Gas-powder Flow For Laser Coaxial Nozzle With Multi-Channels

Coaxial powder feeding nozzle is a key component in Laser Engineered Net Shaping(LENS)processes.The flow conditions of the supplying powders in an LENS process greatly influence the quality and property of the deposits.Therefore,it is of significance to study the gas-powder flow convergence characteristics of a coaxial nozzle.In this paper,through a series of multi-channel coaxial nozzle feeding experiments,divergence phenomena at the exit of the nozzle is studied,and divergence angle and powder focal distance are proposed to characterize the convergence of the gas-powder flow.Different inner surface roughness nozzle tubes are fabricated by Abrasive flow Machining(AFM).The influences of the wall roughness on gas-powder flow convergence and thus on the AM part quality are studied.The main research contents are as follows:(1)Divergence angle and powder focal distance are proposed to characterize the convergence of the gas-powder flow.The effects of feed rate and gas velocity on the gas-powder flow convergence and the powder velocity at the nozzle exit are studied experimentally.(2)A discrete phase model of coaxial powder feeding is developed in FLUENT based on the feed rate and gas velocity measured from experiments.The inlet and wall boundary conditions are set based on the experimental results.The main reasons for the divergence phenomena at the nozzle are analyzed through simulation results.Based on the divergence angle and distribution of feed rate,a mathematical model of gas-powder flow is established to predict the relationships between the gas-powder concentration and the powder focal length.(3)Different AFM parameters are used to polish the nozzle tube inner surface to obtain different roughness.Then the nozzle tubes are used in coaxial feeding experiment.The effect of wall surface roughness on gas-powder flow convergence is studied.(4)Using nozzle tubes with different wall surface roughness,a single track,multi-layer and block AM parts are manufactured by the LENS experiments to study the geometric feature and the microstructure.The effect of gas-powder flow convergence on the AM part qualities is studied.