Study On The Evolution Of Residual Stress In Multi-layer And Multi-pass Laser Cladding

The hob of the shield machine is the core component of rock cutting and rockbreaking in the tunnel excavation process.It is subjected to severe impact and vibration during work,and is easily worn and consumes a lot of money.Therefore,remanufacturing worn hobs can greatly reduce construction costs.Laser cladding remanufacturing technology can endow parts with wear resistance and fatigue resistance through the selection of cladding materials,and can also obtain a certain size of cladding by lapping and stacking cladding materials layer by layer.Layer,which has great potential in the remanufacturing of shield machine hobs.However,in the remanufacturing process of multi-layer and multi-pass laser cladding,the laser energy density is concentrated,the temperature field is extremely non-uniform,and the properties of the cladding materials are different,which ultimately leads to the generation of residual stress in the cladding layer after forming.The high residual stress state can easily cause unfavorable phenomena such as deformation and cracking of parts and components,which is a problem of " neck sticking " in the application of this technology.Aiming at the above problems,this thesis systematically studied the influence mechanism of process parameters and scanning paths on the residual stress distribution and microstructure properties of multi-layer and multi-pass cladding layers,and discussed the method of adjusting the residual stress level and distribution through optimization of process parameters and scanning paths,and applied the optimized process parameters and scanning path to the cladding of the hob surface of the shield machine.The main research work of this thesis includes:(1)The powder M was used to conduct multi-layer and multi-pass laser cladding experiments on the Q345 substrate,and the effects of different process parameters on the residual stress distribution and microstructure and properties of the cladding layer were studied.The results show that as the laser power increases within a certain range,the cladding stress first increases and then decreases,and the tensile stress of the heataffected zone first decreases and then increases.When the scanning speed changes,the residual stress presents the same changing trend.When the laser power is 1.8k W and the scanning speed is 0.02m/s,the average compressive stress of the cladding layer is the largest,the value is-468.85?-437.29 MPa,and the average tensile stress of the heataffected zone is the smallest,the value is 199.06?248.18 MPa,the matrix and the cladding layer and the layer-to-layer bonding are good,the structure is compact and defect-free,the fine-grain strengthening effect is obvious,and the hardness of the cladding layer is increased.(2)Under the same process parameters,the laser scanning path was changed to conduct multi-layer and multi-pass laser cladding experiments,and the influence of different scanning paths on the residual stress distribution and microstructure properties of the cladding layer was studied.The results show that when the scanning path is "twolayer laser vertical cross scanning",the linear accumulation of residual stress in each layer can be effectively avoided.The influence of the scanning path change on the microstructure of the cladding layer is mainly manifested in the growth direction of dendrites and the size of equiaxed grains.When the scanning path is "both layers of laser scanning along the length of the board",the grains of the upper and lower cladding layer fusion areas are refined and the hardness increases.(3)Based on the best process parameters and scanning path,powder N was used to conduct multi-layer and multi-pass laser cladding experiments on the shield hob material H13 steel.The experimental results show that the average compressive stress inside the cladding layer of the hob sample is-452.8?-350.43 MPa,and the surface quality of the cladding layer is good and no cracks are found.As the temperature gradient G and the solidification rate R change,the microstructure of the cladding layer sequentially presents the morphology of plane crystals,cellular crystals,dendrites and equiaxed crystals from bottom to top.Compared with the matrix,the hardness of the cladding layer is significantly improved,up to 900 HV or more.This thesis has 61 pictures,10 tables,and 88 references.