Conintous Equal channel Angular Processing

UNIVERSITY OF WOLLONGONG

FACULTY OF ENGINEERING

FINAL REPORT

ON

CONTINUOUS EQUAL CHANNEL

ANGULAR PRESSING

ADVANCED MANUFACTURING PROCESSES

MECH 934

MAY 2010

AUTHORS

JOGI RAJU MANDAPAKA 3579505

FABRICE PAILLEUX 3794362

SANTHOSH H MALLIKARJUNAPPA 3498384

EXECUTIVE SUMMARY :-

Metals with grain sizes smaller than 1-μm have received much attention in the past

decade. These materials have been classified as ultra fine grain (UFG) materials. Advanced

materials, such as UFG show promise for many industrial applications including aerospace,

automotive, biomaterials, metal-forming etc. Our presentation addresses the production of

BULK UFG metals through the use of Continuous Equal Channel Angular Pressing method

in SEVERE PLASTIC DEFORMATION processing(SPD).

The report clearly explains the different methods of Continuous Equal Channel Angular

Pressing like Equal Channel Angular Pressing – Conform, Continuous Confined Strip Shearing

(C2S2) and explains their mechanics and their effect on the microstructure of the elements.

TABLE OF CONTENTS

1.0 INTRODUCTION.............................................................................................

2.0 SEVERE PLASTIC DEFORMATIOIN..........................................................

2.1 DIFFERENT TECHNIQUES IN SPD...............................................

3.0 EQUAL CHANNEL ANGULAR PRESSING...............................................

3.1 MECHANICS OF ECAP....................................................................

3.2 ADVANTAGES OF ECAP..................................................................

3.3 DISADVANTAGES OF ECAP............................................................

1. INTODUCTION :-

Metals are still the main material of modern engineering. Metals feature ductility and a large

degree of forming, which determines their applications and methods of production. Plastic

deformation results from an internal crystallographic structure and, in turn, affects the structure

of metals. Structural changes during plastic deformation are diverse and outside the traditional

concepts of hardening, grain refinement, forging, densification, connection, and so on.

Mechanical properties of bulk polycrystalline metals depend critically upon the internal

micro structural characteristics and especially upon the grain size. If the grain size of a solid is

reduced, the material will become stronger at ambient temperatures through the Hall –

Petch Relationship .

σy = σ0 + Kyd-1/2

where, Ky is the Hall – Petch slope and d is the mean grain size. An identical relation holds for

the indentation hardness.

The rate of flow in super plastic deformation varies inversely with the grain size raised to a

power of two, reduction in grain size can provide an opportunity for achieving a super plastic

forming capability at elevated temperatures. Both of these trends demonstrate significant

advantages in preparing materials with very fine grain sizes. Langdon (2006).

Raghavan Srinivasan et all (2006) reported that Ultra fine grain (UFG) materials are the

materials with grain size of 100 to 1000-nm, i.e., <1-μm. These materials are larger than Nano –

materials which have a grain sizes less than 100-nm. There is a lot of interest in the

manufacturing of bulk UFG materials over the past two decades, so a number of techniques for

producing UFG have also been developed. There are 2 ways of doing it. The (i) Bottom – Up

approach and the (ii) Top – Down approach. The Top – Down approach uses severe plastic

deformation processes to manufacture ultrafine grained materials.

Bottom – Up Method involves nano particles to be chemically synthesized in a solution

phase, and assembled macroscopic materials. Assembling bulk materials one at a time is not

practical. Whereas Top – Down approach, is more suited to bulk manufacturing of materials. It

uses severe plastic deformation techniques in reducing the grain size of materials without

significantly affecting the gross dimension of the specimen or work piece.

2. SEVERE PLASTIC DEFORMATION (SPD) :-

SPD are a group of techniques which use the top – down approach to manufacture ultrafine

grained materials. In Severe Plastic Deformation techniques the specimen is subjected to intense

strains over and over again resulting in a highly refined grain structure without actually affecting

the dimensions of the work piece.

SPD processes should involve controlled loading history, simple-shear mode, and the

possibility of reaching high strains in bulk products. Moreover, they must satisfy ordinary

specifications, such as uniform deformation, low pressures and forces, simplicity of technical

realization, a low cost, and so on. At present, there are several methods of solving this problem.

A few important ones are Equal Channel Angular Pressing or Equal Channel Angular Extrusion.

High Pressure Torsion, Accumulative Roll Bonding, Friction Stir Processing etc. Controlled

simple shear of the bulk material contributes to the efficiency of SPD processes. Raghavan

Srinivasan et all (2006).

3. EQUAL CHANNELL ANGULAR PRESSING : -

The equal-channel angular extrusion/pressing (ECAE/P) process was developed in Russia

during the 1970s by Segal et al. as a method for introducing large plastic strains in a metal, while

maintaining the outer dimensions of the work piece substantially unchanged. In contrast,

conventional mechanical processing operations, such as extrusion, rolling or forging

impart substantial shape changes to achieve high accumulated plastic strains. Over the past

decade there has been extensive investigations by many research groups all over the world on the

process. Raghavan Srinivasan (2006)

Equal-channel angular (ECA) pressing is a processing procedure whereby an intense plastic

strain is imposed upon a polycrystalline sample by pressing the sample through a special die.

This procedure is capable of producing large fully-dense samples containing an ultrafine grain

size in the sub micrometre or nanometre range.

Fig:1 Schematic of ECAP (Raghavan Srinivasan (2006))

The ECAE/P process shown schematically in Fig:1, pushing or extruding a work piece

though two channels of equal cross-section which meet at an included angle Φ. In principle, if

the fillet radius R is zero (sharp corner) the work piece will undergo simple shear as it passes

through the plane of intersection between the two channels.

3.1 Principles

ECAP bases on introduction of strain into the material by pressing the billet through two

channels of identical diameter intersecting each other at an angle of 90 – 135 degrees. The

sample, which is put in the vertical channel, is pressed by the plunger of a pressing machine to

the horizontal channel as shown on the FIG:1 the sample is bent inside the channel the large

strain is introduced into the material. The strain introduced to the sample in one pressing depends

on die geometry. (The whole process of deformation is quite complicated and will not be

discussed here. Detailed theoretical study of the sample deformation on the intersection of two

channels can be found in (V.M.Segal, Materials Science and Engineering) The theoretical strain

introduced into the sample can be estimated on the basis of the bellow equation

N = (N/3) * (2 cot (/2 + /2) + cosec (/2 + /2))

Where:

N – strain introduced into the material,

N – number of passes through a die,

- angle of channels intersection,

- angle describing an outer corner.

Ruslan Z. Valiev et al. (2006), says that “The small grain sizes and high defect densities

inherent in UFG materials processed by severe plastic deformation lead to much higher strengths

than in their coarse-grained counterparts. Moreover, according to the constitutive relationship for

superplasticity, it is reasonable to expect the appearance in UFG metals of low-temperature

and/or high-rate superplasticity.”

3.2.1 Microstructure evolution during ECAP deformation

As reported by many scientists ECAP severely changes microstructure of a processed

material and that is the main reason why it arouses so much interest.

Figure 2 – Microstructure of pure Al after one passage through a die [13]

Further pressings through a die increased introduced strain and caused gradual

decrease of grain size and diminished band structure. However, a rate of band structure break-

up was dependant on route of ECAP processing. Authors noticed that for route A band

structure of subgrains was still visible after 4 passes. On the other hand, if sample was processed by

route BC the band structure almost vanished after 3 passes and is not visible after 4 (b). Reasons for

observed difference are changed shearing patterns for those two routes (Error: Reference source not

found).

Fig3

Figure 1 – Microstructure of pure aluminium after 4 passes, a) route A, b) route BC

Obviously shearing that occurs during pressing sample with route BC is more efficient in refining

microstructure in this case.

In a nutshell, choice of a route of pressing during ECAP influences a speed of a

microstructure evolution. Moreover, it changes fraction of high angle grain boundaries. But after

several passes similar microstructures are usually obtained regardless the route.

3.3 Change of physical properties due to ECAP

Main purpose of processing material with ECAP is possibility of enhancing its physical properties.

It was reported that ECAP may influence not only mechanical behaviour of the material but also its

fundamental properties including Curie temperature and elastic module .

However, most commonly analyzed parameter is yield stress, UTS and elongation to failure.

What is interesting in case of ECAP is that considerable raise of strength of materials is not accompanied

by drastic loss in ductility For most alloys reduction of plasticity occurs, but it is much smaller than for

regular work hardening case. For some alloys ECAP was reported to lead to high strain rate super-

plasticity.

3.4 FUTURE OF ECAP :-

Processing by ECAP is at present one of the most promising techniques for manufacturing

UFG structures in different metals and alloys and the various parameters associated with the

processing operation have been described in detail in this review. Concerning the economically-

feasible production of UFG metals, there are several tasks which must be addressed during

processing development. Among these tasks are a reduction of the material waste, the fabrication

of bulk billets and semi-products in the form of rods, sheets and wires with a homogeneous UFG

structure and superior properties and, most important, raising the efficiency of the ECAP

processing technique.