Analysis of defects during backward extrusion by the rigid-plastic finite-element method

Abstract

In this paper, the development of a corner cavity and of cracking of the outer surface during backward extrusion are studied by means of the rigid-plastic finite-element method. It is found that: (i) the critical relative thickness of the bottom of the billet for the formation of a corner cavity decreases with increase in the reduction in area; (ii) the critical relative thickness of the bottom of the billet for the formation of a corner cavity increases, but the growth rate of the corner cavity decreases, with increase in the friction factor; (iii) an outer surface crack does not occur as soon as the stroke begins, but occurs only after the ram has moved a particular distance; (iv) a surface crack does not occur when extruding T2. The results that have been computed agree very well with experimental data. Moreover, the Wu-Liang ductile fracture criterion, ∗ ∗ J. Mater. Proc. Technol. 21 (3) (1990) 295. , derived from the concept of damage mechanics, is seen to be applicable for assessing crack formation during the process.