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To provide intelligent control options during sheet metal forming process, magnetorheological (MR) fluid has been applied as a property-adjustable flexible-die. However, the mechanism of the magnetic field effect on the sheet deformation behavior is not clear yet. In this paper, to clarify the deformation mechanism, the bulging behavior of thin-walled Al1060-O sheet was systematically investigated through experiment and numerical simulation. Special attention was paid to the effect of magnetic field on loading curve, bulge dome height, and thickness strain distribution. Results showed that the maximum bulging force increased obviously with the increase of magnetic flux density. The bulge dome height and thickness strain were influenced by magnetic field condition and magnetic particle content synthetically. Appropriate magnetic field condition and magnetic particle content can improve formability. However, excessive magnetic flux intensity and magnetic particle volume fraction, for example, B = 0.20 T and φ = 46%, would lead to premature rupture. Finite element analysis was adopted to assist in explaining the bulging behavior. Detailed analysis indicated that the flow stress variations caused by magnetic field affect the stress states, velocity distribution of MR fluid, and sheet/MR fluid contact condition, further leading to different stress and strain distribution of sheet metal. Finally it causes the great difference in specimen geometry and wall thickness distribution. [ABSTRACT FROM AUTHOR]

In conventional bulging process, it is usually assumed that sheet metal deforms under normal pressure regardless of the frictional force between the sheet and the pressure-carrying medium. However, there is a phenomenon that sheet metal is bulged under the combined action of normal pressure and beneficial frictional force, which is resulted from the special characteristics of the pressure-carrying medium. Effects of constant tangential stress (beneficial frictional force) caused by rubber or urethane have been studied analytically and experimentally in previous works. Yet, the tangential adhesive stress imposed on sheet metal by viscous medium is correlated with its flow velocity due to its rate-dependent property, which can be controlled and measured by loading equipment. This kind of rate-dependent tangential adhesive stress has been taken into account to establish an analytical model to analyze its effects on the viscous pressure bulging (VPB) process. Then a numerical approach is presented to solve the problem, which utilizes incremental strain to start an iteration loop by considering different magnitudes of adhesive stress. Results have shown that rate-dependent beneficial frictional force in VPB process can promote longitudinal deformation of the bulged part to help make the strain distribution more uniform. The location of maximum thickness strain point deviates from the pole the outer radius, which means that when polar strains remain at the same level, higher bulging height can be obtained for larger magnitude of rate-dependent tangential stress; this characteristic mainly accounts for the postponement of fracture instability of sheet metal in VPB. The theoretical predictions of strain components and geometric parameters are analyzed and compared with experimental results. The comparisons have shown good agreement to validate the analytical model. [ABSTRACT FROM AUTHOR]