Thermal Stress Analysis And Mechanical Performance Prediction Of A Metal Structure Repaired By Laser Material Deposition

Due to the surface scratch,wear or cracks,the performance and dimension of high-speed train wheelseat axle reduce to a level that cannot meet the design requirement after a period of service.Reasonable and efficient maintenance of those worn and valuable parts yields both economical and ecological benefit.Laser material deposition(LMD)is capable to repair such metal parts,however,this technology involves the use of rapid melting and solidification,which will result in residual stress in the repaired structure.Since the residual stress cannot be avoided by nature,and its distribution and magnitude are significantly influenced by both the process parameters and geometric morphology of the repaired structure.Nevertheless,the quantitative analysis of residual stress field is the basis of evaluating the mechanical performance of a repaired structure.Therefore,to evaluate the residual stress and mechanical properties of a repaired structure,we conducted some work shown as follows:(1)Based on the standard Gaussian heat source,a combined heat source model is proposed for the thermal simulation of conventional LMD and extreme high-speed laser material deposition(EHLA)processes,and the accuracy of the model is validated by experiments.Compared with the conventional heat source models,the combined heat source model comprehensively takes the characteristics of energy on the workpiece surface and penetration direction as well as the shape of molten pool under different process parameters into consideration,therefore,the numerical results are closer to the experiment.The results show that the portion of energy entering the substrate,the energy weight parameter of Gaussian heat source and the shape parameter of the exponential heat source decrease with the increases of the scanning speed.(2)The temperature characteristics and residual stress filed of LMD and EHLA processes are analyzed based on the thermal-elastic-plastic finite element method,considering the factors such as the movement of heat source,the change of heat exchange interface,the overlap rate,the thermal action of continuous cladding and the latent heat of material phase transformation.Taking the transient temperature results as the load in the structural analysis,the distribution of residual stress field in the structure after cladding is obtained.The results show that the cooling rate of EHLA is about 10 times higher than that of LMD,which is close to that of laser powder bed fusion(LPBF);The numerical results of residual stress are in good agreement with those measured by electronic speckle pattern interferometry assisted hole drilling method.The peak residual tensile stress in the penetration direction appears in the sub-surface of the depisition,and there is compressive residual stress in the outermost cladding layer.(3)A method of directly reconstructing the entire residual stress distribution of the repaired structure by using limited input data is proposed.For the engineering structure with large size and complex geometry,it is both time consuming and difficult to predict the residual stress by incremental method.The eigenstrain method is applied to reconstruct the initial stress state of the whole structure with incomplete data set for welding and laser cladding structure.The order of basis function,weight coefficient,point selection strategy and the amount of data on the construction results is discussed by data testing.The results show that the residual stress field reconstructed by the eigenstrain method is in good agreement with the experimental results and finite element results.Moreover,both its calculation efficiency and accuracy are very high.This method can be used to effectively reconstruct the residual stress field introduced by thermal process.(4)A method for evaluating the mechanical performance of structures with residual stress after LMD or EHLA is proposed.Firstly,the structure is considered as an ideal forged material with initial stress,therefore,residual stress is defined and loaded as in the deposited zone to study the influence of the initial stress field on the mechanical performance of the repaired structure.Secondly,the cladding material is regarded as a new material completely different from the base metal.By defining two different material properties to distinguish the base and cladded material,the mechanical properties of a repaired structure under the working conditions of operation,traction,curve and braking is analyzed.The results illustrate that the mechanical performance of the two methods are very close,and it meets the design requirements.