 cladding fractures are dendritic and quasi-cleavage fracture.The shear fracture in the fusion zone of Ni40 cladding is located along the interface of cladding layer and the substrate,and for Co06 cladding there was not no fracture in the interface,which illustrates the bonding strength of Ni40 cladding layer interface is relatively low.Compared with the substrate,the tensile strength of the Co06 cladding layer is 1.87 times that of the substrate.(4)In the high temperature friction and wear test,the wear resistance of Co06 cladding layer is the best,followed by Ni40 cladding layer,the matrix material is the worst.For the three kinds of material,under the condition of 600 °C,there are oxidizing and adhesive wear,but relative to the substrate steel casting material,Ni40 and Co06 cladding layer show good oxidation resistance at high temperature,and surface density Cr203 oxide film has a very high stability at high temperature,which can protect the substrate from being oxidized.At the same time,the stable oxide film can reduce the spalling of friction disk particles,and can prevent the friction surface becoming rougher and the occurrence of serious abrasive wear.(5)In the inertial braking test of the matrix material and the cladding layer,the torque stabilization coefficient of the Ni40 cladding layer is the lowest during the braking process,and the fluctuation range of the friction coefficient is the largest,which is beyond the standard range.Co06 cladding has the highest torque stability coefficient.In the beginning of 120km/h and 200km/h in the braking velocity,the average heating rate to reach peak temperature were 25.14 ?/s and 51.67?/s,and the heating rate seen initial speed 200km/h is 2.06 times that of 120km/h.For the initial speed braking of 200 km/h,the maximum temperature of the Ni40 cladding friction disc reached 326?,which is the highest temperature in the friction plate of the three materials.The surface temperature of the friction plate is in banded distribution at the beginning of the brake,and in the middle period is uniformly distributed,and in the final period is distributed close the inner side.In the inertia braking test,the surface of the friction surface of the matrix friction plate has serious furrow wear and oxidation wear.Ni40 cladding friction plate has no furrow and oxidation,but there is a lot of adhesive wear,and Co06 cladding friction disc surface is relatively smooth and smooth.Under the initial braking speed of 200km/h,Co06 cladding friction plate has the minimum wear on the brake,the matrix friction plate and the Ni40 cladding friction plate were 1.19 times and 1.53 times the Co06 cladding friction disc,which shows a good match between the Co06 cladding and the friction brake block during the friction and wear process.