Equal channel angular extrusion of difficult-to-work alloys

Abstract

The deformation and failure characteristics of several difficult-to-work alloys during equal channel angular extrusion (ECAE) were established. The behavior of commercial-purity titanium (CP Ti) and AISI 4340 steel (quenched and tempered to R c35) was determined under isothermal conditions at temperatures between 25 and 325°C and ram speeds producing average effective strain rates between 0.002 and 2 s −1. CP Ti was particularly susceptible to shear localization during ECAE; uniform flow occurred only at high temperatures and low strain rates. On the other hand, the 4340 steel deformed uniformly under all conditions except those comprising high temperature and high strain rate. Observations of shear banding and shear failure were interpreted in terms of the tendency for strain concentration as quantified by the flow localization (‘alpha’) parameter, or the ratio of the normalized flow softening rate to the strain-rate sensitivity. Similar observations of shear failure during non-isothermal ECAE of Ti-6Al-4V at hot-working temperatures were also explained in terms of the effect of temperature, strain rate, and deformation mode on the alpha parameter. Physical modeling (visioplasticity) and finite element method (FEM) modeling were applied to elucidate failure modes and the effects of chilling on non-uniform flow during non-isothermal ECAE. The understanding of the effect of material properties on the tendency for flow localization was used to develop processing alternatives to promote uniform flow during ECAE.