Effect Of Fluoride Addition On The Influence Mechanisms Of Forming Quality Of Wire And Arc Additive Manufactured Ti-6Al-4V Titanium Alloy Components

Nowadays,the development trend of Ti-6A1-4V titanium alloy components in aerospace field is high-performance,large-scale,integral and complicated.Wire and arc additive manufacturing(WAAM)is more and more widely used in the production of titanium alloy components.Thus,improving the forming quality of these titanium alloy components has been a major issue of WAAM.The forming quality of WAAM relates to many problems,including the influence factor of surface quality,the evolution law of as-deposited material and so on.On the basis of exploring the influence mechanism of forming accuracy,a new method to control the microstructure and mechanical properties of as-deposited material has been proposed,which will provide the improvements for WAAM technology and forming quality of Ti-6Al-4V titanium alloy components.In order to solve the deterioration in mechanical properties of coarse columnar ? grains of as-deposited titanium alloy components,the main problems researched in this dissertation are as follows:? Influence factor analysis of forming quality of WAAM titanium alloy components;?"Droplet oscillation momentum" and its effect on surface quality;?"Continuous growth restriction factor" and its application in analysis of the local microstructure evolution within the weld pool;?The effect of activating fluxes on forming quality of titanium alloy manufactured by layer-by-layer deposition."Zhang-Chen" theory does not answer question about the influence of the droplet impact on the weld pool deformation.Thus,the analysis conclusion is drawn that "droplet oscillation momentum" has a direct impact on surface quality,on the basis of researching the relationship between droplet momentum,weld pool deformation and surface quality.Since "Rajamaki"theory does not consider the interdependent relationship between grain's nucleation and growth,the analysis conclusion of the weld pool having the characteristics of solidification continuum is reached through the research on the behavior of grain's nucleation and growth under a continuous change of solute concentration.On the basis of above analysis conclusions,the influencing factors and their mechanisms of forming quality of WAAM titanium alloy components are analyzed systematically.To investigate the interaction mechanism between welding current,droplet oscillation and surface quality,it is necessary to find a variable which can reflect the growth of droplet oscillation.The influence of the momentum during droplet growth on the weld pool deformation and surface quality is analyzed,and the method of "droplet oscillation momentum" analyzing the influence mechanism of surface quality is proposed.The mathematical model of "droplet oscillation momentum" is established based on "mass-spring"theory.Subsequently,the relationship between welding current,droplet oscillation and surface quality is founded based on the combination of the mathematical model and the analysis conclusion of "droplet oscillation momentum" having a direct impact on surface quality.And then,it is proposed that "the increasing welding current gives rise to the reduction of droplet oscillation momentum and the advancement of surface quality".The validations of the proposed method and the analysis conclusion are tested by the model calculation,droplet transfer experiment,single-layer single-bead and multi-layer single-bead experiments.Because the model of growth restriction factor cannot objectively reflect the local mcrostructure evolution within the weld pool,the method called"continuous growth restriction factor" to analyze the local microstructure evolution within the weld pool is proposed by the analysis of growth restriction factor development along the solidification pathway within the weld pool under the condition of the continuous change of solute concentration and the interdependent relationship between grain's nucleation and growth.The mathematical model of "continuous growth restriction factor" is established,on the basis of the combination of growth restriction factor and the analysis conclusion of the weld pool having the characteristics of solidification continuum.The model of"continuous growth restriction factor" is used to analyze the evolution of grain size within the weld pool under the given concentrations and weld speeds.It also can be used to analyze the influences of concentration and weld speed on the mean grain size within the weld pool and the planar growth distance adjacent to fusion boundary.The effectiveness of the proposed methods is verified by the experimental results.The main aim of the present dissertation is to refine the coarse columnar ? grains of as-deposited titanium alloy components.In order to achieve the aim,a new method of the application of CaF2/NaF to control as-deposited titanium alloy material is proposed on the basis of researching the influence of CaF2/NaF additions on the evolution law of forming quality of WAAM titanium alloy components.The method adds the process of coating fluoride between the layers during WAAM process,which changes arc characteristics and solidification environment of the weld pool at the same welding specifications.The mechanism of reduction of surface quality after fluoride additions is elucidated by the model of "droplet oscillation momentum".The mechanism of refining ? grains is analyzed by the model of "continuous growth restriction factor".Then,to explain the mechanisms of the refined platelet ?,the quasi-continuous grain boundary a and the increasing volume fraction of ? phase,the dynamics of a phase diffusion growth is researched.The mechanism of material strengthening is discussed after the material structure of as-deposited titanium alloy materials is analyzed.The effects of CaF2 and NaF additions on inhibiting the growth of coarse columnar ? grains and improving the microstructure and properties of as-deposited titanium alloy materials are compared,which test the validation of the proposed method.