Study On Microstructure And Properties Of Cobalt-based Coatings Prepared On Cast Iron Via Plasma Cladding

Compacted graphite cast iron has been widely used in mechanical engineering due to its good mechanical properties and low production costs.However,the service condition of the engine cylinder head is very harsh.It has been affected by high temperature erosion,gas corrosion,heavy load and cyclic load for a long time,and the valve seat ring is prone to wear and failure.The valve seat ring plays a sealing role in the engine and is very important for the normal operation of the engine.The use of plasma cladding surface modification technology to modify and strengthen the surface of the valve seat can improve and enhance the wear resistance and corrosion resistance of the surface of the valve seat and prolong its service life.In this paper,the plasma cladding technology is used to prepare the Stellite-20 alloy cladding layer on the surface of compacted graphite cast iron,optimize the cladding process parameters,and study its structure and properties.First,the effects of cladding current on the microstructure,dilution rate,microhardness,tensile properties and interface bonding properties of single-pass coatings during plasma single-pass cladding were studied,and the tensile fracture of cobalt-based cladding layers was clarified.Meanwhile,the failure mechanisms and coating-substrate interface bonding properties were explored.The research results of the single-pass coating cladding process show that the coating quality is optimal under the process parameters of cladding current 50A,scanning speed 30 mm/min,and powder feeding speed 25 rad/min,and the coating is mainly composed of cellular crystals and columnar dendrites.The phase of the single-pass cladding layer is mainly composed of fcc-Co solid solution matrix phase and hard carbide phases such as Cr₂₃C₆,Cr₇C₃,Co Cx and Mo₂C.The hardness of the coating reaches 412.0±10.14 HV_0.2,the tensile strength of the coating increases to 944.48 MPa compared with the substrate,and the elongation after fracture increases to 13.46%.The fracture mode is quasi-cleavage fracture.Secondly,the bonding strength of the coating-substrate interface reaches414.88 MPa,achieving good metallurgical bonding with the substrate.Then,single-layer multi-layer cladding was adopted to study the effect of cladding current on the structure,hardness,fracture toughness and friction and wear properties of multi-layer coatings,and the wear resistance mechanism of cobalt-based coatings was determined.The research results show that:under the process parameters of cladding current 60A,scanning speed 150 mm/min,powder feeding speed 20 rad/min,and lap rate 30%,the coating quality is the best,and the microhardness of the coating reaches 530.78.HV_0.2.When the current increased to 65A,the crystal grains in the coating were thermally decomposed due to overheating,resulting in a decrease in the microhardness of the coating.The results of nanoindentation test show that the hardness of various carbides is:Cr₂₃C₆<Mo₂C<Cr₇C₃<Cr₃C₂,and their average nanohardness is 13.5,14.6,15.4 and18.5 GPa,which are higher than 8.01 GPa of fcc-Co solid solution matrix phase.The fracture toughness evaluation results show that when the cladding current increases from 50A to 65A,the fracture toughness value of the coating decreases from 45.52±0.92 Mpa·m^?to 33.82±0.57 Mpa·m^?,which is due to the increase of cladding current.The increase in heat input will cause the carbide particles at the grain boundary to grow and merge to form a network structure,that is,the structure changes from a diffuse distribution to a network continuous distribution structure.This structure easily leads to stress concentration and crack propagation inside the coating,thereby reducing the fracture toughness of the coating.The results of multi-layer coating friction and wear tests show that when the cladding current is60 A,the prepared coating exhibits the best wear resistance,and its friction coefficient and mass loss are the lowest,which are 0.60 and 1.46 mg,respectively.Because suitable heat input conditions can promote the precipitation of carbides at the grain boundaries and make them dispersed,this structure helps to improve the wear resistance of the coating.The main forms of coating wear are abrasive wear and adhesive wear,with shallow furrows and more adhesions on the surface.On the other hand,it was found that the overall mass loss of the pin was less than that of the four coatings and the substrate when it was ground against N80.Using the multiple linear regression method in MATLAB 2019b software,the theory between the wear mass loss of multiple coatings and five influencing factors（microhardness,elastic modulus,fracture toughness,tensile strength,elongation after fracture）was established Mapping relationship,and get the preliminary linear prediction equation.At the same time,the independent analysis of the five influencing factors was carried out,and it was found that the elongation after fracture and the tensile strength had the most significant influence on the wear resistance of the cobalt-based coating.In this paper,single-pass and multi-pass cobalt-based cladding layers were prepared by plasma cladding technology,and the failure behavior and mechanism of the cladding current on the single-pass cobalt-based cladding layer-substrate interface under uniaxial tension at room temperature were studied.Cladding current on the friction and wear mechanism of multi-layer cladding layers under normal temperature and high temperature wear conditions,aiming to increase the understanding and cognition of the plasma coating-substrate interface behavior,improve and improve the surface strength and performance of the substrate,in order to provide theoretical and data support for the application of plasma cladding technology in the field of cast iron surface repair and strengthening.