Criterion for prevention of central bursting in forward extrusions through spherical dies using the finite element method

Spherical dies are increasing in popularity in the cold forming industry because of the ease in subsequent secondary operations. This paper presents criteria curves, calculated using the finite element method, to avoid central bursting or internal chevrons in forward extrusions through spherical dies. Critical values of mean stress at the centerline of the extrusion are used as failure criteria to distinguish between acceptable and unacceptable die designs. These failure criteria are conservative in that the critical step for central bursting is considered to be the formation of a microvoid during extrusion, rather than linking of the voids during continued deformation. The resulting process criteria curves are conservative estimates of internal chevron formation during extrusion through spherical dies.