Safety maps in bimetallic extrusions

In this paper, the upper bound method has been applied, in combination with the minimum energy principle, to obtain a fracture condition in the hot extrusion of bimetallic tubes. The simultaneous deformation of material components with quite different plastic properties or under unfavourable geometric conditions may lead to decohesion or fracture. A criterion based on the geometric compatibility of the deformation of each material has been used here. The dependence of the fracture condition on the different extrusion parameters involved in the process is analysed, and several maps containing safety and fracture zones for each parameter are obtained. It is concluded that the yield stress ratio, the die angle and the extrusion ratio appear as the most effective factors in the failure control. The fracture condition is promoted by an increase in the yield stress ratio, in the extrusion die angle, or by decreasing the extrusion ratio. A die angle of 54 degrees or higher, as well as a reduction of 29% or lower, implies that the extrusion operation is only possible at yield stress ratios lower than 2. This limit has many practical implications in the selection of suitable material combinations.