Simulation Study Of Thermal-flow Field In Laser Powder Bed Fusion Process

Laser powder bed additive manufacturing technology is widely used in automotive industry,aerospace and nuclear power because of its high forming accuracy,low surface roughness,flexible design and high material utilization rate.The laser powder bed fusion technology involves many physical phenomena which are difficult to be characterized by experiment in the forming process.The determination of the optimum process parameters depends on a large number of high-cost trial and error experiments.The computer numerical simulation has the advantages of flexibility,safety,low cost and high efficiency,which provides a way to solve the above problems.In this paper,the multi-physical field mesoscopic scale model of laser powder bed melting additive manufacturing was established by introducing computer aided method from the powder bed and the powder melting process.The formation of metal powder bed,the molten pool generated by the powder melting,the temperature field and the flow field were numerically simulated,and the influence of process parameters on the temperature field and flow field was expounded.Firstly,the Discrete Element Method(DEM)was used to establish a powder bed with a random distribution size of 1 mm×0.4 mm×0.006 mm.Microsoft Visual Studio software was used to compile the Heat source and Rsq governing equation for loading the Gaussian laser heat source and the motion trajectories on the surface of the powder bed,so as to realize the secondary development in FLOW-3D,and establish the numerical model of laser powder bed melting at mesoscopic scale.Secondly,Computer Fluid Dynamics(CFD)software FLOW-3D was used to study the effects of scanning speed and laser power on the morphology,size,temperature field and flow field of molten pool,and to predict the optimal processing parameters of laser powder bed.Taking 316 L stainless steel as the research object,the molten pool size and morphology of experimental and numerical simulation under the same parameters were compared,observed and analyzed.The experimental results are in good agreement with the simulation results.Finally,in the multi-channel numerical simulation,the optimized scanning speed and laser power in a single pass were selected to study the influence of different scanning spacing on the forming quality of the solidified surface and the influence of the connectivity between the molten pool passes.By adapting the governing equation of laser trajectory,two different scanning strategies of "S" shape and "Z" shape are realized,and the different states of the fuse under the two scanning strategies are compared and analyzed.The influence of process parameters on temperature field and flow field of molten pool was described.The results show that with the change of process parameters,the temperature field and flow field of the molten pool will be changed,which will affect the morphology and size of the molten pool,and then affect the quality of the forming component.It is found that when the scanning interval H=90 μm,the lap effect of two fusion channels is good.Under the two methods,the beginning and end of the fuse are identical,while there is no significant difference in the middle part.When Z-shaped scanning is adopted,the high end is close together,while the low end is close together;In the case of S-shaped scan,there may be crossover in the high and low directions,but compared with Z-shaped scan,there is a greater inclination trend.