Study On Processing Optimization And Performance Improving Of NiCrBSi Alloy Fabricated By Laser Deposition

Generally,the main chemical compositions of NiCrBSi alloy include Ni,Cr,B,Si,Fe,and C,etc.Due to its superior hardness,wear and corrosion resistance at both room temperature and high temperature,the alloy has been widely used in the preparation of coatings on metal parts for wear and corrosion protection.A typical example of NiCrBSi alloy is Ni60(Cr16.0 wt.%,B 3.2 wt.%,Si 4.0 wt.%,Fe<15.0 wt.%,C 0.8 wt.%,Ni Bal.),which hardness of the coating prepared by Ni60 can reach 60 HRC.With the rapid development of additive manufacturing technology,Ni60 alloy has shown great potential in the fields of mould surface strengthening and repairing of damaged metal parts.In this study,the parameters of laser deposit Ni60 alloy and properties of deposited specimens were studied systematically.The formation control method was developed and the process parameters were optimized.The phase composition,microstructural characteristics and the influence of microstructures on the properties were investigated.In order to lower the cracks susceptibility in laser deposit Ni60 alloy,nano-Nb and nano-ZrC were added in the alloy respectively.The results show that the microstructures of the laser deposit Ni60 alloy was changed and macrocracks were eliminated by both of the methods.The parameter investigation was conducted according to the order of specimen complexity.The formation control methods of single-line deposition,thin-wall parts,plate parts and block parts were studied in sequence.In the research of laser deposition process,among the large number of factors that influence the formation,parameters of powder feeding gas flow rate,protecting gas flow rate and working distance were optimized at first for their slight influence during the deposition.Under the optimized parameters of these factors,the influences of laser power,scanning speed and powder feed rate on the specimen formation and powder utilization rate were investigated.According to the experimental conditions in this thesis,the recommended parameters for laser deposition of Ni60 alloy are as follows,laser power:400?1000 W,scanning speed:2?14 mm/s,powder feed rate:7.3?18.1 g/min.In the study of the microstructures and properties,the phase constitution and microstructural characteristics under the best optimized parameters were firstly investigated.Then the influences of the laser power and alloy constitution on the microstructures and properties were studied.Finally,to identify the characteristics of the laser deposit Ni60alloy,the microstructures and properties of casting Ni60 alloy were investigated for comparison.The phase constitution of laser deposit Ni60 alloy include:?-Ni,Ni₃Fe,CrB,Cr₅B₃,Ni₃B,Ni₃Si and Cr₇C₃.The microstructures of the alloy consist of Ni grains,Ni-B-Si eutectics and Crhard phases.Under the best optimized parameters(with the laser power of400 W),the microstructures of the alloy differ obviously with the change of cooling rate.In the middle area of the deposition layer,the cooling rate is the slowest in the specimen.So the hard phases in the area are block shape and possess large size.The ratio of Ni grains is relatively high and the ratio of eutectic structures is relatively low.The hardness of the area is about 800 HV.In the top and bottom areas of the deposition layer,the cooling rate is relatively fast and there are a lot of Ni dendrites and hard phase dendrites in the area.The ratio of eutectics is relatively high and the hardness of the areas is about 850 HV.The cooling rate at the edge area is the fastest in the specimen.So the hard phases in the area are much finer and more uniformly distributed than that in other areas.The proportion of eutectic structure of the area is the highest in the specimen,and the hardness of the area is about 900 HV.The tensile strength of the specimen is about 350 MPa,and the fracture mode demonstrates brittle characteristics.When the laser power increases and other parameters remain unchanged,the heat input of the specimen increases and the cooling rate decreases.As a result,the number and length of dendrite structures decrease and the size of hard phases increases.With the increase of laser power,the uniformity of the microstructures is improved,but the hardness decreases.The hardness of the specimen with the laser power of 700W is about 822 HV,and that of specimen with 1000 W is about 800 HV.When the laser power improves,the heat input of the substrate and the specimen increases,leading to the increment of the stress after cooling.So the crack susceptibility of the specimen remains the same even though the microstructures uniformity of the specimen is improved.As one type of NiCrBSi alloys,Ni45 has lower hardness and wear resistance than that of Ni60 alloy.It has the same chemical compositions as Ni60 alloy,but the fractions of Cr,B,S are relatively lower.The phase composition of laser deposit Ni45 alloy is almost the same as Ni60 alloy,but the ratio of Ni phases is much higher than that of Ni60 alloy.There are a large amount of Ni dendrites in the microstructures and the hardness of the specimen is about 527 HV.The tensile strength of the specimen is about 723 MPa,and the fracture mode demonstrates brittle characteristics.The crack susceptibility of the specimens is much better than that of Ni60 alloy,indicating that the Ni grain is the main ductile phase in NiCrBSi alloy.In order to identify the characteristics of laser deposit Ni60 alloy,the microstructures and properties of casting Ni60 alloy were investigated in compare.The cast specimen was prepared by vacuum melting furnace at 1100?and held for 2 hours.The microstructures of the cast Ni60 alloy is relatively uniform,but there are pore defects in the middle part of the specimen.Under the condition of slow cooling,there are no dendrite structures in the alloy and the Ni grains are cross distributed with eutectics.The hardness of the specimen is about 700 HV.In order to lower the cracks susceptibility in the specimen,different proportions of nano-Nb particles were added in Ni60 alloy by ball milling.Doped with nano-Nb,the crack susceptibility of Ni60 alloy is lowered significantly.During laser deposition,Nb C was formed in situ,which provides nucleate sites for hard phase and promote the transformation of dendrite structures into equiaxed ones.With the addition of nano-Nb particles,the eutectic structure transforms from continuous distribution into dispersed distribution which inhibit the initiation and propagation of microcracks thus lowers the crack susceptibility.With the increase of Nb proportion,the hardness of the specimen decreases and the tensile property improves.In order to lower the cracks susceptibility in the specimen,different proportions of nano-ZrC particles were added in Ni60 alloy by ball milling.Doped with nano-ZrC,the crack susceptibility of Ni60 alloy is lowered significantly.During laser deposition,ZrC reacted with oxygen in the air,so ZrC did not provide nucleation sites for hard phase.Zrand Ni formed Ni₅Zrand Ni₇Zr₂by eutectic and peritectic reactions.With the increase ZrC proportion,the morphology and size of hard phase mostly remain the same.The ratio of Ni grain increases.The ratio of Ni-B-Si eutectic decreases.The hardness and tensile properties of the specimen decrease.The study systematically investigated the laser deposition process and properties of Ni60 alloy,and improves the crack susceptibility of Ni60 alloy by changing the structures of Ni grain and Ni-B-Si eutectic innovatively,providing primarily theories and practical experience for further application of NiCrBSi alloys in additive manufacturing.