T. RADETI] et al.: INFLUENCE OF COLD-ROLLING DEFORMATION AND ANNEALING TEMPERATURE ...
85–90
INFLUENCE OF COLD-ROLLING DEFORMATION AND
ANNEALING TEMPERATURE ON THE GRAIN GROWTH
OF Al-4%Mg-Mn ALLOY
VPLIV DEFORMACIJE MED HLADNIM V ALJANJEM IN
TEMPERATURO @ARENJA NA RAST ZRN ZLITINE Al-4%Mg-Mn
Tamara Radeti}
*
, Miljana Popovi}, Endre Romhanji
Faculty of Technology & Metallurgy, University of Belgrade, Karnegijeva 4, 11 120 Belgrade, Serbia
Prejem rokopisa – received: 2019-06-21; sprejem za objavo – accepted for publication: 2019-08-08
doi:10.17222/mit.2019.134
The propensity of an alloy toward an abnormal grain growth (AGG) severely limits the annealing temperature range, capability
for hot forming, and deteriorates mechanical properties. In this work, the results of a study of the effect of thermo-mechanical
processing (TMP) of Al-Mg alloy AA5182 on the AGG are reported. TMP included cold rolling with a reduction range of
40–85 % followed by isochronal annealing (1 h) in a temperature range of 350–520 °C or isothermal treatment at 480 °C for
different periods of time. The microstructure was characterized using optical microscopy in polarized light and an FEG SEM.
The results showed that an increase in the degree of reduction lowers the annealing temperature for the onset of the AGG, while
an increase in the annealing temperature increases the extent of the AGG for a given cold reduction. The abnormal grains tended
to form in the region close to the surface of the plates, and with extended annealing bands, develope parallel to the surfaces in
the rolling direction. However, no appreciable grain growth occurred in the plate center. The AGG and grain-boundary mobility
showed a strong anisotropy with a much faster lateral growth than the motion of the growth front normal to the plate surface.
Such anisotropy was attributed to the rod-like shape and alignment of Al 6Mn dispersoids through Zener pinning.
Keywords: abnormal grain growth, thermo-mechanical processing, Al-Mg-Mn alloy
Nagnjenost zlitin na osnovi aluminija k abnormalni rasti kristalnih zrn (AGG) mo~no omejuje temperaturno obmo~je, v katerem
se lahko izvaja njeno rekristalizacijsko `arjenje in sposobnost za njeno preoblikovanje v vro~em, ker vse skupaj poslab{uje
njene mehanske lastnosti. V prispevku avtorji predstavljajo {tudijo vpliva termo-mehanske obdelave (TMP) na abnormalno rast
kristalnih zrn Al-Mg zlitine vrste AA5182. TMP je bila sestavljena iz hladnega valjanja s stopnjo redukcije med 40 % in 85 % in
eno-urnega izohronega `arjenja v temperaturnem obmo~ju med 350 °C in 520 °C ali razli~no dolge izotermne obdelave pri
480 °C. Mikrostrukturo zlitine so preiskovali z opti~nim mikroskopom v polarizirani svetlobi in v FEG SEM. Rezultati ka`ejo,
da pove~ana stopnja redukcije zni`uje temperaturo `arjenja za za~etek AGG medtem, ko zvi{anje temperature `arjenja povi{uje
obseg AGG za dano stopnjo redukcije. Abnormalna kristalna zrna nastajajo pod povr{ino plo{~ in s podalj{evanjem ~asa
`arjenja nastajajo njihovi trakovi vzporedno s povr{ino v smeri valjanja. Vendar pa niso zaznali znatne rasti zrn v sredini plo{~e
oz. vzorca. AGG in mobilnost mej kristalnih zrn sta mo~no anizotropni z mnogo hitrej{o bo~no rastjo kot je hitrost rasti ~ela
pravokotno na povr{ino plo{~e. Tak{no anizotropijo lahko pripi{emo Zenerjevemu pripenjanju usmerjenih pali~astih izlo~kov
Al 6Mn.
Klju~ne besede: abnormalna rast zrn, termo-mehanska obdelava, zlitina na osnovi Al-Mg-Mn
1 INTRODUCTION
Grain growth was the subject of a copious studies for
over half a century, but there is still an ongoing research
interest in the fundamental aspects of the phenomenon as
well as in its practical aspect realized through the deve-
lopment of new processing routes.
1–5
Some key issues
remain unresolved, one of them being the initiation and
mechanisms of abnormal grain growth (AGG)
1
.A G G
represents a development of the microstructure of large
grains surrounded by finer, i.e., bimodal grains. It is
generally accepted that in metals, AGG takes place when
a normal grain growth is inhibited by particles, texture or
surface effects, and certain grains have a growth
advantage over the neighboring grains.
6
A G Gi su n -
wanted as it severely deteriorates the mechanical pro-
perties of alloys, with the exception of Fe-Si alloys, used
for electrical applications.
6
The propensity of an alloy
toward AGG at certain temperatures severely limits the
annealing-temperature range as well as the capability of
an alloy for hot forming.
7
In spite of the susceptibility of aluminum alloys to
AGG, the number of studies on the conditions for its
occurrence is limited.
8–15
Studies of particle-containing
Al-Mg-Mn,
8,9
Al-Mn
10
and Al-Cu
11
alloys showed that
the occurrence of AGG strongly depends on the anneal-
ing temperature. It takes place at temperatures
sufficiently high to provide for the grain-boundary
mobility, but below the solvus, and at which the volume
fraction of pinning particles is small. Though dimi-
nishing Zener pinning is believed to have a critical
role,
8–11
some of the studies showed that the texture and
special grain boundaries might play important parts.
8,10
The texture importance is stressed in the studies on
Materiali in tehnologije / Materials and technology 54 (2020) 1, 85–90 85
UDK 620.19:669.018.255:621.785.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(1)85(2020)
*Corresponding author's e-mail:
tradetic@tmf.bg.ac.rs (Tamara Radeti})

low-alloyed Al-alloys with an insignificant dispersoid
fraction
12–14
. The AGG in AA5182 is reported to take
place at temperatures above 480 °C, albeit with different
kinetics.
8,9
According to both studies, dispersoid coarsen-
ing and dissolution were the causes of the AGG, whereas
its initiation was attributed to the size advantage of the
grains with a certain texture.
8
The aim of this study was a systematic investigation
of the influence of cold work and annealing temperature
on the occurrence of AGG. Due to the detrimental effect
of AGG on mechanical properties, a strategy for avoid-
ing it during high-temperature annealing was examined.
2 EXPERIMENTAL PART
The material used for the study was a commercial
AA5182 aluminum alloy provided by Impol Seval Alu-
minium Mills. The industrial processing of the alloy,
whose chemical composition is shown in Table 1,
included DC casting, homogenization at 550 °C and hot
rolling.
Thermo-mechanical processing (TMP) of the
as-received hot band included cold rolling up to
reductions of 40–85 % followed by annealing at
350–520 °C for 1 h. In addition to the isochronal anneal-
ing, specimens cold-rolled to a 64-% reduction were
isothermally annealed at 480 °C for different periods; the
exact conditions are shown in Table 2.
The microstructure of the as-received and thermo-
mechanically processed material was characterized with
optical microscopy in polarized light and scanning
electron microscopy using a FEG SEM Tescan Mira at
20 kV. Specimens in the RD-ND orientation were
mechanically polished and electrolytically etched in
Barker’s reagent. Composite images of large specimen
areas were created by digitally stitching together indi-
vidual micrographs. Image processing and an analysis
were conducted using the ImageJ software.
3 RESULTS
The study of the effect of the annealing temperature
on recrystallization and grain growth showed that
annealing up to 450 °C resulted in a recrystallized micro-
structure with equiaxed grains. There is a grain size
gradient through the plate thickness: finer grains are
closer to the surface and coarser grains are in the middle
of the plate. A very small, if any, change in the grain size
T. RADETI] et al.: INFLUENCE OF COLD-ROLLING DEFORMATION AND ANNEALING TEMPERATURE ...
86 Materiali in tehnologije / Materials and technology 54 (2020) 1, 85–90
Table 1: Chemical composition of the AA5182 alloy (w/%)
Mg Mn Si Fe Ti Cu Zn Cr Zn Other
4.04 0.371 0.0732 0.186 0.0019 0.011 0.0397 0.011 0.0223 0.0082
Table 2: Annealing conditions for the specimens cold rolled to a 64-% reduction
Isochronal (1 h) t, °C 350 400 440 470 480 500 520
Isothermal (480 °C)  , min 0 5 15 30 45 60 180
Table 3: Grain-size and shape parameters of abnormal grains after isochronal annealing (1 h) in the temperature range of 470–520 °C
Ann. T,°C
Normalized area of
AGG*, mm
2
/mm
Eq. diameter, μm Roundness Aspect ratio
470 °C 0.72 443 0.53 2.3
480 °C 1.63 743 0.45 2.7
500 °C 2.03 703 0.47 2.5
520 °C 2.2 435 0.50 2.4
*The normalized area was calculated as the area covered by abnormal grains per unit length in RD. It corresponds to the mean length of the
propagation front of AGG in ND.
Figure 1: Effect of the temperature on AGG during isochronal
annealing (1h): a) 470 °C, b) 520 °C

and shape was observed in the specimens annealed at a
temperature of 350–450 °C. However, an increase in the
annealing temperature to 470 °C gave rise to AGG (Fig-
ure 1a). Here, we have a sandwich-like microstructure
with a strip of fine recrystallized grains between the two
bands of abnormally large grains next to the plate
surfaces parallel to the rolling direction (RD).
The area transformed by AGG increased with a
further increase in the annealing temperature, but the
mean size of abnormal grains showed a more complex
behavior. The initial increase in the size was followed by
a decline since more abnormal grains were nucleated as
the temperature rose (Table 3). Abnormal grains formed
throughout the specimen at 520 °C (Figure 1b).
The microstructure upon reaching 480 °C, the tempe-
rature of the isothermal-annealing experiments, showed
recrystallized, equiaxed grains without evidence of
AGG. After only 5 min of the annealing, grains larger
than average were observed in the region up to 700 μm
from the plate edge (Figure 2a). The smallest of the
incipient abnormal grains tend to have a more equiaxed
shape while the larger grains are elongated (Figure 2a),
the tendency confirmed by the size-shape distribution
plot (Figure 2b). The abnormal grains remain elongated
after forming a continuous band (Figure 2c), with the
longer axis parallel to RD and the mean aspect ratio
increasing from  2t o 2.7.
Apparently, the growth rate and grain-boundary
mobility are anisotropic, i.e., the grain-boundary mi-
gration is faster in the direction parallel to the RD than in
ND. If the only force influencing the grain growth
decreases in the interface area, i. e. capillary force, such
a trend would be unexpected; the migration of the
boundaries of the abnormal grains in the ND decreases
the total grain-boundary area since the smaller grains are
consumed, while the motion of the boundaries in the RD
leaves the total boundary area effectively unchanged
until a complete collapse of the intermediate grains. It is
likely that the observed anisotropy of the grain mobility
is caused by the shape and anisotropic alignment of the
Al
6
Mn particles (Figure 3) that can limit the mobility
through Zener pinning.
16
In contrast to the dramatic changes to the outer
regions during annealing, the grain size in the center is
steadier, with the mean grain size changing from
14.5 μm after 5 min to 17.7 μm after1ha t4 8 0° C .
In addition to the annealing temperature, the
propensity toward abnormal grain growth strongly de-
pends on the cold-rolling reduction as illustrated in
Figure 4. The specimen annealed at 480 °C for 1 h,
having undergone a 40-% reduction, exhibited recrys-
tallized equiaxed grains (Figure 4a), but an increase in
the reduction to 50 % resulted in a few abnormal grains
in the microstructure near the plate surface (Figure 4b).
A further increase in the reduction to 65 %, under the
same annealing conditions, gave rise to two bands of
abnormal grains (Figure 4c). In the case of a cold
T. RADETI] et al.: INFLUENCE OF COLD-ROLLING DEFORMATION AND ANNEALING TEMPERATURE ...
Materiali in tehnologije / Materials and technology 54 (2020) 1, 85–90 87
Figure 3: Rod-like Al
6
Mn particles are more efficient at pinning grain
boundaries in the RD due to the longer axis aligned parallel to the RD
(5 min/480 °C)
Figure 2: Effect of the annealing time on AGG during annealing at
480 °C: a) 5 min, b) size (eq. diameter) vs shape (roundness) of abnor-
mal grains after 5-min annealing, c) 15-min annealing

reduction of 85 %, abnormal grains covered the plate,
although some finer grains were left behind (Figure 4d).
There is an inverse relationship between the reduction
degree and the onset temperature for AGG: the temper-
ature of 440 °C is required for the reduction of 85 %,
while lowering the reduction to 40 % requires 500 °C for
the start. Clearly, an increase in the reduction degree
promotes AGG. Furthermore, as the reduction increases,
incipient abnormal grains form deeper, close to the plate
center. For example, abnormal grains are confined to the
region near the surface when deformed by 50 % (Figure
4b), but after a 64-% reduction, they form as far as 1.5
mm from the edge.
The complexity of the factors inducing AGG is
demonstrated by a two-stage-annealing experiment. In
contrast to single-stage annealing at 480 °C (Figure 4c),
low-temperature annealing for 48 h at 220°C followed by
high-temperature annealing for1ha t4 8 0° Cg a v erise to
a microstructure without an abnormal grain growth (Fig-
ure 5).
4 DISCUSSION
The effect of the cold-rolling reduction and annealing
temperature on the grain growth is summarized in the
plot of Figure 6.
The results show that AGG can occur at a lower tem-
perature, 440 °C, than previously reported for AA5182,
+480 °C.
8,9
The lower temperature questions the attrib-
ution of AGG to diminishing Zener pinning pressure due
to the coarsening and dissolution of dispersoids since the
dissolution temperature is reported to be close to
500 °C.
8,9
The particle coarsening/dissolution is not
critical for AGG and it is suggested, based on the obser-
T. RADETI] et al.: INFLUENCE OF COLD-ROLLING DEFORMATION AND ANNEALING TEMPERATURE ...
88 Materiali in tehnologije / Materials and technology 54 (2020) 1, 85–90
Figure 4: Effect of the reduction on the grain growth during annealing at 480 °C for 1 h: (a) 40 %; (b) 50 %; (c) 64 %; (d) 85 %
Figure 5: AGG is absent after a 65-% deformation and two-stage
annealing: 48 h/220 °C + 1 h/480 °C

vations, that AGG occurs after a 64-% reduction and not
after a 40-% reduction under the same annealing
conditions. It is evident (Figure 6) that the degree of
cold reduction has a strong influence on the temperature
of the onset of AGG. To the best of the authors’ knowl-
edge, there is no published work that investigates the
effect of cold deformation on the temperature, at which
AGG occurs in aluminum alloys. For a low-alloyed steel,
a similar dependence of the AGG onset temperature
versus the reduction degree was reported
17
with an
interpretation in terms of a higher dislocation density
providing fast diffusion paths for a dissolution. However,
in the reduction range of interest to this work, i.e., over
40 %, a saturation with dislocations is expected to be
reached, so it is unlikely that a further increase in defor-
mation would significantly boost diffusion. Indeed, even
for steel, the onset temperature remains nearly constant
in a reduction range of 40–75 %.
17
Moreover, our charac-
terization showed that the microstructure is already
recrystallized upon reaching the annealing temperature,
so the grains should be nearly dislocation free. The
presence of recrystallized grains also rules out the
"pinch-off" mechanism proposed by Taleff’s group.
7
The texture and special grain boundaries with a high
mobility are also considered as the key factors for
AGG.
6,10
The grain-size gradient across the plate, the
band of abnormal grains at the plate surface and the
increasing depth of incipient abnormal grains with
reduction as well as stable grains in the center suggest
that the shear texture and texture gradient are responsible
for the onset of AGG in this work, albeit without a text-
ure measurement.
The suppression of AGG in the two-stage annealing
experiment (Figure 6) highlights a possible role of
subgrain boundaries. The activation of low-temperature
recovery mechanisms during the annealing at 220 °C can
prevent a high-temperature recovery and recrystallization
due to the subgrain growth.
5,18
As a result, during the
annealing at 480 °C, the recrystallization due to the
nucleation and growth may take place allowing a more
random texture
5
and the absence of AGG. Hence, the
AGG during the one-stage annealing could be a con-
sequence of a discontinuous subgrain growth.
18
The
mechanism of AGG, based on energy considerations and
solid-state wetting,
19,20
assumes that low-angle subgrain
boundaries of abnormal grains penetrate into the matrix
by means of repeated events of triple-junction wetting.
Although morphological features such as small island
grains and small grain clusters within abnormal grains
(Figures 1 and 4) resemble the morphology typical for
triple-junction wetting,
19
they are commonly observed
during AGG
9–11,19
and ascribed to dispersoid pinning.
11
Whether the critical factor for AGG is the grain-boun-
dary mobility, i.e., grains with mobile high-angle boun-
daries consuming the surrounding grains with low-angle
low-mobility boundaries,
10
or the energy criterion and
solid-state wetting
20
at this point remains a conjecture.
5 CONCLUSIONS
A systematic investigation of the influence of TMP
parameters on the occurrence of abnormal grain growth
(AGG) showed that an increase in cold deformation and
annealing temperature increases the propensity toward
AGG. The results show that lowering the reduction
degree or two-stage annealing temperature can suppress
AGG during annealing in a high-temperature range.
AGG appears to be affected by multiple factors. A
confinement of incipient abnormal grains to the region
close to the plate’s surface and an absence of AGG in the
center indicate that the initiation of AGG is controlled by
the texture and grain-boundary character gradients. The
anisotropy of the growth of abnormal grains is related to
the shape and anisotropic alignment of Al
6
Mn particles
due to Zener pinning, while the absence of AGG in two-
stage annealing points at the role of recovery processes.
Acknowledgment
This work was supported by the Ministry of
Education, Science and Technological Development of
the Republic of Serbia and Impol Seval Aluminium
Mills, under contract No. TR 34018.
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T. RADETI] et al.: INFLUENCE OF COLD-ROLLING DEFORMATION AND ANNEALING TEMPERATURE ...
90 Materiali in tehnologije / Materials and technology 54 (2020) 1, 85–90