Study On Microstructure And Properties Of Laser Composite Repair Technology On Punch Surface Of Automobile Universal Joint

H13 steel is widely used in the extrusion die of automobile transmission parts.In actual working condition,the die surface is often damaged by wear,erosion and thermal fatigue because of high intensity alternating stress and cyclic thermo-mechanical load.The traditional repairing technology not only causes a large amount of thermal deformation to die,but also pollutes the environment.Therefore,laser cladding（LC）as a new repair technique,can effectively repair the damaged surface of die.However,it tends to promote the crack sensitivity and reduce the fatigue strength due to the large residual tensile stress existing in the cladding coating.As a surface strengthening technology,laser shock peening（LSP）can effectively improve the stress distribution,microstructure and wear resistance of the cladding coating.Therefore,laser hybrid repairing technology was adopted to prepare Ni25/Fe104 composite coating.The research conclusions are listed as follows:（1）The influence of laser processing parameters on the macroscopic and microscopic morphology of Ni25 coating was investigated,and the microstructure of Ni25 coating was characterized prepared with the optimal parameters.The results showed that increasing the laser power or decreasing the scanning speed can increase the heat input per unit time,and the increase of the heat input tends to contribute to the dilution rate and internal thermal stress of the coating.Excessive thermal stress will also lead to cracking of the coating.The optimal Ni25 coating specimen can be obtained when the laser power is 1600 W and the scanning speed is12 mm/s,which is well bonded with the substrate without any obvious defects.It can be observed that equiaxed grains,columnar grains and cellular grains are distributed from top to bottom of the Ni25 coating.The microstructure of the Ni25 coating is composed of dendritic and interdendric structure,and the main composition of them isγ-Ni,with a small amount of M₇C₃（where M represents Fe and Cr）carbides in the interdendritic structure.（2）By comparing the microstructure and phase composition of the Fe104 coating with or without Ni25 transition layer,the element diffusion phenomenon in the interlayer region and its influence mechanism on the microstructure evolution were investigated.The bonding quality between pure Fe104 coating and substrate is relatively poor with a few microcracks in the bonding region.The coating is divided intoα-Fe,γ-Fe and a small amount of M₂₃C₆ and M₇C₃ carbides.The microstructure of the cladding layer is also dendritic structure,and the dendritic structure mainly contains lamellar martensite structure.The interdendritic structure is a mass structure with high density combined with martensite and carbide.For Fe104/Ni25 coating,due to the good wettability and fluidity of Ni25 alloy,good metallurgical bonding occurs in the bonding region between Ni25 layer and substrate,which effectively restrains the growth of potential cracks.In addition,a smooth boundary is observed at the Fe104/Ni25 interface,which indicates that the interlaminar bonding between the Ni25 alloy and the Fe104alloy is good.The results showed that a large number of Ni elements are diffused from the Ni25 layer to the Fe104 layer due to the Marangoni convection,resulting in a large number of columnar dendrites with fully grown secondary dendrite arms in the middle region,and a large number of equiaxed grains in the top region of the Fe104 layer.At the same time,γ（Fe,Ni）becomes the main component of Fe104 layer due to the suppression of martensitic transformation by Ni diffusion.In addition,a small quantity of Fe-Cr intermetallic compounds with the same orientation distributed in theγ（Fe,Ni）matrix.（3）The microstructure,residual stress,microhardness and wear performance of Ni25/Fe104 coating before and after LSP treatment were compared,and the strengthening mechanism of LSP on the microstructure and mechanical properties of the coating was explored.Under the mechanical action of LSP,the stress state in the Ni25/Fe104 coating changes from tensile stress to high-level compressive stress with the peak value is 396.5 MPa at a depth of 100μm,and the maximum affected depth of LSP on the residual stress is 750μm.In addition,the shock wave generated by LSP induced a strong plastic deformation in the Ni25/Fe104 coating to form a hardened layer with a thickness of appropriately 800μm,where high-density dislocation structures（dislocation entanglement and dislocation cell）and a large number of nano-twins generate.Therefore,the grains in the surface layer are refined to nano-crystallization and the microhardness is increased by about 31.8%.The wear mechanism of Ni25/Fe104 coating before LSP is mainly abrasive wear,accompanied with a small amount of adhesive wear.After LSP treatment,the coefficient of friction and wear rate of Ni25/Fe104 coating are reduced,and the scale of fatigue spalling is also decreased.The strengthening mechanism of LSP on Ni25/Fe104 cladding layer includes three strengthening ways:nano-grain strengthening,nano-twinning strengthening and dislocation strengthening,and the refined Fe-Cr intermetallic compounds in the interdendritic structure also contributes to the improvement of the wear resistance.In summary,the research content of this paper provides a reference for the study of the laser hybrid repairing damaged molds and the influence of hybrid technology on the microstructure and wear resistance of composite coating.