| TC4 titanium alloy is one of the most widely used two-phase titanium alloys currently.The microstructure morphology,grain size and structure of two-phase titanium alloys are different due to the differences of alloy composition,deformation and heat treatment process.Microstructure changes of the alloy can be achieved by using a suitable thermal deformation process.Multi-directional forging technology is a representative process of severe plastic deformation.The forging direction changes constantly during the multi-directional forging,the samples are constantly compressed and elongated in three directions to achieve the purpose of improving microstructure of the alloy.In this paper,finite element simulation experiment and physical experiment are used to reveal the deformation.The deformation behavior and deformation law of TC4 titanium alloy during the multi-directional forging are revealed and studied using finite element simulation method.Multi-directional forging test of TC4 titanium alloy at deformation temperature of 700? and strain rate of 0.001s-1 was investigated by Gleeble-3500 Thermal Simulation Machine,combined with modern material analysis methods,the evolution of microstructure of titanium alloy have been studied.The damage value,maximum principal stress,effective stress,effective strain and deformation load during the multi-directional forging are analyzed by Deform-3D finite element numerical simulation software.The effect of deformation temperature,deformation rate and forging step on multi-directional forging deformation was revealed.Simulation results show that the reduce of deformation rate and increase of deformation temperature are favorable to the reduction of deformation damage value.The increase of deformation rate and reduce of deformation temperature can lead to the increase of the maximum tensile stress and the maximum compressive stress after the deformation.The increase of deformation rate,the decrease of deformation temperature and the increase of forging step could lead to the increase of effective strain value of titanium alloy after the deformation.The difference of the maximum principal stress and effective strain distributed parallel to the compression axis and perpendicular to the compression axis indicates that the alloy deforms unevenly.The change of deformation load is the result of deformation rate and deformation temperature.The deformation load could be reduced by reducing the deformation rate and improving the deformation temperature.The results of physical experiment show that the flow stress-strain curve shows dynamic recrystallization characteristic during the first step of multi-directional forging.The flow stress increases rapidly and then remains almost constant during the second step.During the third step,the flow stress increases constantly,with a large increase rate at the initial stage,then the increase rate decreases and nearly remain constant after a certain degree of deformation.After three steps of forging,intergranular ? phase and ?/? cluster of the center part of titanium alloy disappear while large size of ? laths appear,and the original ? phase had split into different lengths.The degree of microstructure transformation becomes smaller at the edge part of the alloy,the process of microstructure transformation will first take place at the grain boundaries.According to high magnification SEM image,the original lamellar microstructure appears to bend and fragment,and equiaxed ? grains were observed.The globularization process of lamellar structure is that: subgrain boundaries were formed because of the bent of lamellae or the effect of local shear stress,and grooves would appear at the place of subgrain boundaries;then ? lamellae split into a small one due to the wedge of ? phase;the interface of fragmented lamellae diffuses and migrates to minimize the surface energy,equiaxed structure is formed at last. |
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