Study On Superplastic Deformation Behavior Of Additive Manufacturing Ti-6Al-4V

TC4(Ti-6Al-4V)titanium alloy was widely used in aerospace,biomedical,marine and other fields due to its good comprehensive mechanical properties,corrosion resistance and excellent superplastic properties.It was difficult to process through conventional processing methods considering the large deformation resistance and poor thermal conductivity of TC4 titanium alloy.The above problems can be effectively solved by additive manufacturing(AM).While the additive manufacturing microstructure was obtained by rapid solidification,which was not conducive to the excellent comprehensive mechanical properties of the part.Superplastic deformation after additive manufacturing allows material to obtain better micro structure during the forming process.Studying the superplastic deformation mechanism of titanium alloys can provide theoretical bases for additive manufacturing combined with superplastic deformation process.In this paper,the microstructures and properties of the deposited materials obtained by electron beam additive manufacturing were studied,and the samples were taken in both vertical and horizontal directions and the high temperature tensile tests were conducted under different process parameters.The microstructure evolution law and deformation mechanism of the additive manufacturing materials were analyzed under superplastic conditions,and the main results obtained were as follows:(1)Columnar grains can be clearly observed in the macro structure of electron beam additive manufacturing materials.The thickness of the microstructure decreased along the building direction from the bottom to the upper layer due to the accumulation of heat cycle.Hot isostatic pressing process can effectively eliminate the unwelded defects in the asdeposited state,but it also led to the microstructure coarsening.(2)A maximum elongation of 451.3%can be obtained at 900℃,5×10-4s-1,which showed that the additive manufacturing material has a certain superplasticity.As the temperature increased,the material flow stress decreased,and as the strain rate increased,the material flow stress increased.The Arrhenius equation and the Johnson-Cook equation were used to establish the high temperature deformation constitutive model of the electron beam additive material.The modified Johnson-Cook model considering strain was selected,and the model was verified to have high precision at low strain rates.(3)Microstructure analysis showed that with the increase of temperature,dynamic recovery and dynamic recrystallization were more likely to occur,the grain size after equiaxification was larger,and the proportion of β phase increased;as the strain rate decreased,the grain size was larger,the beta ratio increased;as the strain increased,the microstructure gradually became more equiaxed and induced grain growth.(4)In the process of high temperature tensile deformation of electron beam additive materials,the main deformation mechanism in the initial stage was dislocation behavior,a large number of dislocations form the substructures,the behavior not only contributed to deformation,but also served as a deformation coordination mechanism.The α-lamellar was disintegrated and equiaxed under the coupling of dynamic recrystallization and βphase incorporation into the a phase.When the equiaxification was completed,the main deformation mechanism of the material became grain boundary sliding.