Whether macro-defect appears or not during the cup-cup combined extrusion

In this paper, whether the macro-defect (cavitation of metal) appears in the chamber corner has been analyzed during extrusion by using the minimum energy principle and the upper-bound approach. The shape of workpiece is cup-cup with a different outer radius. It is found that cavitation does not appear during extrusion in spite of the final deforming stage. This conclusion is further demonstrated according to the viewpoint of the transformation of the deformation patterns varying with the relative thickness of the billet bottom. The conclusion is confirmed by experiments.