Simulation Study Of Molten Pool Morphology And Pore Defects In Laser Powder Bed Fusion Via Inconel 718 Alloy

Laser Powder Bed Fusion(LPBF)is one of the most reliable and feasible metal additive manufacturing technologies,which show advantages of simple process,high material utilization and short production cycle for the complex structure with thin-walled surface.However,the LPBF process is usually accompanied by a rapid phase transition,which results in a large temperature gradient and leads to large thermal stresses in the heat affect zone,generating pores,cracks and ballings,reducing the quality of the LPBF printed part.In this paper,simulations and experiments on random powder bed,molten pool morphology and pore defects of LPBF process were conducted to develop the framework of the LPBF process.The surface morphology and the dynamic evolution of the molten pool,and the formation mechanism of pore defects during LPBF were investigated,and the corresponding experiments were designed to verify and optimize the process parameters to improve the quality of LPBF printed parts.For the powder deposition process,the discrete element method(DEM)was employed to reproduce the random powder bed.The variation of the relative density of the powder bed with different scraping speed and different scraper shapes was analyzed to obtain a denser random powder bed for the subsequent LPBF process simulation.For the simulation of molten pool and pore defects in LPBF process,a three-dimensional transient LPBF model was developed by computational fluid dynamics(CFD)method.The dynamic evolution via the size and the morphology of the molten pool and the formation mechanism of pore defects with different process parameters were investigated.It was found that as the laser energy was insufficient,the surface ripple morphology and irregular pore defects shown in the molten track,while as the laser energy was too high,the interaction between the laser and the melt pool at the laser center is intense,leading to evaporate of the melts which generated a downward recoil pressure,producing keyhole defects.For the optimization and experimental verification of LPBF process,a set of experiments were carried out to observe the surface roughness,molten track width,surface spheroidization and microstructure of the LPBF printed parts.It was found that when the laser energy was insufficient,the ripples appeared on the surface of the molten track,while when the laser energy was too high,and the molten tracks was warped and deformed by the more concentrated heat,which increased the surface roughness and burned the surface of the parts.The molten track widths measured in the experiments met good agreement with the simulation results,and the pore defects which were observed in the microstructure,also verified the simulation results.With the laser power of 180 W,the scanning speed of800 mm/s and the hatching space of 0.08 mm,there were almost no obvious defects in the microstructure of the LPBF printed parts which shown the best surface quality.