| Laser cladding technology is typically used in the repair process.Due to its high energy density and processing accuracy,wide material selection and good interface metallurgical bonding,it is one of the key technologies for the repair of damaged parts.It can extend the service life of high valued parts and improve the surface performance of ex-service parts,which is in line with the national strategic needs of building a circular economy.Nickel-based superalloys have a wide range of applications in aerospace,marine,nuclear,and petrochemical industries due to their good high temperature mechanical properties and corrosion resistance.Laser cladding technology has unique advantages in the rapid repair of complex parts based on nickel.In addition,the laser cladding layer has a good grain growth orientation and is superior to conventional casting alloys.However,research on the phenomenon of dendritic turning growth behavior and elemental segregation control between dendrites during laser rapid solidification is limited,which impedes the control of the orientation of alloy crystals and the properties of integrated alloys in laser additive manufacturing.In this paper,the lasercladded nickel-base superalloy IN718 is studied.The main research contents include the following: preparation and microstructure analysis of laser cladded IN718 layer.The diverging growth morphology and crystal orientation of the top dendrite of the cladding layer are investigated.Combined the experimental and numerical simulation results,the thermal evolution of the molten pool at the top of the cladding layer is studied.The results of Electron Backscattered Diffraction(EBSD)revealed the origin of crystal grain in the top dendritic transition region.Through the electroless plating of nickel on the surface of carbon nanotubes(CNTs),the CNTs reinforced IN718 laser cladding layer was fabricated using nickel-plated CNTs(NiCNTs)with two types of coating thicknesses.The structural evolution of CNTs under the high energy irradiation of laser beam and high hermal heating of the molten pool,and the effect of CNTs addition on the element segregation and inhibition of Laves phase during laser rapid solidification were studied.The effect of NiCNTs content on corrosion resistance,wear resistance and tensile properties of laser cladding layer were also analyzed.The specific research contents and results are as follows:(1)Laser cladding of IN718 alloy with good forming,low dilution rate,no defect and good metallurgical bonding with matrix can be prepared by optimizing the process parameters.By the characterization of the dendritic microstructure,and EBSD crystal orientation of the laser cladding layer,it is proved that the top dendritic morphology transition of the IN718 laser cladding layer is attributed to the enlarged angle between the growth direction of the dendrite and the local temperature gradient,the dendritic crystals are largely deviated from the opposite direction of heat flow which is not traditionally considered dendritic columnar to equiaxed transition.(2)Through the numerical simulation combined with the experimental results,the growth transition mechanism of the dendrites at the top of the cladding layer was analyzed,and the thermal evolution process of the molten pool formed by the top turning dendrites was revealed.It is found that in the last stage of solidification of the molten pool,the direction of the maximum temperature gradient is near vertical to the laser scanning direction at the bottom of the molten pool,whereas it is vear pallel to it at the top of the molten pool,resulting in the transformation of the dendritic topography of the laser cladding layer.The finite element simulation results show that for laser cladding of IN718 alloy,the critical angle of deflection of dendritic growth direction is 70°~80°.The EBSD test results show that the crystal grain of the top transition dendrites is mainly origined from the re-nucleationsites in the top supercooled liquid at the last stage of solidification.(3)By electroless nickel plating on the surface of CNTs,two different thicknesses of nickel-plated carbon nanotubes Ni1 CNTs and Ni0.5CNTs were obtained and incorporated into the laser cladded composite alloy.Microstructure analysis show that the addition of CNTsis beneficial to the grain refinement of IN718 alloy.The addition of CNTs can effectively inhibit the element segregation and Laves phase formation during the solidification of IN718 alloy,and the inhibition effect increase with the increase of the electrolessly plated nickel layer.(4)Analyze the effect of Ni0.5CNTs content on the corrosion resistance,wear resistance and tensile properties of laser cladded Ni0.5CNTs / IN718 composite alloy.With 50 wt.% Ni0.5CNTs addition,the laser cladded IN718 composite alloy exhibits the highest corrosion resistance.The electrochemical corrosion resistance of the cladding alloy is mainly affected both by the beneficial effect of CNTs and their nano products in NiCNTs composite powder,and the harmful dilution of the Ni element.The IN718 laser cladding alloy with 30 wt.% Ni0.5CNTs has the highest wear resistance.CNTs have good self-lubricating properties,and their nano products under laser irradiation are pressed and adhered to the surface of the composite coating during sliding wear to form a lubricating layer.The lubricating layer reduces the direct contact between the abrasive material and the composite coating,thereby reducing the coefficient of friction and wear rate of the composite coating.The tensile strength and yield strength are related to the addition of Ni0.5CNTs.By the addition of Ni0.5CNTs,the tensile strength and yield strength of the composite alloy are both improved,which is attributed to the unique structural characteristics and composite materials of the nano products of CNTs,as well as the finer microstructure. |
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