Study On The Molten-pool Geometry And Solidification Parameters Of Laser Additive Manufactured Inconel 718

Laser additive manufacturing(LAM)technology,also known as 3D printing,is one of the most promising manufacturing technologies.LAM has the ability of fabricating complex-shape parts layer by layer and it has broad application prospects in aerospace,marine,biomedical and other fields.However,it is still a great challenge to fabricate parts with high dimensional accuracy,refined microstructure and excellent mechanical properties due to the complicated physical process and a lot of processing parameters involved in LAM.Until now,there are still some problems that have to be faced,such as metallurgical defects and uncontrolled microstructures,which are closely related to the dynamic characteristics and thermal behaviors of the molten pool.In this thesis,the LAM process was studied as the subject and Inconel 718 powder was selected as the forming material.The physical process,including the laser-power interaction,heat and mass transfer of the molten pool was analyzed and simplified.The three-dimensional transient numerical model of LAM was established to study the effect of process parameters on the gas-liquid interface,the temperature parameters and the solidification parameters(temperature gradient G,solidification rate R and cooling rate G×R)of the molten pool.This thesis aims to provide a guidance for process optimization and the foundation of fabricating LAM parts.The detailed research contents and results are as follows:(1)The force of the gas-liquid interface was analyzed and the influence of each force on the gas-liquid interface was studied.The energy control equilibrium equation of free surface was established.The results show that the surface tension is the main force to maintain the gas-liquid interface morphology under equilibrium conditions.The evolution of the gas-liquid interface was simulated.The simulated morphology and size of the deposit match well with the experimental results,which verifies the accuracy of the model and the control equation.(2)The main factors determining the molten-pool shape were analyzed and the process window was also optimized.The results indicate that laser power is the main factor that affects the width and the depth of the molten pool.Scanning speed affects the height of molten pool.The optimized process window for laser power is in the rage of 600W-900W and for scanning speed is in the rage of 6mm/s-16mm/s.(3)The temperature parameters and solidification characteristic parameters at different positions along the solidification interface of the molten pool were analyzed.The solidification microstructure was also analyzed.The results show that the temperature gradient G gradually increases,while the solidification growth rate R decreases,from the top to bottom of the solidification interface of the molten pool.The cooling rate G X R decreases from the top to bottom of the molten pool.The morphological parameters G/R increases slowly from the top to bottom of the molten pool,and rapidly increases at the bottom.The distribution of predicted microstructure was nonuniform.The dendritic structure is fine at the top of the deposit while it is coarser at the bottom.(4)The influence of process parameters on the temperature parameters and solidification parameters of the molten pool was investigated.Further,the effect of process parameters on solidification microstructure was predicted.The results indicate that the temperature gradient G and the cooling rate G×R decrease with the increase of laser power.The high laser power result in the coarsening of dendritic structure.The temperature gradient G,the solidification rate R,and the cooling rate G×R increase with the increase of scanning speed.The high scanning speed result in the refinement of dendritic structure.The laser power shows a more significant on temperature gradient G than the scanning speed.Whereas the cooling rate G× R is mainly determined by scanning speed.The powder feeding rate shows less effect on the solidification parameters of the molten pool.The growth direction of the dendrite is greatly affected by the scanning speed.Whereas the growth direction of the dendrite shows little dependence on the laser power and the powder feed rate.