Useful properties of twist extrusion

doi:10.1016/j.msea.2007.12.055

Materials Science and Engineering: A

Volume 503, Issues 1–2, 15 March 2009, Pages 14-17

https://doi.org/10.1016/j.msea.2007.12.055 Get rights and content

Abstract

We present an experimental study of the kinematics of twist extrusion (TE) and show that TE has the following properties: (i) as in equal-channel angular pressing (ECAP), the mode of deformation in twist extrusion is simple shear. Unlike in ECAP, there are two shear planes; one of them is perpendicular and the other is parallel to the specimen axis. (ii) The following processes are present during twist extrusion: vortex-like flow with large strain gradient, stretching and mixing of metal particles. We argue that, due to these properties, TE opens possibilities for investigating and forming new microstructures. It has already been successfully used to obtain ultrafine-grained microstructures with good properties in Al, Cu and Ti alloys.

Introduction

Severe plastic deformation (SPD) is a family of metal forming techniques that use extensive hydrostatic pressure to impose a very high strain on bulk solids, producing exceptional grain refinement without introducing any significant change in the overall dimensions of the sample [1]. Several different SPD processing techniques are now available including high-pressure torsion (HPT) [2], equal-channel angular pressing (ECAP) [3], multi-directional forging (MDF) [4], accumulative roll-bonding (ARB) [5], repetitive corrugation and strengthening (RCS) [6] and twist extrusion (TE) [7], [8].

Each process has unique properties determining its use in research and practice. This paper presents several properties of TE, which open possibilities for investigating and forming new microstructures.

Section snippets

Basics of twist extrusion

TE is based on pressing out a prism specimen through a die with a profile consisting of two prismatic regions separated by a twist part (Fig. 1). As the specimen is processed, it undergoes severe deformation while maintaining its original cross-section. This property allows the specimen to be extruded repeatedly in order to accumulate the value of deformation, which changes in the microstructure and properties of the specimen.

TE is performed under high hydrostatic pressure in the center of

Experimental investigation of TE kinematics

To experimentally study the kinematics of metal flow during TE, we use a specimen with nine fibers embedded along its main axis.¹ The specimen was pressed through a built-up twist die until stationary flow was reached. It was then removed from the die and cut perpendicularly to its main axis with an interval of 0.5 mm, starting from its end. The nine markers in the obtained cross-sections (shown

Two shear planes and four deformation zones in TE

There are four sufficiently well separated deformation zones observed when processing different materials with twist extrusion.

Deformation zones 1 and 2 are located at the two ends of the twist part of the die (see Fig. 1). The mode of deformation in these zones is simple shear in the transversal plane (TP), as in HPT. The shears in the two zones have opposite directions, since the prism specimen is twisted in zone 1 and straightened in zone 2 being restored to its original shape. Each zone

Two main routes of TE

There are two types of twist dies: clockwise (CD) and counter-clockwise (CCD). When transitioning from CD to CCD, the shears in each of the four deformation zones reverse its sign. This gives us two main routes of TE:

Route I: CD + CD (or CCD + CCD),
Route II: CD + CCD (or CCD + CD).

Fig. 3 schematically shows the sign-sensitive change in the shift deformation along the transversal (γ_T) and the longitudinal (γ_L) planes during TE along routes I and II. It is easy to see that in the transversal plane, route

Structure and properties during multi-pass TE

Refs. [10], [11], [12], [13], [14], [15], [16] present experimental results with TE for different materials (Al, Cu, Ti, and their alloys; powders of different composition). Using optical microscopy, it is shown that cross-sections typically exhibit a characteristic macrostructure with structural elements elongated along the direction of a vortex centered at the extrusion axis. In the longitudinal cross-section this macrostructure resembles a turbulent flow [9], [13]. The underlying

New possibilities coming from TE

The main routes of TE can be combined with any SPD or metal forming processes (e.g., ECAP, rolling, and extrusion) to broaden the space of possible loading paths. The field of equivalent strain under TE has a large gradient. This is of interest for investigating the effects of strain gradient on the evolution of the material's microstructure, as well as obtaining microstructural gradients.

Strain distribution and deformation zone boundaries strongly depend on the geometry of the die

Conclusion

Twist extrusion has the following properties, which open new possibilities for forming and investigating new shapes and microstructures:

(i)
As in ECAP, the deformation mode in twist extrusion is simple shear. Unlike in ECAP, there are two shear planes; one of them is perpendicular and the other is parallel to the specimen axis.
(ii)
Two orientations of the twist die lead to two main routes of TE: CD + CD (or CCD + CCD)—Route I; CD + CCD (or CCD + CD)—Route II.
(iii)
The following processes are present during twist

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Useful properties of twist extrusion

Abstract

Introduction

Section snippets

Basics of twist extrusion

Experimental investigation of TE kinematics

Two shear planes and four deformation zones in TE

Two main routes of TE

Structure and properties during multi-pass TE

New possibilities coming from TE

Conclusion

Acta Mater.

Acta Mater.

Mater. Sci. Eng. A

Mech. Mater.

JOM

Fiz. Metal. Metalloved.

Russ. Metall. (Metally)

J. Mater. Sci.

Phys. Technol. High-Press.