Fatigue Life Evaluation On The Laser Cladding Remanufactured Product Coupled With Thermal Damage

The phenomenon of heavy mechanical fatigue and wear between wheel and rail is particularly prominent as high speed and heavy haul trains are widely used in China railways with the rapid development of rail transit industry.Huge economic losses were caused because of large number of condemned wheels.However,if these scrapped wheels could be remanufactured by new technologies,not only the pollution is reduced while saving energy,but also manufacturing time and labor costs could be saved.In addition,significant economic value could be created.There are many methods to realize remanufacturing including laser cladding technology which causes widespread concern by scholars at home and abroad.Laser cladding technologycould form strong metallurgical bond between the cladding material and the substrate resulting in very small heat affected zone with low dilution rate and small thermal expansion.In addition,the cladding process not only can be used for many materials but also iseasy to realize industrial automation.Theoretical and experimental analysis were carried out on the fatigue life prediction method for laser-cladding remanufactured products in the thesis.This advanced point and main conclusions are summarized as follows:(1)The formation mechanism of laser cladding thermal damage was studied through numerical simulation and experimental.Ni60 alloy powder was used to repair artificial defects created on the specimen of Q345 R steel,and the residual stress,static and fatigue tensile properties of the specimens were studied.The result indicates that cladding thermal damage really exists because there are larger tensile stress in cladding specimens,and tensile and elongation of cladding specimen was less than the Q345 R steel substrate specimen,and the fatigue life of cladding specimens was drastically reduced compared with standard specimen.(2)Chaboche's model of nonlinear continuum fatigue damage was modified based on the impact of crack closure effect on fatigue damage.A cumulative fatigue damage model was developed accordingly to predict the remaining life for condemned products to be remanufactured.The related parameters were concluded from fully reversed fatigue experiment.To verify the proposed model,tension-compression fatigue tests under two levels of load spectrum(high-low loading and low-high loading)were carried out.The results showed that the calculated value and experimental value were in good agreement.(3)Chaboche nonlinear continuum damage model was modified considering the residual stress and crack closure effect.Related parameters in the revised model were obtained through the tension-compression fatigue test to build quantitativelythe initial thermal damage model and the thermal-fatigue interactive damage model.(4)The metal magnetic memory technology was tried to detect the thermal damage and the thermal-fatigue interactive damage of the laser cladding remanufactured product.Parameters based on the magnetic memory signal were proposed for quantitative characterization of the initial thermal damage and the thermal-fatigue interactive damage.(5)The modified nonlinear continuum damage model and energy method were employed to evaluate the fatigue life of laser-cladding remanufactured specimens.Two fatigue life models are proposed,which could be used to predict the remained life of laser-cladding remanufactured wheels for high-speed railway vehicle.The results show that the proposed models can serve the engineering applicationwell.