Study On Electrically Assisted V-bending Macro And Micro Forming Law Of Ti55 Alloy Sheet

With the continuous development of aerospace technology,the demand for high-temperature titanium alloys is increasing.Ti55 alloy is a nearly?-type two-phase titanium alloy with high specific strength,low density and high temperature resistance,and is widely used in the aerospace field.Ti55 titanium alloy is a nearly alpha titanium alloy independently researched and developed by my country.It is added with rare earth element Nd.It has good strength,creep resistance and thermal stability,and can be used for a long time at 550°C.Titanium alloy is a kind of metal that is difficult to form at room temperature,and titanium alloy sheet is easy to crack during cold stamping and requires a large forming force;and due to its high strength and low elastic modulus,the high elastic recovery results in the forming of Ti55 sheet.Produces a large springback,which affects the forming accuracy of the material.Electric-assisted forming technology has attracted more and more attention because of its high efficiency and good forming quality.Electrically assisted forming refers to a forming method in which pulse current is introduced in the process of metal plastic forming,and the electroplastic effect of the current is used to reduce the flow stress and increase the forming limit.Therefore,the electric assisted V-bending process is combined with Ti55 alloy to study the influence of this process on the macro-micro forming law of Ti55 alloy,which has important theoretical and engineering significance.In order to obtain the V-bending forming law of electrically assisted Ti55alloy sheet,a simulation model for the whole process of V-bending and springback of electrothermal-mechanical coupling of Ti55 alloy sheet was established based on the finite element analysis software ABAQUS.The results show that the billet temperature rises rapidly at first and then tends to be gentle,and the rate of temperature rise is proportional to the square of the current density.The stress and strain are mainly concentrated at the bend,and the stress and strain gradually increase from the neutral layer to the inner and outer sides.The thickness of billet varies the most at the bend.By means of orthogonal experiment,it is found that the bending Angle and relative bending radius have significant effects on the springback angle through range analysis.When the bending Angle changes from 60°to 120°and the relative bending radius changes from 2 to 8,the proportion of elastic deformation increases,and the springback quantity increases.When the effective current density is from 3 A/mm2 to 5A/mm2,the higher the forming temperature,the smaller the springback Angle.As the forming speed increases from 0.1mm/s to 1 mm/s,the springback Angle decreases first and then increases under the joint action of the strain rate and the forming temperature.The electrically assisted plate V-bend can effectively reduce the springback Angle of billet,which is lower on average than that of mechanical V-bending.The microhardness,OM and EBSD tests of electrically assisted V-bending Ti55 were studied.The microstructure evolution laws,such as microstructure morphology,grain orientation difference and grain boundary evolution,before and after the electrically assisted V-bending were studied.The results show that with the increase of the effective current density,the content of?phase decreases gradually,and the microstructure of the?photo layer of the sample is refined to different degrees,which is related to the thermal effect and non-thermal effect of the current to promote the phase transition.When the effective current density is5A/mm~2,all martensite structures are formed by non-diffusing phase transformation during the rapid cooling process.The hardness of the samples increased with the increase of the effective current density,which was related to grain refinement and phase transformation.The grain orientation increased obviously and tended to be more uniform after the electro-assisted deformation.Under the effective current density of 4A/mm~2,the small-angle grain boundary ratio of the inner layer and the neutral layer decreased significantly compared with the initial sample,while the mean grain boundary orientation difference increased.The small-angle grain boundary ratio of the outer layer increased,and decreased with the increase of the effective current density to4.5A/mm~2.After electrically assisted bending,the maximum polar density of the sample electrode diagram decreases,and the internal microstructure uniformity of the sample is enhanced.After electrically assisted V-bending,the specimen texture strength decreases,which is mainly related to the recrystallization process.