Research On Deformation Behavior Of Variable Viscosity Viscous Pressure Bulging Of Sheet Metal

The process of flexible-die forming is an important branch in sheet metalforming, in which hydroforming using liquid medium，viscous pressure formingusing semi-solid medium and polyurethane rubber forming using solid-state mediumhave been widely used in aviation, aerospace, automotive and other fields for theirunique advantages. It will join the advantages together if we can invite a newforming method, in which the state of medium can be changed during the formingprocess. It is also the starting point of this study.Firstly, to improve the formability of sheet metal, the idea of variable viscosityduring viscous pressure bulging forming is proposed, we can choose differentviscosity medium for different sheet metal, or change the viscosity of the mediumduring different stages of the forming process. Strength coefficient and strain ratesensitivity exponent are used to represent the viscosity of viscous medium. Bycomparing bulging using rigid dies, hydraulic bulging and viscous pressure bulgingthrough finite element analysis, the variation of necking points during viscouspressure bulging process is obtained. Next, we study the change law of position ofnecking points by changing the parameters of viscous medium during differentstages of forming process. We come to the conclusion that changing viscosity ofmedium properly can impact the position of necking points significantly.Secondly, the variable viscosity bulging device based on magnetorheologicalfluids for sheet metal is designed, including the magnetic field generating deviceand bulging device. By combining finite element analysis and experimentalmeasurements, the distribution of magnetic field strength of the working area isdetermined, and the influence of magnetorheological fluid to the magnetic fieldstrength of the working area is also studied; peak phenomenon ofmagnetorheological fluid under the magnetic field is observed, which illustrate theviscosity of magnetorheological fluid can change in the experimental condition andverify the feasibility of the test principle, by which the force field and the magneticfield during the test can be applied at the same time.Finally, the bulging test is selected to verify the feasibility of method ofvariable viscosity and the possibility of using magnetorheological fluid as a formingmedium. During magnetorheological fluid bulging test, contrast tests are carried out to obtain the strain distribution, one group is done under different stroke with thesame magnet strength, the other is done under different magnet strength with thesame stroke. The result shows that using magnetorheological fluid, which viscositycan change, as forming medium in bulging test influence the critical bulging heightand strain distribution of specimens markedly.