Stable and Unstable Flow in Materials Processed by Equal-Channel Angular Pressing with an Emphasis on Magnesium Alloys

Magnesium alloys such as ZK60 exhibit strain softening when processed by equal-channel angular pressing (ECAP). Finite element modeling (FEM) was used to examine the flow process during ECAP with an emphasis on the importance of the strain-rate sensitivity m. The simulations show there is unstable flow and shear localization for values of m of 0 and 0.01, but the flow is stable for values of 0.05 and 0.1. The flow softening reduces the cross-sectional area of the billet and leads to an enhanced accumulation of damage at the upper surface. The simulations show that the presence of a back pressure increases the ability of the billet to fill the exit channel but does not remove the development of plastic instabilities such as shear concentrations. It is shown that an imposed back pressure reduces the level of the maximum principal stresses in the area in which deformation takes place, and this reduces the tendency for cracking of the billet during the pressing operation.