Journal of Minera ls & Materials Ch ar ac te ri zatio n & Engineeri ng, Vol. 9, No.8, pp.763-773, 2010 Printed in the USA. All rights reserved
Effects of Heat Treatment on Strength and Ductility of Rolled and
Forged Aluminum 6063 Alloy
S. O. Adeosun1*, S.A. Balogun1, O.I. Sekunowo1, M.A.Usman2
1Department of Metallurgical and Materials Engineering, 2Department of Chemical
Engineering University of Lagos, Akoka -Yaba, Lagos, Nigeria
*Corresponding Author:
This work examines the effect of heat treatment on tensile strength and ductile
responses of rolled and forged AA6063 aluminum alloy. Some cast samples were
rolled while some were forged at ambient temperature (320C). The deformed
samples were subjected to heat treatment processes. The tensile strengths of rolled
(212 MPa) and forged (127 MPa) samples are enhanced at ambient temperature
but with poor elongation responses. A combination of improved strength and
elongation (127 MPa, 24%) can be obtained in rolled sample when solution heat
treatment (SHT) is applied after deformation and cooling in water. The forged
sample when homogenized, solution treated and water quenched has elongation of
about 24% with improved strength of 137 MPa. These results were obtained
because of the development of very fine AlFeSi texture in the matrix and along the
grain boundaries.
Key words: aluminum alloy, forging, rolling, heat treatment, strength, ductility.
The solution heat treatment (SHT) of heat treatable AA6063 aluminum alloy consists
of solution treatment, quenching, and aging. During casting, however, the slow
cooling rate of the alloy allows for the strengthening Mg2Si phase to precipitate out of
solution and grow into large incoherent phases within the matrix. In the as-cast
structure, the large, incoherent nature of the Mg2Si phase does little to increase the
strength of the alloy .To obtain finely dispersed Mg2Si, a solution heat treatment
needs be conducted on the alloy. The process raises its temperature to near the
melting point of the alloy but not high enough to initiate partial melting. At the
764 S. O. Adeosun, S.A. Balogun, O.I. Sekunowo, M.A.Usman Vol.9, No.8
solution temperature (~540ºC for Al-Si-Mg alloys), the Mg2Si dissolves back into
solution with the aluminum matrix. During quenching the alloy quickly cools from
solution, locking the strengthening elements within the aluminum matrix. The alloy
after quenching becomes meta-stable, where some silicon and magnesium crystals
attempt to precipitate out as Mg2Si but cannot, since at room temperature, there is not
enough energy for precipitation to occur. In aging, the part is raised to a high enough
temperature to initiate precipitation of the Mg2Si. During aging, the Mg2Si
precipitates out as finely dispersed phases which anchor the matrix and impede
deformation resulting in a significant increase in strength [1].
Totik et al investigated (2003) the effect of homogenization treatment conditions on
cold deformation of aluminum alloys AA 2014 and AA 6063 by the torsion test. The
alloys were homogenized at 320, 370, 420 and 470oC for 8 hours [2]. Torsion tests
were performed on as-cast and homogenized alloys at room temperature and at a
strain rate of 2×102 s1. It was observed that the homogenization treatment affected
the microstructure of the specimen tested. The secondary phases, which were large
and distributed on grain boundaries before the homogenization treatment were small
and spread through the grains after it. Compared to as-cast, the homogenization
treatment improved the degree of cold deformation, depending on the kind of
aluminum alloy and homogenization temperature.
It has been found that small amount of manganese in AA6063 aluminum alloy
significantly helps in homogenizing and transforming the plate-like β-AlFeSi phase to
more rounded α-AlFeSi phase, which increases the ductility of the material [3, 4].
Aluminum 6063 alloy was processed by upset forging and cold rolling at ambient
temperatures and the tensile, ductile and hardness (HRN) properties of the samples
were studied [5]. The UTS and HRN increased as the range of reduction from
processing increases from 0 to 50 percent at room temperature. However, the ductility
decreases correspondingly, thus indicating a low strain-hardening exponent.
The wear behavior of age-hardened AA6063 aluminum alloy under dry sliding
conditions shows that artificial aging produces the harder structure that is attributable
to acceleration of precipitation of Mg2Si and other phases such as CuAl2 and AlFeSi.
The microstructure was also altered by the aging treatment, as it was observed that the
precipitates in the structure dispersed finely with increasing aging time [6]. The effect
of precipitation on the tensile strength, yield strength, hardness, ductility and number
of cycles required to fail the alloy at constant stress was investigated [7]. The
variation in time and temperature improved the mechanical properties of the AA6063
aluminum alloy, whereas the ductility decreased. This study investigates the effect of
homogenization, solution treatment, annealing, normalizing, quenching in water and
aging on the mechanical properties of forged and rolled AA6063 aluminum alloy.
Vol.9, No.8 Effects of Heat Treatment on Strength and Ductility 765
The ingot of AA6063 aluminum alloy used for this study was obtained from
Aluminum Rolling Mills (ARM), Ota, Ogun State Nigeria and its chemical
composition is given in Table 1.
Table 1. Chemical Composition of AA6063 aluminum alloy.
Element Mg Si Mn Cu Zn Ti Fe Na B Sn Al
0.482 0.432 0.029 0.003 0.017 0.007 0.209 0.0003 0.001 0.0012 98.82
The AA6063 aluminum alloy ingot was melted in a pit furnace and cast into 5.96 mm
diameter x21 mm cylindrical samples using a metal mould. Fourteen cast samples
were produced; seven were up-set forged in five passes using a pneumatic hammer of
56.31 kPa and driven by a 4.5-HP electric motor. The rest were rolled in fifteen passes
using a two-high mill. One sample from each group was labeled as-cast without
thermo mechanical processing. Forging and rolling were carried out on the samples to
60% cumulative thickness reduction at ambient temperature (320C). Six of the forged
samples were homogenized at 5150C for 22 hours and five from each group were
solution-treated at 5250C for 12 hours. After solutionizing the forged samples, one
sample was normalized, one was quenched in water, one was annealed, and one was
quenched in water and aged at 2100C. The same treatment was given to solutionized
rolled samples. Tensile test was carried out on heat treated forged and rolled samples
and the control samples. The test samples were prepared in accordance with BS
standard for non-proportional rectangular test pieces having corner radius of 1 mm
and tensile tested on a table top Instron Electromechanical testing system, 3369 model
at strain rate of 10 mm/min. The shape and dimensions of the test samples are shown
in Figure 1.
Figure 1: Tensile test specimen.
The microstructural states of control and coldworked specimens were investigated
using standard metallographic procedures. Each sample was ground and polished
before being etched in 2% Sodium Hydroxide (NaOH) solution for 20 seconds.
Photographs of these structures were obtained using the digital metallurgical
microscope at a magnification of X100 with α-aluminum phase as white, Mg2Si
crystal as dark and the AlFeSi crystal as brown.
766 S. O. Adeosun, S.A. Balogun, O.I. Sekunowo, M.A.Usman Vol.9, No.8
The sample rolled at ambient temperature to a cumulative thickness reduction of 60%
has superior UTS (212 MPa) to the forged sample (127 MPa) similarly deformed (see
Figures 2-5 and Table 2).
Table 2. Ultimate Tensile Strength (UTS) and Elongation Characteristics of processed
AA6063 Aluminum alloy.
Thermal processing UTS (MPa) Elongation
ForgedRolled Forged Rolled
Deformed at ambient temperature. 126.98 212.4 2.49 6.1
Deformed and Homogenized. 99.3 104.5 13 14
Deformed and homogenized, SHT and normalized. 73.7 126.7 7.1 19
Deformed, homogenized, SHT and annealed. 102.9 99.7 18 12
Deformed, homogenized, SHT and water quenched 131.8 127.1 25 24
Deformed, homogenized, SHT,water quenched and
107.76 93.8 17.2 9.8
As-cast 114.8 114.8 10.6 10.6
However, both samples show poor elongation responses as the values are far less
(6.1% and 2.5% respectively) than that obtained in the as-cast sample (10.6%). In the
as-rolled sample, the Mg2Si crystals are found at grain boundaries, matrix surface and
in the rolling direction, with fine distribution, which serves as obstacles to the motion
of dislocation during plastic deformation.
Vol.9, No.8 Effects of Heat Treatment on Strength and Ductility 767
768 S. O. Adeosun, S.A. Balogun, O.I. Sekunowo, M.A.Usman Vol.9, No.8
Other intermetallic crystals are fine and of smaller volume fraction (see Plate 2a).
These features promote strength while sacrificing ductility. In the as-forged sample,
Mg2Si crystals which are predominantly needle-like in shape are scattered over the
matrix surface. The volume of the AlFeSi phase precipitated exceeds that of Mg2Si
that has some of its crystals diffused into the matrix consequent upon the applied
forging load (see Plate 1a).
Homogenizing the forged sample at 515oC and even normalizing after treatment at
525oC does not promote the precipitation of strength enhancing phase over that
observed in the as-cast structure. The percent elongation at these heat treatment
procedures however, increases significantly over the as-cast. There is a decline in
strength of forged sample whereas the property improved in the rolled sample. The
volume of Mg2Si and other intermetallics precipitated decreased with Mg2Si crystal
distribution in similar pattern to solution treated sample (see Plate 1b). During
normalization of homogenized rolled sample, Mg2Si and other intermetallic crystals
are precipitated over the homogenized sample. This gave rise to strength and
Vol.9, No.8 Effects of Heat Treatment on Strength and Ductility 769
elongation increases (see Plate 1c). Normalizing the homogenized forged sample did
not improve the precipitation and distribution of Mg2Si. There was incoherent
clustering of Mg2Si crystals in the matrix resulting in a further reduction in strength
(see Plate 1c).
Subjecting deformed samples after homogenizing at 515oC to water quenching at
525oC resulted in strength evolution similar to as-cast but with elongation that is twice
(25%) as much as the as-cast (11%). A striking observation here is that the strength
and elongation of forged and rolled samples are quite similar. If the sample is
quenched in water instead of air cooling, the volume of precipitated Mg2Si in rolled
sample does not significantly increase. Rather, there is an increase in volume fraction
of other intermetallics giving rise to enhanced ductility (see Plate 1d).There is an
increase in Mg2Si precipitated in the matrix at the grain boundaries and more of the
other intermetallics are produced to enhance elongation response of the sample (see
Plate 1b).
Annealing the sample instead of quenching in water does not increase the volume of
precipitated Mg2Si and other intermetallics (see Plate 1d).
a b c
770 S. O. Adeosun, S.A. Balogun, O.I. Sekunowo, M.A.Usman Vol.9, No.8
d e f
Plate 1. Structural morphology of rolled samples (a) deformed at ambient (b)
deformed and homogenized at 5150C (c) deformed, homogenized at 5150C and
normalized at 5250C (d) deformed, homogenized at 5150C and water quenched at
5250C (e) deformed, homogenized at 5150C and annealed at 5250C (f) deformed,
homogenized at 5150C, quenched and aged at 2100C (g) as-cast.
Vol.9, No.8 Effects of Heat Treatment on Strength and Ductility 771
a b c
d e f
Plate 2. Structural morphology of forged samples (a) deformed at ambient (b)
deformed and homogenized at 5150C (c) deformed, homogenized at 5150C and
normalized at 5250C (d) deformed, homogenized at 5150C and water quenched at
5250C (e) deformed, homogenized at 5150C and annealed at 5250C (f) deformed,
homogenized at 5150C, quenched and aged at 2100C ( g) as-cast.
Annealing forged sample after homogenization does not produce significant
improvement in strength (103 MPa) over the homogenized sample (99MPa). But the
elongation improves (18% as against 13%) due to the increased precipitation of other
intermetallics (see Plate 2e). Annealing after SHT at 5250C, water quenching and
ageing at 2100C and homogenization treatments did not enhance the tensile strength
of both forged and rolled samples. However ductility improved. Ageing the quench
sample at 2100C does not improve the precipitation of the strengthening phase (see
Plate 2f). At 60% cumulative reduction and in the range of heat treatment considered,
rolled samples allowed for the evolution of strengthening precipitates, permit increase
772 S. O. Adeosun, S.A. Balogun, O.I. Sekunowo, M.A.Usman Vol.9, No.8
in crystal–size and generate mobile dislocations for superior ductility over the as-cast
structure. The precipitation of other intermetallics is similarly affected (see Plate 1f).
The as-cast structure contains fine equiaxed crystals of the phases in roughly equal
amounts in the matrix (see Plate 2g). The Mg2Si crystals are visible at the grain
The peak ductility of forged (25%) and rolled (24%) samples obtained after
deformation, homogenization, solution heat treatment and water quenched processes
in this study are superior to the results obtained for the same alloy by the works of
Balogun et al (2007), Siddiqui et al (2000) and Al-Marahleh (2006) [5,7,8]. The
reason for this improve elongation can be attributed to the transformation of plate-
like β-AlFeSi phase to round α- AlFeSi phase which increases the ductility of the
material [3,4]. This was ensured in the study of Zajai et al (1993) [3] by the addition
of small amount of manganese. However, there is similarity in the elongation of cast
sample homogenized at 580oC for 6 hours and followed by extrusion (28%) in the
study by Al-Marahleh (2006) [8] with the results obtained in this study.
The study has shown that the ultimate tensile strength (UTS) and ductility of forged
aluminum 6063 alloy can be improved after deformation processing by quenching in
water after solution treatment at 525oC. The elongation of 25% at ultimate strength of
132 MPa is superior to the as-forged condition (2.5%). The results are consequent
upon the increase in fineness of the texture of AlFeSi and its precipitation at the grain
boundaries. The incoherent precipitates of Mg2Si enhance the ductility of the sample
as dislocation movements are permitted.
In the rolled alloy, superior strength and high ductility are conferred after rolling by
solution treatment followed by normalizing or quenching in water. However, if it is
desired to produce a material possessing high strength with little ductility, the alloy
may be cold rolled without any other treatments. If it is desired to have texture
changes in the sample microstructure, solution treatment of rolled sample followed by
normalizing can be applied to improve the as-cast texture, which shows fine crystals
of well distributed phases. Deformation (rolling) combined with thermal treatment
such as homogenization, SHT quenching in water and or ageing will produce large
volume of AlFeSi crystals in α-Aluminum matrix for enhanced ductility.
Vol.9, No.8 Effects of Heat Treatment on Strength and Ductility 773
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