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Advanced Manufacturing
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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.