Study On Microstructure And Interface Bonding Mechanism Of In-situ Carbide Reinforced Laser Cladded Ni Based Coating

With the rapid development of automobile industry,lightweight design is required urgently to realize environmental protection and energy economy for both new energy automobiles and conventional cars.The steel plate of high strength and high elongation replacing ordinary carbon steel is a basic way to realize lightweight design,but the application of ultra-high-strength steel plates leads to more serious wear of mould,which brings a series of problems to the design and use of automobile mould.As a strengthening and surface repair technology,laser cladding is widely used in the repairing of automobile mold,especially the metal matrix composite(MMC)coatings reinforced by ceramic particles can improve the mechanical properties and wear resistance of automobile mold.In this study,to solve the low hardness and poor wear resistance of automobile mould in the service process,MMC coatings were made by laser cladding on die steel Cr12Mo V.On the basis of the optimized process parameters,Nb C particles were in-situ synthesized in the Ni45 coatings,the optimized theoretical designed content of Nb C was determined by the microstructure,hardness and wear resistance;then(Ti,Nb)C/Ni composite coatings were obtained by replacing Nb with equimolar Ti,the microstructure,mechanical properties,wear resistance and(Ti,Nb)C/?-Ni interface were investigated;At last,(Ti,Nb)C/Ni coatings were modified by different content of Ce O₂ and changing the cooling rate.The results are expected to provide guidance for repairing worn surface of automobile die.The main contents and conclusions are shown as follows:Firstly,the effect of process parameters on the height,width,depth and dilution rate of coatings was studied.When the laser power,scanning speed and powder feeding rate was 2000 W,4 mm/s and 15 g/min,respectively,the height,width,depth and dilution rate of coatings were 1.340 mm,4.791 mm,0.135 mm and 9.15%,respectively,the coatings were well,and metallurgical bonding were also achieved.Based on the process parameters,Nb C/Ni coatings were made on Cr12Mo V with Ni45,Cr₃C₂and Nb powders.The different designed content has great influence on the size,shape and distribution of strengthening phases,Nb C particles changed from lump to petal with the increase design content of Nb C.When the designed content was 20 wt.%,in-situ Nb C particles were regular and distributed homogeneously,the diameter of Nb C particles was highly concentrated between 0.3?m?0.8?m.Moreover,the improvement microhardness of coatings was attributed to the Nb C,Cr₃C₂,Cr₂₃C₆and Cr₇C₃.In the wear test,Nb C and chromium carbides possessed the high ability to resist plastic deformation,strong toughness and crack propagation,which significantly improved the wear resistance(4.43 times)than that of Ni45coatings.Based on the optimized designed content of 20 wt.%Nb C,different Nb:Ti mole ratio of(Ti,Nb)C/Ni composite coatings were obtained by replacing Nb with equimolar Ti.Ti C and Nb C have similar lattice constant and same crystal structure,as well as many matched surfaces with lower surface energy,it could be concluded that(Ti,Nb)C composite carbides can be easily synthesized in the melt pool.In addition,Nb C has higher nucleation ability in comparison to Ti C,which led the lattice parameter of(Ti,Nb)C closing to Nb C.When the ratio of Nb:Ti was 1:1,the wear resistance of coating was slightly better than other coatings.According to the first principles,(Ti,Nb)C possessed good mechanical stability,and the hardness of Nb_0.5Ti_0.5C was the largest.Based on the anisotropic index and three-dimensional surface contours,Nb C and Nb_0.75Ti_0.25C showed stronger anisotropy.(Ti,Nb)C compounds were a mixture of metallic and covalent bonds,and Debye temperature increased with the increase of Ti content.In the (Ti,Nb)C/Ni composite coatings with Nb:Ti ratio of 1:1,it was found that there were more lower free surfaces of Ni,which can be matched with other phases,such as Ni(100),Ni(211)and Ni(110).Based on the interface orientation relationship of?-Ni(202)/(Ti,Nb)C(220)obtained by TEM and EDS,ten interface models were modeled.It found that the stacking sites of Nb atom have much influence than the substitution position of Ti,the center-site stacking interfaces shower stronger interaction,interfacial adhesion and thermodynamic stability.The mechanical failure of(Ti,Nb)C/?-Ni may initiate at the interface or close to?-Ni.In addition,(Ti,Nb)C/?-Ni was a mixture of covalent bond and partial ionic bond,and the electron transfer was mainly concentrated at and near the interface.Lastly,(Ti,Nb)C/Ni coatings were modified by different content of Ce O₂ and changing the cooling rate.It can be concluded that Ce from decomposed Ce O₂ can remove impurities and purify coatings.It has also the effect of reducing the interfacial energy and surface tension,suppressing the growth of crystals,which refined the microstructure of coatings.Moreover,the undecomposed Ce O₂ particles have become the substrate of(Ti,Nb)C and chromium carbides(Cr₂₃C₆ and Cr₇C₃)with heterogeneous nucleation.The coating with 5 wt.%Ce O₂ showed higher UTS and excellent tensile properties.The microstructure of coatings with the liquid nitrogen cooling was remarkably refined,the solubility of Fe,Cr,Nb and Ti elements in the?-Ni matrix increased,resulted in higher hardness and elastic modulus.In addition,the wear resistance was 73%higher than that of air cooling coating,the tribochemical reaction was occurred in both coatings,and the worn surface roughness of liquid nitrogen cooling coating was small.