Study On Deformation And Failure Mechanism Of TC4 Titanium Alloy By Additive Manufacturing

Titanium alloys have high strength,low density,high fracture toughness,corrosion resistance,and good biocompatibility,and are widely used in fields such as aerospace,biomedical,and agricultural machinery.In the field of agricultural machinery,using titanium alloy materials to manufacture components such as agricultural machinery engines,chassis,and suspension systems can reduce the overall weight of the system and effectively improve the service life and production efficiency of agricultural machinery equipment.Applying new materials and processes to the production and manufacturing of core components of agricultural machinery has become one of the main directions for the future development of new agricultural machinery equipment.Additive manufacturing technology has various advantages,such as reducing production steps,high flexibility,high material utilization efficiency,and near-net forming production.These characteristics of additive manufacturing enable it to design and manufacture agricultural machinery components with complex geometric shapes,minimize waste generation in the production process,shorten delivery time,achieve integration and integration of multiple components of agricultural machinery structures,lightweight structure,and improve performance,laying the foundation for the widespread application of titanium alloys in agricultural machinery engineering.The application of titanium alloy materials and the development of additive manufacturing technology can provide important support for the research and manufacturing of high-end agricultural machinery and equipment in China.Firstly,to study the damage failure process of additively manufactured titanium alloy TC4 under different stress states,uniaxial tensile,pure shear and notched tensile specimens with different initial stress triaxiality were designed in this paper.Digital image correlation methods were applied to observe the deformation,crack sprouting,and extension processes of the material under different loading conditions,and the effect of the additive manufacturing forming direction on the mechanical properties of titanium alloy TC4 was investigated.The experimental results show that the yield strength and tensile strength of the specimens formed along the vertical direction of the additively fabricated titanium alloy TC4 are slightly lower than those of the horizontal specimens,but the elongation at the fracture of the vertical specimens is larger and the plasticity is better.The fracture morphology of the specimens after failure was analyzed,and it was found experimentally that defects and stress states have a large influence on the damage and failure behavior of the additively manufactured titanium alloy TC4.For specimens with different forming directions,the tensile stress has an opening effect on the defects in the vertical specimens,and the defects in the horizontal specimens tend to close under the load.Under tensile loading,the main failure mechanism of the additively manufactured titanium alloy TC4 is the nucleation,growth,and convergence of voids resulting in crack sprouting and expansion behavior,and the shear mechanism of the internal voids of the titanium alloy under shear stress dominates the failure damage of the specimen.Secondly,the deformation and damage failure mechanism of rolled titanium alloy TC4 under tensile loading was investigated.The strain field on the surface of the specimen observation area was obtained by using the digital image correlation method.Experimentally,an obvious strain localization phenomenon was observed on the specimen before fracture,and the strain variation curves of the specimen surface at different locations with loading were obtained by the analysis of the strain field.After the fracture of the specimen,the fracture surfaces show a certain angle relative to the loading axis,which indicates that the sprouting and expansion of cracks in the specimens are influenced by the shear stress.In order to more accurately describe the plastic deformation of rolled titanium alloys,the Linear-Swift-Voce(LSV)hardening model is proposed in this paper to describe the strain-hardening behavior of rolled TC4,and the results show that the LSV hardening model can better describe the stressstrain relationship of the material.The effect of stress triaxiality on the damage failure of rolled titanium alloy TC4 was analyzed by observing the fractures of specimens using an electron scanning microscope.The results showed that the higher stress triaxiality promoted the nucleation,growth,and convergence of voids,which led to the sprouting of cracks in the center of the specimens,and the crack expansion eventually led to the failure damage of the tensile specimens.Finally,based on uniaxial and notched tensile experiments and finite element analysis of titanium alloys,an inverse identification method for determining the parameters of strainhardening and damage models of titanium alloys is proposed.For the deformation characteristics of titanium alloy for additive manufacturing,this paper proposes the LudwikHockett-Sherby(LHS)hardening model,which describes the hardening behavior of titanium alloy before and after necking using segmentation functions and has good applicability and flexibility.The stress-strain curve,specimen width,and numerical evolution of strain were used as evaluation parameters to invert the parameters for identifying the strain-hardening model.A comparison of numerical simulations and experimental results showed that the proposed method can effectively characterize the strain-hardening behavior of titanium alloys in the whole deformation range.The main parameters of the Johnson-Cook and GTN damage models were identified by inversion using the load-displacement curves and the equivalent variation in the local area.The results show that the fracture strains predicted by the fracture model are consistent with the experimental results,further validating the feasibility and effectiveness of the proposed method.The method can be further applied to the characterization of plasticity as well as the fracture behavior of other metallic materials.