Principles of Nonequal Channel Angular Pressing

Arman Hasani

László S. Tóth

Benoît Beausir

Laboratoire de Physique et Mécanique des Matériaux,

Université Paul Verlaine de Metz,

Ile du Saulcy,

57045 Metz, France

1 Introduction

Principles of Nonequal Channel Angular Pressing

A variant of the equal channel angular pressing (ECAP) process is examined in this paper where the channels are of rectangular shape with different thicknesses while the widths of the channels are the same. The process is named nonequal channel angular pressing and it is similar to the earlier introduced dissimilar channel angular pressing (DCAP) process. In DCAP, however, the diameters are near values, with the exit channel being slightly larger, while in NECAP, the exit channel is much smaller attributing sev eral advantages to nonequal channel angular pressing (NECAP) with respect to ECAP. In this work an analysis is performed to determine the strain mode in a 90 deg NECAP die. A new flow line function is also presented to better describe the deformation field. The proposed flow line function is validated using finite element simulations. A comparison is made between ECAP and NECAP. Finally, texture predictions are presented for NECAP of fcc polycrystals. The advantages of this severe plastic deformation process are the following: (i) significantly larger strains can be obtained in one pass with respect to the classical ECAP process, (ii) grains become more elongated that enhances their fragmen tation, and (iii) large hydrostatic stresses develop that improve the stability of the defor mation process for difficult-to-work materials. The results obtained concerning the de formation field are also applicable in the machining process for the plastic strains that imparted into the chips.  DOI: 10.1115/1.4001261 

Keywords: ECAP, DCAP, simple shear, flow line modeling, texture   = pc + cp 2 

Equal channel angular pressing  ECAP  is a severe plastic de formation process that was invented by Segal  1–3 . It has re ceived real attention only since the beginning of the 1990s when it was shown that this severe plastic deformation  SPD  process can produce bulk materials with very fine grain sizes. The ECAP pro cess involves imposing very large plastic strains on a work piece without changing the cross-sectional dimensions, which ulti mately refines the grain size down to  200 nm with improved mechanical and physical properties, see the review of Valiev and Langdon  4 . Its main principle is that a bulk sample is extruded through two channels with equal cross sections that are connected to each other by an angle between 60 deg and 135 deg. The work piece is extruded with the help of a piston. As there is no change in the cross section, the sample can be re-introduced into the inlet channel and can be extruded several times. Difficult-to-work ma terials can be also extruded with the help of a back pressure that is exerted by a second piston in the outgoing channel  5 .

In the present work, such angular extrusion is considered in which there is no rounding of the corners at the intersection of the channels. A modification of the ECAP process was first examined by Lee  6  in which the thicknesses of the two channels were considered to be different  NECAP . Lee  6  carried out an upper bound analysis to estimate the pressing stress and obtained also a formula for the shear strain in one pass:

  = cot   + cot    1 

where   and   are the angles of the intersection plane with the inlet and outlet channels, respectively. Equation  1  can be rewrit ten for a 90 deg die as

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received January 27, 2009; final manuscript received February 1, 2010; published online June 15, 2010. Assoc. Editor: Georges Cailletaud.

where p and c are the thicknesses of the inlet and exit channels, respectively. DCAP was then applied in the continuous confined strip shearing  C2S2  process of metal sheets that aims to develop good textures in aluminum to improve formability  7–11 . These DCAP applications were limited to a 120 deg die and relatively small differences in the diameters of the channels; the ratio K of the inlet and exit channel diameters was only K=t0 /tf =0.935. Note that the exiting sheet had larger thickness. Actually, it is also the case for the chip formation in machining where the chip di ameter is always larger than the depth of cutting. Lee et al.  8  also proposed a formula for the total strain in DCAP, which is the following for one pass:

  = 2K2 cot 2  3 

where   is the die angle. Using again p and c for the channel diameters, Eq.  3  takes the following form for a 90 deg die:   = 2  pc 2 4 

Obviously, Eq.  4  is different from Eq.  2 . The difference is large; for example, for the typical ratio of p/c=0.935 used in DCAP, Eq.  3  gives  =1.748, while from Eq.  2 , we get   =2.005. The latter value is almost the same as the value of  =2 known for a 90 deg ECAP die  12  while the first is significantly smaller. It is important to use the right formula for the estimation of the strain in DCAP or NECAP, which will be one of the objec tives of the present work. The differences come from the fact that the deformation mode in ECAP is still not commonly understood or agreed in the scientific community. For example, Eq.  4  is obtained in Ref.  8  by assuming that simple shear takes place parallel to the exit channel. Another assumption was made in Ref.  13  where it was taken to be perpendicular to the intersection plane of the channels. However, other analyses have shown that the shear is actually parallel to the intersection plane of the chan