processing of alluminium and al-mg alloys by...

METAL 2008 13. – 15. 5. 2008, Hradec nad Moravicí

PROCESSING OF ALLUMINIUM AND Al-Mg ALLOYS

BY SEVERE PLASTIC DEFORMATION

Radu COMANECI, Ph.D., Romeu CHELARIU, Ph.D., Luchian ZAHARIA, Ph.D.

Technical University Gh. Asachi Iassy

Faculty of Material Science & Engineering

D. Mangeron, 63, Iasi, 700050, Romania

E-mail: [email protected]

Abstract

Alluminum alloys and Alluminum itself are very popular materials using for production of

ultrafine-grained and nanomaterials by Severe Plastic Deformation (SPD). Equal Channel Angular

Pressing (ECAP) is one of the most promissing process among the different methods of intense plastic

deformation procedures. Understanding both the contact phenomenon at the interface between die

and the workpiece in terms of material flow and phenomena associated with strain and forming load

requirement in ECAP process, becomes important. In this context, the paper presents some

technological and FEA aspects concerning processing of Alluminum and Al-Mg alloys by ECAP.

1. INTRODUCTION

Materials with ultrafine – grains (UFG) or nanometric structures (NS) offer significant advantages

in terms of large strength, hardness and ductility or high strain rate superplasticity. These materials are

generally improved physical and mechanical properties, compared with conventional grain sizes. A

number of processes are used for the refinement of grains. Advanced TMTs not exceed 1 µm in terms

of grain size. For obtaining UFG/NS materials, not conventional techniques are necessary, (Figure 1).

There are two fundamental approaches to

fabricating nanomaterials. The “bottom-up”

approach represents the concept of constructing a

nanomaterial from basic building blocks, such as

atoms or molecules. The second approach, the “top-

down” method, involves restructuring a bulk

material in order to create a nanostructure. The most

important procedure “top-down” remain Severe

Plastic Deformation (SPD), (OLEJNIK, 2005).

Applying a large level of plastic strain to a course

grain precursor we obtaining small grains in metals.

The traditional picture of plastic deformation

behavior and its mechanical response (i.e. hardness

increase due to work hardening) seem to be no

longer valid while reducing grain size far into submicron scale. The most effective technique for

subjecting a bulk material to SPD is Equal Channel Angular Pressing (ECAP).

Bulk nanostructured materials have great impact in biomedical, electronics, military, aerospace,

automotive. Industries and academics have shown great interest in fabrication of NS materials with

high performance to weight ratio such Al, Mg, Al-Mg etc. The purpose of this research is to simulate

the process and to provide technological information to confirm FEA results and finally to improve the

die design for achieving satisfactory UFG/NS materials by ECAP.

2. ECAP PROCESS

Equal channel angular pressing (ECAP), a most promising method among various SPD methods

(SEGAL, 1995), involves large plastic deformation in a deforming workpiece by moving through a die

containing two intersecting channels - usually at an angle of 90° or 120° - of identical cross sections,

see Figure 2. In the vertical channel, the billet moves as a rigid body while all deformation is localised

Conventional grain size

TMTs

Advanced TMTs Superalloys

Syntesis, Powders

PVD, CVD, SPD

Atomic Fe 0,268nm

Amorphous

Nanostructure

10µm 1µm 100nm 10nm 1nm

Polycristals

UFG

Figure 1. Schematic illustration for the

refinement of grain size


in the small area around the channel’s meeting line. The metal is subjected to a simple shear strain

under relative low pressure compared to the traditional extrusion process. The die geometry is

presented in Figure 3.

The total strain ε of workpiece in N passes through the die is given by (FURUKAWA, 2001):

++

+=

22cos

222

3

ψφψ

ψφε ecctg

NN

(1)

where the significance of terms are revealed in Figure 3. It is revealed that the introduction of an outer radius

transition of the extrusion channels decreasing both the strain (according to Eq. (1)) and load level by improving

the material flow. For average strain rate the following relation is derived:

Ψ

+Ψ+

+=w

vecctg

2

22cos

222

3

1 ψφψφε& (2)

where w [mm] is a diameter or width of the workpiece and v [mm/s] is the punch speed.

3. EXPERIMENTAL PROCEDURE

The material used for this study was Al 99.5 and Al-Mg alloys (see Table 1). The specimens were

machined at the10x10x60 [mm]. ECAP testing was carried out with 8.75 mm/s at room temperature

through route BC (the sample was rotated by 90° in the same sense around the longitudinal axis after

each pass). This processing route was adopted because it leads most expeditiously to equiaxed grain

structures consisting of high angle boundaries (ZHU, 2000). In order to reduce the friction, a solid

lubricant was used. ECAP was performed on Al and Al-Mg alloys for eight and four passes

respectively.

Table 1. Chemical composition (weight, %) of Al-Mg alloys

Alloy Chemical composition, %

Mg Si Fe Mn Cr Ni Pb

Al-Mg 1.35 1.35 0.11 0.10 0.008 0.001 0.10 0.05

Al-Mg 3.40 3.40 0.09 0.12 0.015 0.001 0.09 0.09

Al-Mg 5.90 5.91 0.09 0.10 0.024 0.001 0.09 0.04

The die for ECAP was based on a system of two symmetrical half dies, machined from two blocks

of steel, heat treated to achieve a nominal hardness of 60 HRC. Equal cross-section channel of

10x10 mm2 were machined. An outer and inner radius transition (R = 4 and r = 2 mm respectively)

between the two channels intersecting at 90° was machined. Experimental equipment includes a

hydraulic press with maximum load of 750 kN, compression load cell with signal conditioner, pressure

and displacement transducers and data acquisition equipment from National Instruments, USA.

Figure 2. ECAP principle (channel intersecting at 120° Figure 3. The die geometry.

and 90° respectively).


4. FINIT ELEMENT ANALYSIS

4.1. Conditions

The processing takes place at room temperature. The workpiece is condidered a plastic body in

whole deformation process. The punch is moved under constant speed of 8.75mm/s. The workpiece is

dicretized in 8000 tetrahedral elements. The tolerance, positioning of both workpiece and top/bottom

die, convergence criteria, re-meshing conditions, and boundary conditions must be specified before the

execution of the simulation process.

4.2. Friction Summarizing the evolution of forces in ECAP

process (Figure 4), we can observe the decrease of

forces thanks to the continuum reducing of friction in

the first channel, so an experimental relation for

tangential stress can be derived. The tangential stress is:

A

F∆=τ (3)

where ∆F = F1-F2 and A=4a(s2-s1) is the contact area

between die and workpiece in the first channel. So:

)(4 12

21

ssa

FF

−

−=τ (4)

F1, F2, s1, s2 can be determinated from the experimental

curves and a is the dimension of transversal section. If

we consider the constant friction law τ = m⋅k, (FU, 2006) where 0 ≤ m ≤ 1 is a frictional factor and k

= σY/2 (after Tresca), with relation (3) it is possible to determine frictional factor m. For aluminium σY

= 100 MPa (SEGAL, 1995), so for the first pass results m = 0,12…0,13. We take m = 0.12.

4.3. Die design

Three design scenarios are analysed by FEA to reveal the deformation behaviors and their

relationship with the design configuration:

A - there is no arc transition, the channels are intersected at a sharp corner: R, r = 0 mm (90_0_0)

B - there is only one arc (outer) transition at one side of the channels: R = 4 mm; r = 0 mm (90_4_0)

C - there are two arc transitions at both sides of the extrusion channels: R = 4 mm; r = 2 mm (90_4_2)

5. RESULTS AND DISCUSSIONS

Figure 5 (a) shows the deformation status at the last extrusion stage in pass one (simulation). The

experimental samples are also shown (Figure 5, b). There is good match between simulation and

experiment. The design scenarios can be now evaluated.

Forc

e

F1

F2

s1 s2 Stroke

Figure 4. The schematic representation of

forces evolution during ECAP

a. b.

Figure 5. a) The sample after first pass (simulation); b). Alluminium samples – ECAP90°, 8 passes


5.1. Deformation load

For this research, the deformation loads are not so critical because of quite small billet dimensions.

But the matching of simulated and experimental results this is important for the validation of the

model that we are using in this study. Figure 6 shows deformation loads for the three scenarios.

Naturally, the 90_0_0 scenario determines the highest level of deformation load. It gets to a maximum

after about 10-12 mm of travel stroke. The situation correspond the time range where the die channel

fills with material. Further, the deformation load decreases as the entire blank goes through the

deformation firing, marked by the bisecting plan of the two channels of equal sections. The lack of the

fillet radii implies, as expected, a bigger effort. The presence of inner radius transition (r = 2 mm)

between channels not spectacular reduces the load and not decrease the strain (see next).

As is seen, there is a good matching between simulation and experimental, not only for alluminium,

but for Al-Mg alloys too (see next, chapter 5.3). The model had surpassed the second check.

90_0_0

90_4_0

90_4_2

Figure 6. Deformation load, Al 99.5, RT, ECAP90°– simulation (left); experimental (right)


5.2. Strain distribution

To evaluate the strain distribution, a few tracking points are defined in longitudinal section of the

workpiece, corresponding to the most sensible deformation area, that is mean in the bisect region of

the two channels. From the Figure 7 it can be found that the effective strains for the three points in the

first pass of ECAP simulation (for Al 99.5) have different evolution. For 90_0_0 case, it is shown that

P3 has a maximum strain value. This behavior is due to the friction and especially to the difficulty of

the material to fill the sharp corner of the die. For 90_4_0 and 90_4_2 scenarios, P1 become the point

of maximum effective strain. But in these cases the strain distribution is keeping balance.

For all scenarios it is shown that maximum strain is not achieved for all the three points

simultaneous. In addition, the maximum strains are reached after the material had passed out the

bisecting plan of the channels. This is in concordance with stress distribution, Figure 8. Furthermore

from practical experience we know that the samples fracture occur at the contact with inner channels.

The nanostructuring is related to accumulated strain. This is used as a criteria for evaluation of the

process in this study. From this point of view, the scenarios 90_4_0 and 90_4_2 seems to be similarly,

all the more as forces are closes. The FEA of strain distribution tell us that for obtaining a

homogenous UFG/NS it is necessary to rotate the workpiece with 180 degrees along the velocity axis.

A combined BC and C routes in ECAP process could be effective.

90_0_0

90_4_0


90_4_2

Figure 7. The strain distribution (FEA) – ECAP90°, 90_0_0; 90_4_0; 90_4_2 scenarios

Figure 8. The stress distribution (FEA) – ECAP90°, 90_4_0; 90_4_2 scenarios


5.3. Model validation. Processing Al-Mg alloys by ECAP. From FEA of strain distribution and deformation load, the optimum die design was obtained. For

validation of model simulation, testing samples of Al-Mg alloys was processing by ECAP90°. Figure 9

and 10 shows the predicted and experimental loads respectively, for the first pass. As is seen, there is a

good matching between simulation level and experimental values.

ECAP, 90_4_2, Al-Mg 1.35 ECAP, 90_4_2, Al-Mg 3.40

ECAP, 90_4_2, Al-Mg 5.90

Figure 9. Predicted loads – ECAP, 90_4_2, Al-Mg alloys, first pass

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70

Deplasarea [mm]

Fort

a [kN

]

Al-Mg 1.35 (1)

Al-Mg 3.40 (1)

Al-Mg 5.91 (1)

ECAP90

Figure 9. Experimental loads – ECAP, 90_4_2, Al-Mg alloys, first pass


As is expected, magnesium leads to increasing of load level over 50kN. The testing lot was

deformed for a complete cycle of four passes, route BC, for further investigations on the efficienty of

the structure refinement by ECAP. However, no expected strain hardening have been reported out of

first or second pass of ECAP processing.

6. CONCLUSIONS Three dimensional ECAP simulation of the rectangular-shape sample was carried out to investigate

the effect of die design on the behavior of Al and Al-Mg alloys processed by ECAP. It was found that

the deformation load requirements and the strain distribution in the deformed material are clearly

sensitive to the die geometry. The estimated data through FEA were experimentally confirmed,

sustaining the validity of the performed analysis. The present study of the deformation behavior in

ECAP process provides basic and useful information for optimizing die design and process

determination.

REFERENCES FU, M. W., YONG, M. S., PEI, Q. and HNG, H. H., 2006, Deformation Behavior Study of Multi-

Pass ECAE Process for Fabrication of Ultrafine or Nanostructured Bulk Materials, Materials and

Manufacturing Processes, 21, 501–506.

FURUKAWA, M.; NEMOTO, M. & HORITA, Z., 2001, Processing of Metals by Equal-Channel

Angular Pressing, Journal of Mat. Science, vol. 36, 2001, pp. 2835-2843.

OLEJNIK L., ROSOCHOWSKI A., 2005, Methods of fabricating metals for nano-technology,

Bulletin of the Polish Academy of Sciences, Technical sciences, 53, no. 4.

SEGAL, V.M.,1995, Materials processing by simple shear, Mat. Sci. Eng, A 197, 1995, pp. 157-164.

ZHU, Y.T., LOWE, T.C., 2000, Observations and issues on mechanism of grain refinement during

ECAP process, Mat. Sci. and Eng, A291 pp. 46-53.

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