Numerical Study On Deposition Layer And Molten Pool Characteristics Of IN718 Coaxial Powder Feeding Laser Deposition

In the coaxial powder feeding laser directed energy deposition process,the mass transfer,heat transfer and fluid flow in the molten pool are closely related to the surface and internal quality of the part.Numerical simulation technology can provide effective means for studying this series of complex physical phenomena.In this paper,a coupled mathematical model that can accurately describe the formation and development of the deposited layer and the physical processes in the molten pool was established,and then the forming processes of single-pass,double-pass overlap,single-layer multi-pass,and multi-layer multi-pass of coaxial powder feeding laser deposition IN718 were separately calculated and verified.The results obtained can be used to analyze and predict the shape of the deposited layer,the topography of the molten pool and the temperature gradient and the solidification rate on the solidification interface,which provides help for understanding the deposition mechanism of the coaxial powderfeeding laser deposition process,improving the dimensional accuracy of the deposited layer and realizing the prediction and control of the solidification structure,so that the laser deposition technology will develop towards high dimensional accuracy and high internal quality.The main research contents and conclusions are as follows:Firstly,by combining the VOF method with the powder feeding equation,a deposition layer calculation model was proposed.The process of coaxial powder feeding laser deposition IN718 was computed by appending into Fluent software with UDF of some calculation modules such as Gaussian heat source,powder feeding,heat convection and heat radiation,surface tension of molten pool and material properties amendment.The simulated results are in the good agreement with the experimental in terms of the morphology and size of the deposited layer and molten pool for single-pass and double-pass overlapping,subsequently the single-layer multi-channel and multilayer multi-channel processes were computed.It is proved that this model can be used for the computation,prediction and optimization of the deposition layer and molten pool shape of actual laser deposition manufacturing.Secondly,based on the good agreement between the simulation and experimental results of the size of the single-pass deposition layer and the molten pool morphology,the temperature field and solidification characteristics in the single-pass deposition process were analyzed.The “tailing” phenomenon of temperature field distribution and molten pool morphology in a single deposition process was explained from both sensible heat and latent heat,and the significant influence of latent heat on the heating rate and cooling rate of the single-pass processing was analyzed.The temperature gradient(G)and the solidification rate(R)on the solid-liquid interface were calculated.The changes of temperature gradient(G),solidification rate(R),cooling rate(G×R)and solidification shape factor(G/R)at different depth on the solid-liquid interface were analyzed.According to the solidification theory,the development trend of crystal grains from the bottom to the surface of the molten pool is developed from coarser columnar and cellular crystals to finer equiaxed crystals.The results provide a method for realizing the prediction and control of microstructure formation in laser deposition.Finally,based on the good agreement between the simulation and experimental results of the size of the double-pass overlap deposition layer and the molten pool morphology,the temperature characteristics of the double-pass and multi-pass overlap processing are analyzed.In one-way parallel double-pass overlap scanning,affected by the heat accumulation of the first deposition channel,the molten pool during overlap processing is larger than that during single-pass processing.However,due to the influence of latent heat,the temperature increase of the molten pool of the overlapping deposition channel will be suppressed,and the maximum temperature may even decrease.At the same time,the first deposition channel is also affected by the high temperature state of the overlapping deposition channel,and there are different degrees of temperature rise at different positions.The temperature rise is 100K?300K occurring in a scope of 1000K?1600K.In the multi-pass processing,the temperature of one deposition channel will be continuously affected by the heat input of the subsequent channels.The temperature recovery will occur several times while the recovery range will decrease sequentially.In this case,the temperature recovery will affect the solid phase transition after solidification of the IN718 alloy with the solid phase transition temperature of 800K?1500K.