MODELING VOID NUCLEATION AND GROWTH IN AXISYMMETRICAL EXTRUSION

Elastic-viscoplastic constitutive equations, with kinematic and isotro pic hardening, are employed to model the deformation behaviour of an a luminium alloy in extrusion. Constitutive equations are also employed for void nucleation and growth, which are fully coupled with the defor mation behaviour. The material model is employed to investigate the ro les of void nucleation and growth in extrusion defect formation. It ha s been shown that central bursting is a void growth controlled process . The existence of nucleated voids only leads to central burst formati on with the existence of appropriate stress states which lead to void growth. The results obtained show excellent agreement with well-establ ished limit diagrams, obtained analytically and experimentally. The re sults also show that for a given combination of area reduction and sem i-cone die angle, the introduction of friction tends to inhibit the fo rmation of central bursting, but increases the likelihood of surface t earing/cracking. The tendency to inhibit central burst formation with increasing friction results from the reduction in the levels of tensil e hydrostatic stress, which therefore reduce the rate of void growth. A comparison of the results obtained using kinematic and isotropic har dening in the extrusion process showed that significantly different re sidual stress fields are obtained for the two cases. This is of import ance, for example, in the case of multipass extrusion or where the res idual stress field is to be used subsequently in design analysis.