The Research On The Microstructure And Properties Of The Laser Cladding Layer Of Co-base Alloy On Ductile Cast Iron

Ductile cast iron,with high strength,toughness and plasticity,wear resistance,shock absorption,easy cutting,and crack insensitivity,can replace the carbon steel and alloy steel widely being used to manufacture those parts of complex load bearing and high strength,toughness and wear resistance being required,such as the traction wheel,the crankshaft,the cylinder sleeve and the cylinder roller.But in the harsh environment,ductile cast iron can often fail to work due to the surface serious wear.At present,the repair methods for the damaged parts are mainly thermal spraying,deposit welding and other methods,but the properties and application effect of the repaired parts need to be improved.In order to significantly improve the surface hardness and wear resistance of ductile iron,the cladding layer of Co-based alloy,5%TiC/Co based alloy,10%TiC/Co based alloy,30%TiC /Co based alloy on the surface of nodular cast iron has been made by laser cladding.And the preparing technique of the cladding layer of TiC/Co-based alloy on the ductile cast iron is studied.The microstructure,composition and phases,hardness on the cross section of the cladding layer and abrasion resistance of the cladding layer are systematically investigated and analyzed by GX71 O lympus metallurgical microscope,Zeiss-Sigma scanning electron microscope,X 'Pert MPD Pro X-ray diffraction,TEM,MHV2000 digital micro hardness tester and MM200 abrasion tester.The effect of TiC reinforced phase on the microstructure and properties of laser cladding layer is mainly studied in this paper and the optimal process parameters of laser cladding are obtained.The effects of the composition of the cladding alloy and the laser cladding parameters on the cladding layer are studied and the wear mechanism of the cladding layer is revealed.The research results obtained are as follows:?1?Laser cladding layer of the ductile cast iron is well-formed without the slag inclusion,pores,cracks and other defects.The cladding layer can be divided into melting zone,bind ing zone and heat-affected zone and it is integrated in the metallurgical bonding with the ductile cast iron matrix.The microstructure of the cladding layer is consisted of a surface layer of dendritic crystals and an internal cellular crystal and the structural transformation occurs in the heat affected zone,martensite forming and the spherical graphite dissolved partly whose diameter becomes smaller.?2?There exist the TiC particles in the cladding layer in the melted and incompletely melted present whose content increases gradually from the substrate surface to the cladding layer surface.?3?The laser cladding layer of Co-based alloy is mainly composed of ?-Co,CoCx and a small amount of Cr₇C₃.And there exists TiC phase in the laser cladding layer of TiC /Co-based alloy besides containing ?-Co,CoCx and a small amount of Cr₇C₃.?4?The hardness on the cross section of the cladding layer of Co-based alloy or TiC /Co-based alloy increases with the increase of the distance from the surfaces of the substrate.The hardness increases significantly in the heat-affected zone and increases slowly in the cladding layer and finally,increases suddenly in the cladding surface layer.?5?Compared with the hardness of laser cladding layer of Co-based alloy,the hardness of the cladding layer of TiC/Co-based alloy is significantly improved,and it increases with the increase of TiC content.The maximal hardness of the cladding layer of Co-based alloy is 1077HV0.2,up to over 4 times the hardness of ductile cast iron matrix,while at the content of TiC reaching to 30%,the maximal hardness of the cladding layer is 1278.8HV0.2,up to over 5 times the hardness of ductile cast iron.?6?The worn surface morphology of the cladding layer of 30%TiC/Co-based alloy is basically smooth without the obvious sharp grooves and granular protuberances on which the wear resistance is up to over 210 times of the substrate of ductile cast iron and the mainly wearing mechanism is abrasive wear with the average friction coefficient of 0.2315.The worn surface morphology of the cladding layer of Co-based alloy has got obvious furrows and local avulsions on which the wear resistance is up to over 74 times of the substrate of ductile cast iron and its mainly wearing mechanism is abrasive wear and adhesive wear with the average friction coefficient of 0.4914.?7?In the scope of this thesis,the optimizing process parameters of the Co-based alloy coating laser cladding on the ductile cast iron are: Laser power P=3kW,cladding speed V=350mm/min,spot diameter is 2 mm,overlapping rate 1.5,while,the optimal process parameters of the TiC /Co-based alloy cladding coating on ductile cast iron are: Laser power P=3.4kW,cladding speed V=400mm/min,spot diameter is 2 mm,overlapping rate 1.5.