Study On Microstructure Regulation And Fracture Mechanism Of EA4T Axle Remanufactured By Laser Cladding

In the course of service,the axle is subjected to a variety of stresses and corrosion effects,resulting in varying degrees of damage,which leads to axle scrapping.To reduce the cost of manufacture,the remanufacturing of axles becomes an important subject.Laser cladding technology has the advantages of low heat input and high degree of automation,which is a promising technology in axle remanufacturing.In this work,the effects of powder and its preparing technique on the performance of EA4T axle after remanufacturing was investigated by theoretical analysis,experimental method and numerical simulation.The reaction thermodynamics of each element in iron-based alloy powder was calculated and analyzed during solidification.Then,the powders for laser cladding were designed and Fe Cr Ni Mo RE alloy powder was proposed.The laser cladding remanufacturing technique of EA4T axle was optimized using the iron-based alloy powders,and the microstructure and interface composition of cladding layer were regulated.With the aid of optical microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）and X-ray Diffraction（XRD）,the microstructural evolution of remanufactured sample was investigated.The performance of remanufactured sample of EA4T axle was further analyzed by tensile test,micro-shear test,hardness test and fatigue test.Some results were observed and summarized as follows:1.Computational analyses of the thermodynamical reactions of each element in cladding layer found that,Cr B₂,Mo C and VC can be formed through adding element M（B,Mo and V）in Fe-Cr-Ni-C alloy.The formation of MC phase can refine the microstructure of cladding layer.Based on the matching principle of strength and toughness with substrate,the EA4T axle was remanufactured using laser cladding with Fe314（No.1）and Fe Cr Ni Mo RE alloy powder（No.3）,on the condition of 2400W laser power,5.0 mm/sec scanning speed and 4.0V powder feeding voltage.High quality metallurgical bonding was formed between the cladding layer and the substrate.The maximum temperature gradient direction in the cladding layer varied from the direction perpendicular to the substrate surface at the bottom of the cladding layer to the direction parallel to the laser travels at the top of the cladding layer,thus forming the gradient structure.In the bonding zone,the tissue grew slowly from the bottom to the surface at a planar interface.The cladding zone was mainly composed of cellular crystals,columnar crystals,dendritic crystals and equiaxial crystals from the bottom to top.2.The rare earth elements took part in the metallurgical reaction of Fe Cr Ni Mo RE alloy powder,and purified the melt with the combination of O and S.The formed rare-earth compounds can promote the heterogenous nucleation,and also prevented the growth of grain;therefore,the grain of the cladding layer was refined,and the strength and toughness of remanufactured sample was improved.3.Fe Cr Ni Mo RE laser cladding alloy powder with excellent mechanical properties and fatigue resistance was designed to repair the damage in the axle.The strengthening mechanism of the cladding layer was revealed by thermodynamical calculation and non-equilibrium solidification theory.Compared to the substrate,the properties of the remanufactured Fe Cr Ni Mo RE alloy powder sample were improved;for instance,the tensile strength was increased from 828MPa to 932MPa,the yield strength was increased from670MPa to 735MPa,the strain hardening exponent improved from 0.1389 to 0.1433,and the fatigue limitσ_-1 reached 429MPa comparing with its original value of 412MPa.4.The fatigue strength of matrix and remanufactured sample was investigated by rotating bending fatigue method.The mathematical model of S-N relationship curve was established,and the fatigue damage initiation mode and damage evolution process were revealed.Nf was expressed by the formulas,_Nf（28）26.?10⁹（S_a-400）^2-,N_f（28）.82?10⁸（S_a-375）^2-andN_f（28）.41?10⁸（S_a-425）^2-for the substrate,Fe314 powder remanufactured sample,and Fe Cr Ni Mo RE alloy powder remanufactured sample,respectively.The fatigue limit of substrate was lower than that of sample repaired by Fe Cr Ni Mo RE alloy powder,while it was higher than that of Fe314 powder remanufactured sample.When the stress ratio R was 0.1,the fatigue crack growth rate of Fe314 powder remanufactured sample was basically accordance with that of substrate sample at stable growth stage（II）,but the fatigue crack growth rate of Fe Cr Ni Mo RE alloy powder remanufactured sample was slightly lower than that of substrate sample.5.EA4T axles were remanufactured by laser cladding with Fe314 powder and Fe Cr Ni Mo RE alloy powder,and their properties were compared with substrate sample.The results showed that the properties of laser cladding remanufactured sample with Fe314powder were slightly lower than those of substrate,but the properties of laser cladding remanufactured sample with Fe Cr Ni Mo RE alloy powder were similar to substrate.This remanufacturing technology can help to prolong the life of EA4T axles.EA4T axles were remanufactured by laser cladding with Fe314 powder and Fe Cr Ni Mo RE alloy powder,and their properties were compared with matrix sample.The results show that the properties of sample laser cladding remanufactured by Fe314 powder are slightly lower than those of matrix sample,but Fe Cr Ni Mo RE alloy powder laser cladding remanufactured sample are close to matrix sample.This remanufacturing technology can help to prolong the life of EA4T axles.