Microstructure And Mechanical Properties Of Laser Cladded 316L Stainless Steel By Hard Turning(HT)and Ultrasonic Surface Rolling(USR)

Laser cladding technology has the advantages of simple operation,convenient automation and increasing material utilization.It has become a hot topic of laser surface modification research at home and abroad.However,the parts remanufactured by laser cladding can't meet the requirements of size and precision,which limits the application of laser cladding technology.Therefore,it is of important significance to study the post-treatment process of laser cladding technology.The 45#steel stirring shaft is adopted by Jiangxi Yinhai mining co.,LTD,which is repaired by laser cladding technology after wear and tear.The repaired stirring shaft has low size accuracy,poor surface performance,residual thermal stress and other defects.For such problems,the post-treatment process of laser cladding technology is discussed in this paper.And it is expected that the parts repaired by laser cladding technology can not only meet the requirements of size,but also reduce the thermal stress and improve the surface performance.The Cr-Ni alloy powder was deposited by laser cladding(LC)on the surface of 45#shaft and then processed by hard turning(HT)and combined HT with ultrasonic surface rolling(USR).USR was introduced to improve surface properties of LC layer by exerting severe plastic deformation.Microstructure,surface roughness,surface residual stress,and mechanical properties(microhardness,elastic modulus,surface fracture toughness and wear resistance)of the cladding layer after LC,HT and combined HT with USR were investigated by optical microscope(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD),roughness measuring instrument,nano-indenter and wear testing machine.The results reveal that USR can neither change the element distribution on the surface of laser cladding layer nor induce the formation of new phases.After USR,the texture on the surface is weakened.Fine particles layers with thickness of 2.5 ?m and 2?m are observed on the topmost surface of USR and HT samples,and the particle sizes are 245 nm and 510 nm respectively,but not on the surface of LC samples.However,obvious grain refinement and change of grain morphology in the subsurface and interior of cladding layer are not observed in this work.The surface properties of the USR sample are greatly improved.Compared with HT,the surface roughness decreases by 88.5%,the surface microhardness increases by 34.6%,the surface residual stress changes from tension(449.6 MPa)to compression(-334 MPa),the surface elastic modulus changes little,and the surface fracture toughness increases by 106%.However,the wear resistance of USR samples become worse and the corrosion resistance changes little.Under the experimental parameters,the hardened depth of y-z cross section is about 140 ?m.Above results indicate that USR can improve the surface properties of laser cladding layer as a post-treatment process,which has a certain grain refinement,work hardening effect and transforms residual tensile stress into residual compressive stress.