Microscopic Simulation And Experimental Study Of Temperature Field And Flow Field Of Inconel 718 Alloy By Selective Laser Melting

Selective Laser Melting(SLM),as one of the most promising additive manufacturing methods,has broad application prospects in the future.Inconel 718 alloy has been widely used in aerospace,petrochemical,medical materials and other fields due to its good structural stability and comprehensive mechanical properties under high temperature conditions.At present,in the study of selective laser melting process,there is no suitable instrument to capture the complex physical,chemical and metallurgical phenomena occurring in the laser-powder interaction zone.Applying simulation to reproduce the process and realizing the adjustment and optimization of process parameters is of great practical significance for academic research and industrial application in the field of additive manufacturing.In this paper,the microscopic phenomenon in the molten pool of Inconel 718 nickel-base superalloy is formed by selective laser melting.A three-dimensional transient finite element model is established by using the heat transfer module and laminar flow module in COMSOL Multiphysics.The influence of the process parameters on the temperature field,flow field,size and shape of the molten pool was verified by the selective laser melting experiment.In the simulation several special phenomenon characterized with SLM process was considered,such as the interaction between the laser and material with stochastic porosity distribution on powder bed,the nonlinear change of thermophysical properties due to the state change of material,the influence of the Marangoni effect in the molten pool and the phenomenon of powder material melting-evaporation-solidification under laser energy.The results show that with the continuous laser scanning and heat input,the t internal temperature of the molten pool is gradually increased.When the temperature in the molten pool is higher than the boiling point,the evaporative heat flow drives the molten liquid to flow obliquely above the laser and forms eddy current at the front of the molten pool.At this time,the melt with the largest flow rate is located at the center of the molten pool,and then some of the heat is lost to the surrounding with the heat of evaporation,and the peak temperature of the molten pool decreases slightly.As the temperature in the molten pool decreases,the evaporation rate decreases,and the melt flow rate decreases,which reduces the heat dissipation of the molten pool.The heat in the molten pool increases again and the temperature rises again.When the temperature is cooled between the melting point and the boiling point,the Marangoni convection will replace evaporation,predominating in the transition zone of the molten pool tail where melt flow occurs.Through the cyclic reciprocation of the above process and the accumulation of heat in the powder bed,the peak temperature of the molten pool eventually fluctuates and tends to be stable after the third scanning track.In the process of molten pool formation,increasing laser power(from 150 W to 230W)or decreasing scanning speed(from 1600mm/s to 800mm/s)can increase energy density input per unit time,thus increasing the evaporation and Marangoni convection,thereby accelerating the melt flow in the molten pool,and ultimately improving the heat transfer and mass transfer efficiency in the molten pool.Too high laser power(higher than 210W)or too low scanning speed(less than 1000mm/s)will cause excessive temperature in the molten pool,resulting in overheating,boiling and even evaporation.Under the action of the evaporating heat flow,the eddy current in the molten pool increases,and the molten metal causes defects such as splashing and denudation,which deteriorates the surface quality of the formed part.When the overlap rate is 20%,t the metallurgical bond of the formed part is good,and the forming quality is high.Excessive overlap rate(above 40%)will cause the collapse,pores and sharp corners of the surface track.Excessive low overlap rate(less than 20%)will cause obvious ravines between adjacent scanning tracks.In the case where the forming parameters such as laser power,scanning speed and overlap rate are constant,as the powder layer thickness increases(from 20?m to 60?m),the depth of molten pool grows rapidly and the depthwidth ratio of molten pool increases.When the powder layer thickness is too high(above 50?m),the depth of molten pool cannot reach either the whole powder layer or not deepen to the underlined substrate,no fully metallurgical bonding can be obtained.The experimental results are in good agreement with the simulation results.By comparing the size and shape of the molten pool of the simulated and experimental samples,it is found that although the surface of the molten pool is curved due to the surface tension effect of the actual processed samples,the experimental results are in good agreement with the simulation results,and the simulation results can roughly show the trend of the size and morphology of the molten pool changing with the process parameters.The research on the microscopic phenomena of heat and mass transfer in the molten pool during selective laser melting of Inconel 718 alloy lays a foundation for further understanding the phenomenon of selective laser melting process,controlling the manufacturing process and exploring the technological parameters of selective laser melting of Inconel 718 alloy material.