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Abstract. Piercing or surface-crack defect occurs in combination-extruding the support pin. There has been so far insufficient knowledge in avoiding such forming defects in forming the similar products. In this paper, a combination-extrusion experiment, with various combinations of forward and backward extrusion ratios, was conducted to observe the occurrence of the defects. The result shows that the defects occur at low level of both forward extrusion ratio and backward extrusion ratio used in the combination-extrusion process. The low forming level causes the onset of insufficient plastic metal-flow, which is similar to that of inhomogeneous deformation in the unidirectional extrusion or drawing process. A forming limit diagram was constructed and can be used in the defect prediction for the future development of similar products.

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