Numerical Simulation And Experimental Study Of Gas Solid Two Phase Flow In Laser Direct Metal Deposition Forming

The coaxial powder feeding laser direct metal deposition forming technology can directly realize the manufacturing and remanufacturing of three-dimensional parts.As a key part of powder transmission,powder feeding nozzle directly affects the quality of powder transmission and deposition molding.In order to improve the molding quality and optimize the nozzle structure,this paper first developed a new double-ring coaxial nozzle,and then based on the gas-solid two-phase flow theory,systematically developed the nozzle aerodynamic characteristics,powder transmission characteristics,coupling of light powder characteristics Numerical simulation and experimental study of deposition modeling.The collision behavior,particle trajectory and velocity distribution of the powder particles in the nozzle,the structure and convergence characteristics of the powder jet outside the nozzle are mainly studied.Three direct internal correlations between the particle transport behavior in the nozzle,the powder jet structure outside the nozzle and the deposition molding characteristics are discussed.The main conclusions are as follows:(1)Developed a new type of dual-ring coaxial nozzle,which can be divided into four parts according to functionality,relying on thread connection.Among them,the two internal parts are hollow,including the laser beam channel.The outer part is divided into upper and lower parts.The upper part contains four-way cylindrical powder flow channels,the lower part contains cooling water circulation channels,and the gap between the outer part and the inner part forms an annular powder flow channel.The overall taper decreases.The nozzle entity is manufactured through the 3D model,and the powder-flow transmission model is established using the gas-solid two-phase flow theory,and the powder flow morphology obtained by numerical calculation are in good agreement with the experimental data.(2)The influence of the collision behavior of powder particles on powder transport was studied.The results show that the existence of momentum loss between powder particles(Kn=0.9)can improve the convergence of the powder flow outside the nozzle,which is manifested in the concentration of the powder flow in the cylindrical area where the particle trajectory density is higher,the concentration curve peak is higher,and the radial concentration distribution Uniformity;it can make the overall dispersion degree of the trajectory rate curve of powder particles reduce.At the same time,collision is the cause of the"jump"phenomenon in the trajectory rate curve.Its frequency is proportional to the powder feed rate.Its direction and span are mainly determined by the angle between the speed direction of the powder particles and the direction of the gas velocity streamline when the powder particles rebound.Less than 90°presents an upward“jump”,and a downward“jump”occurs when the angle is greater than 90°.The closer the angle is to 0°or 180°,the greater the“jump”span.(3)The effects of carry gas flow rate and powder feeding rate on powder transmission were studied.The results show that with the increase of the carrier gas flow rate,the trajectory velocity curve of the powder particles in the nozzle is increasing and the concentration is stronger,which is conducive to the stability of the powder flow.The overall convergence of the powder flow outside the nozzle appears to move up,and the peak concentration Increase first and then decrease.Among them,the decrease in concentration is due to the"double peak"phenomenon;the effect of the powder feed rate on the powder flow is mainly reflected in the concentration distribution,and the influence on the shape of the powder flow is small.When the powder feed rate gradually increases to 3.6 g/min,the"double peak"peak value reaches the maximum,about 3 kg/m~3.If the powder feed rate continues to increase,the powder flow will diverge,causing the peak concentration to decrease.(4)The spatial geometric relationship between the converging interval of the powder flow and the laser focusing spot and the coupling state of the powder flow in the spot are studied.The results show that when the carrier gas flow rate is 2.0 L/min,the powder flow converging interval is below the laser focusing plane,and the radial concentration in the spot is"M"-like distribution;when the carrier gas flow rate increases to 3.0 L/min,convergence occurs upward shifting phenomenon makes the radial concentration in the laser focusing spot show"?"-like distribution;when the carrier gas flow rate is 4.0 L/min,the laser focused spot is located in the cylindrical region where the powder flow converges,and the radial concentration in the spot shows a Gaussian distribution.(5)The effects of carrier gas flow rate and powder feeding rate on the variation of deposition forming characteristic parameters were studied.The results show that keeping the carrier gas flow rate constant,with the increase of the powder feeding rate,the height and contact angle of the deposited layer increase,and the width changes slightly and tends to decrease.Among them,the maximum powder utilization rate is about 54.2%when the powder feed rate is 2.4 g/min,and the molding size is better when the powder feed rate is 2.8 g/min;Keeping the powder feed rate is unchanged,as the carrier gas flow increases,The height and contact angle of the coating first increase and then remain unchanged,and the width first increases and then decreases and tends to be stable.The utilization rate of the powder is within the range of smaller carrier gas flow rate of 2.0?3.0L/min,which increases linearly with the increase of air flow.In the range of 3.0 L/min to 4.0 L/min,due to the divergence of the powder flow,the powder utilization rate decreased from 0.45 at 3.0 L/min to 0.43 at 4.0L/min.