Three-dimensional Numerical Simulation Of Thermal And Fluid Behaviors In GMA-based Additive Manufacturing

Gas metal arc(GMA)based additive manufacturing is extensively used in fabricating and repairing large-scale metal parts due to its low equipment cost,high deposition efficiency,and good mechanical properties.However,owing to high heat input and many disturbing factors,it is difficult to ensure the uniform size and shape of the deposited layers in GMA-based additive manufacturing.Futhermore,the deposited layer shape and size are a consequence of heat transfer and fluid flow in the molten pool.In this paper,a three-dimensional numerical model was developed to analyze heat transfer and fluid flow of the molten pool in GMA-based additive manufacturing,and predict the size and shape of deposited layers.Moreover,the influences of the process parameters on the thermal field and flow field distribution were discussed.The purpose is to reveal heat and mass transfer characteristics in the molten pool during the forming process,and to optimize the process parameters and predict the dimensions of deposited layer profile.A three-dimensional weak coupling modeling method of the arc and metal transport is developed to simulate the molten pool dynamic in stainless steel GMA-based additive manufacturing based on software FLUENT.The droplet transfer process is considered as the process of flowing into the molten pool from the upper specific area of the molten pool and the periodic function of liquid metal flow velocity versus time function is used to characterize the droplet transition frequency.The characteristics of thermal field and flow field in the transient deposition process of stainless steel GMA-based additive were discussed.Experiments were performed to verify the numerical results.The results show that as the deposition process progresses,the temperatures on the substrate and molten pool gradually increase,and the high temperature region increases.The temperature gradient of the substrate decreases,and the fluid flow rate of the molten pool increases with the deposition process proceeding.The main flow pattern is in the counterclockwise direction with the flow dominated by Marangoni forces.As the number of the deposited layer increases,the maximum temperature in the molten pool increases,and the length of the molten pool gradually increases.When the first,second and third layers are deposited,the highest temperatures near the midpoint of the molten pool are 2355 K,2397 K and 2933 K,respectively,and the length of the weld pool is 10.8 mm,11.8 mm and 12.4 mm,respectively.When the deposition process is completed and cooling to the room temperature,the start edge of the layers tends to higher than the end edge.Moreover,the effects of travel speed and deposition current on the thermal field and flow field of GMA-based additive in molten pool were studied.The simulation results show that the isotherm distribution in front of the heat source is denser and the temperature gradient is larger as the travel speed increases.With the increasing travel speed,the flow field distribution range and flow rate of the molten pool all decrease,and the width and height of the layer decrease.When the travel speed changes from 0.01 m/s to 0.015 m/s and 0.02 m/s,the maximum flow velocity in the molten pool varies from 0.113 m/s to 0.105 m/s and 0.095 m/s as the second layer is 10 mm long.As the current increases,the maximum temperature and flow velocity in the molten pool increases,and the depth,length and width of the molten pool are enlarged.The fluctuation on the upper surface is weakened with the increasing current.When the deposition current changes from150 A to 200 A,the maximum flow rate varies form 0.095 m/s to 0.13 m/s as the second layer is 10 mm long.