| TC4 necking tube has the advantages of high strength and light weight,and is widely used in aerospace field.However,TC4 alloy has poor formability at room temperature,so it is difficult to manufacture parts with large deformation and high forming precision.At present,the large-size necking tube are mainly manufactured by“forging+welding”,which cannot meet the development needs in terms of reliability,weight reduction and advancement.Based on this,combining the technical advantages of necking spinning and superplastic bulging,this paper puts forward a necking spinning/superplastic bulging forming technology which firstly carries out necking spinning on small-sized cylindrical parts,and then carries out superplastic bulging on the spun parts.ABAQUS software is used to simulate the necking spinning/superplastic bulging forming process of TC4 alloy,and the simulation results are verified by experiments.The deformation behavior of equiaxed fine grain TC4 alloy under deformation temperature of 800?950?and strain rate of 0.01?10 s-1 was studied by hot compression experiment,and the Arrhenius constitutive equation and hot processing map were established.Based on the true stress-strain curves obtained by experiment,the strain compensation is modified to the constitutive equation.The true stress decreases with the increase of temperature and the decrease of strain rate.The correlation coefficient between the predicted true stress and the experimental value R is 0.985,and the relative error AARE is 6.8%.Combined with the hot processing map and the electron backscatter diffraction(EBSD)analysis of the corresponding region,it can be seen that the temperature in the instability zone is between 875?950?,the strain rate is between 0.3?10 s-1,and the microstructure features are long strip grains.The optimum temperature in the processing area is between 800?875?,the strain rate is between 0.01?0.3s-1,and the microstructure is equiaxed fine grain.The numerical model of equiaxed fine grain TC4 alloy necking spinning is established by ABAQUS numerical simulation software in the optimum temperature range.The numerical simulation results show that the optimum strain rate of equiaxed fine grain TC4alloy necking spinning process window is as follows:spindle speed 200 rpm/min,feed speed100 mm/min and feed ratio 0.5 mm/rpm.By comparing the necking spinning of different tube blank diameters,the optimum diameter of necking spinning is determined to be 120 mm;The stress,strain and thickness distribution after multi-pass necking spinning are analyzed under the optimum processing parameters:with the increasing number of spinning passes,the stress value increases continuously,and the maximum value of stress value appears at the port;The strain value also increases continuously,and the maximum value appears in the straight line from the port to the tapered par.With the increase of passes,the thickness of the port part will increase continuously,the maximum thickness is 17 mm,the fixed end will be thinned by0.85 mm,and the rebound phenomenon will be alleviated after thickening;With the increase of spinning passes,the metal flows towards the free end of the port,resulting in the overall elongation of the tube,with the final length of 136.08 mm and the increment of 16.08 mm.The experimental results are basically consistent with the numerical simulation results,with a relative error of ARRE of 5.15%.The numerical model can reliably predict the deformation.The observation of microstructure shows that the microstructure after necking spinning has no obvious orientation,and the grains are equiaxed fine grain,with an average grain size of 12.6?m.The spun microstructure has superplasticity,thus can be superplastically formed.Through superplastic tensile test,the optimum superplastic bulging temperature of TC4alloy after necking spinning is 850?,and the strain rate is 0.0005 s-1.Under this parameter,the elongation of TC4 alloy is 788%,and the strain rate sensitivity coefficient m is 0.60.The superplastic rheological constitutive model of TC4 alloy Backofen established by true stress-strain curve was applied to the superplastic bulging numerical model established by ABAQUS numerical simulation software.The results of stress,strain and thickness distribution after superplastic bulging were analyzed.The results show that stress,strain and thickness are symmetrically distributed during superplastic bulging.Due to the difference of die sticking sequence,the top area of the tube piece is stuck first,the middle necking section and the transition area at the top are stuck last,and the maximum value of stress,strain and minimum value of thickness all appear in the transition area.The maximum pressure of pressure-keeping bulging is 1.90 MPa.The experimental results are basically consistent with the numerical simulation results,and the relative error ARRE is 6.98%.The numerical model can well predict the deformation.The microstructure after superplastic bulging is observed.The results show that the grain size of TC4 alloy tube necking part grows obviously after superplastic bulging,which is 17.9?m at the top and 14.7?m at the middle.Due to the different deformation directions in different parts,the orientation of grains changes:The grains at the port and top have orientation in the[0001]direction,while the grains in the middle have obvious orientation in the [2(?)(?)0]direction.A large number of dislocations proliferate in the?grain at the port with small deformation.Under strain induction,needle martensite is produced in the?grain at the middle necking section with the laege deformation,and lath martensite is produced at the top with the largest deformation.Mechanical tests show that the tensile strength of the straight tube section at the top of the weak zone is improved,while the yield strength and elongation are slightly decreased compared with the original tube blank. |
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