Study On Hot Bending Process Of 2A97 Aluminum Lithium Alloy Sheet

With the increasing demand for lightweight in aerospace field,aluminum lithium alloy has been greatly developed.In addition to greatly reducing the mass of aircraft and space vehicles,the service life of structural components can also be prolonged if aluminum-lithium alloy is used instead of conventional aluminum alloy.Moreover,Al-Li alloy inherits the technology of aluminum alloy and can be used to make parts with complex shapes.Therefore,Al-Li alloy is considered to be one of the most potential structural materials in aerospace field.2A97 alloy is the third generation of new Al Li alloy,which was developed in China and can be strengthened by heat treatment.The original sheet of the alloy is supplied in H14 state.It is found that 2A97 alloy can get sufficient solid solution at 520?for 1.5h,the ratio of grain length to width is reduced,and the microstructure is more uniform.The hardness peak value of 170 HV/0.5 can be reached by artificial aging at 165?for 45hours,which is higher than the equilibrium value of 115.8 HV/0.5 at natural aging.When the strain rate is constant and the temperature rises between 25-540?,the strength gradually decreases,while the plasticity first increases and then decreases,reaching the peak value at 500?.When the strain rate rises at constant temperature,the strength is improved,and its plasticity decreases gradually in contrast to its strength.At 540?,the strength and plasticity decreased sharply.In addition,the Arrhenius and Grosman constitutive models in the range of 460-520?,0.01-0.5 s^-1were also established.The V-shaped hot bending process of 2A97 alloy can be divided into three parts:forward bending,reverse alignment and pressure retaining.The temperature drops very little in the process of hot forming and rapidly in the process of pressure retaining.Increasing the temperature can improve the formability of 2A97 alloy,and there will be positive and negative springback.Under high temperature,different bending angles of die are likely to change from positive to negative springback.When the bending angle rises,the transition temperature decreases gradually,which is because the proportion of denatured area decreases gradually with the increase of angle.When the relative bending radius is greater than 6.4,almost all the springback is positive at different temperatures.When the die clearance is close to the blank thickness,it is easier to form and the springback is relatively small.2A97 Al-Li alloy sheet stringer can be formed by pre bending and then final forming.When the initial temperature of blank is 505?,the temperature of pre bending punch and die can be increased to 200?,and the temperature of preformed blank can be maintained above 425?.The thinning of the final formed part is serious at the corner side wall of the inverted few,especially at the arc bending of the axis.Friction should be reduced and holding pressure should be reduced during the forming of 2A97 aluminum lithium alloy stringer.The final forming simulation under different conditions at 400? shows that the thinning rate is less than 11%,which makes the shape and part size of 2A97 sheet stringer satisfied.The gradual curvature arc can be formed by increasing the allowance and keeping the die surface level.Increasing the forming temperature to 500?can not only reduce the forming force,but also reduce the springback.Reducing the friction in the process of arc forming with gradual curvature will reduce the relative resistance between the sheet and the die,so that the smaller the internal stress and the smaller the springback after unloading and cooling.When the gradual curvature arc is formed by one time of sheet metal gap,the forming force increases moderately and the springback is relatively small.