Fundmental Study Of Synchronous Mechanical Vibration Augmented Wire Arc Additive Manufacturing Of AA2219 Aluminum Alloy

AA2219 is a ternary Al-Cu-Mn high-strength aluminum alloy with excellent comprehensive performances,and is one of the most important lightweight materials used in aerospace large-scale structures.Compared with laser additive manufacturing,wire arc additive manufacturing(WAAM)process would be more suitable to fulfill near net shape of large components.However,before current study was carried out,the problems such as porosity,coarse microstructures and poor properties of WAAMed AA2219 alloy have not been well solved.In this dissertation,a novel method integrating synchronous mechanical vibration(SMV)and WAAM process was developed to improve the deposition morphology,metallurgical defects,microstructures,and mechanical properties of WAAMed AA2219 alloy.The main findings are as follows.The influences of process parameters such as arc current,polarity ratio,welding speed and argon flow rate on the deposition morphology,porosity and tensile properties of WAAMed AA2219 alloy were studied and optimized.Using these optimized parameters,the effective width ratio of thin wall would be increased to 83.1%,the unilateral machining allowance reduced to 0.71 mm,and the porosity decreased to 0.54%.The maximum yield strength(YS),ultimate tensile strength(UTS),and elongation(EL)reach123.7 MPa,268.5 MPa and 15.4%respectively,which are 110%,98.7%and 104.1%of the best results reported in the existing literatures.In addition,the prediction model of deposition morphology was established based on multiple linear regression method.The error rates of effective width and forming height are within 3.3%and 6%,respectively,which can predict accurately the deposition size in the actual production.SMV can effectively reduce porosity and refine grain size of WAAMed samples.When the vibration acceleration is 10 m/s~2,the porosity would be decreased to 0.27%,which is 50%lower than that of the sample without SMV.Besides,the average diameter of all grains and columnar dendrite size are 68.6 and 127μm,respectively,which are 27.4%and 62.2%lower than that of the sample without SMV.The main reason for the decrease of porosity is that SMV can enhance the fluidity of molten pool and make it easier for the aggregated pores to overcome the hindrance of columnar dendrites and escape from molten pool.The grain refinement mechanism lies in that,on the one hand,SMV enhances the convection intensity of liquid metals and lowers the temperature gradient between different components in the molten pool,inhibiting the nucleation of columnar dendrites.On the other hand,the vibration behavior breaks columnar dendrites and increases the nucleation of equiaxed grains,promoting the columnar to equiaxed transition.The tensile properties of WAAMed AA2219 alloy with SMV can be improved significantly due to the refining grains and decreasing porosity.Under the optimized parameters,the YS,UTS and EL in the vertical direction of the sample with SMV are131.6 MPa,275.4 MPa and 14.7%respectively,which are 14.1%,10.3%and 23.6%higher than those of the sample without SMV.The YS,UTS and EL in the horizontal direction are 133.9 MPa,280.4 MPa and 17.3%respectively,which are 8.2%,4.4%and12.3%higher than those of the sample without SMV.The anisotropies of YS,UTS and EL decrease from 6.8%,7.0%and 22.7%to 1.7%,1.8%and 15.2%,respectively.On the basis of microstructures observation and strengthening mechanisms calculation,the properties enhancement can be ascribed to three main reasons.Firstly,SMV reduces the rate of micro-cracks propagation by refining the grains.Secondly,the plastic deformation is improved by increasing dislocation density and entanglement degree.Finally,reducing porosity helps to inhibit the micro-cracks initiation.The effects of solution temperature,solution time,aging temperature and aging time on the eutectic phases and tensile properties were studied.A suitable heat treatment specification for WAAMed AA2219 alloy was established based on the mechanical properties.It can be found that the fine-grain strengthening induced by SMV was inherited during the heat treatment process.Furthermore,the maximum YS,UTS and EL of WAAMed sample applied with this specification are 306.9 MPa,451.3 MPa and 11.9%respectively,which are 1.3%,8%and 16.7%higher than the best results reported in the existing literatures,and exceed the forging standard used in aerospace AA2219-T6 alloy(AMS4143F).By the microstructures analysis and strengthening mechanisms calculation,the needle nanometerθ"phase precipitated during aging treatment forms coherent strain strengthening withα-Al matrix,and its strong resistances to dislocation movement are the main reason of properties enhancement.