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Journal of Light Metals 2 (2002) 247–252
www.elsevier.com/locate/lightmetals
Hot and warm forming of 2618 aluminium alloy
P. Cavaliere
*
Department of Ingegneria dellÕInnovazione, University of Lecce, Via per Arnesano, I-73100 Lecce, Italy
Abstract
The hot and warm formability of 2618 aluminium alloy, in the as-solutioned condition, was investigated in extended ranges of
temperature and strain rate by means of torsion testing. Precipitation was found to occur during deformation. The effect of the
precipitation of second phase particles, occurring during deformation, on the flow curve shape and on the stress level was evaluated.
At the lowest temperatures, precipitation, during deformation, resulted in a continuous increase of flow stress; by contrast, when
testing temperature exceeded 250 C, precipitation and subsequent coarsening of precipitates, during deformation, resulted in a peak
of flow curves, followed by softening. The constitutive equations of the material were calculated and the ductility of the alloy was
evaluated as strain to fracture values in all the tested condition.
2003 Elsevier Science Ltd. All rights reserved.
Keywords: Hot and warm forming; Flow curves; Dynamic precipitation; Constitutive equations; Strain rate sensitivity
1. Introduction
Hot forming of aluminium alloys is widely used in the
modern industry and have been investigated by scientists. Warm forming process has been widely investigated [1–4]. The interest towards such an innovative
process derives from the reduction in flow stress, increase in ductility, reduction in work hardening and
increase in toughness of the material when compared
with cold forming; moreover, temperatures lower than
those involved during hot forging make easier the obtaining of close tolerances and high surface finish [5–10].
The 2xxx aluminium alloys, widely used for aeronautical applications, are frequently formed by hot
rolling or forging; the intermediate and high forming
temperatures result in a number of different microstructural processes that significantly influence the final
mechanical response of the alloy. A solution treatment,
followed by a proper artificial ageing, are then necessary
to obtain a good combination of mechanical properties
[11–13].
In this context some studies took into consideration
the possibility of forging the alloys in the solution
treated condition. The process, even if results in higher
flow stresses, in principle could significantly reduce the
*
Tel.: +39-0832320324; fax: +39-0832320349.
E-mail address: [email protected] (P. Cavaliere).
cost of the component, by eliminating the need for postforging solution and ageing treatments.
Thus, other authors has proposed a thermomechanical processing consisting of a solution treatment at 530
C for 4 h, followed by water quenching and then by a
forging operation at temperature ranging from 150–500
C [14]. The precipitation of a fine dispersion of second
phase particles induced by heating at the warm forming
temperature of 250 C and subsequent precipitation
allowed the obtaining of forgings with mechanical
properties, in terms of strength and ductility, almost
coincident with those obtained via a traditional forging
sequence, followed by a T6 treatment but with the advantage of eliminating such an artificial ageing treatment [9,14].
In some aluminium alloys, developed for structural
applications, most of strengthening produced by natural
ageing at room temperature in the as-solutioned condition occurs within few days. The very short ageing time
makes such alloys very attractive for warm forging even
if the low formability in such conditions could reduce
the shape complexity of the forgings. The typical precipitation sequence of the 2xxx series alloys (Al–Cu–Mg
alloys) consists in the formation of GP zones, which
after prolongated ageing at low temperature dissolve
and is replaced by precipitating in form of semi-coherent
S0 (Al2 CuMg) precipitates [15]. After longer time of
exposure the formation of S (Al2 CuMg) stable phase
results in overageing [16,17].
1471-5317/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S1471-5317(03)00008-7
P. Cavaliere / Journal of Light Metals 2 (2002) 247–252
2. Experimental procedure
The alloy, supplied in form of extruded rods, had the
following chemical composition (wt.%): Cu ¼ 2:3%,
Mg ¼ 1:6%, Fe ¼ 1:1%, Ni ¼ 1:0%, Ti ¼ 0:07%, Si ¼
0:18%, Al ¼ bal. Before testing, the alloy was solution
treated at 530 C for 2 h and then water quenched to
room temperature (W temper state). The hot and warm
formability of AA2618 was studied by means of torsion
tests in the ranges of temperature (T ) and strain rate (e_)
varying from 150 to 500 C and from 103 to 1 s1 . The
deformation testing was carried out to fracture on a
computer-controlled torsion machine. Samples with a
gauge diameter of 10 mm and a gauge length of 20 mm
were used. The samples were heated by a high frequency
induction coil rapidly (3 C/s); a thermocouple was inserted inside the specimen to control temperature during
the tests. A computer program acquired the torque (C)
vs. twist angle (h) data. They were converted to surface
shear stress (s) and strain (c) according to the relationships [18–20]:
s¼
C
ð3 þ n0 þ m0 Þ
2pp3
c¼
Rh
L
gauge length. Light microscopy (LM) was used to
evaluate microstructural behaviour during hot and
warm deformation. Grain evolution was followed by
LM in several experimental conditions to establish the
occurrence of dynamic phenomena such as dynamic
recrystallization.
3. Results
Typical flow curves of AA2618-W aluminium alloys,
in warm and hot forming condition are shown in Figs. 1
and 2.
In the low temperature regime (150–200 C) the r–e
curves monotonically increase up to fracture; at intermediate temperatures (200–300 C) the curves increase
up to the peak followed by a moderate decrease in
equivalent stress. In the high temperature regime the
decrease in equivalent stress becomes more and more
pronounced, and it transforms in a continuous reduction in r, approaching zero. The temperature effect on
400
200 C 10 - 2 s - 1
-1
200 °C 10 s -1
200 °C 1 s-1
True Stress [MPa]
The hot and warm formability of 2618 aluminium
alloy was investigated, the constitutive equations relating flow stress, temperature and strain rate, were
calculated with the aim to correlate the mechanical response (ductility or temperature-normalized strain rate)
to the microstructure (subgrain size, grain size, dislocation distribution). Strain rate sensitivity of the material
was evaluated in all the deformation conditions and the
obtained values were correlated to the ductility of the
alloy in terms of strain to fracture.
300
200
100
ð1Þ
where n0 ð¼ d logC log C=hjh_;T Þ is the work hardening
rate and m0 ð¼ d logC log C=h_jh;T Þ is the revolution rate
sensitivity coefficient of torque. The value of n0 is assumed to be zero: this hypothesis is true at the peak of
the C–h curve; in the other parts of the curve, it leads to
negligible errors. Conversely, m0 values were calculated
by plotting torque vs. revolution rate (h) data on a
logarithmic scale at different temperatures for each h
value: torque increases with revolution rate, having a
remarkable effect at higher temperatures. Equivalent
stresses (r) and strains (e) were derived from surface
shear stresses and strains by means of the Von Mises
criterion:
p
p
r ¼ s 3 e ¼ c= 3
ð2Þ
The temperature of the samples was measured by an
internal thermocouple in close proximity of the gauge
section. Preliminary measurements, in fact, confirmed
that this configuration of the control thermocouple did
not result in significant temperature gradients along the
0
0.00
0.15
0.30
0.45
0.60
0.75
True Strain
400
-1
200 °C 10 s- 1
True Stress [MPa]
248
300
- 1 -1
250 °C 10 s
200
-1
300 °C 10 s-1
100
0
0.00
0.25
0.50
0.75
1.00
1.25
1.50
Fig. 1. Representative equivalent stress vs. equivalent strain curves
obtained by testing the 2618 alloy after solution-treatment at 200 C in
all the strain rates investigated.
P. Cavaliere / Journal of Light Metals 2 (2002) 247–252
and hot forming conditions, the flow curves show the
typical behaviour with an increase in flow stress with
increasing strain up to a maximum followed by a flow
softening until fracture, this phenomenon is more
marked at lower temperatures than at higher ones.
The hot and warm forming behaviour of AA2618
aluminium alloys was modelled by correlating flow
stress to strain rate and temperature according to the
well known constitutive equation [21–23]:
Q
ð3Þ
¼ A½sinhðarÞ n
Z ¼ e_ exp
RT
75
True Stress [MPa]
60
450 °C 1 s- 1
45
450 °C 10
-1
s- 1
30
450 °C 10- 2 s- 1
15
0
0
5
(a)
10
15
20
True Strain
75
True Stress [MPa]
60
400 °C 10- 1 s- 1
45
where Z is the Zener–Hollomon parameter representing
the temperature modified strain rate, Q is the activation
energy related to the deformation mechanism taking
place in the material during the process, R is the universal gas constant, T is the absolute temperature, e_ is
the strain rate, A, n and a are material parameters.
The activation energy of the deformation process was
calculated by the relationship:
450 °C 10 - 1 s- 1
Q ¼ R
30
o lne_
oð1=T Þ
-1
500 °C 10 s-1
¼ 2:3R
15
0
0
5
10
15
249
20
Fig. 2. Representative equivalent stress vs. equivalent strain curves
obtained by testing the 2618 alloy after solution-treatment at 450 C
(a), in all the strain rates investigated, and at 101 s1 (b) in all the hot
forming temperatures.
the equivalent stress–equivalent strain curves at high
strain rates is less marked than that at low e_. In all warm
r
o loge_
o log sinhðarÞ
e;T
o log sinhðarÞ
oð1000=T Þ
ð4Þ
e;e_
The plot used to calculate Q were, for a given strain, the
slope of the log e_ vs. log½sinhðarÞ and log½sinhðarÞ vs.
1000=T plots at different temperatures and strain rates.
At different temperatures and strain rates, the mean
values of the slopes of the previous curves lead to value
of the activation energy of 161 kJ/mol very close to the
one calculated for self-diffusion in aluminium alloys
(Fig. 3).
The data were obtained from compression tests. The
stress multiplier a was calculated by an optimisation
Fig. 3. Activation energy calculation for the studied 2618 aluminium alloy.
250
P. Cavaliere / Journal of Light Metals 2 (2002) 247–252
procedure: the a value with the best correlation coefficient for the linear relationships log e_ vs. log½sinhðarÞ
and log½sinhðarÞ vs. 1000=T was 0.02 for 2618 respectively.
The ductility of AA2618-W alloy in hot and warm
conditions, measured as equivalent strain to failure (ef )
is shown in Fig. 4.
The strain rate sensitivity coefficient ðmÞ of the material was calculated using the following equation [24–
27]:
m¼
o logr
o loge_
ð5Þ
e;T
The m value was calculated by interpolating the data
obtained in torsion tests at the peak stress value.
The map obtained shows the variation of the strain
rate sensitivity coefficient with temperature and strain
rate (Fig. 5), giving the behaviour of the material in all
the testing conditions.
Equivalent strain to failure
1.5
0.001 1/s
0.01 1/s
0.1 1/s
1 1/s
1
0.5
0
125
150
175
200
225
250
275
300
325
Temperature, (˚C)
Equivalent strain to fracture
18
0.01 /s1
16
0.1 1/s
1 1/s
14
12
10
8
6
350
400
450
500
550
Temperature (˚C)
Fig. 4. Ductility of AA 2618-W alloy in warm and hot forming conditions.
Fig. 5. Strain rate sensitivity map for all the strain rates and temperatures investigated.
Higher is the value of the m parameter, better the
material dissipate the flow energy through metallurgical
transformation.
The room temperature mechanical behaviour, in
terms of yield strength and ductility, of the warm and
hot formed AA2618-W alloy vs. the forming temperature can be related to the different hardening and
softening mechanisms, occurring before and during
forming.
The ductility of the alloy in warm forming conditions,
measured as equivalent strain to failure (ef ) exhibits
lower values in comparison with those measured for the
same alloy in hot forming condition. The equivalent
strain to failure at low strain rates (e_ ¼ 0:001 and
e_ ¼ 102 s1 ) shows a decrease with increasing temperature up to a minimum value obtained at about 250 C,
and then ef increases with further increases in temperature. At e_ ¼ 101 s1 the equivalent strain to failure vs.
temperature curve is rather scattered even if the ef is
generally higher than at lower strain rates and the behaviour appears to be similar to that observed at lower
strain rates. Finally, at the highest strain rate investigated (e_ ¼ 1 s1 ), ef monotonically increases with increasing forming temperature from the lowest to the
highest value experienced in this investigation. As a
consequence, the forming temperature of 300 C appears to be the most interesting from the formability
point of view in the warm conditions. In hot forming
conditions ef shows the highest values at 450 C for all
the strain rates investigated reaching the highest levels
for the strain rate 101 s1 . The results on formability
were used, as a general guideline, to select the process
parameter for warm and hot forming.
Hardness measurements performed on samples after
warm forming and natural aging were compared with
P. Cavaliere / Journal of Light Metals 2 (2002) 247–252
the values obtained on AA2618 alloy subjected to the T6
treatment. It was observed that the HRB value measured after warm forging at 300 C 1 s1 (72) is very
close to the one measured on the T6 treated sample (73).
Also the value measured after warm forming at 150 C
0.1 s1 (76) is quite similar to the T6 one whilst the HRB
values measured at 200 and 250 C (86 and 84 respectively) are remarkably higher than that of the T6 treatment.
251
tremely similar to that produced by solution-treatment
followed by cold-working ageing [15–17].
4. Discussion
The behaviour shown by AA2618-W alloy in warm
forming conditions is a consequence of the dynamic
nature of the precipitation processes taking place during
deformation.
In such conditions the microstructural evolution does
not depend only on the applied stress, but is the product
of complex interactions between the kinetics of dislocation generation and annihilation, nucleation of new
phases and zones, and growth of the precipitates (Fig.
6). At high temperatures, the kinetic of precipitation
becomes faster and a longer time of exposure produces a
marked coarsening of the second phase particles precipitated during heating and subsequent deformation
(Fig. 7). As a consequence, low strain rates are required
to produce strong strengthening effects during warm
forming at low temperatures, high strain rates are
needed at high temperatures.
The calculation of the strain rate sensitivity leads to
the conclusion that m increase with increasing temperature in all the strain rate regimes investigated, at the
same time, the calculated parameter increases with decreasing strain rate in hot forming conditions.
Fig. 8 shows the microstructure of the sample tested
at 400 C and 103 s1 ; precipitates are densely distributed in the grain interior the microstructure being ex-
Fig. 6. Optical micrography of the 2618 aluminium alloy torsion tested
at 200 C 101 s1 showing grain elongated in the flow direction.
Fig. 7. Optical micrography of the 2618 aluminium alloy torsion tested
at 300 C 101 s1 showing grain elongated in the flow direction.
Fig. 8. Optical micrography of the 2618 aluminium alloy torsion tested
at 400 C 101 s1 showing equiaxed recrystallized grains.
Fig. 9. Optical micrography of the 2618 aluminium alloy torsion tested
at 500 C 101 s1 showing equiaxed recrystallized grains.
252
P. Cavaliere / Journal of Light Metals 2 (2002) 247–252
Fig. 9 shows the microstructure of the studied material at 500 C with all recrystallized grains.
rupture. In any case, straining greatly enhanced the kinetics of precipitation.
5. Conclusions
References
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temperature, the stress–strain curves exhibited a peak,
followed by softening.
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• The forging sequence including the warm forging operation at about 250 C, after the natural ageing, provides forgings with yield strength and ductility values
that are very similar to those obtained after the conventional sequences whilst eliminating the expensive
and time consuming precipitation hardening treatment (T6).
• At low warm forging temperatures (150–200 C), the
increase in the duration of the warm forging operation enhances the hardening effect produced by the
dynamic precipitation. This can be obtained by lowering the die speed and/or by acting on the initial
geometry of the billet. At temperatures similar to
the optimal value for warm forging (250 C), the
hardening effect is almost independent of the duration of the process.
• The forging sequence involving hot forging requires
the lowest load level whilst in warm forging at temperatures ranging from 150 to 200 C the loads are
similar to those experienced in cold forging notwithstanding the higher temperatures involved. When
temperature exceeds 200 C, the warm forging loads
become significantly lower than those measured in
cold forging.
• The consequence of precipitation was that the strainrate did not appreciably affect the magnitude of peakstress, since, in particular at low temperature, the
stress–strain curves almost overlap.
At low temperatures the formation of coherent precipitates led to a continuous increase in flow stress up to
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