Generation of a Fine Grain Layer in the Vicinity of Frictional Interfaces in Direct Extrusion of AZ31 Alloy

The present paper deals with the generation of hard layers in the vicinity of frictional interfaces in metal forming processes. The primary objective of the paper is to introduce a general approach to relate the strain rate intensity factor and parameters that characterize the microstructure and thickness of such layers. This approach is used in conjunction with axisymmetric direct extrusion of an AZ31 alloy. The thickness of the hard layer is determined experimentally. Also determined is the distribution of average grain size and hardness near the friction surface. The strain rate intensity factor is found using an available semi-analytical solution.

Effect of Plastic Anisotropy on the Strain Rate Intensity Factor: A Simple Analytic Solution

Solutions for many rigid/plastic models are singular in the vicinity of maximum friction surfaces. In particular, the magnitude of the equivalent strain rate near such surfaces is controlled by the strain rate intensity factor. This factor is the coefficient of the leading singular term is a series expansion of the equivalent strain rate in the vicinity of maximum friction surfaces. Since the equivalent strain rate has a great effect of material properties, it is of important to reveal the dependence of the strain rate intensity factor on parameters characterizing material models. In the present paper, quite a general model of anisotropic plasticity under plane strain conditions is adopted. Then, using an analytic solution for instantaneous compression of a layer of plastic material between two parallel plates the effect of the shape of the yield locus on the asymptotic behavior of the equivalent strain rate in the vicinity of the friction surface is demonstrated.

Modelling of Damage Evolution in the Vicinity of Frictional Interfaces in Metal Forming

Conventional ductile fracture criteria are not applicable in the vicinity of maximum friction surfaces for several rigid plastic material models because the equivalent strain rate (second invariant of the strain rate tensor) approaches infinity near such surfaces. In the present paper, a non-local ductile fracture criterion generalizing the modified Cockroft-Latham ductile fracture criterion is proposed to overcome this difficulty with the use of conventional local ductile fracture criteria. The final form of the new ductile fracture criterion involves the strain rate intensity factor which is the coefficient of the principal singular term in a series expansion of the equivalent strain rate in the vicinity of maximum friction surfaces. When the velocity field is not singular, the new ductile fracture criterion reduces to the modified Cockroft-Latham criterion. The strain rate intensity factor cannot be found by means of commercial finite element packages since the corresponding velocity field is singular. In the present paper, the new fracture criterion is illustrated with the use of an approximate semi-analytical solution for plane strain drawing. It is shown that the prediction is in qualitative agreement with physical expectations.