| 摘 要： | In this paper,the kinematically admissible velocity field with surface crack on forwardextruding bar is put forward during the axisymmetric cup-bar combined extrusion process,in accordance with the results of model experiments.On the basis of velocity field,the necessary condition for surface crack formation onthe forward extruding bar is derived,with the help of upper bound theorem and theminimum energy principle.Meanwhile,the relationships between surface crack formationand combination of reduction in area for the part of forward and backward extursions(εb,εf),relative residual thickness of billet(T/R_0),frictional factor(m)or relativeland length of ram and chamber(l_b/R_0,l_f/R_0)are calculated during the extrusion process.Therefore,whether the surface crack on forward exturding bar occurs can be predictedbefore extruding the lower-plasticity metals for axisymmetric cup-bar combined extrusionprocess.The analytical results agree very well with experimental results of aluminium alloyLY12 (ASTM2024)an
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