| The brake system is one of the most important components of the high-speed train safety system.During long-term operation,the brake disc surface will be damaged by friction and wear and high temperature thermal fatigue.Because the wear of the brake disc or the thermal fatigue failure usually only occurs on the surface and near surface of the friction surface,it is of great application value to enhance the anti-wear energy of the brake disc surface and the thermal stability at high temperature by surface strengthening technology.In the surface strengthening technology,laser cladding is widely used due to its high interface bonding strength,heat-affected zone and small thermal deformation.A metal-based composite material suitable for brake disc brake conditions is developed by laser cladding technology to improve the friction and wear of the brake disc and the high-temperature thermal stability,In turn,the service life of the brake disc is extended,and the maintenance cost of the train can be reduced.Due to the repeated heat input and the characteristics of fast cooling and fast heat in the process of laser coating,the crack problem is one of the most difficult problems in this technology,and the crack of the coating layer is mainly related to the morphology of the single-channel coating layer and the residual stress distribution of the coating layer.In this paper,according to the choice of cladding powder material,the morphology optimization of single pass cladding layer,the influence of process parameters on the stress of multi-layer and multi-pass cladding layer,the technological parameters of laser cladding and the strategy of stress regulation are determined.The microstructure,hardness,residual stress distribution,element distribution,etc.of cladding layer and matrix were analyzed by means of micro metallography,microhardness,X-ray tester,X-ray diffraction?XRD?,scanning electron microscope?SEM?,X-ray energy spectrum?EDS?.According to the engineering application background of brake disc in this paper,the mechanical properties,high temperature oxidation resistance and thermal fatigue properties of the coating were evaluated by micro shear test,high temperature oxidation test and high temperature thermal fatigue test.The thermal fatigue performance of the coating is optimized by the regulation of residual stress and the heat treatment process to meet the braking conditions of the brake disc.The nonlinear fitting between the process parameters and the morphology of the single channel coating is carried out by neural network.The morphology of the coating layer under different technological conditions is predicted accurately,so as to achieve the purpose of optimizing the morphology.The ELM?Extreme Learning Machine?algorithm can solve the problems of large prediction errors and unstable prediction results in the traditional BP neural network.Finally,the single-pass process parameters with better forming are selected as power2000 W,powder feeding voltage 3 V,powder carrier gas flow rate 3.5 L/min,and scanning speed 280 mm/min.Through the study of the distribution of residual stress in the coating,it can be obtained that the stress peak of the first cladding layer is the largest.With the superposition of the number of layers,the stress peak shifts to the top in a translational manner.The stress peaks of different thicknesses in the cladding layer have stratification,and the layer is remelted between layers.The regional stress peak is significantly improved;when multi-layer cladding is used,the cross-scanning method in the laser scanning path,the control laser energy ratio in the range of 103 J/mm2-143 J/mm2,and the preheating temperature of 100°C can effectively reduce the melting.The cladding has a residual stress.Comparative study on high temperature oxidation resistance and thermal fatigue properties of cladding layer and base metal It can be concluded that the high temperature oxidation resistance of the cladding layer is far superior to the matrix material.and its oxidation index is 1.41,which is higher than 1.03 of the base material,and the oxidation rate is 1.42?10-3,which is much lower than 7.59?10-3 of the base material.After the high temperature oxidation of the matrix material,a porous oxide film is formed on the surface.The oxide film on the surface of the cladding layer is divided into two layers.The first layer is a dense oxide film composed of Cr2O3 and SiO2,which prevents further corrosion of oxygen atoms.The second layer is a polyhedral FeCr2O4 spinel compound,which further enhances the high temperature oxidation resistance.In the fatigue crack propagation of the specimen with the cladding layer,the oxidized corrosion pit will be generated when the crack passes through the interface,and the transverse crack region is formed at the interface,the crack growth rate is greatly reduced,Finally,the crack tip is gradually passivated,and the crack propagation is almost stagnant.The cladding layer is mainly a network of cell crystals,and there is serious segregation.The M7C3 metastable phase exists in the eutectic formation at the grain boundary.There is a small amount of granular M23C6 in the crystal.In the process of thermal cycling,the supersaturated metastable phase M7C3 at the network boundary becomes a crack propagation channel,making it extremely prone to brittle fracture and causing extremely poor thermal fatigue performance.Through the cross scan and the preheating temperature of 100°C to regulate its microstructure and stress,the thermal fatigue performance can be slightly improved,but it still has a big gap with the thermal fatigue properties of the matrix material;It is heat treated at 850°C for 5 h.The process can homogenize the element distribution,eliminate dendrite segregation and internal stress,and transform M7C3 into a relatively stable M23C6,greatly optimize the thermal fatigue performance of the cladding layer,and no cracks are found in the 2000 thermal fatigue tests.Its thermal fatigue performance is far superior to the matrix material. |
PDF Full Text Request