Microstructure And Properties Of The Laser Cladding Layer Of Co-Base Alloy Reinforced By TiC On Nodular Cast Iron

In this paper,laser cladding layer of TiC/Co based alloy on the surface of ductile cast iron is fabricated using the 10%TiC/Co based alloy powders with different particle size by the fiber laser.The effects of the laser technical parameters and the particle size of TiC added to TiC/Co based alloy powders are mainly studied on the microstructure,microhardness and the morphology and distribution of TiC in the layer,optimal process parameters of laser cladding obtained.The results obtained by laser cladding layer by CO₂ laser are compared with those by the fiber laser.The conclusions are drawn as following:（1）The cladding layer of TiC/Co base alloy on ductile cast iron is shaped well with no defects such as pores and cracks,while the cladding layer is bound metallurgically with the matrix of the cast iron.The microstructure of the cladding layer is made up of the developed dendrites in the combining zone,the equiaxed dendritic crystals in the middle layer and the tiny equiaxed dendritic crystals at the surface layer and its phase composition is mainly composed ofγ-Co、TiC、CoC_x、Ni-Cr-Fe and a small amount of Cr₇C₃.（2）TiC particles in cladding layer exist in the shape of particles incompletely dissolved or dendrites precipitated after dissolving.With the particle size of TiC added in the alloy increasing,the microstructure of cladding layer is refined and the amount of TiC particle incompletely dissolved increases with the more uniform distribution.（3）The microhardness on the cross-section cladding layer increases with the distance from the matrix surface increasing and sharply increases near the surface layer of the cladding layer.The microhardness in the near surface of the cladding layer increases with the laser power decreasing or scanning speed increasing or TiC particle size increasing.In this experiment,the maximal microhardness of the cladding layer is 1120.1HV_0.2,which is up to 3.7 times higher than the matrix.（4）The optimizing process parameters of laser cladding the TiC/Co based alloy coating by fiber laser are:the particle size of TiC added of 80-100μm,laser power P=1.0kW,scanning speed v=4mm/s,spot diameter D=3mm,overlapping rate at 50%.（5）The optimal process parameters of laser cladding（P,v）by CO₂ laser are higher than those by fiber laser when the same powders are cladded.Compared with the cladding layer by the fiber laser,there are developed columnar dendritic crystals in the near surface layer of the laser cladding layer by CO₂ laser without equiaxed dendritic.TiC are mostly dissolved to small particles during laser cladding by CO₂ laser and there is more dispersive distribution in the layer.Compared with fiber laser,the Cr-Ni-Fe-C phase exists in the microstructure without CoC_x phase and Cr-Fe-Ni phase.（6）The microhardness increases significantly in the near surface of cladding layer fabricated by CO₂ laser compared with fiber laser when the same powders are cladded.And the maximal microhardness is 1246.6HV_0.2,up to 4.2 times higher than the matrix,which is higher than the maximum of the layer by fiber laser(1087.4HV_0.2).