Effect of magnetic field on tension–tension side of forming limit diagram in sheet metal forming using a magnetorheological fluid

Sheet flexible-die forming using a magnetorheological (MR) fluid, which can change the mechanical property of flexible-die during the forming process, has become an innovative technology with great application potential in manufacturing thin-walled complex components. Although it has been proved that using an MR fluid as a flexible-die can improve the sheet formability, there is still a lack of in-depth understanding and systematic research. In this paper, the bulging process using elliptical dies with six kinds of ellipticities was adopted to achieve different strain paths and obtain the tension-tension side of forming limit diagram (FLD). Experiments using three kinds of MR fluids and four magnetic field conditions were conducted to systematically study their effects on the tension–tension side of FLD. The effect of the magnetic field on the fracture pattern of the specimen and its mechanism were also discussed. Results showed that magnetic particle content and magnetic field condition have a comprehensive impact on the forming limit diagram. When an MR fluid with a magnetic particle content of 20% is used, the forming limit under different magnetic field conditions has little difference. When the magnetic particle content increases to 30%, the forming limit curve gradually increases with increasing magnetic flux density. But when it reaches 43%, the forming limit curve increased first and then decreased with the increase of magnetic flux density. Besides, the extended FLD of the Al1060-O sheet was established, in which the conventional forming limit diagram is expanded to consider the influence of the magnetic field condition. These results can provide guidance in the parameter selection of magnetic flux density and magnetic particle content in sheet flexible-die forming using an MR fluid.