| Surface modification technology can not only save resources but also repair and significantly improve the surface properties of work pieces.Laser cladding technology has been widely studied and applied in surface modification technology due to its characteristics such as high density concentration of heat source energy,low dilution rate,thickness of cladding layer,wide range of optional cladding powder and good metallurgical properties.In recent years,the microstructure of laser cladding layer prepared by in situ auto-genous reaction method is compact and uniform,and presents excellent comprehensive mechanical properties such as high hardness and high wear resistance.In this paper,the ZG230-450 steel was used as substrate material,Stellite20 blended powder of cobalt base alloy powder and pure Nb was used as cladding powder material and cross-flow continuous Nd:2kw synchronous send powder laser was used for cladding of different composition Stellite20-Nb alloy powder mixture in ZG230-450 cast steel specimen surface.Laser cladding layer with high hardness and high wear resistance was prepared by changing the laser process parameters.Analyses on macro morphology,microstructure,microhardness and wear resistance performance of Stellite20 alloy cladding layer shows that when the laser power is 1200w,cladding layer was continuous,stable,smooth and good dilution rate was obtained.Planar crystal,the cellular crystal,columnar dendrites and isometric with characteristics of typical rapid cooling organization were obtained.The phases generated in Stellite20 laser cladding layer wereComposed of matrixy-co M7C3(Cr7C3、(Cr,Fe)7C3、Fe 7C3)、M23C6(Cr23C6、(Cr,Fe)23C6)、Cr3C2、Co6W6C、WC、Co3W、FeCrphases.The microhardness and wear resistance of the cladding layer were significantly improved,which were about 8 times and 3 times of ZG230-450 matrix.The laser cladding of Stellite20+Nb alloy powders on the surface of ZG230-450 steel castings was applied to study the macroscopic morphology,microstructure and mechanical properties of the cladding layer.Experimental results show that in the cladding layer,in-situ formation of NbC,CoxNb,NbCr2 carbide and hard phase occurred.When the laser power was 1200w,with the increase of Nb addition,the microstructure of the cladding layer was significantly refined,so that the cellular crystals and columnar dendrites in the cladding layer gradually decreased,and the dense and uniform equiaxed crystals gradually increased.When Nb contents was 1%～3%,the microstructure was dominated by frstly precipitated dendrites and interdendritic eutectic.When the addition amount of Nb is 5%～13%,NbC and matrix with different shapes generated by the in-situ spontaneous reaction of the cladding layer play a leading role in the characteristics under the condition of off-equilibrium solidification.Morphology changes of in situ fromed NbC in Stellite20+Nb cladding layer show that in matrix powder with strong carbide forming elements Nb can take precedence over c-reactive generated high melting point carbide of high hardness,When Nb content is 13%,NbC wasdistributed in cladding layer in block,diamond,square and butterfly,high melting point carbide of high hardness was firstly generated and it could be used as a heterogeneous nucleation of cladding layer.Refined cladding layer with uniform density was obtained.Microhardness and wear resistance were improved significantly.The microhardness and wear resistance of the cladding layer also gradually increased with the increase of Nb content When the Nb content increased by 13%,the microhardness of the cladding layer reached 1050HV0.5,which was about 10 times and 2 times of the substrate material and the pure Stellite20 cladding layer respectively,and the highest wear resistance was 10 times and 8 times of the substrate material and the pure Stellite20 cladding layer respectively. |
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