Indian Journal of Engineering & Materials Sciences

Vol. 25, April 2018, pp. 113-121

Comprehensive study of effect of process parameters in equal channel

angular pressing

Atul Dayal, Ankit Sahai, K Hans Raj & Rahul Swarup Sharma*

Department of Mechanical Engineering, Faculty of Engineering, Dayalbagh Educational Institute, Agra 282 005, India

Received 23 February 2017; accepted 5 February 2018

In present work, comprehensive study of equal channel angular pressing (ECAP) process is attempted for analyzing the

influence of process parameters, viz., channel angle (Φ), the angle of curvature (Ψ), plunger velocity (v), processing routes

(R), shear friction (m) at die and billet interface and the number of ECAP passes (N). This study will help to understand

ECAP process with a better insight into influence of multiple process parameters and develop improved ECAP process with

two objectives, i.e., (i) high and uniform distribution of equivalent strain in ECAPed billet and (ii) minimum pressing

requirement during the process along with selection of best process parameters without committing to expensive tooling and

machinery. ECAP study is performed on the AA6061 aluminum alloy. ECAPed billet is mechanically tested. The usefulness

of experimental and FEM analysis in developing practical ECAP process is demonstrated in this paper. The outcome of this

work certainly provides momentum in the commercialization of ECAP process.

Keywords: Equal channel angular pressing, Finite element method, Severe Plastic Deformation

Ultrafine grain materials/bulk nano materials are among the interest areas of research today to meet

present engineering challenges. Traditionally bulk

nano materials are made by nano scale synthesis, i.e.,

self-assembly, non-traditional lithography, template

growth and biomimetics. In these processes,

nanoparticles, delivered in the form of nanotubes,

nano powders, quantum-dots, and biomaterials are

staked into final product in a designed way. These

processes are quite expensive, time consuming and

chemically unstable1. Recently, a top down grain

refinement approach known as severe plastic

deformation (SPD)2 has been adopted to produce bulk

nano material at a commercial level. Among different

SPD approaches, one approach known as equal

channel angular pressing (ECAP) (Fig. 1)3 attracted a

lot of attention recently because of its potential to be

used at an industrial level.

Though ECAP process has a potential to be

employed at a commercial level, but still its

applications are restricted to laboratory level only.

Larger pressing load, heavier dies, large experimental

setup and labour intensiveness are the major hurdles

before the commercialization of an ECAP process4

It is observed that ECAP process efficiency and

industrial feasibility is influenced by two process

constraints, i.e., equivalent strain distribution5 and

pressing force6 These constraints depend on six

process parameters7-14

, i.e., die geometry15,16

, plunger

velocity, repetitive ECAP passes , die billet interface

friction and processing routes (Fig. 2).

This study is a step forward in selecting process

parameters in order to get uniformly distributed

average equivalent strain across the ECAPed billet

with lower pressing force. In first step process

parameters are numerically analysed on the basis of

ECAP constraints. Best value of each process

parameter is transferred to next set (Fig. 3), on the

—————

*Corresponding author (E-mail: rahulswarup@dei.ac.in)

Fig. 1 — Schematic of equal channel angular pressing process

INDIAN J. ENG. MATER. SCI., APRIL 2018

114

basis of best process parameters experimental ECAP

dies are designed and tested.

Further, result validation and mechanical testing

are performed. It is confirmed by the experimental

results that yield strength and ultimate tensile strength

of AA6061 aluminium alloy processed with best

process parameters increased up to 49% after 4

repetitive ECAP passes at an average pressing load of

8 tonnes.

Finite element modelling

An industrial version of FE modelling software

“FORGE2013” is used for FEM analysis. Five

intersection angles (i.e., 90°,105°,120°,135° and

150°), four outer arc angles (Ψ = 0°, 20°, 30°, 40°),

three plunger velocities (i.e., 1 mm/s, 5 mm/s, 10

mm/s), two shear frictions (i.e., medium (m = 0.1) and

high (m = 0.4)), four processing routes were

numerically modelled and analysed in this work.

FEA predictions give useful information regarding

pressing load and equivalent strain; these results will

be used further for experimental setup and die

designing.

Experimental Procedure

Billet fabrications for experimental analysis

Circular cross-section billets with 10 mm diameter

and 100 mm length are used for analysis. Tolerance

between plunger and ECAP channel kept very low to

prevent material flow through the joint cavity. To

avoid cup formation at top portion of the billet, top

and bottom portion is grinded into a conical shape.

Experimental setup

Split ECAP dies are used for the experimentation.

Since AA6061 is ductile in nature, all forming

experiments are carried out at room temperature.

Plunger speed kept low (2 mm/s), to avoid heating up

the material, which could lead to the grain growth. A

computerized hydraulic press with a pressing capacity

up to 100 tonnes is used for experimentation analysis.

A 80 kg weight cast iron fixture is used for holding

the vertically split ECAP die, This fixture had added

Fig. 2 — ECAP process parameters

Fig. 3 — Process parameters data flow during ECAP process

DAYAL et al.: EFFECT OF PROCESS PARAMETERS IN EQUAL CHANNEL ANGULAR PRESSING

115

extra strength to the ECAP die for hard material to

processed. The fixture also facilitates the rapid

engagement and disengagement of an ECAP die for

multiple passes. All experiments are carried out a room

temperature with or without molybdenum disulfide

(MoS2) lubricant.

Results and Discussion Equivalent strain distribution along the length of billet based

on intersection angle (Φ) and repetitive passes

Equivalent strain distribution in ECAPed billet

processed through different ECAP dies with

intersection angle 90°, 105°, 120°, 135°, 150° and

constant outer arc angle (Ψ) = 0° at medium shear

friction coefficient (m = 0.1), 5 mm/s plunger velocity

and ambient temperature is shown in Fig. 4, graphs

revealed that for smaller intersection angle (i.e., 90°

and 105°) higher variation in equivalent strain

distribution is achieved. With an increase in

intersection angle Φ=120°, 135° and 150° variation in

equivalent strain distribution reduces and strain gets

more uniformly distributed along the length of the

billet. Plots revealed that with repetitive ECAP pass

better uniformity in equivalent strain distribution has

been achieved for higher intersection angles (i.e.,

Φ=120°, 135° and 150°) ECAP dies. FE results

(Fig. 5) indicated that first two intersection angles

(Φ = 90°, 105°) poses a non-uniform distribution of

equivalent strain across the top middle and bottom

sections. However, billet processed through dies with

intersection angle 120° or greater shows more

Fig. 4 — Equivalent strain distribution along the length of billet based on intersection angle (Φ)

Fig. 5 — Equivalent strain distribution at top, middle and bottom

layer with increasing intersection angle (Φ = 90°, 105°, 120°) and

constant outer arc angle (Ψ=0°) at shear friction coefficient m =

0.1, after first pass

INDIAN J. ENG. MATER. SCI., APRIL 2018

116

uniform distribution of equivalent strain throughout

the cross-section.

Dead zone formation at the bottom surface of ECAP

die is the main reason behind this non uniformity in

equivalent strain distribution. Larger strains get

accumulated near the dead zone at the bottom segment

of the billet and thus cause the uneven distribution of

equivalent strain across the cross section.

The dead zone formation is associated with an

occurrence of corner gap at the junction of inlet and exit

channel (Fig. 6). From FE results it can be observed that

occurrences of this corner gap formation reduces with an

increase in intersection angle (for Φ > 105°), thus leads

to the uniform distribution of equivalent strain.

From FEM results it is evident that corner gap

depends on outer arc angle (Ψ), with an increase in outer

arc angle formation of corner gap reduces (Fig. 7). Billet

processed with four outer arc angles (i.e., 0°, 10°, 20°

and 30°) are presented; it can be observed because of

large curvature at corner, billet can easily slide and

chances of corner gap formation reduces. At outer arc

angle Ψ = 30° equivalent strain is more uniformly

distributed across the cross-section of the billet at top

middle and bottom layers.

Experimental Procedure Average equivalent strain value for five intersection

angles (i.e., Φ = 90°, 105°, 120°, 135°, 150°) at shear

friction m=0.1 and at three plunger velocities (i.e., 1, 5

and 10 mm/s) are given in Table 1.

Variation of pressing force distribution along the length of billet

based on intersection angle (Φ) and repetitive ECAP passes

The pressing force flow direction and variation at

different intersection angle dies are shown in Figs 8 and

9, respectively. It is depicted that initially large pressing

force is required as the plunger tries to push the billet

towards a different channel. With continued pressing,

shear deformation occurs as billet slides along the length

of exit cannel, and value of pressing force decreases

continuously as the end of the process is reached.

Nearly, same trend is observed for all the channel angles

(Table 2). The numerical result of variations of pressing

force for multiple ECAP passes are shown in (Fig. 10).

It was observed that with repetitive ECAP passes peak

pressing load increases because of strain hardening.

During experimentation, contradictory results have

Fig. 7 — Effect of outer arc angle (Ψ) on corner gap formation

Fig. 6 — Dead zone and corner gap formation

Table 1 — Average equivalent strain obtained for different

intersection angles (i.e., Φ: 90°, 105°, 120°, 135°, 150°), at

3 plunger velocities and shear friction (m) = 0.1

Angle(Φ) Ram velocity mm/s Ist IInd IIIrd IVth

90° 1 1.3 2.32 3.3 4.1

5 1.4 2.5 3.4 4.2

10 1.42 2.6 3.52 4.34

105° 1 0.9 1.85 2.62 3.7

5 0.98 1.92 2.8 3.72

10 0.98 2 3 3.78

120° 1 0.72 1.6 2.5 3.55

5 0.8 1.77 2.62 3.63

10 0.87 1.85 2.9 3.7

135° 1 0.42 1.75 2.1 3.1

5 0.63 1.75 2.3 3.23

10 0.71 1.8 2.72 3.4

150° 1 0.24 0.7 1.6 2.5

5 0.32 0.82 1.72 2.62

10 0.5 1.2 2.2 2.91

DAYAL et al.: EFFECT OF PROCESS PARAMETERS IN EQUAL CHANNEL ANGULAR PRESSING

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been observed. It was observed that with repetitive

ECAP passes pressing load has decreased, it happened

because pressing force required for extruding the

material billet depends on billet length, with larger

length more pressing load is required to extrude the

billet, but with continued passes the length of billet has

been decreased (because of material removal and flash

generation) thus peak pressing load also reduced.

Effect of plunger velocity and die billet interface friction

Effect of friction on the deformation behaviour of the

processed material is studied by many researchers17,18

.

Many of the reported results are contradicting. Yang et

al.19

have reported that strain is independent of friction.

Domlin et al.20

and Suo et al.21

have reported that strain

homogeneity increases with friction. Prangnellet et al.22

has reported that strain concentration and

inhomogeneous deformation increases with friction. In

present work a shear friction (m) model is adopted for

FE analysis. FE results concluded that for medium die

billet interface friction (m = 0.1) at die angles (Φ = 90°

and Ψ = 0°), after fourth ECAP pass, average equivalent

strain obtained is 4.2 at pressing load of 8 tonnes. FEM

iso-contours of the same combination also indicate the

dead zone formation with the accumulation of strain at

the bottom layer. Alloy processed with ECAP die with

angles (Φ = 90° and Ψ = 30°) at die billet interface

friction (m = 0.1) gives approximately similar average

equivalent strain (ε = 4) at pressing load of 7.2 tonnes,

but with this combination uniformity in equivalent strain

distribution has considerably improved. FE results

indicated that at higher plunger velocity (v = 10 mm/s)

slightly higher value of average equivalent strain is

obtained, but experimentally plunger velocity v = 10

mm/s is more prone to plunger and die failure. For

safety and economic point of view 1-5 mm/s plunger

velocity is preferable.

Effect of multiple passes

AA6061 billet is numerically simulated for 8

repetitive ECAP passes through ECAP die with Φ = 90°

and Ψ = 30° with processing route (BC) at medium shear

friction coefficient (m) and 1 mm/s plunger velocity. It is

Fig. 8 — Pressing force flow direction along the length of billet

Fig. 9 — Pressing force distribution along the length of the billet

at different intersection angle (Φ)

Fig. 10 — Pressing force distribution along the length of billet

based on 4 repetitive ECAP passes

Table 2 — Pressing load (tonnes) obtained at different

intersection angles (i.e., Φ: 90°, 105°, 120°, 135°, 150°) at three

plunger velocities and shear friction (m) = 0.1

Angle (Φ) Ram velocitymm/s Ist IInd IIIrd IVth

90° 1 6.8 7 7.4 7.81

5 6.78 7.1 7.42 7.9

10 6.82 7.23 7.5 8

105° 1 5.1 5.61 6 6.23

5 5.23 5.7 6.21 6.42

10 5.3 5.81 6.4 6.7

120° 1 4.23 5.1 5.61 5.9

5 4.4 5.23 5.7 6

10 4.48 5.4 5.8 6.12

135° 1 2.6 3.1 3.41 3.81

5 2.78 3.23 3.57 4.12

10 3 3.4 3.72 4.2

150° 1 0.9 1.31 1.81 2.41

5 1.2 1.4 2.11 2.6

10 1.21 1.48 2.3 2.62

INDIAN J. ENG. MATER. SCI., APRIL 2018

118

observed that average equivalent strain induced in the

billet increases with each pass until it reached to the

saturation limit (for this case saturation point obtained at

7th pass (strain =7.2). Further increase in ECAP passes

after saturation point, results no further increase in

equivalent strain rather gives incorrect hypothetical

values. This phenomenon is also experimentally

analysed with similar experimental conditions,

experimentally it was noticed that billet after 4th pass

losses its strength and during 6th ECAP pass it get

fractured. It is apparent that FEM and experimental

results are in agreement and both results show an

excellent agreement with Hall-Petch relationship24

Processing routes

ECAP process has four basic routes (Fig. 11); these

routes induced different slip system in the ECAPed

billet. In route ‘A’, sample is processed between two

consecutive passes without rotation. In route ‘BA’ the

sample is rotated 90° in alternate directions between

consecutive passes. In route ‘BC’ the sample is rotated

by 90°

in the same direction (clockwise or

anticlockwise) between each passes. In route ‘C’ sample

is rotated 180° between the consecutive passes in the

same direction. Numerically, no significant effect of

pressing route have been observed, but microstructural

and mechanical testing results indicate that processing

route ‘BC’ helps in getting equiaxed grain with high

angle grain boundaries and also helps in retaining the

ductility.

Experimental results

Commercially available AA6061 aluminum alloy is

used for numerical and experimental analysis. Circular

cross-section billets with 10 mm diameter and 100 mm

length are used for analysis. Raw specimen billet has

been grinded up to 0.5-1.0 mm in diameter to permit

loose fit in the die channel. It facilitates billet to slide

properly into the tightly closed ECAP die. Before

placing the processed billet into die for next pass,

processed billet needs to be finished properly and extra

flash is removed by filing, as shown in (Fig. 12 (a)),

removal of the flash is necessary because this flash gets

inserted into the die’s joint cavities , and tightly sticks

with the die and leads to plunger failure.

Extrusion process has been carried out using a tool

steel plunger guided by a hydraulic press. Tolerance

between plunger and ECAP channel kept very low to

prevent material flow through the joint cavity and cup

formation at top portion of the billet. To avoid cup

formation at top portion of a billet, top and bottom

portion of billet is grinded into a conical shape as shown

in Fig. 12 (b).

Mechanical testing

The engineering stress-strain curves of AA6061

alloy before ECAP and after ECAP are presented

(Fig. 13). Ductility and strength are measured by

Fig. 12 — Billet fabrications for experimental analysis

Fig. 13 — Engineering stress-strain curve for AA6061 aluminium

alloy, extruded Rod, ECAPed billet after 2 passes with route (BC) and

route (A), ECAPed billet after 4 passes with route (BC) and (A)

Fig. 11— ECAP processing routes

DAYAL et al.: EFFECT OF PROCESS PARAMETERS IN EQUAL CHANNEL ANGULAR PRESSING

119

uniaxial tensile test. As illustrated in stress-strain curve,

yield strength increases after two ECAP passes but

ductility decreases. However, alloy processed with

route (BC) shows slight retention in ductility in

comparison with route (A). There is no further

reduction in ductility with addition ECAP passes but

strength and ductility both increases. This satisfies the

effect called “Paradox of Strength and Ductility in SPD

processed material”23,24

. It is also observed that

pressing route also helps in retaining ductility; route

(BC) shows greater retention of ductility than route (A).

Microstructural analysis

Scanning electron microscope is employed to study

the microstructure produced in the processed billet.

For microstructural analysis, a segment is cut from the

processed billet and properly electro-polished.

Microstructural results of unprocessed billet before

ECAP and ECAPed billet after 4 passes with route

‘BC’ are presented in Fig. 14. Results showed that the

Fig. 14 — Microstructural images of (a) unprocessed billet and

(b) processed billet

Fig. 15 — Micro-hardness of ECAPed billet after 4th pass with

processing route ‘BC’

Fig. 16 — Schematic illustration of top middle and bottom surface

for taking micro-hardness measurements after ECAP

Fig. 17 — Surface roughness profile for AA6061 specimen

INDIAN J. ENG. MATER. SCI., APRIL 2018

120

unprocessed billet (extruded rod) contains grain with size

of 10 to 15 µm and after four passes with processing route

‘BC’, the average grain size reduces to 0.4 to 0.5 µm.

Hardness measurements

In this work evolution of hardness measurement in

AA6061 aluminium alloy after 4 repetitive ECAP

passes with route BC are presented. The Vickers

microhardness (Hv) recorded at top, middle and

bottom layers of processed billet along longitudinal

direction (Fig. 16). In every traverse there was

approximately 25 readings taken along 50 mm billet

length for top, middle and bottom surface.

The dash line in each plot represents the

microhardness of the unprocessed billet. The plot

Fig. 18 — Selected process parameters chart

DAYAL et al.: EFFECT OF PROCESS PARAMETERS IN EQUAL CHANNEL ANGULAR PRESSING

121

indicate that hardness increases from an initial value

80 Hv to 135 Hv after four repetitive ECAP passes

with processing route BC, an increase of

approximately 68% (Fig. 15).

Surface profile analysis

The surface finish of a processed billet is important

not only for dimensional correctness, but also for other

physical properties such as corrosion resistance and

fatigue life. In present work, the surface roughness (Ra)

was measured up to 3 mm length for the billet

subjected to different ECAP pass and route (BC). The

surface profiles of unprocessed and processed billet are

shown in Fig.17. Result indicates that with increase in

ECAP passes surface roughness decreases.

Conclusions In this paper ECAP process is modeled and

analyzed for five intersection angles (Φ), four outer

arc angles (Ψ), three plunger velocities, two shear

frictions for producing ultra-fined grained materials.

FE results obtained are experimentally validated.

Each process parameter is tested by making other

process parameters constant and the optimal value at

which better objective parameter are achieved, is

transferred to next set of simulation (Fig.18). The

research outcomes are summarized as follows:

Numerical and experimental analysis performed in

this work indicates that for attempting ECAP on

AA6061 aluminum alloy the best shortlisted

parameters are: Ф = 90°, Ψ = 30°, plunger velocity (v)

= 2 mm/s, friction = medium (m = 0.1) , processing

route = ‘BC’ and number of passes is = 4. With these

parameters the average equivalent strain induced in

the billet is 4.2 at pressing load of 8 tonnes.

Tensile test results indicate that the yield strength

and ultimate tensile strength of ECAPed AA6061

alloy has increased to 470 and 550 MPa respectively

after 4th ECAP pass. It was an increase of 49%. It was

also observed that processing route BC shows better

retention of ductility (15% elongation) as compared to

route A (12.5% elongation) and with microstructural

analysis, it was also observed that route ‘BC’ shows

the presence of equiaxed grain with high angle grain

boundaries.

Micro-hardness measurements and surface profile

analysis indicates that after 4 ECAP passes with

processing route ‘BC’ the hardness (Vickers hardness)

is improved from initial value of 80 Hv to 135 Hv,

and also smoothness in surface profile after 4th pass is

considerably improved.

Acknowledgements

Support for this research from the MHRD,

Government of India under National Mission on

Education through Information and Communication

Technology – Virtual Labs and Department of Science

& Technology (DST), Government of India under

grant No. SR/S3/MERC/104/2008 is deeply

appreciated.

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