INTRODUCTION AND LITERATURE REVIEW

INTRODUCTION

Extrusion is a plastic deformation process in which a block of metal (billet) is forced to flow

by compression through the die opening of a smaller cross-sectional area than that of the

original billet as shown in Fig. 1. Extrusion is an indirect-compression process. Indirect-

compressive

Fig. 1 Definition and principle of extrusion

forces are developed by the reaction of the workpiece (billet) with the container and die;

these forces reach high values. The reaction of the billet with the container and die results

in high compressive stresses that are effective in reducing the cracking of the billet material

during primary breakdown from the billet (Gleiter,1981). Extrusion is the best method for

breaking down the cast structure of the billet because the billet is subjected to compressive

forces only. Extrusion can be cold or hot, depending on the alloy and the method used. In

hot extrusion, the billet is preheated to facilitate plastic deformation.

Classification of Extrusion Processes

The two basic types of extrusion are direct and indirect, which are commonly used in

aluminum industries as shown in Fig. 2

Fig. 2: Types of extrusion process (a) Direct Extrusion and (b) Indirect Extrusion

Conventional Direct Extrusion

The most important and common method used in aluminum extrusion is the direct process.

Figure 1 shows the principle of direct extrusion where the billet is placed in the container

and pushed through the die by the ram pressure. Direct extrusion finds application in the

manufacture of solid rods, bars, hollow tubes, and hollow and solid sections according to

the design and shape of the die. In direct extrusion, the direction of metal flow will be in the

same direction as ram travel. During this process, the billet slides relative to the walls of the

container. The resulting frictional force increases the ram pressure considerably. During

direct extrusion, the load or pressure-displacement curve most commonly has the form

shown in Fig. 3. Traditionally, the process has been described as having three distinct

regions:

1. The billet is upset, and pressure rises rapidly to its peak value.

2. The pressure decreases, and what is termed “steady state” extrusion proceeds.

3. The pressure reaches its minimum value followed by a sharp rise as the “discard” is

compacted.

Fig. 3: Variation of load or pressure with ram travel for both direct andIndirect extrusion process.

Indirect Extrusion

In indirect extrusion, the die at the front end of the hollow stem moves relative to the

container, but there is no relative displacement between the billet and the container as

shown in Fig. 6. Therefore, this process is characterized by the absence of friction between

the billet surface and the container, and there is no displacement of the billet center relative

to the peripheral regions.

AIM OF THE RESEARCH

The aim of the project is to describe the plastic deformation executed by combined torsion

and ECAE of AA6063 to achieve significant improvement of strength of investigated

material.

JUSTIFICATION

Aluminium alloys, like Al 6063, are some of the most widely used materials today which

spans the entire range of industries. They are used in many consumer products, including

pipes, railings, furniture, architectural extrusions, irrigation pipes, and transportation. The

aircraft and aerospace industry uses aluminium alloys because it is much lighter than steel

and every kilogram of weight reduction results in greater fuel savings and higher payloads.

The car industry has increased its use of aluminium over the years as the price of gasoline

has increased and the need to reduce vehicle weight has been of paramount importance.

Today, much of aluminium’s use is to reduce the weight of the item being produced, but it

has always been popular because it is easy to machine, cast, extrude, roll, etc. and many

alloys are age-hardenable [Sterie et al 2008, Panait et al 2009]. Because of the widespread

use of these alloys, it is important to understand their mechanical behaviour when exposed

to different loading conditions, strain rates and temperatures, and to be able to model the

behaviour and later, to predict the behaviour for any of these conditions. In order to

improve mechanical properties of these alloys many processing routes can be applied.

During the last decade, the equal-channel angular pressing (ECAP) route proved to produce

ultrafine-grained bulk samples in a fully dense condition without changing the cross-

sectional dimensions of the samples. During ECAP, significant grain refinement occurs

together with dislocation strengthening, resulting in a significant enhancement in the

strength of the alloys [Ferrasse et al 1997, Horita et al 2007, Chakkingal et al 1999).

BACKGROUND

Current processing methods that are used to develop fine-grained superplastic

microstructures in aluminum alloys involve extensive hot and cold deformation steps,

usually in the form of hot and cold rolling. This approach has distinct limitations that can

have a significant influence on the cost and quality of superplasticity-forming (SPF)-grade

aluminum sheet. First, the extensive cold rolling required for SPF aluminum sheet typically

results in substantial edge cracking and overall yield losses. The second limitation is that the

high levels of hot and cold work necessary to achieve the desired microstructure requires

starting with very large ingot size, while the final product is limited to thin sheet gage

thickness.

The ECAE process offers several potential advantages in the processing of SPF-grade

aluminium alloys. The ability of the ECAE process to achieve high levels of work through

localized shearing can provide a mechanism for distributing the eutectic constituent

particles and dispersoids that play a critical role in the recrystallization process and resulting

thermally stable fine grain size. In addition, with ECAE there is the unique ability to achieve

these desirable microstructures without reducing the dimensions of the starting material, as

is the case in conventional processing of SPF materials (Macheret etal, 1999).

For the nanostructured processing of bulk materials, the current effective approaches

include powder synthesis, amorphous casting and severe plastic deformation. These

approaches produce the bulk materials into crystalline or quasicrystalline state with grain

size ranging from 1 to 100 nm. Among these approaches, severe plastic deformation (SPD) is

the easiest and most cost-effective way as the process and die structure are simple. The

severe plastic deformation approach for producing bulk nanostructured materials is usually

implemented via equal channel angular extrusion (ECAE) process. The process allows the

bulk materials to undergo severe plastic deformation and break down the original texture

into ultra-fine structure after a few passes of extrusion and finally nanostructure is obtained.

Currently, significant research interest has been expressed in this area and a lot of efforts

have been invested into the researches, focusing on investigation of the material

microstructure and mechanical properties related to ECAE process (Herian et al 2008,

Iwahashi et al 1996). All of these have proven that the ECAE process is an effective approach

for producing bulk nanostructured materials. However, the in-depth research on plastic

deformation mechanics, process configuration and optimisation in ECAE process has not

been fully addressed yet. This kind of research is critical as it can provide fundamental

information for improving process and die performance and finally obtaining satisfactory

ultra-fine structure

LITERATURE REVIEW

DESCRIPTION OF EQUAL CHANNEL ANGULAR PRESSING (ECAP)

“Equal Channel Angular Pressing” (ECAP), also known as “Equal Channel Angular Extrusion”

(ECAE). This technique has originally been developed by Segal et. al. A billet of the test

material is pressed through a die consisting of two channels with identical cross sections,

intersecting at an angle φ, usually 60° < φ < 135° and often φ =90° (fig. 1). Some dies have a

rounded corner with angle ψ, others have ψ= 0.

Principle of this method consists in severe deformation of massive samples realised by shear

without change of cross section. The sample is pressed through a die, in which two channels

intersect, forming an angle usually of 900. Pressing is made either at room or at increased

temperature. Equivalent deformation can achieve the value of 10 or even higher. The most

critical for development of microstructure and resulting properties of samples is above all

number of passes and selection of deformation route (manner of turning of the sample after

each pass). It was established from the analysis of shear characteristics at various

deformation routes that turning of the sample by 900 was optimal. Many works, dealing

with optimisation of the laboratory ECAP equipment, were published. Promising

modifications for production of ultra fine-grained massive semi-products in industrial

practice have appeared (Herian et al 2008, Iwahashi et al 1996).

Fig. 3 A laboratory scale ECAP die.

Because the channels have an identical cross section, the dimensions of the billet remain

unchanged, and the process can in principle be repeated any given number of times. In spite

of the actual popularity of the technique, some drawbacks of ECAP must be recognized.

ECAP is a discontinuous process with limitations in up-scaling potential. Moreover, the

volume fraction of useful material (with uniform microstructure and without cracks) can be

rather low because only the portion of the billet that has passed through the shear zone,

will receive the desired deformation and grain refinement. (Barber et. al., 2004).

There exists a variety of methods to impose large plastic strains on materials in order to

produce fine-grained microstructures. Forging, extrusion, drawing, and rolling have been

used for this purpose, but they all have significant drawbacks. Multiple reductions of the

initial billet cross-section are limited by the geometrical change of the work piece, require

high loads, and result in a nonuniform deformation. In rolling, for example, the strain levels

needed to achieve the formation of ultra-fine grain structures are only reached in thin foils.

These problems pose significant limitations for production of larger parts or synthesis and

processing of new materials with the objective of developing special microstructures and

properties. Many limitations associated with the conventional metal deformation

techniques can be overcome by ECAE, a method was developed and patented in the former

Soviet Union.1

The benefits of ECAE come from its ability to impose intense simple shear deformation

through innovative die design. Unlike conventional extrusion processes, the cross-section of

billets extruded via ECAE is not reduced. This process, therefore, can be applied repeatedly

through multi-pass operations to achieve strains of significant magnitudes while preserving

the billet size. Each consecutive pass through a 90-degree die arrangement increases the

strain intensity by 1.15. Table 1 lists the effective strain intensity and equivalent reduction

for multiple passes. There is no geometric restriction on the strain magnitude that can be

achieved. It has been shown that four or more passes creates a homogeneous and stable

microstructure.2 In addition, by choosing appropriate billet orientations as it enters the

extrusion die on each pass, a variety of microstructures and textures can be developed.

ECAE, therefore, has a potential to emerge as an important metal processing method for

obtaining ultra-fine grain structures in alloys in bulk forms. Figure 4 illustrates the ECAE

process (Macheret etal, 1999).

The severe plastic deformation (SPD) is an effective approach for producing bulk

nanostructured materials. The equal channel angular extrusion (ECAE) is the most efficient

SPD solution for material nanostructuring as material billet undergoes severe and large

deformation in ECAE process. To improve material nanostructuring, the ECAE die design and

process configuration are critical. The deformation behaviours study in ECAE process

provides information for optimising die design and process determination.

Equal-channel angular pressing (ECAP) has widely been investigated due to its ability of

producing billets sufficiently large for industrial applications in functional or structural

components. The significant strength increase based on grain refinement is typically

accompanied by a significant decrease in ductility and toughness.

Fig. 4 Schematic Diagram of the ECAE process

The ECAP method makes it possible to obtain the grain size of several hundreds of

nanometres (Kurzydlowski, 2004). Materials with sub-micron size of sub-grains/grains

(d=0.1-1 nm) are usually classified as ultra fine-grained materials. The ECAP method was so

far unsuccessful at attempts of obtaining nanometric materials.

Depending on the billet rotation, different deformation routes can be applied. Route A has

no rotation of the billet, route BA is rotated counter clockwise 90° on even number of

passes and clockwise 90° on odd number of passes, route BC is rotated counter clockwise

90° after every pass, and route C is rotated 180° after every pass [Furukawa et al, 1998].

This technique can be applied to commercial pure metals and metal alloys, with FCC, BCC

and HCP crystal structures, with coarse grains to fabricate ultra-fine grained materials that

have no porosity and higher strength than the non-processed material [Mukai et al, 2001].

In the last ten years a large number of investigations regarding a severe plastic deformation

(SPD) technique called Equal Channel Angular Pressing (ECAP), were published in the wake

of the pioneering work by Valiev et al. and Segal. Justification for this interest lies in the fact

that ECAP- deformed metals and alloys exhibit a very small grain size and consequently,

their tensile strength is remarkably improved. All relevant work on this SPD technique has

been recently summarized by Valiev and Langdon in a comprehensive review. The

microstructural evolution of metals and alloys subjected to low and medium plastic

deformation has been discussed by Liu and Hansen4, and Bay et al.5. The outcome of these

studies is a model describing how, in severely deformed metals with high to moderate

stacking fault energy, grain subdivision takes place by the formation of cell blocks separated

by arrays of geometrically necessary dislocations. Within these cells there are regions

relatively free from dislocations, bounded by low angle boundaries. The more severe the

deformation, the narrow the cell blocks become, until the cell boundaries transform into

high angle boundaries. This sequence has been often observed in ECAP - deformed metals

and alloys and seems to explain the formation of very small grains (Furukawa, 2006) . A still

active dispute is how the high angle dislocation boundaries transform into grain boundaries;

on this respect, a study by Chang et al.8 identified three types of boundaries in commercially

pure Al subjected to ECA - deformation: i) polygonized dislocation walls of the type

described by Bay et al.5; ii) partially transformed boundaries, and iii) proper grain

boundaries. The evolution of type i to type iii take place by the dissociation of lattice

dislocations and their absorption into the boundaries. This process decreases the free

energy of the system since the resulting grain boundaries are of the equilibrium type.

ADVANTAGES/USES OF ECAP

Compared to classical deformation processes, the big advantage of SPD techniques

(represented in particular by equal channel angular pressing (ECAP) is the lack of shape-

change deformation and the consequent possibility to impart extremely large strain. SPD

has received enormous interest over the last two decades as a method capable of producing

fully dense and bulk submicrocrystalline and nanocrystalline materials. Significant grain

refinement obtained by SPD leads to improvement of mechanical, microstructural and

physical properties [Cojocaru et al, 2010].

Advantages and uses of ECAE include:

• Impartation of tremendous increase in material strength (Semenova et al., 2004; Amol et

al., 2005).

• Production of Ultrafine-equiaxed grained (UFG) materials has been achieved e.g. grain size

less than 1 μm (Mathis et al., 2004)

• Unusual properties appear such as high tensile strength, ductility and the possibility to get

super plasticity at low temperatures (Horital et al., 2000; Komura et al., 2001; Xu et al.,

2004).

• Ability to control crystallographic and mechanical texture during multi-pass ECAE by

judicious rotation of the workpiece between passes (Toth et al., 2004).

• Uniform and unidirectional deformations can be produced under relatively lowpressure

and load for massive products (Park et al., 2005).

• Formation of complex microstructures: equiaxed, laminar and fibrous textures (Ferrasse et

al., 2004; Li et al., 2004).

• Increase in hardness (Yu et al., 2005).

• Achievement of powder consolidation/

• compaction (Senkov et al., 2005; Mathis et al., 2004).

SHORTCOMINGS

• Buckling instability of the extruding ram.

• The cross-section of the billet becomes smaller with the number of passes.

• Surface defects such as cracks, pores are also common.

DEFINITION OF VARIOUS ROUTES USED IN ECAE

Routes here mean the directions followed by the billet or test sample.

• ROUTE A: 0o, all passes; the billet is not rotated between successive passes.

• ROUTE B or BA: 90o, N even, 270o N odd; the billet is rotated 90o clockwise and

counterclockwise alternatively.

• ROUTE C: 180o, all passes; the billet is rotated 180o.

• ROUTE D or BC: 90o, all passes; the billet is rotated 90o clockwise.

Fig. 5 Rotation schemes of the four ECAP routes

INDUSTRIAL SIGNIFICANCE

Nanotechnology is another upward technology trend in engineering science. From

engineering material perspective, the bulk nanostructured processing technology is a

promising and practical application of nanotechnology, as it directly processes

microstructured material into a nanostructured one. In bulk nanostructured process, severe

plastic deformation is a straightforward method and it directly breaks down grains in

microstructure into ultra-fine structure and finally nanostructure. To implement SPD, the

ECAE extrusion is the most efficient and economic approach. The research reveals indepth

deformation phenomena and behaviours in the process, which provides basic and

fundamental information for process determination and configuration and tooling design of

ECAE process to improve nanostructuring performance of die and process. The research also

paves the way for potentially wide application of the technology in the near future.

REFERENCES1. R.E. Barber, T. Dudo, P.B. Yasskin and T. Hartwig, in “Ultrafine Grained Materials III”, ed. Y.T. Zhu et. al., TMS, 2004, 667-6722. H. Gleiter, Deformation of Polycrystals: Mechanisms and Microstructures, ed. N. Hansen et al., Riso National Laboratory, 1981.

3. W.D. Callister, Materials Science and Engineering – an Introduction, John Wiley & Sons, 2000.

4. J. Adamczyk, A. Grajcar, Heat treatment and low- carbon steel with dual-phase microstructure, Journal of Achievements in Materials and Manufacturing Engineering 22/1 (2007) 13-20.

5. R.Z. Valiev, Some Trends in SPD Processing for Fabrication of Bulk Nanostructured Materials, Materials Science Forum 503-504 (2006) 3-7.

6. W.Sitek, Employment of rough data for modelling of materials properties, Journal of Achievements in Materials and Manufacturing Engineering 21/2 (2007) 65-68.

7. J. Herian, R. Sulkowski, Quantitative evaluation of the structure and properties of hot rolled products of continuous ingots made of low-carbon steel, Journal of Achievements in Materials and Manufacturing Engineering 28/1 (2008) 15-18.

8. Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, G. Langdon, Principle of Equal Channel Angular Pressing of Ultra Fine Grained Materials, Scripta Materialia 35 (1996) 143-148.

9. K.J. Kurzydlowski, Microstructural refinement and properties of metals processed by severe plastic deformation, Bulletin of the Polish Academy of Sciences, Technical Sciences 4 (2004) 301-309.

10. R. Sterie, N. Panait, P. Moldovan, Study and research concerning the quality of cast billets of 6XXX aluminium alloys, U.P.B. Sci. Bull., Series B, Vol. 70, 2008, 4, 85-92

11. N. Panait, M. Ciurdas, R. Stephan, Microstructural characterization of AA - 7050 alloy slabs (part I), U.P.B. Sci. Bull., Series B, Vol. 71, 2009, 3, 147-151.

12. S. Ferrasse, V.M. Segal, Microstructure and properties of copper and aluminium alloy 3003 heavily worked by equal channel angular extrusion, Metallurgical and Materials Transactions, 1997, A 28A, 1047–1057

13. Z. Horita, T. Fujinami, Equal-channel angular pressing of commercial aluminium alloys: grain refinement, thermal stability and tensile properties, Metallurgical and Materials Transactions, 2000, A 31A, 691–701

14. U. Chakkingal, A.B. Suriadi, P.F. Thomson, The development of microstructure and the influence of processing route during equal channel angular drawing of pure aluminium,Materials Science and Engineering: A, 1999, 266, 241–249

15. V.M. Segal, V.I. Reznikov, A.E. Drobyshevskiy, V.I. Kopylov, Plastic working of metals by simple shear, Russ. Metall. (Engl. Trans.), 1981, 1, 99–105

16. R.Z. Valiev, A.V. Korznikov, R.R. Mulyukov, Structure and properties of ultrafine-grainedmaterials produced by severe plastic deformation, Materials Science and Engineering: A,1993, 168, 141–148

17. V.M. Segal, Materials processed by simple shear, Materials Science and Engineering: A, 1995,197, 157–164

18. M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, The shearing characteristics associated with equal-channel angular pressing, Materials Science and Engineering: A, 1998, 257, 328–332

19. V.V. Stolyarov, Y.T. Zhu, T.C. Lowe, R.Z. Valiev, Microstructure and properties of pure Ti processed by ECAP and cold extrusion, Materials Science and Engineering: A, 2001, 303,82–89

20. M. Mukai, M. Yamanoi, H. Watanabe, K. Higashi, Ductility enhancement in AZ31 magnesium alloy by controlling its grain structure, Scripta Materialia, 2001, 45, 89–94

21. W.J. Kim, C.W. An, Y.S. Kim, S.I. Hong, Mechanical properties and microstructures of an AZ61 Mg alloy produced by equal channel angular pressing, Scripta Materialia, 2002a, 47,39–44

22. W.J. Kim, J.K. Kim, Y.T. Park, S.I. Hong, I.D. Kim, Y.S. Kim, J.D. Lee, Enhancement of strength and superplasticity in a 6061 Al alloy processed by equal-channel-angularpressing, Metallurgical and Materials Transactions, 2002b, 33A, 3155–3164

23. Semenova, I. P., Raab, G. I., Saitova, L. R. and Valiev, R. Z. (2004) ‘The effect of equal-channel angular pressing on the structure and mechanical behavior of Ti–6Al–4V alloy’ Materials Science and Engineering A, Vol. 387-389, Pp 805-808.

24. Mathis, K., Gubicza and Nam, N. H. (2005) ‘Microstructure and mechanical behavior of AZ91 Mg alloy processed by equal channel angular pressing’ Journal of Alloys and Compounds, Vol. 394, no. 1-2, Pp 194-199.

25. Horita, Z., Furukawa, M., Nemoto, M., Barnes, A. J. and Langdon, T. G. (2000) ‘Superplastic forming at high strain rates after severe plastic deformation’Acta Materialia, Vol. 48, Issue 14, Pp 3633-3640.

26. Komura, S., Furukawa, M., Horita, Z., Nemoto, M. and Langdon, T. G. (2001) ‘Optimizing the procedure of equal-channel angular pressing for maximum superplasticity’ Materials Science and Engineering A, Vol. 297, Issues 1-2, Pp 111-118.

27. Xu, C., Furukawa, M., Horita, Z. and Langdon, T. G. (2004) ‘Severe plastic deformation as a processing tool for developing superplastic metals’ Journal of Alloys and Compounds, Vol. 378, Issues 1-2, Pp 27- 34.

28. Park, K., Lee, H., Lee, C. S. and Shin, D. H. (2005)‘Effect of post-rolling after ECAP on deformation behavior of ECAPed commercial Al–Mg alloy at 723 K’ Materials Science and Engineering A, Vol. 393, Issues 1-2, Pp 118-124.

29. Iwahashi, Y., Wang, J., Horita, Z., Nemoto, M., and Langdon, T. G. (1996) “Principle of equal-channel angular pressing for the processing of ultra-fine grained materials” Scripta Materialia, Vol. 35, Issue 2, Pp 143-146.

30. Segal, V. M. (1995) “Materials processing by simple shear” Materials Science and Engineering A, Vol. 197, Issue 2, Pp 157-164.

31. C. Xu, M. Furukawa, Horita Z, T.G. Langdon, “Developing a model for grain refinement in equal channel angular pressing”. Materials Science Forum 2006; 503-504:19-24.

32. V.D. Cojocaru, D. Raducanu, N. Serban, I. Cinca, R. Saban, Mechanical behavior comparisonbetween un-processed and ECAP processed 6063-T835 aluminum alloy, U.P.B. Sci. Bull.,Series B, Vol. 72, 2010, 3, 193-202

33. J. Macheret, G. E. Korth, T. M. Lillo, A. D. Watkins, J. E. Flinn “Equal Channel AngularExtrusion”, Progress Report for March 1998-May 1999, Idaho National Engineering and Environmental Laboratory, Bechtel BWXT Idaho, LLC. (1999)