Microstructure Control And Its Mechanisms During Laser Additive Manufacturing Of Inconel 718

Laser additive manufacturing(LAM),as a typical rapid prototyping process,has the advantage of fabricating complex shaped components that are difficult or impossible through conventional manufacturing methods and gained popular attentions to repair worn parts,due to its unique processing method of layer by layer.Nickel-based superalloy has been widely used in aerospace,navigation,nuclear industry and petrochemical industry due to its pronounced combination of excellent mechanical properties and good corrosion-oxidation resistance.However,it is a great challenge to obtain the fully controlled dimensional accuracy and meachnical properties of LAM processed parts,due to the complexity of the physical process in LAM and the involved numerous processing parameters.There are many problems which restrict the development and industrial application of LAM of Nickel-based superalloy,such as the serious composition segregation,the anisotropy in microstructure and performance,and the poor controllability of solidification microstructure of LAM processed parts.In this thesis,the LAM process was studied as the subject and Inconel 718 powder was selected as the forming material.The above problems were investigated using comprehensive research methods of temperature measurement,microstructral characterization,numerical simulation and analysis of solidification theory model.This thesis aims to reveal the mechanism of microstructural formation,to establish the relation among process,microstructure and mechanical properties,and further to control the solidification microstructure during LAM of Inconel 718.The detailed research contents and results are as follows:(1)Molten-pool temperature was monitored using a two-color pyrometer during continuous-wave(CW)LAM of Inconel 718.The effect of processing parameters,such as laser power and scanning speed,on molten-pool cooling rate,dendritic microstructure,Nb segregation,Laves-phase formation and mechanical properties were investigated.The quantitative relationship between process and solidification was established.The results showed that the microstructure of CW-LAM processed Inconel 718 was featured by highly ordered columnar dendrites,Nb segregation and continuously distributed Laves phase.The dendritic microstructure was refined and the volume fraction of Laves-phase were reduced by enhancing the cooling rate of the molten pool using the low laser power and the high scanning speed.Compared with laser power,scanning speed played a more important role on cooling rate of the molten pool.dendritic microstructure.Nb segregation and Laves-phase formation.The new powers function relation between primary dendritic arm spacing and cooling rate was determined,in which the constant term was 438.5 and the index was-0.577.The Nb segregation ratio and the volume fraction of Laves-phase decreased with the increase of cooling rate due to decreased diffusion channels and increased solute trapping under high cooling rate and short solidification time.(2)The quasi-continuous-wave(QCW)laser processing mode was proposed to control the solidification microstructure during LAM.The effects of pulse parameters,such as pulse frequency and duty ratio on the microstructure and mechanical property were investigated.Compare to CW-LAM,the solidification condition of the molten pool was significantly modified during QCW-LAM.The hardness and tensile performance of the sample before and after heat-treatment were significantly improved due to the refined dendrites,the supressed Nb segregation and the reduced Laves-phase.The high pulse frequency favored the formation of fine columnar dendrites,attended linear-distributed Laves-phase and highly orientated crystallographic texture,while the high pulse frequency tended to produce fine equiaxed dendrites and discrete-distributed Laves phase and more random crystallographic texture.The significantly modified solidification conditions,the molten-pool boundary and the periodic remelting contributed to the microstructural formation.The low duty ratio promoted the refinement of grain and the inclination of grains to the scanning direction due to the obtained higher cooling rate of molten pool and the more gentle molten-pool boundary.(3)The instantaneously three-dimensional mathematical model of the molten pool was established to simulate the evolution of' molten-pool morphology,the temperature distribution and the varition of solidification parameters under different processing conditions.In addition,the formation mechanism of solidification microstructure under two different laser processing modes was analyzed by the combination of molten-pool mathematical model and solidification theory.The results revealed that the solidification conditions of the molten pool were significantly modified using QCW-LAM,such as the peridically varition of molten-pool morphology and temperature,the incresed temperature gradient(G).solidification rate(R)and cooling rate of the molten pool.The solidification microstructure was predicated by the combination of the molten-pool temperature field and the dendrite growth model.Compare to the high pulse frequency,the low puls frequency tended to reduce the remelting frequency,to increase the curvature of molten pool interface and to enhance the cooling rate of the molten pool.The reduced G/R ratio at top of the molten pool resulted in the formation of equiaxed dendrite.The high scanning speed and the low pusle frequency refined the solidification microstructure and promoted the formation of stray or equiaxed dendrites due to the improved cooling rate and the reduced G/R ratio of the molten pool.(4)Effect of scanning paths on the solidification microstructure and crystallographic texture was investigated and its influencing mechanism was also analyzed.The feasibility of precisely tairing the local microstructure of a part using the LAM process is explored.The results showed that the unidirectional path developed a fiber texture,while the backward and forward scanning pattern induced the epitaxial growth of the second dendrite in the preexisted layer and developed a rotated cube texture in the deposit.In addition,the cross path produced a more random orientation with a cubic texture.The combination of a backward and forward path and a high pulse frequency developed full columnar dendrites and produced the near single crystal texture.The formation of different textures attributed to the modified molten-pool boundary,the periodic remelting and the changed solidification conditions under different processing conditions.A good response of microstructure was obtained when the customized process parameters were used.On the premise of the consistent outline of parts,the site-specific solidification microstructure could be obtained using the site-specific process parameters.The above results confirmed the feasibility of tairing the local microstructure of a part using the LAM.