Study On Impact Hydroforming Characteristics And Mechanisms Of Formability Improvement And Springback Reduction For AA2024 Aluminum Alloy Sheets

In order to realize the lightweight of aircraft,2xxx series aluminum alloys are widely used in manufacturing of aerospace sheet metal parts.With the gradual increased design complication of aerospace parts with aluminum alloy sheet,such as spatial surfaces,deep cavities,small fillets and other features,the traditional aerospace manufacturing technology is facing two manufacturing problems at room temperature,namely,insufficient formability and springback.These cause issues such as low product yield,poor dimensional accuracy,and assembly difficulty.As a high strain rate forming technology,impact hydroforming technology has the advantages of simple tools,high operability and good applicability.Previous studies have indicated that impact hydroforming has obvious formability improvement and springback reduction effect on the forming of metal materials.However,at present,the mechanism of formability improvement and springback reduction of metal materials is still not clear under impact hydroforming technology,which limits the further development and application of impact hydroforming technology.Therefore,this dissertation takes AA2024 aluminum alloy for aerospace as the research object,and firstly studies the characteristics and inertia effect of impact hydroforming on complex sheet metal parts.Secondly,in order to explore the formability improvement and springback reduction mechanism of AA2024 aluminum alloy under impact hydroforming,evolution of dislocation configuration under different strain rates and springback behavior under different loading modes were also studied respectively.Based on uniaxial tensile tests under different strain rates,the influence of strain rates on the mechanical properties of AA2024 were studied.The results show that the well accurate parts can be achieved under the process parameters of deep drawing ratio of 1.91,impact speed of 35m/s and binder gap of 0.2mm.In addition,under the constraint of the die,the effect of inertia effect in the process of impact hydroforming is affected by the shape of the part.For the symmetrical section,the deformation sequence is "parabola" type with material tending to flow from both ends to the central area.For the asymmetric section,the deformation sequence is"flat" type,where the material flows from the center to both ends.Such featured deformation sequence promotes the uniformity of wall thickness distribution of parts and reduces the thinning ratio.The evolution of dislocation slips and configuration at high strain rates was studied by means of EBSD and TEM.The results show that high strain rate increases the density and nucleation rate of movable dislocations,and strongly interacts with barriers such as grain boundaries and S phase,resulting in the increase of yield stress.In addition,although the insufficient operation of cross slip at high strain rate reduces the degree of dynamic softening,the large-size grains of AA2024 can be divided into multiple small deformation regions by S phase.Dislocation accumulated in the S phase at high strain rate can bypass the S phase and continue to move,providing more space for dislocation movement.This contributed to secondary hardening and improving the successive hardening capacities of AA2024.The coordinated deformation among the small regions improved the deformation uniformity,which avoided the premature strain localization.Therefore,the elongation of AA2024 was improved.The springback behavior of AA2024 under different loading modes and bending angles was studied.It is found that the springback angle of AA2024 greatly reduces by amount of 80.11%under high-speed stamping loading compared with conventional stamping loading.Specifically,the maximum decremental of springback angle is 113.90%under impact hydroforming,meaning that springback is almost totally eliminated.Under the stamping mode,reason for the springback reduction caused by high-speed loading is that the inertia at high speed makes the sheet showing the cyclic deformation characteristics of "bending,separation and reverse bending",which reduces the stress difference between the inner and outer sides of the sheet.However,the reason for the springback reduction under the loading mode of high-speed impact hydroforming is that the sheet changes from bending deformation to shear deformation under pressure,and the plastic deformation area is expanded.This leads to more obvious springback reduction under impact hydroforming,even to the extent of eliminating springback.