Study On Fiber Laser Cladding Ni-based Alloy Coatings Of Cr12 Surface

Cr12 is an important cold work die steel which is used widely in cold stamping parts.Due to its harsh working conditions?high load,high friction?,the wearing and tearing is often occurred.Conventional repairing methods such as arc surfacing,spraying,etc.,which can't guarantee their service life.Laser cladding technology is playing more and more important role in mold repairing area for its advantages of good reparability and high flexibility.In this paper,three types of cladding coatings are fabricated on Cr12 mold steel surface,which are Ni-based alloy coatings,Ni-xCr3C2?x=0%,5%,10%,15%,20%?coatings and Ni-10%Cr3C2-yCe?y=0%,0.5%,1.0%,1.5%,2.0%?coatings.Using optical microscopy?OM?,scanning electron microscopy?SEM?,energy dispersive spectroscopy?EDS?,X-ray diffractometry?XRD?and microhardness tester to systematic analyze the microstructure,element distribution,phase composition and microhardness of cladding layers.In addition,Gibbs free energy of phases which detected by XRD are calculated to estimate the degree of spontaneity of reactions and infer the precipitation sequence of each phase combining with the phase diagram.Research indicates:?1?The microstructures of cladding layer in middle area are mainly fine dendrites and the size of dendrites become thicker and thicker as laser powder increasing.The microstructures of cladding layer can be refined when adding a certain proportion of Cr₃C₂and Ce.Regarding experiments on the preparation of Ni-xCr3C2 composite coatings,the distance between secondary dendrites is 1.11?m with the optimum content of 15%Cr₃C₂.Regarding experiments on the preparation of Ni-10%Cr3C2-yCe composite coatings,the distance between secondary dendrites is 0.96?m with the optimum content of 1%Ce.?2?The microhardness of Ni-based,Ni-xCr3C2,Ni-10%Cr3C2-yCe cladding layers are significant higher than that of substrate.Microhardness of cladding layers decrease gradually with the increasing of laser powder,when the laser powder is 600W,the microhardness is highest,with an average of 544HV_0.2;The microhardness of Ni-based cladding layer can be significantly improved by adding ceramic-phase Cr₃C₂,with an average of 623.3HV_0.2,but the microhardness is bad-distributed;The microhardness become well-distributed by adding rare-earth Ce,with an average of 607.1HV0.2.?3?The main phases of Ni-based cladding layer are?Fe,Ni?,Fe₃C,NiB₁₂,?Cr,Fe?₇C₃,Ni₃Fe.Result shows that new phases of Cr₂₃C₆,Cr₂Fe₁₄C and SiC emerge after adding Cr₃C₂and new phases of CeB₆ and CeC₂ emerge after adding Ce.The dendritic structure is mainly composed of?Fe,Ni?,the netted eutectic structure is mainly composed of Fe₃C,Cr₇C₃,Cr₂₃C₆,Cr₂Fe₁₄C etc.,and phases of CeB₆ and CeC₂ are mainly segregated at grain boundaries.?4?Gibbs free energy calculation shows that the three phases of Fe₃C,Cr₇C₃ and Cr₂₃C₆can be spontaneously reacted in thermodynamics.The major carbide phase is Cr7C3 in Ni-based cladding layer without adding Cr₃C₂,but the major carbide phase becomes Cr₂₃C₆after adding Cr₃C₂.