Microstructure And Properties Of Ni-based Composite Coatings Prepared By Laser Cladding Under High Frequency And Light Vibration

In this paper,Ni/WC composite coatings have been prepared on Ti-6Al-4V alloy substrate from mixed powder of nickel-based alloy powder,WO₃,Al powder and graphite under laser cladding and high frequency light vibration assisted laser cladding process,respectively.The Ni matrix composites fabricated by in situ reaction and the influence of laser cladding process parameters on the forming quality and dilution rate of the coating was explored.Microstructure,phase composition,hardness and wear resistance of coatings were analyzed.Finally,the influence of the content of mixed powder on the microstructure and phase composition of the coating was studied under the optimal laser processing parameters and vibration frequency conditions.The microhardness and wear resistance of the coating were further tested.Optimization results of process parameters show that there are no obvious crack,porosity and unmelted particles in the coating surface,the cladding layer is smooth and the forming quality is best when the thickness of preset powder is 1mm,spot diameter is6mm,scanning speed is 600mm/min,laser power is 1.5KW and vibration frequency is998Hz.With the increase of laser power and vibration frequency,the depth and dilution rate of cladding layer show a significant upward trend,and the increase of scanning speed will result in a decrease of dilution rate.By repeating the best process,the multi-pass laser cladding test was carried out with 40%lap ratio.The surface of the coating was corrugated and the metal glossiness was high.Compared with the direct laser cladding layer,the bonding area and the overall quality of the coating is better.Under the condition of different vibration frequency,microstructure,hardness and wear resistance of nickel-based alloy coating show that the appropriate increase of the vibration frequency is helpful to the refinement of the grain and the densification of structure.The coating is mainly composed of fine dendrites.The main types of the enhanced phase are W₂C and WC,and the network structure of the laser cladding layer is eutectic from the supersaturated?Ti?Ni solid solution.In addition to the partial reaction of producing WC and W₂C,some W and C react with Ti to form eutectic carbides?Ti,W?C.In the appropriate vibration scope,the increase of the frequency is beneficial to the improvement of the hardness of the coating and the optimization of the performance.The maximum hardness of the coating is 921HV_0.2,the microhardness of titanium alloy is about360?380HV_0.2,and the average microhardness of the coating is nearly 3 times that of the matrix.The lower vibration frequency has no effect on the hardness and wear resistance of the coating,the highest friction coefficient appears in the absence of vibration,and the lowest friction coefficient appears at the vibration frequency of 998Hz.Analyses of microstructure and properties of cladding layer with different contents of mixed powder show that the microstructure of the coating with different powder proportions is similar,which is composed of mesh structure,network encircling area and white bright particle.With the increase of content of the mixed powder,the number of hard phase in-situ synthesized in the laser cladding layer increased significantly,and the eutectic structure of the netted structure had been coarsened.The network encircling area is?Ti?Ni solid solution,and the main component of the network structure is the eutectic carbide?Ti,W?C formed by W,C and Ti elements.The composite coating is mainly composed of Ni Ti,Ni Ti₂,W₂C,and WC,?Ti,W?C.The microhardness of the cladding layer is increased with increasing WC content.When the content of mixed powder of nickel-based alloy powder,WO₃,Al powder and graphite is 30%,the microhardness of the cladding layer reach 1267HV_0.2,which is about three times higher than that of the substrate.The uniformity of the enhanced phase distribution is poor when the content of mixed powder is higher than 20%wt.Friction and wear curves have four stages including running-in period moderate growth,low,rapid growth and stable period.With the increase of the content of mixed powder,the friction coefficient and the wear loss of the coating decreased obviously,and the wear resistance is greatly improved.