Mechanics of continuous equal-channel angular extrusion

Abstract

The paper discusses possible approaches for realization of continuous equal-channel angular extrusion (ECAE). A practical interest presents Conform ECAE (CECAE) converting an inlet channel into a friction press. Slip line solutions and plastic zones are analyzed for different cases of active and passive frictions. It is shown that CECAE provides lower extrusion pressure, higher effective strains and better approximation to simple shear deformation mode than ordinary ECAE. Specific distributions of plastic friction depending on a slip between material and roll are introduced for the beginning stage, steady CECAE and during slippage. The paper describes a few designs of CECAE. Optimization of the main characteristics is performed for the concept of CECAE with an additional coining roll.

Introduction

Equal-channel angular extrusion of batch billets is now a recognized technique in materials processing for properties. Continuous equal-channel angular extrusion is an attractive alternative to ordinary ECAE owing to high productivity, low material waste and fabrication of long semi-finished products. Instead of the punch pressure in ordinary ECAE, an extrusion load in continuous ECAE may be developed by pulling force at the outlet billet end, pushing force at the inlet billet end and partly pushing and pulling forces at both billet ends. The first concept was realized by Chakkingal et al. (1998) in equal-channel angular drawing (ECAD). Alkorta et al. (2002) show that the stress–strain state during ECAD is similar to tensile bending around a sharp channel corner that leads to material thinning, near pure shear deformation mode and local fracture. It is notable that typical characteristics of severe plastic deformation such as shear band localization, new spatially distributed high angle boundaries and structure refinement were not detected after multi-pass ECAD. Therefore, application of the pulling force is not practical for continuous ECAE.

More effective are the pushing forces developed by contact friction along the inlet billet end such as in Extrolling (Fig. 1a) (Avitzur, 1975), extrusion Conform (Fig. 1b) (Green, 1973) and friction extrusion Linex (Fig. 1c) (Voorhes, 1975). The Extrolling process is effective when a contact area between billet and rolls is sufficiently large and an extrusion die can be inserted into a gap between rolls. As ECAE does not change the billet cross-section and due to geometrical restrictions on dimensions a, A and D (Fig. 1a), the contact area of active friction between billet and rolls is relatively small in comparison with contact area of passive friction between billet and channels. On these reasons, a single pair of rolls used by Chung et al. (2003) or multiple rolls and conveyer mechanisms applied by Stecher and Thomson (2003) and Chaundhury et al. (2005) cannot develop the extrusion pressure that is usually necessary for ECAE. On the other hand, the Linex extrusion process (Fig. 1c) is complicated and unreliable.

In contrast to Extrolling and Linex, the Conform extrusion (Fig. 1b) has a significant industrial potential and found numerous industrial applications. The first Conform press for ECAE was developed yet in 1976 (Segal et al., 1994). The same concept with some modifications was applied in a few recent patents and publications. However, specifics of the Conform ECAE (CECAE) have not been investigated, and known approaches are empiric and intuitive. Thus, the critical role plays a contact area length between billet and driven rolls providing stable processing for particular friction conditions. If the contact length is small, steady processing is interrupted by material slippage. For the large contact area, excessive friction increases power, pressure and tool wear. Huang (2005) used a large roll diameter-to-billet thickness ratio to have a superfluous contact length and stable CECAE. Zhu et al. (2006) increased the contact length by winging the billet around a larger part of the roll circumference. Lee et al. (2002) induced the roll with surface irregularities enhancing contact friction. In all these cases, problems of stability, minimization of power, material overheating and tool wear remain unresolved. Utsunomiya et al. (2001) suggested a conshearing process as a combination of Extrolling and Conform ECAE with low contact friction and a large angle between channels. Under these circumstances, the extrusion pressure is significantly lower than the material yield stress, the material within the inlet channel experiences elastic bifurcation, and the main role plays Extrolling providing stable ECAE of thin sheets with relatively small shear strains. However, this technique cannot be applied for intensive processing of billets having rectangular, square or round cross-sections. Utsunomiya et al. (2001) also considered mechanics of the conshearing process. For non-zero friction, their analysis is not correct because the extrusion pressure was not evaluated independently, and equilibrium equations themselves lead to a statically infinite problem.¹ Another shortcoming is the use of Coulomb friction that is typical for conditions of elastic but not for plastic contact. In result, this analysis does not include a contact length between material and roll which is the important parameter of CECAE.

Like in other techniques of severe plastic deformation, diversity in realization of CECAE results in different processing characteristics, material structures and properties. Therefore, an analysis of the mechanics of CECAE is equally important from scientific and practical points; however, it was not performed in the previous works. This paper for the first time considers plastic zone and contact friction defining specifics of CECAE. A simple mathematical model and process optimization are also suggested.