Research On Process,microstructures And Properties Of Tungsten Carbide Particles Reinforced Wear Resistant Coatings Fabricated By Laser Wire Deposition

Owning to the excellent wear resistance of tungsten carbides reinforced wear resistant coatings,they have been widely applied for surface repairing and strengthening of mechanical components.Compared with the traditional methods including arc welding and thermal spraying,laser wire deposition has several advantages including higher material utilization and improved deposition efficiency.However,the consumables are almost the metal solid wire in the field of laser deposition,so the composite coatings will not be obtained directly.It is of great importance to develop the wire-based laser deposition method to produce ceramic particles reinforced wear resistant coatings.Based on the previous studies,the method for fabricating tungsten carbides reinforced coatings by laser deposition with the tubular cored wire feeding is proposed in this study.The processing characteristics,microstructural evolution and the wear resistance of the coatings have been analyzed.Firstly,surface coating experiments have been carried out on Q345 steel by laser wire deposition with the Fe-based and Ni-based tubular cored wire feeding,combined with the technology of high-speed camera,the wire melting behaviors during laser wire deposition have been observed and analyzed.The results show that when the front feeding is applied and the wire tip is placed at the front region of the molten pool,three typical wire transfer modes including liquid spreading transfer,wire plunging transfer and wire stubbing transfer can be obtained under different combinations of processing parameters.The process windows of Fe-based and Ni-based coatings fabricated by laser wire deposition have been established,and the geometrical characteristics of these coatings have been statistically investigated.It is found that the continuous smooth coatings with low dilution ratio and high-volume fraction of retained particles can only be obtained under the wire plunging transfer mode.The microstructures of Fe-based and Ni-based coatings fabricated by laser wire deposition under different wire transfer modes have been systematically investigated.（1）Fe-based coatings are mainly composed of retained WC/W₂C particles and the Fe-based solid solution,the precipitations formed in matrix are mainly the M₆C carbides with blocky and herringbone shape.Most of the retained particles in the coatings show the typical dissolution characteristic of reaction-diffusion pattern,the WC/W₂C particles can react with Fe-based matrix in the molten pool,which results in the M₆C reaction layer and the M₆C carbides around the retained particles.（2）Ni-based coatings are mainly composed retained particles and the matrix（including the nickel-based dendrites and interdendritic Ni/Ni₃B eutectic structure）.The precipitations in these coatings are mainly the in-situ WC particles and M₆C carbides.Most of the retained particles in the Ni-based coatings show the typical dissolution characteristic of decomposition-diffusion pattern.The W₂C phase in the WC/W₂C particles can be preferentially decomposed into fine particulates and finally be dissolved into the molten pool,while the WC phase in the WC/W₂C particles can not be dissolved obviously but remained intact,resulting in the WC interfacial layers around the retained particles.（3）The content of M₆C carbides in the Ni-based coatings is increased when the wire pluning transfer is changed to the liquid spreading transfer mode.On the basis laser wire deposition,laser hot-wrie deposition is introduced and the effects of heating currents（I_p）on the appearance and microstructures of the coatings have been investigated.The wire stubbing transfer and poor formation of the coating can be obtained when the insufficient heat input is employed for the given feeding material.When I_p is in the range of 55A～80A,the wire stubbing transfer can be changed to the stable wire plunging transfer mode,and the unmelted defects can be successfully eliminated.The microstructures of Fe-based coatings are not obvious different from that obtained in laser cold wire deposition.However,the in-situ reinforcements in Ni-based coatings are changed when I_p is increased.When I_p is improved to 55A～80A,in-situ W₂C particles are homogenously distributed in the coatings.As I_p is increased to80A～100A,the dominated carbides precipitated in the coatings are the M₆C phases.According to the microstructural characteristics of Ni-based coatings,laser remelting has also been used to regulate the precipitations in Ni-based coatings,and the effects of different heat inputs on the microstructures of remelted coatings have been investigated.The in-situ W₂C particles homogeneously distributes in the as-deposited Ni-based coating fabricated by laser hot-wire deposition under the wire plunging transfer mode.When the as-deposited coatings are remelted by the low-level heat inputs（800J/mm,1000J/mm）,the in-situ W₂C particles are redissolved in the molten pool and the in-situ WC particles are formed and homogeneously distributed in the remelted coatings.When the as-deposited coatings are remelted by the high-level heat inputs（1200J/mm,1400J/mm）,the precipitations in the remelted coatings are mainly the M₆C carbides.Combined with TEM characterizations,the theory of crystal morphology evolution and the first principle calculations,the typical morphologies of the in-situ W₂C and WC particles in the Ni-based coatings have been investigated.The results show that the in-situ W₂C particles have the morphology of regular hexagonal prism,while the in-situ WC particles show the morphology of triangular prism.The wear resistance of Fe-based and Ni-based coatings have been investigated,and the wear mechanisms are analyzed.Experimental results show that the wear resistance of Fe-based and Ni-based coatings prepared by laser wire deposition is about14.2～32.9 and 19.0～39.0 times higher than that of Q345 substrate,respectively.There are two major wear mechanisms including abrasive wear and the adhesive wear in the wear process.When the wire plunging transfer is changed to the liquid spreading transfer mode,the coating wear resistance is decreased,and the main wear mechanism is changed from abrasive wear to adhesive wear.When the method of laser hot wire deposition is employed to fabricated Ni-based coatings when I_p is in the range of55A～70A,the wear resistance of these as-deposited coatings obtained under wire plunging transfer mode is about 34.7～41.7 times higher than that of substrate.When these as-deposited coatings are remelted by the relatively low heat inputs（800J/mm,1000J/mm）,due to the homogeneously distributed in-situ WC particles in the matrix,the wear resistance of these remelted coatings is further increased to 1.1 and 1.2 times higher than that of the as-deposited coatings.While when the relatively high heat inputs（1200J/mm,1400J/mm）are used to remelt the as-deposited coatings,the relative wear resistance of these remelted coatings is decreased by 50%and 57%,respectively.Based on laser wire deposition,the high-quality Ni-based coatings with uniform distribution of retained WC/W₂C particles and in-situ WC particles can be successfully fabricated by introducing the technologies including laser hot-wire deposition and laser remelting,which can significantly improve the wear resistance of steel and definitely own expansive potential applications.