The rapid response of forming medium's properties to variable loading types of magnetic field and consequent field-dependent sheet formability

Previous studies show that forming medium with adjustable mechanical properties may be used to achieve the controllable sheet formability. Magneto-rheological (MR) fluid was once adopted as property-adjustable forming medium to achieve the variable sheet formability. Yet, as a key process parameter in this process, the loading types of magnetic field may affect the sheet formability significantly, which is emphatically evaluated in this paper. Finite element analysis of bulge tests using three kinds of MR fluids with different mechanical properties were carried out. The field-dependence of sheet formability in this process is clarified by the finite element results. On the basis of that, bulge tests of Al1060-O sheet under different loading types of magnetic field were implemented. The sudden drop and swift recovery of forming load were observed, which is used to indicate the rapid response of forming load to external magnetic field. Deviation of forming load curves under different loading types of magnetic field was verified to be influenced by iron particle content of MR fluid. Geometry of bulged specimens and thickness strain under different loading types of magnetic field were measured. Forming limit surface was presented to show the variation of sheet forming limit. The loading types of magnetic field are considered to be responsible for the extent of formability increase in MRPF process. Optimized types of magnetic field are essential for the practical application of this process.