This paper describes the superplastic behavior of the commercial 2024 aluminum alloy. The investigated alloy was processed in a 3 mm thick sheet form. The superplastic properties of the alloy were investigated using an uniaxial tensile testing, with a constant strain rate in the range 8 x 10-4 ÷ 1 x 10-2 s-1, at temperatures in the range 450 ÷ 480 oC. The investigations included the determination of the truestrain, the true-stress caracteristics, the elongation to failure, the strain-rate sensitivity exponent m and the aspect of the alloy microstructure. Elongations to failure longer than 200% for the fine grained 2024 aluminum alloy were obtained at 460o and lower strain rates and at 480 oC and a higher strain rate

Dan v

Ionut MARIAN

Luciana RUS

Marius TINTELECAN

Monica SAS-BOCA

The Annals of “Dunarea de Jos” University of Galati Fascicle IX Metallurgy and Materials Science

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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 3 – 2011, ISSN 1453 – 083X 
 
 
 
 
SUPERPLASTIC BEHAVIOR OF 2024 ALUMINUM ALLOY SHEET 
SUBJECTED TO THERMOMECHANICAL PROCESSING  
 
Luciana RUS, Marius TINTELECAN, Monica SAS-BOCA,  
Dan NOVEANU, Ionut MARIAN 
Tehnical University of Cluj-Napoca 
email: Luciana.Rus@ipm.utcluj.ro 
 
ABSTRACT 
 
This paper describes the superplastic behavior of the commercial 2024 
aluminum alloy. The investigated alloy was processed in a 3 mm thick sheet form. 
The superplastic properties of the alloy were investigated using an uniaxial tensile 
testing, with a constant strain rate in the range 8x10-4÷1x10-2 s-1, at temperatures in 
the range 450÷480oC. The investigations included the determination of the true-
strain, the true-stress caracteristics, the elongation to failure, the strain-rate 
sensitivity exponent m and the aspect of the alloy microstructure. Elongations to 
failure longer than 200% for the fine grained 2024 aluminum alloy were obtained at 
4600 and lower strain rates and  at  4800C and a higher strain rate. 
 
KEYWORDS: 2024 aluminum alloy, elongation to failure, strain-rate 
sensitivity exponent, superplastic properties, thermomechanical processing 
 
1. Introduction 
 
The attractive characteristics of aluminum alloys 
have become of great interest both in the academic 
and in the industrial research fields. Their high 
specific strength can be functional for most of the 
structural purposes. The exceptional elongation can be 
reached only by the grain refinement technique: 
powder-metallurgy processing, thermomechanical 
processing, sever plastic deformation [1]. One method 
to produce a very fine grain size usually consists of 
rolling and then carrying out a heat treatment which 
causes a precipitation of another phase or phases in a 
finely divided form, pins at the grain boundaries of 
the simultaneously recrystallizing matrix [2]. This is 
the technique used for producing commercial 
superplastic aluminum alloys. 
However, the basic requirement of fine grain 
size (<10 μm) is a necessary but not always a 
sufficient condition to obtain superplasticity [3]. If the 
fine grain microstructure is not stable at high 
temperatures, superplastic elongation will be 
significantly reduced. 
Superplasticity is defined as the ability of a 
polycrystalline material to exhibit, in a generally 
isotropic manner, very high tensile elongations prior 
to failure [4]. There are two basic requirements for 
achieving superplasticity: first, a very small and stable 
grain size, typically less than ~10 μm; second, a high 
testing temperature, typically ≥ 0,5 Tm (where Tm 
represents the absolute melting temperature of the 
material), so that diffusion-controlled processes are 
reasonably rapid. Superplastic elongations are 
achieved over a limited range of strain rate in the 
superplastic region II (10-5÷10-3 s-1) [5, 6]. 
 
2. Material and experimental procedure 
 
The material studied in this work is an extruded 
bar (22 mm in diameter) of commercial aluminum 
alloy 2024-T3, produced by S.C. ALPROM S.A. 
Slatina, Romania, with the following chemical 
composition (in wt %): 4.955 Cu; 1.322 Mg; 0.437 
Mn; 0.358 Zn; 0.300 Fe; 0.119 Si; 0.017 Cr; 0.005 Sn; 
0.005 Sb; 0.005 Pb; 0.003 Ni; Al-balance. 
Because the ductility of the received 2024 
aluminum alloy samples is small (tensile tested at 
temperatures and strain rates used in this research, the 
tensile elongation is smaller than 108% for round 
samples of 25 mm gauge length and 5 mm diameter) 
[7], the material was processed thermomechanically 
(a method meant to increase the ductility through 
grain refinement). 
The processing route for the 2024 aluminum 
alloy is shown in Figure 1. The specimens from the 
received material (60 mm length and 22 mm 
diameter) were homogenized at 500oC for 8 hours 
before rolling. Hot rolling was carried out 
unidirectionally to obtain a thickness reduction of 
68% and prepare a 7 mm thick strip which was 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 3 – 2011, ISSN 1453 – 083X 
 
 
quenched. Then the samples were cold rolled (a final 
3 mm thickness, with a thickness reduction of 57%), 
fast heated up to 480oC, soaked for 15 minutes and 
quenched in water. The rolled plates were then soaked 
at 350oC for 30 minutes, in order to make the fine-
grained structure stable. 
 
 
 
Fig. 1. Schematic thermomechanical processing route for 2024 aluminum alloy. 
 
The tensile samples were prepared with a 25 
mm gauge length, 6.6 mm width and 3 mm thickness, 
all of them being machined with the tensile stress 
direction parallel to the rolling direction.  
The tensile tests were conducted on the testing 
equipment that was interfaced with a computer to 
provide complete control of the strain rate. After 
heating the specimens in the electrical horizontal 
furnace (Carbolite CTF 12/75/700) to the necessary 
temperatures, for 25÷30 minutes, each specimen was 
submited to a tensile axial load which produced the 
deformation until fracture. The tensile load and the 
elongation were recorded simultaneously. 
Constant strain rate tests were conducted over a 
range of strain rates of 8x10-4 ÷5x10-3 s-1 and test 
temperatures of 450÷480oC. After fracture the 
samples were coaled in water to keep the resulting 
structure during the test. The behavior of the material 
is described by the constitutive equation [3] mKεσ &=  
(1), where σ is the true flow stress, K is a constant, 
ε&  is the true strain rate and m is the strain rate 
sensitivity exponent.  
The logarithms equation (1) to obtain a 
relationship with the general form 
ii mK εσ &lnlnln +=  (2), where i = 1…N 
representing the number of experimental tests. 
The characteristics of the material, m and K, are 
determined by minimizing the function f (m, lnk) 
expressed according to the method of least squares, 
calculating first order derivatives of function f and 
solving the system of equations [7]. The as-received 
and as-processed aluminum samples were cut in 
transversally, polydol mounted, and mechanically 
polished. Keller’s reagent (a composition of 3 ml 
HCl, 2 ml HF, 5 ml HNO3 and 190 ml of distilled 
water) was used to reveal the microstructure of this 
material by the intercept method. 
 
3. Experimental results 
 
3.1. Microstructural characteristics 
Figure 2 shows the as-received alloy and the 
microstructural changes during the thermomechanical 
processing of 2024 aluminum alloy. 
 
 
 
 
 
 
 
 
 
 
                                     a)                                                                                       b)                                           
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 3 – 2011, ISSN 1453 – 083X 
 
 
 
 
 
 
 
 
 
 
 
 
 
                                        c)                                                                         d) 
 
Fig. 2. Optical micrographs of 2024 aluminum alloy: a) as-received; b) homogenizing sample;  
c) hot and cold rolled sample; d) recrystallized sample. 
 
The microstructure of the as-received alloy 
(figure 2.a) shows a structure consisting in 
supersaturated in Cu and Mg solid solution, with large 
grains of 36 μm average size. It is obvious that the 
chemical composition of the alloy is non-
homogeneous and it is necessary to make a 
homogenization thermal treatment before rolling, by 
means of which a recrystallized structure is obtained 
[Figure 2.b)]. During the heating, the soluble 
components (as CuAl2) undergo in solid solution and, 
as a result of diffusion, the equalization of alloying 
compounds concentration take place at the grain 
boundaries. During the cooling that follows, the solid 
solution is decomposed and chemical compounds are 
separated, refined and uniformly distributed in the 
alloy matrix. After the hot rolling with a 68% 
reduction and cold rolling with a 57% reduction, 
elongated grains in the main direction, alongside the 
equiaxed grains were observed [Figure 2.c)]. 
Figure 2.d) shows the microstructure of a 
processed specimen after recrystallization and 
stabilization, consisting of equiaxed recrystallized 
fine grains; at the grain boundaries, the presence of 
insoluble compounds based on Fe (AlFeSi) and Mn 
(Al6Mn) can be identified. The 2024 aluminum alloy 
contains Mn, as the refining element, and Al-Mn pins 
at the grain boundaries at higher temperatures; these 
might have contributed to the microstructural stability 
observed at temperatures below 480oC. The average 
grain size at the center of the processing region was 
found to be about 5÷8 μm. 
 
3.2. Superplastic behavior 
 
Figure 3 is a plot of true stress versus true 
plastic strain for the samples tested at given strain 
rates at 4600C.  
 
 
Fig. 3. True stress versus true strain at 460oC for samples of 2024 aluminum alloy 
thermomechanically processed. 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 3 – 2011, ISSN 1453 – 083X 
 
 
As can be seen in Figure 3, the material revealed 
strain hardening regime at high strain rates (2.5x10-3, 
5x10-3 and 1x10-2 s-1), followed by strain softening. At 
lower strain rates, low hardening or softening is 
observed and a straight zone where the true stress is 
approximately constant. 
The strain rate can have a substantial effect on 
the true stress. Under conditions of deformation at 
high temperatures and low strain rates, the resulted 
curves are almost horizontal (perfectly plastic zone) 
due to diffusion, recovery and recrystallization 
mechanisms, having time to counterbalance strain-
hardening effects [8]. At these strain rates, the 
superplastic behavior of 2024 aluminum alloy can be 
observed.  
At high strain rates, there is no longer sufficient 
time for that to occur and stress increases as a 
function of strain. 
Figure 4 presents ductility values plotted against 
temperatures, obtained for different strain rates (8x10-
4÷1x10-2 s-1). A maximum elongation of 269% was 
measured in the superplastic condition of 4800C and a 
5x10-3 s-1 strain rate, whereas at 4600C, values more 
than 200% were obtained at lower strain rates (8x10-
4÷1.5x10-3 s-1) (218%, 245.5% and 220.5% at 8x10-4 s-
1, 1x10-1 s-1 and 1.5x10-3 s-1 strain rates). 
It may be mentioned that 4600C (at which 
superplastic behavior at the lowest strain rates is 
obtained), is lower than the conventional temperature 
(500oC) for superplasticity in aluminum alloys [1]. 
 
 
Fig. 4. Variation of elongation to failure with temperatures at different strain rates. 
 
The strain rate sensitivity exponent, m, was 
determined by a computer program developed in C++ 
[7], during hot tensile tests, with constant strain rate 
and temperature for a given temperature, a given true 
strain (0.1) and a range of strain rates (8x10-4÷1.5x10-
3 s-1) and flow stress resulted from experimental tests 
(Fig. 5). In general, the higher values of the strain rate 
sensitivity exponent at high temperatures are 
attributed to increased speed of heat of activated 
processes such as dislocation climb and grain 
boundary sliding. 
 
 
Fig. 5. Strain rate sensitivity exponent at different temperatures for 2024 aluminum alloy. 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 3 – 2011, ISSN 1453 – 083X 
 
 
The value of the strain rate sensitivity exponent 
increases with temperature, but at a temperature of 
4600C, where the alloy shows superplastic behavior, a 
value of m greater than at other temperatures is 
obtained, although still to the lower limit of the 
requirement for the micrograin superplasticity 
(m>0.3) (m = 0.3462). 
Figure 6 shows hemispheres, obtained from 
circular membranes of fine grained 2024 aluminum 
alloy, that were free-bulged under low air pressure. 
 
 
 
Fig. 6. Hemispheres obtained through free-bulging under low pressure. 
 
4. Conclusions 
 
- Superplasticity can be achieved in a commercial 
2024 aluminum alloy by termomechanical processing. 
- The maximum elongation of 269% was obtained at 
4800C and a strain rate of 5x10-3 s-1 in the 
thermomechanically processed alloy. 
- Elongations higher than 200% were also achieved in 
tensile testing at 4600C (218%, 245.5% and 220.5% at 
strain rates of 8x10-4s-1, 1x10-3 s-1 and 1.5x10-3 s-1). 
- An average grain size of 5÷8 μm can be achieved by 
control of recrystallization. 
- In 2024 aluminum alloy, the value of the strain rate 
sensitivity exponent to a true strain of 0.1 has the 
highest value - 0.3462 at 4600C and a strain rates 
range of 8x10-4÷1.5x10-3 s-1, where the aluminum 
alloy shows superplastic behavior. 
- By gas pressure blow forming tests, the possibility 
of obtaining hemispheres from 2024 aluminum alloy 
sheets, thermomechanically processed by hot rolling, 
cold rolling and recrystallization was demonstrated. 
 
References 
 
[1]. Charit, I. and Mishra, R.S. - High Strain Rate Superplasticity 
in a Commercial 2024 Al Alloy Via Friction Stir Processing. In: 
Materials Science and Engineering A, volume 359, Issues 1-2, 
2003, pp. 290-296. 
[2]. Perce, R. - Sheet Metal Forming, Institute of Physics 
Publishing, Philadelphia: Hilger, 1991, pp. 122-127. 
[3]. Pilling, J. and Ridley, N. - Superplasticity in Crystalline 
Solids. The Institute of Metals, London, 1989, pp. 3-8. 
[4]. Langdon, T.G., Wadsworth, J. - Superplasticity in Advanced 
Materials, The Japan Society for Research on Superplasticity, 
Osaka (Japan), 1991, p. 847. 
[5]. Langdon, T.G. - Mechanisms of Superplastic Flow. In: 
Superplasticity: 60 Years after Pearson, Edited by Norman Ridley, 
The Institute of Materials, London, 1995, pp. 9-13. 
[6]. Carrino, L., Giuliano, G. and Palmieri, C. - On the 
Optimisation of Superplastic Forming Processes by the Finite-
Element Method. In: Journal of Materials Processing Technology, 
volumes 143-144, 2003, pp. 373-377. 
[7]. Rus A. L. - Cercetari privind comportarea superplastica a 
materialelor metalice (Researches Concerning Superplastic 
Behaviour of Metallic Materials) - Ph.D. Thesis - Technical 
University of Cluj-Napoca, 2006. 
[8]. Guillard, S. - Workpiece Materials Datebase. In: 
Metalworking: Bulk Forming, volume 14 A, ASM International, 
The Materials Information Society, 2005, pp. 651-659. 
 
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Superplastic Behavior of 2024 Aluminum Alloy Sheet Subjected to Thermomechanical Processing

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Superplastic Behavior of 2024 Aluminum Alloy Sheet Subjected to Thermomechanical Processing

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