Numerical Simulation Of In-flight Behavior Of WC-Ni Particles Near Sample Surface By HVOF

WC-Ni cemented carbide coatings prepared by HVOF have excellent comprehensive properties,and are widely used in automotive industry,aerospace and other fields.The characteristics of the coating are largely determined by the velocity and temperature of the spray particles,and the velocity and temperature of the particles depend on the spray process parameters.The purpose of this paper is to establish a visual analysis of the process of HVOF spraying samples with different shapes by means of numerical analysis,and to provide theoretical guidance for subsequent experiments.In this paper,the 3D model of woka-610-si spray gun with WC-12 Ni as spray powder is established based on CFD software Fluent.k-? Turbulence model and eddy dissipation model were used to simulate the gas-solid two-phase flow field inside and outside the spray gun under the condition of specific fuel ratio.The flame flow distribution and particle flight behavior were obtained under the condition of no spray sample.The accuracy of the model was verified by comparing with the experimental results of particle temperature measurement and velocity measurement;The influence of different shapes of samples on the flame distribution and the corresponding particle flight behavior are analyzed.The results show that:(1)Under the condition of no sample,the velocity and temperature of free jet flame are Gaussian distribution in radial direction,that is,the value of flame core is high,and the value of flame core decreases gradually.The maximum temperature of combustion chamber can reach 3357 ?,the pressure can reach 5.7 atm,and the peak velocity of spray gun outlet is2851 m/s.At the actual spraying distance of 350 mm,the flame core velocity is 650 m/s and the flame core temperature is 1050 ?.(2)With the addition of particles without sample,small particles are more likely to be in the periphery of the flame,and the velocity and temperature of particles are higher than those of particles in the inner flame,and the numerical fluctuation is larger;The larger the particle size is,the lower the axial velocity and temperature are,and the smaller the decay rate is.The suitable particle size range is 15?30 ?m.Within the distance of 200?400 mm from the exit of the spray gun,the velocity and temperature of the particles first increase and then decrease with the increase of the distance,and the velocities are 900 m/s,901 m/s,903 m/s,892 m/s and 847 m/s,respectively.The errors with the experimental results are 0.77 %,1.58 %,1.46 %,0.78 % and 8.87 %,respectively;The temperature is 1903 ?,1922 ?,1907 ?,1852 ? and 1756 ? respectively,and the error is 2.96 %,0.26 %,3.25 %,0.82 % and3.20 % respectively.It shows that the simulation results are in good agreement with the particle temperature and velocity measurement experiments.(3)90° The results show that the velocity and temperature of the flame flow in the steady jet stage are lower than those in the non sample condition,and the Gaussian distribution is no longer observed at 1 mm near the surface of the sample.The flame velocity begins to decay rapidly at 328 mm from the exit of the spray gun,and decreases to 43 m/s when it reaches the surface of the sample;The "edge effect" appears at 1 mm near the sample surface: the maximum flame velocity is 450 m/s at the edge of the sample strip,and the minimum flame velocity is 14 m/s at the center;The maximum temperature of flame flow is 1100 ? in the center,and the temperature drops sharply from 963 ? to 850 ? in the edge.After adding the particles,under the condition of mixed particle size,The particles with 16?25 ?m particle size are more concentrated in the center of flame flow,and the trajectory deviation is smaller,With the increase of distance,the proportion of high-temperature and high-speed particles decreases,and the difference of velocity and temperature between particles decreases;Under the condition of single particle size,the smaller the particle size is,the more uniform the particle distribution is,but the distribution area is also larger,and the suitable particle size range is 20?30 ?m?(4)Under the condition of spraying cylindrical sample,the flow around the cylinder is generated when the flame passes through the cylinder.A low velocity zone is generated in the center of the surface directly opposite the spraying,and a high velocity zone is generated on both sides of the cylinder.After the addition of particles,the particle size in the central region of the deposition is concentrated in 25?35 ?m,Small particles and parts of big particles are Scattered on the periphery.In addition,the velocity of deposition particles in the central region is low and the temperature is high.(5)Under the condition of spraying inner hole sample,There are two low velocity zones in the inner hole: the entrance of the sprayed side and the inner wall of the unpainted side.The larger the aperture is,the larger the proportion of low velocity zone is,and the smaller the aperture is,the higher the flame velocity is.There will be a low temperature region in the large pore,so the temperature drop in the pore is faster than that in the small pore.The smaller the spraying distance,the higher the flame velocity and temperature in the inner hole.After adding the particles,the large particles are distributed in the periphery of the deposition area,while the small particles are distributed in the center of the area and the depth of the inner pore.With the increase of spraying distance,the area covered by particle deposition increases,the particle spacing increases,the particle velocity decreases and the overall temperature difference decreases;The particle deposition velocity and temperature of the large pore are lower than those of the small pore,and the particle deposition area of the large pore is smaller,and the inner pore is more concentrated.