Study On In-situ Synthesized Mechanism And Property Of Tungsten Carbide Reinforced Ni-based Coating By Laser Cladding

The coating of ceramic phase particles reinforced metal matrix composite is widely employed in industry sectors,such as mechanic,electronic,aerospace,energy and petroleum,etc.,because of its high hardness,high wear resistance and mechanical behavior under high temperature.Nowadays,the preparation process of laser cladding draws much attention from specialists and scholars,because it has low heat input,less deformation on the workpiece,and good metallurgical combination between coating and substrate.However,due to the great difference of the thermophysical properties between the ceramic phase and the metal matrix,it is easy for the prepared coating to generate high thermal stress during the process of rapid heating and cooling by laser cladding.Therefore,the high stress concentration is liable to occur,which can cause crack initiation,especially at the interface between the ceramic phase and the metal matrix.These facts greatly hinder the further application of composite coating in industrial field.In-situ synthesis of metal matrix composite depends on the alloy component design,which utilizes chemical reaction to synthesize one or more ceramic or intermetallic compounds reinforcement phases in the alloy system.These synthesized reinforcement phases have high hardness and high elastic modulus,which plays an important role in strengthening the metal matrix composite.Meanwhile,the interface combination between the in-situ synthesized ceramic phase and metal matrix phase is clean.The interface can be more mutually accommodating after adjusting the atomic arrangement between interfaces in the solidification process.Therefore,the coating possesses strong interface combination,and the thermal stress can be relieved with crack being avoided as well.The synthesized reinforcement phase particles distribute homogeneously in the coating.This preparation technology provides a new way for the application of ceramic particles reinforced composite coating.The ceramic phase particle WC reinforced Ni-based coating is prepared by in-situ method of laser cladding in this paper.Many measures,such as laser cladding parameters,preparing technology,powder mixture ratio,rare earth doping,are used to investigate the coating.The morphology characteristic of in-situ synthesized WC and the evolution rule are revealed.The effect of process parameters on the microstructure and properties of the coating is elucidated.The interface characteristic of WC particle and Ni matrix phase is revealed.The mechanical properties of in situ synthesized WC reinforced Ni coatings are characterized.The main wear mechanism of the composite coatings is disclosed.The research results of this paper are as follows:?1?The intrinsic characteristic of WC and the electronic structure and bonding characteristic of WC/Ni interface are studied by using the first principles.It is found that the WC crystals mainly consist of covalent bonds,and some of them are metal bonds and ionic bonds.The covalent bonding mainly consists of the hybridization between W-5d with C-2p,while partial electron filled in the W-5d valence band is the main reason of the metallic bond formation.The ionic bond of W-C is ascribed to the electric charge transformation from the W atom to the C atom.Six kinds of interfaces constructed by OT,MT,and HCP configurations combined with W-termination and C-termination are investigated.The result shows that the highest interface energy is W-termination with HCP structure.It is also shown that the interface energy of W-termination with HCP structure is less than that of C-termination with HCP structure over the chemical potential range of?_C^slab-?_C^bulk,indicating that WC crystal has preferential formation with W-termination and HCP configuration.The research on the charge density distribution,the differential charge density distribution,and the density of states of each atom indicates that the interface charge redistribution presents obvious localization characteristics.The charge redistribution mainly exists on both sides of the interface.The formation of chemical bonding is mainly contributed by the electrons of Ni-d,W-d and C-p orbitals.?2?Based on Gibbs-Wulff theory,crystal growth thermodynamic law and Bravais-Friedel-Donnay-Harker?BFDH?crystal growth kinetic model,growth rate of each WC crystal face is calculated.Results show that the growth rate of each grain face accords with the order{0 0 1}<{1^—1 0}={1 1^—0}<{1^—1 1}={1 1^—1}<{1^—1 2}??.Combined with the calculated results of Ni?1 1 1?/WC?1^—1 0?and Ni?1 1 1?/WC?11^—0?interface energy,the evolution rule of WC morphology with“spherical polyhedron?polyhedron?hexagonal prism?triangular prism”is illustrated.?3?Based on the preparation technology of multilayer cladding combined with the character of WC particles being easy to deposit in the laser welding pool,5 cladding layers are prepared according to the initial powder mass ratio,which consists of W powder and C powder according to the 30%?40%?50%?60%and 70%mass ratio orders.The result shows that the area fraction of in-situ WC particles can reach 81%.By contrast,for the initial powder mass ratio with 70%based on single-layer laser cladding technology,the area fraction of in-situ WC particles is 68%.The maximum hardness can be up to 942 HV in the prepared cladding coating with high area fraction WC particles,indicating that an excellent mechanical property is obtained.?4?In-situ synthesized WC reinforced Ni coating has been modified by adding rare earth CeO₂ with different mass ratio.Results show that the size of WC particles gradually gets smaller,and the morphology of WC gradually changes from sharp corner to be smooth with the increasing mass ratio of CeO₂.The calculated results of Ni/WC interface by rare earth Ce doping show that the charge loss of Ce atom has the obvious direction,which in turn,indicates that W atoms and C atoms have the strong bonding interaction,changing the interface energy of Ni/WC interface.The above results reveal the evolution mechanism that WC crystal morphology changes from much angles to being smooth.?5?The friction and wear behaviors of the following coating are investigated,such as the in-situ synthesized WC reinforced coating,the ex-situ WC reinforced coating,and the rare earth doping modified in-situ synthesized WC reinforced coating.The main friction and wear mechanism of the reinforced phase particles are revealed for these reinforced coatings.Results show that the main wear mechanism is micro-cutting effect of abrasive particles applied on the coatings associated with abrasive wear and adhesive wear mechanisms.The hard phase WC mainly resists the friction force and the load,while the soft Ni matrix phase counteracts the action of external force by plastic deformation.Due to the low interface bonding strength between WC particles and matrix for the ex-situ coating,WC particles are easy to fall off the coating under high loads and then changes to be hard phase abrasives between the friction pare and coating.That result leads to an improvement of abrasive wear mechanism,which aggravates the wear of coating.The friction cutting force of the rare earth doping modified coating is more homogenous because the WC particles change to be smooth,which effectively enhances the anti-wear property.