| The lower control arm is a key component of the suspension of wheeled armored vehicles,suspension bearing capacity not only needs to meet the requirements of the vehicle equipped with guns,turrets,draped armor and vehicle self-rescue equipment,but also to resist the high-frequency large impact loads brought about by the harsh road conditions.Ti-6Al-4V titanium alloy is an α+βtype two-phase titanium alloy,which has the advantages of light mass and high specific strength,and is one of the preferred materials for the lower control arms of wheeled armored vehicles.At present,there are many problems in forming Ti-6Al-4V titanium alloy lower control arm prefabricated parts by free forging in China,such as low dimensional accuracy,high material consumption,poor process stability and low production efficiency.In this paper,the cross wedge rolling process to form the prefabricated parts of the lower control arm of vehicle titanium alloy,and conducts the research around the scientific problems and technical difficulties such as its dimensional forming accuracy,microstructure evolution,core defects and mechanical properties.Firstly,based on the isothermal compression,tensile and high-temperature phase transformation experiments of Ti-6Al-4V titanium alloy,the high-temperature deformation behavior,dynamic recrystallization behavior,damage evolution mechanism and phase transformation behavior were studied respectively.The results showed that the high temperature flow stress curve of Ti-6Al-4V titanium alloy with flow softening characteristics.Dynamic recrystallization is an important reason for flow softening of Ti-6Al-4V titanium alloy with equiaxed microstructure during high temperature deformation.The fracture morphology of high temperature tension showed ductile damage,which first occurred at the interface of two phases.The change trend of fracture strength is opposite to that of temperature,while the change trend of fracture strain is the same.The β phase transition temperature is 992℃.Based on the high-temperature deformation behavior and microstructure evolution mechanism of Ti-6Al-4V titanium alloy,a set of unified viscoplastic constitutive model considering the phase volume fraction,dynamic recrystallization volume fraction and plastic temperature rise was established.The constitutive model established can better reflect the rheological stress characteristics of the material.And the evolution of its microstructure can also be accurately described.Through the redevelopment of Deform-3 D numerical simulation platform,a finite element model with multi-field coupling of heat-force-microstructure of Ti-6Al-4V titanium alloy formed by cross wedge rolling was established.Based on this model,a synergistic control study of dimensional accuracy and tissue properties of Ti-6Al-4V titanium alloy wedge cross-roll forming was carried out.Secondly,the forming mechanism of necking and out of roundness during cross wedge rolling of Ti-6Al-4V titanium alloy shaft was analyzed by numerical simulation,theoretical analysis and experimental verification.The analysis of the effect on the diameter dimension deviation shows that the Ti-6Al-4V titanium alloy has flow stress softening characteristics,which contributes to its greater susceptibility to necking during cross wedge rolling.When the deformation temperature is higher,the deformation resistance of the material decreases,and the axial force is also lower,so it is not easy to produce necking.When the stretching angle is reduced,the forming angle is increased and the area reduction is increased,the axial force borne by the rolled workpiece from the wedge plane is increased,and the diameter dimension deviation is increased.The analysis of the influence on ellipticity shows that when the deformation temperature increases or the roll speed decreases,the transverse flow of Ti-6Al-4V titanium alloy will be enhanced,and the ellipticity value will increase.By changing the forming angle,stretching angle and area reduction,the contact area between the die and the workpiece is changed,prompting different volume changes in the flow of metal along the axial and tangential directions,which in turn affects the ellipticity of Ti-6Al-4V titanium alloy shaft.The influence of cross wedge rolling process parameters on the evolution of a phase volume fraction during the plastic forming of Ti-6Al-4V titanium alloy shafts was revealed by multi-field coupled finite element model analysis and experimental verification.The basic mechanism of the influence of phase composition and morphology on the mechanical properties of Ti-6Al-4V titanium alloy was analyzed through the mechanical property test results.The results showed that the deformation temperature,area reduction and rolling speed have a significant effect on the volume fraction of a phase.The geometric parameters of the die have little influence on it.The extra equiaxed grains promote the strength of Ti-6Al-4V titanium alloy,and the coarse lath α phase reduces the elongation of the material.When the volume fraction of primary α phase is in the range of 0.35～0.4 can ensure the rolled parts with high strength and good elongation level.The fracture morphology of the high temperature tensile specimen of the rolled piece shows good ductile fracture characteristics.The formation mechanism of central defects in cross wedge rolling was analyzed by experiment and numerical simulation.A mixed damage criterion was established by considering the influence of stress state,temperature and stress triaxiality on the damage of rolled workpiece.The finite element model based on the coupled mixed damage criterion reveals the influence of cross wedge rolling process parameters on the core damage.The results showed that reducing the forming angle,increasing the stretching angle,and increasing the deformation temperature will increase the ellipticity of the rolled workpiece,and the"kneading" effect will obviously increase the damage value.Finally,the process parameters were optimized by response surface method with diameter dimension deviation,ellipticity and volume fraction of a phase as optimization objectives.The proportional part of the lower control arm preform of a vehicle was successfully rolled.The diameter deviation of the rolled workpiece is in the range of+0.11 to-0.06mm,the maximum ellipticity is 0.058mm,and the volume fraction of a phase is 0.465.The lowest tensile strength,elongation and reduction area are 980MPa,18%and 48%respectively,which all meet the technical requirements.This research provides theoretical guidance for cross wedge rolling forming of Ti-6Al-4V titanium alloy lower control arm preforms and its microstructure property regulation. |
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