Research On Springback Behavior Of Viscous Pressure Forming Based On Experimental-Numerical Hybrid Method

Springback is a stress-driven phenomenon during unloading,so the stress behavior during unloading is of significance to springback analysis.However,the accuracy of stress field after loading procedure in FEM,as the basis of springback simulation,is greatly affected by the assumptions of material model and boundary condition.Moreover,the selection of constraints in springback simulation is crucial and highly subjective,thus affecting the authenticity of results.If the experimental data is used as the boundary condition of the model,the defects caused by the model hypothesis can be avoided.It objectively reflects the influence of real material parameters and geometric parameters on springback,so the calculation results of springback stress conforms to the objective and practical conditions.At the same time,the viscous pressure forming(VPF)can reduce wall thickness thinning,inhibit wrinkling and improve sheet formability.However,the springback during unloading process in VPF has not been systematically studied.In view of this,this paper studies the springback behavior of viscous pressure forming based on the experimental numerical hybrid method.The experimental numerical hybrid method of elastic stress is established.The springback stress of aluminum alloy 6K21 sheet during the springback process of viscous pressure bulging(VPB)is calculated.The change law of springback stress under different springback conditions is studied.The corresponding relationship between the magnitude and direction of springback and springback stress vector is revealed,and the change mechanism of springback stress is clarified.The viscous medium pressure forming method of hollow blade is proposed,and the high dimensional precision forming of hollow blade is realized.The research of this paper can provide a new perspective for analyzing and understanding springback and provide a basis for springback control.An experimental numerical analysis hybrid method for calculating elastic stress based on experimental data is established.The digital image correlation method and the finite element method are combined to form the experimental and numerical hybrid method.The configuration of the specimen before and after deformation is obtained by digital speckle optical measurement.The smoothing functional based on least square fitting is constructed to smooth the experimental data.And the finite element method is used to solve the functional extremum.The calculation formula of elastic stress of space membrane element is deduced,and the corresponding calculation program is compiled.Using the experimental data as the boundary condition of numerical calculation,the deformation information in line with the objective experimental conditions can be obtained.The springback stress of sheet metal in the springback process of viscous medium bulging is calculated by experimental numerical hybrid method.Different viscous mediums and loading speeds are used in the viscous pressure bugling(VPB)experiments.The bulging process and unloading springback process are measured in real time by DIC optical measurement technology.The effects of viscous medium with different properties parameters and different loading speeds on springback are studied.The configuration and stress of bulging surface are analyzed.The results show that 1)molecular weight of viscous medium and loading speed have influence on springback.The viscous medium simultaneously possessing good srtain rate sensitivity and viscosity can well control the springback.2)And higher loading speed helps to reduce the overall springback.3)The distribution of springback stress and the spatial orientation of springback stress vector are calculated by using the experimental measurement data.Through comparison and verification,it is proved that the calculation result of springback stress is reliable.Based on the analysis of finite element and experimental results,the springback law and mechanism of springback control by viscous medium are revealed.The finite element analysis of springback in viscous pressure bulging is carried out by unloading the sheet and viscous medium together.The effect of viscous medium properties on springback and characteristics and variation law of non-uniform pressure distribution of viscous medium are studied.The influence of viscous medium on the change of stress path of sheet during loading and unloading is discussed,and the characteristics of interaction between viscous medium and sheet during unloading are analyzed.The experimental law of springback of VPB is analyzed,and the stress and bending moment of bulging specimen are calculated and discussed.The results show that 1)the springback decreases with the increase of material constant(c)and strain rate sensitivity index(m).2)During the loading and unloading,the viscous medium pressure is non-uniform,which changes the bulging shape of the plate,affects the stress path of each point on the plate and the stress state of the plate in the process of loading and unloading.3)The coupling effect of non-uniform normal force and tangential adhesion can promote the uniformity of stress distribution of the whole sheet,and reduce the bending moment of sheet near the die corner.As a result,the springback of bulging sheet is controlled efficaciously.The variation law of springback stress of sheet metal in the springback process of VPB is studied,the relationship between springback and springback principal stress and principal direction is established.The mechanism of springback stress variation and the springback reduction are analyzed.Results show that 1)the springback stress conforming to objective and practical conditions can be obtained,and it changes regularly with springback.2)The viscous medium affects the shape and stress of sheet,causing the change of springback stress.3)Furthermore,the change of springback stress reduces the reverse bending moment to restrain the resulting springback.The viscous pressure forming method is applied to the manufacture of thin-walled twisted hollow blade.The non-uniform pressure of the viscous medium acting on the sheet and stress state of sheet during corner filling process are theoretically analyzed.The effect of the properties parameters of the viscous medium on the wall thickness and springback is studied.The experimental device for viscous pressure forming of hollow blade is designed and the experimental research is carried out.The results show that 1)the non-uniform pressure of viscous medium can promote the coordinated deformation of tube blank by controlling the parameters of viscous medium and the loading speed of die.2)The formed hollow blade has low wall thickness reduction and small springback.The forming of hollow blade with high dimensional accuracy without springback compensation is realized.