| Lightweight design is one of the key technologies in the field of rail transit,which can optimize the rail train in many aspects.At present,aluminum alloy is mainly used in the manufacturing to realize the lightweight of train,but the application of aluminum alloy in rail transit has become a bottleneck.As a metal structure material,magnesium alloy has the advantages of low density,high specific strength and specific stiffness,and good cutting performance.Moreover,magnesium resources are rich,which can meet the development and utilization.However,the poor corrosion resistance of magnesium alloy is the main problem limiting its application in rail transit.Cold spraying coating preparation technology can effectively prevent magnesium alloy from being corroded,and the technology has low energy consumption,recyclable particles and no pollution to the environment.However,the traditional cold spraying technology has high requirements for powder quality and high cost.In this paper,Laval nozzle,an important part of cold spraying technology,was optimized to explore the application of low-cost powder cold spraying.Pure Al coating was prepared on Mg-Zn-Y-Ca.the corrosion resistance of low-cost powder cold spraying pure Al coating was analyzed by electrochemical test and immersion corrosion test.The main research contents and results are as follows:Firstly,the core component of cold spray technology Laval nozzle is designed.The numerical simulation of single-phase and gas-solid two-phase flow of different structure spray guns under different inlet parameters（gas inlet temperature,pressure,velocity and particle inlet pressure and velocity）is carried out by Fluent software.The structure and inlet parameters of the spray gun are analyzed to simulate the gas and solid particles The influence law of mouth velocity.According to the simulation results,the exit velocity is used as the criterion to optimize the Laval nozzle and design the better structure of Laval nozzle.That is,the nozzle inlet diameter is 20mm,the length of the contraction section is20mm,the throat diameter is 2mm,the length of the first expansion section is 20mm,and the expansion angle of the first section is 5°,The second expansion section is 28mm long,the outlet diameter is 8mm,and the particle inlet angle is 11°。Pure Al coating was prepared by cold spraying on Mg-Zn-Y-Ca alloy substrate with low cost pure Al powder.The microstructure and corrosion resistance of the coating were analyzed.It is found that the smaller the scanning speed is,the higher the working gas temperature is,the thicker the Al coating is,the smaller the porosity is,and the better the corrosion resistance is.Under the working gas temperature of 600℃and scanning speed of1mm/s,the pure Al coating prepared by cold spraying has the best corrosion resistance to Mg-Zn-Y-Ca alloy.The results show that the thickness of Al coating is 970μm,the porosity is 0.938vol%,and the self corrosion current density is 2.427×10-6A/cm2.When the Al coating is immersed in 5%NaCl solution for 24 hours and the working gas is 600℃,the corrosion products on the surface of Al coating are the least.The corrosion resistance of Al coating can be improved by appropriately increasing the working gas temperature. |
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