Effect of Hot Accumulative Roll Bonding Process on the Mechanical Properties of AA5083

doi:10.4236/ojmetal.2011.11002

Paper Menu >>

Journal Menu >>

Open Journal of Metal, 2011, 1, 12-15

doi:10.4236/ojmetal.2011.11002 Published Online September 2011 (http://www.SciRP.org/journal/ojmetal)

Effect of Hot Accumulative Roll Bonding Process on the

Mechanical Properties of AA5083

Hassan Sheikh, Ebrahim Paimozd

Materials Engineering Department, Malek Ashtar University of Technology, Shahin Shahr, Iran

E-mail: [email protected]

Received August 10, 2011; revised September 16, 2011; accepted September 25, 2011

Abstract

In this work, accumulative roll bonding (ARB) process as a severe plastic deformation was applied on an

AA5083 sheet up to 6 cycles at the temperature of 300◦C and at the strain rate of 50 s−1. The results of tensile

tests show that the values of the yield stress and the ultimate tensile strength don’t change considerably after

4 cycles. Also, transmission electron microscopy (TEM) micrograph confirmed that the microstructure has

fine (sub) grains with the size of 200 nm - 400 nm.

Keywords: Grain refinement; Aaccumulative Roll Bonding; Mechanical Characterization; Aluminum Alloys

1. Introduction

Reducing the grain size of polycrystalline metallic mate-

rials is an economic way to improve the mechanical pro-

perties such as strength, and toughness [1,2]. The mi-

crostructure with stable fine grains is crucial in the su-

perplastic deformation of materials [3]. Since it is prac-

tically difficult to reduce the grain size of many metallic

materials such as aluminum alloys by a conventional co-

ld working and recrystallization process, several new

methods such as severe plastic deformation (SPD) are

developed to manufacture ultra-fine grain (UFG) materi-

als [2]. SPD processing techniques produce relatively

small quantities of materials, are very difficult to scale

up, and are unlikely to be able to produce at low cost.

The principle of a SPD process includes increasing the

dislocation density and transforming low angle grain

boundaries (LAGBs) into high angle grain boundaries

(HAGBs) [4].The net shape of the sample during proc-

essing is approximately constant, so that there is no

geometric limitation on the applied strain [5].

Among SPD processes, accumulative roll bonding

(ARB) can be performed by a conventional rolling mill

without any special die. Also, the productivity of this

process is relatively high because this process implies the

potential of industrial up-scaling to a continuous produc-

tion of UFGed metallic sheets or plates [6]. An ARB

process is practically limited by technological constraints

such as the occurrence of edge cracking. In this process,

the thickness of the sheet varies between fixed limits,

and very high strains can be accumulated in the material

by repeating the procedure; as a result, a significant

structural refinement can be achieved [6-8]. Saito [6]

used the ARB process at 200◦C to reach a microstructure

with ultra-fine grains for AA5083. Although this method

arised 551 MPa for the ultimate tensile strength, it con-

siderably reduced the value of ductility to 6%. From his

TEM results, it can be found that there is a high density

of dislocations in grains. Indeed, the true effect of the

grain size on the mechanical properties of this UFG ma-

terial was obscured by the presence of a high density of

dislocations. By applying a heat treatment at the tem-

perature of 180 ◦C for a long time, the density of disloca-

tions decreases considerably [9]. Because of high

strength and low ductility of this alloy at low tempera-

tures, applying the ARB at high strain rates and low

temperatures has many difficulties such as edge cracking

and the necessity of a high power rolling mill. For thin

sheets with the thickness of 1 mm, a high strain can be

applied by using the hot ARB process at a high strain

rate to reach a fine microstructure. This method can be

helpful to scale up the materials with a desirable micro-

structure. Therefore, studying the effect of the hot ARB

on the microstructure and mechanical properties of

AA5083 is the prime objective of the present work.

2. Material and Experimental Procedure

The as-received material used in this study was an

AA5083 sheet of the chemical composition presented in

H. SHEIKH ET AL.

Table 1. Figure 1 shows the unique grain color map

recorded on the RD-ND cross-section of the initial sheet

with a mean grain size of 13µm. It can be observed that

the material exhibits an equi-axed polygonal type grain

structure. To achieve a good bonding, the surfaces of the

two sheets with dimensions of 100mm×30mm×1mm

were degreased (in acetone) and wire brushed (by a

stainless steel brush with wires of 0.4mm in diameter).

Plane strain rolling was conducted along the longest di-

mension. After surface treatment, the two sheets were

stacked to make a 2-mm thick specimen and then roll

bonded by 50% reduction in thickness (this corresponds

to a von Misses equivalent strain of 0.8 per pass). After-

wards, the rolled sheet was cut into two pieces along the

transverse direction which produced two sheets of the

length approximately corresponding to the initial length

before rolling. This procedure was repeated up to 6 cy-

cles so that the total equivalent strain of εvM = 4.8 was

achieved. Between the rolling passes, the sheets were

preheated to the temperature of 300◦C for 5 min and the

rolling was performed without any lubrication. The roll

diameter was 150mm which produced the mean strain

rate of 50 s−1 during the rolling. The most severe prob-

lem in the roll bonding step was small edge cracks. In

this study, in order to avoid the propagation of small

edge cracks in the subsequent cycles, the edges of the

roll-bonded sheets were trimmed off.

To appear the interfaces located among the layers, the

RD-ND section of the sample processed by 6 cycles was

heated at the temperature of 125◦C for 12 h, and then it

was etched in a solution of 10% phosphoric acid in

ethanol. By doing so, Mg2Al3 precipitates locate at high

energy places such as interfaces and the etchant can cor-

rode the precipices; as a result, the interfaces can appear

sharply.

Transmission electron microscopy (TEM) micrograph

was also obtained from the sample processed by 6 cycles

of the ARB process. For doing so, thin foil was prepared

by twin-jet polishing from the mid-thickness plane of the

sample.

The Vickers macrohardness test was taken on the sur-

face using 30kg load. The mechanical properties of the

starting material and the ARBed samples were measured

by tensile tests at room temperature and the strain rate of

0.002 s−1. The test specimens were prepared with the

tensile axis parallel to the rolling direction. The dimen-

sions of the specimens were chosen according to the

ASTM-E8 standard.

Table 1. Chemical composition of the material used in this

work (wt%).

Mg Mn Fe Si Cr Cu

4.5 0.71 0.33 0.19 0.058 0.037

3. Results and Discussion

Figure 2 shows the optical micrograph observed in the

RD-ND section of the sample produced by 6 ARB cycles;

the bonded interfaces must be observed across the thick-

ness of the sample. The arrow in Figure 2 shows the

center of thickness. No part of interfaces was not seen

without applying the heat treatment mentioned for etch-

ing. This indicates a good bonding with no delamination

between sheets at each cycle under the present ARB

conditions.

A TEM micrograph related to mid-thickness plane (RD-

TD) of 6 ARB cycles has been represented in Figure 3.

The TEM image shows a feature of (sub) grains with the

Figure 1. Microstructure of the starting material 3.

Figure 2. Optical micrograph of the longitudinal cross-se-

ction of AA5083 sheet after 6 ARB cycles.

14 H. SHEIKH ET AL.

size of 200 nm - 400 nm in which the density of disloca-

tions is extremely low. Because of small grain sizes of

severe plastic deformed materials, many previous inves-

tigations have relied on the TEM to study the grain

structures after processing. The selected area diffraction

(SAD) pattern consists of rings of diffraction spots sug-

gesting that the most grain boundaries are separated by

boundaries with high angles of misorientation. It must be

considered that many of the grain sizes quoted from TEM

analysis are underestimates as they include sub-grains [6].

Figure 4 shows the stress-strain curves and the corre-

sponding tensile properties of the ARBed processed

AA5083 sheets. Ultimate tensile strength increases up to

325MPa after two cycles, which is more than 1.25 times

as high in comparison to the starting material. The tensile

and the yield strengths (according to elongation of 0.2%)

increase during the following cycles, but with very

smaller increments for the last cycles. The yield strength

reaches 265MPa after 6 cycles, which is almost 1.7 times

as high as the annealed material. The shapes of the

stress-strain curves are similar in all samples and the

ARBed specimens reach their peak of strength followed

by a small value of necking strain. After 6 ARB cycles,

the elongation decreases to 11.4%, which means that the

material still shows a sufficient ductility. The combina-

tion of elongation, yield and ultimate tensile strength

points out that this material has desirable toughness for

special applications.

The variation of hardness shown in Figure 5 confirms

the results of the tensile tests. Rapid increase in the

hardness and the strength at the strain of up to 1.6 is

mainly due to the work hardening caused by an increase

in dislocation density and formation of sub-grains [10].

Figure 3. TEM micrograph of the sample processed by 6 cy

cles of ARB process.

Figure 4. Nominal stress-strain curves of the AA5083 sheets

processed at different ARB cycles.

Figure 5. Macrohardness variation of AA5083 with increa-

sing the number of ARB process.

During the following cycles, the dislocation density is

almost constant. The occurrence of static and dynamic

recoveries (as two flow softening mechanisms during the

hot rolling and reheating at the temperature of 300◦C)

causes that the generation and annihilation of disloca-

tions reach an equilibrium state [10]. The increase in

strength after cycle 2 can be attributed to the decrease of

grain size and an increase in the volume fraction of

HAGBs and misorientation. As a result, the strength of

the ARBed samples can be considered as the sum of dis-

location strengthening from LAGBs and grain size

strengthening from HAGBs.

By using the below empirical relationship, the cell size

which is the same value of the (sub) grain size can be

estimated [11]:







 (1)

H. SHEIKH ET AL.

[1] P. B. Prangnell, J. R. Bowen and A. Gholinia, “The For-

mation of Submicron and Nanocrystalline Grain Structure

by Severe Deformation,” Proceedings of the 22nd Risø´

International Symposium on Materials Science, Roskilde,

2001, pp. 105-126.

Table 2. The values used for the present estimation and the

cell size predicted from Equations 1and 2.

G(GPa) (MPa)σ (MPa)τ (nm)δ K

27 260 104 5 360

[2] Y. S. Kim, S. H. Kang and D. H. Shin, “Effect of Rolling

Direction on the Microstructure and Mechanical Proper-

ties of Accumulative Roll-Bonding (ARB) Processed

Commercially Pure 1050 Aluminum Alloy,” Materials

Science Forum, Vol. 503-504, 2006, pp. 681-686.

doi:10.4028/www.scientific.net/MSF.503-504.681

0.9lnG









(2)

Where δ is the cell size, G is the shear modulus, b is the

burgers vector, τ is the resolved shear stress, and τ0 is the

friction stress which is usually neglected since 0



.

The value of τ is calculated by the following equation [11]:

[3] K. F. Zhang and H. H. Yan, “Deformation Behavior of

Fine-Grained 5083 Al Alloy at Elevated Temperature,”

Transactions of Nonferrous Metals Society of China,Vol.

19, 2009, pp. 307-311.

doi:10.1016/S1003-6326(10)60060-X



(3)





[4] J. Lee and H. Seok, “Microstructural Evolutions of the Al

strip Prepared by Cold Rolling and Continuous Equal

Channel Angular Pressing,” Acta Materialia, Vol. 50,

2002, pp. 4005-4019.

doi:10.1016/S1359-6454(02)00200-8

where M and σ are the Taylor factor ( the value of 2.5 is

used for the present work) and the flow stress, respec-

tively. The (sub) grain size is predicted from equation. 1

and 2. The values used for the present estimation are

listed in Table 2. According to these values for the sam-

ple processed by 6 cycles of the hot ARB process; the

predicted grain size is 360 nm which has a good agree-

ment with the TEM result. Also, it is possible to ap-

proximate the dislocation density retaining in the grain

interior. The flow stress is related to the dislocation den-

sity (ρ) by the following equation [11]:

[5] R. K. Islamgaliev, N. F. Yunusova, R. Z. Valiev, N. K.

Tsenev, V. N. Perevezentsev and T. G. Langdon, “Char-

acteristics of Superplasticity in an Ultrafine-Grained

Aluminum Alloy Processed by ECA Pressing,” Scripta

Materialia, Vol. 49, 2003, pp. 467-472.

doi:10.1016/S1359-6462(03)00291-4

[6] Y. Saito, H. Utsunomiya, N. Tsuji and T. Sakai, “Novel

Ultra-High Straining Process for Bulk Materials Devel-

opment of the Accumulative Roll-Bonding (ARB) Proc-

ess,” Acta Materialia, Vol. 47, 1999, pp. 579-583.

doi:10.1016/S1359-6454(98)00365-6

0Gb







 (4)

where 0



is strain insensitive and can be neglected as

described previously. The dislocation densities for the

present work and Saito’s study were estimated 0.72 ×

1015 and 3.23 × 1015 m

–2, respectively. These results

show that a large part of the obtained yield stress for

Saito’s work can be attributed to the high density of dis-

locations.

[7] N

. Tsuji, Y. Saito, S. H. Lee and Y. Minamino, “ARB

(Accumulative Roll-Bonding) and Other New Techniques

to Produce Bulk Ultrafine Grained Materials,” Advanced

Engineering Materials, Vol. 5, 2003, pp. 338-344.

doi:10.1002/adem.200310077

[8] X. Huang, N. Tsuji, N. Hansen and Y. Minamino, “Mi

Crostructural Evolution During Accumulative Roll-Bond-

ing of Commercial Purity Aluminum,” Materials Science

and Engineering: A, Vol. 340, 2003, pp. 265-271.

doi:10.1016/S0921-5093(02)00182-X

4. Conclusions

[9] K. T. Park, J. H. Park, Y. S. Lee and W. J. Nam, “Com-

parison of Compressive Deformation of Ultrafine-Grain-

ed 5083 Al Alloy at 77 and 298 K,” Metallurgical and

Materials Transactions: A, Vol. 36, 2005, pp. 1365-1368.

doi: 10.1007/s11661-005-0227-8

In this study, a hot ARB process was performed on an

AA5083 sheet up to 6 cycles and equivalent strain of 4.8

was successfully achieved without severe edge cracking.

The results confirmed that the process is effective for the

grain refinement of AA5083. [10] K. T. Park, H. J. Kwon and W. J. Kim, “Microstructural

Characteristics and Thermal Stability of Ultrafine

Grained 6061 Al Alloy Fabricated by Accumulative Roll

Bonding Process,” Materials Science and Engineering: A,

Vol. 316, 2001, pp. 145-152.

doi:10.1016/S0921-5093(01)01261-8

The mechanical properties revealed that the strength of

the sheet considerably increased by the first two ARB

cycles which is attributed to work hardening caused by

an increase in dislocation density and sub-grains. After 6

cycles, (sub) grains with a low internal dislocation den-

sity have a size with the range of 200 - 400 nm.

[11] K. T. Park, J. H. Park, Y. S. Lee and W. J. Nam, “Micro-

structures Developed by Compressive Deformation of

Coarse Grained and Ultrafine Grained 5083 Al Alloys at

77 K and 298 K,” Materials Science and Engineering: A,

Vol. 408, 2005, pp. 102-109.

doi:10.1016/j.msea.2005.07.040

5. References