Property-adjustable forming medium induced extension of sheet metal formability under variable magnetic field

Deformation states of sheet metals are determined by the in-plane stress distribution, which is closely related to forming load transmitted by forming medium. Mechanical properties of conventional forming medium, such as mineral oil, gas, and polyurethane, can hardly be controlled during the whole forming process. However, if the mechanical properties of forming medium can be freely altered, the stress state of sheet metal may become controllable. As a result, the magneto-rheological (MR) fluid was employed as a property-adjustable forming medium in this paper, which attempted to extend sheet formability in a controllable way. Effects of magnetic flux density (B) and iron particle content (φ) on yield stress of MR fluid in compression mode were analyzed at first. Magneto-rheological pressure bulging (MRPB) tests for Al1060-O sheet were conducted under different sets of parameters. Effects of B and φ on forming load were presented to show the adjustable mechanical properties of forming medium. Extended forming limit diagram (e-FLD) was proposed to describe the variation of sheet forming limit. The results indicate that sheet forming limit can be extended by monotonously increasing B in a certain range. The appropriate range of B is correlated to φ. The through-thickness shear stress caused by the transverse friction between MR fluid and sheet metal is considered to be responsible for this increase. The proposed process is demonstrated to be an efficient method to control the sheet formability, and more practical application may be expectable.