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NEW HEAT
TREATMENTS
FOR AGE-HARDENABLE ALUMINUM ALLOYS
The positive effects of
secondary aging have
been exploited to develop
two heat treatments (T6IX)
that are applicable
to all age-hardenable
aluminum alloys and offer
the potential to improve
alloy properties and/or
reduce processing costs.
It may also be possible to
incorporate paint-bake
cycles and other
product-specific thermal
treatments into the new
processes.
by Roger N. Lumley,
Allan J. Morton, and
Robert G. O’Donnell
Manufacturing & Infrastructure
Technology
Commonwealth Scientific &
Industrial Research Organisation
(CSIRO)
Clayton, Victoria, Australia
Ian J. Polmear
School of Physics & Materials
Engineering
Monash University
Melbourne, Victoria, Australia
N
ew heat treatments that may
be applied to all age-hardenable aluminum alloys have
been recently developed at
Commonwealth Scientific & Industrial Research Organisation (CSIRO)
in Australia. The heat treatments
may be tailored to increase the
strength and toughness of mill products, castings, and forgings, or used
to reduce the duration (and therefore
cost) of elevated-temperature aging.
In certain applications, where paint
baking is an integral part of the manufacturing process, the treatments
can be incorporated into the bake
cycle to both enhance mechanical
properties and reduce processing
costs. This article provides details.
Conventional Aging Methods
Multistage heat treatments provide a means for improving the
properties of aged aluminum alloys
by modifying the size, composition,
species, and distribution of precipitate particles.1–6 An example of a
multistage heat treatment is the T73
temper in which artificial aging at
one temperature (100°C, for example) is followed by a second treatment at a higher temperature (160°C,
for example). The T73 temper increases the stress-corrosion resistance of 7000 series (Al-Zn-Mg-Cu)
wrought alloys by modifying the microstructure. However, there is a significant sacrifice in tensile properties, compared with the single-stage
T6 temper.1,2
Another multistage (duplex) treatment is to naturally age the alloy at
room temperature after quenching
and before artificial aging. Such a
delay period, which may be unavoidable during normal industrial
processing, can increase the strength
of some 7000 series alloys1 and
casting alloys such as 356 and 357,
while reducing the tensile properties
HEAT TREATING PROGRESS • MARCH/APRIL 2005
Coauthor Roger Lumley loading solution
treated and quenched, extruded aluminum
alloy 6061 tubes into a circulating air furnace for the first artificial aging stage of a
T6IX temper.
of some 6000 series wrought alloys.3
In other alloys, a duplex treatment
of this kind causes retrogression (resolution), in which fine clusters or
precipitates formed at low temperatures redissolve on elevated temperature aging.
Secondary aging: Following aging
at elevated temperatures, the conventional wisdom holds that the microstructure and mechanical properties of an age-hardened alloy
remain unchanged for an indefinite
period if used in service at close to
the ambient temperature. However,
recent observations on a supersaturated lithium-containing alloy aged
to peak properties, and subsequently
exposed to slightly elevated temperatures (60 to 135°C), indicate that socalled “secondary precipitation”
(and hardening) may occur, which,
23
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Table 1 — Interrupted-aging temper variants
Temper and process
T6I4 tempers
T6I4
Solution treat, quench, underage at (Ta), quench, age at 25–65°C (Tb).
T77I4 Solution treat, quench, age at (Ta1), age at (Ta2), where Ta2 > Ta1, quench,
age at 25–65°C (Tb).
Temperature, T
Solution treatment
Quench
Age at
temperature Ta
Quench
T8I4
Solution treat, quench, cold work, underage at (Ta), quench, age at 25–65°C
(Tb).
T9I4
Solution treat, quench, underage at (Ta), cold work, age at 25–65°C (Tb).
T6I6 tempers
Age at
temperature Tc
T6I6
Age at
temperature
Tb
Solution treat, quench, underage at (Ta), quench, age at 25–65°C (Tb), re-age
at artificial aging temperature (Tc), where Tc ≤ Ta.
T6I76 Solution treat, quench, underage at (Ta), quench, age at 25–65°C (Tb), re-age
at artificial aging temperature (Tc), where Tc > Ta.
Time, t
T8I6
Fig. 1 — Schematic representation of interrupted-aging tempers (T6I4 and T6I6) for
heat treatable aluminum alloys. Specific
processes are outlined in Table 1.
Solution treat, quench, cold work, underage at (Ta), quench, age at 25–65°C
(Tb), re-age at artificial aging temperature (Tc), where Tc ≤ Ta.
T9I6
Solution treat, quench, underage at (Ta), cold work, age at 25–65°C (Tb), re-age
at artificial aging temperature (Tc), where Tc ≤ Ta.
in this case, leads to embrittlement.7
More recently, work at CSIRO has
shown that secondary precipitation
may also occur at ambient temperatures but with beneficial effects on
mechanical properties.8–13
Secondary aging has been observed if age-hardenable aluminum
alloys are first underaged at an elevated temperature (150°C, for example), quenched, and then exposed
to a lower temperature (such as 25 to
65°C). At these lower temperatures,
nucleation of fine precipitates occurs
that further depletes the microstructure of solute elements and gives rise
to additional strengthening. We have
named this treatment the T6I4
temper (I = interrupted), and alloys
so treated normally develop tensile
properties close to those for their respective T6 tempers, together with
enhanced fracture toughness.
If these alloys are then aged again
at an elevated temperature to produce the T6I6 temper, further increases in tensile properties of as
much as 10 to 15% are possible, again
usually with simultaneous increases
in fracture toughness.9,14
The CSIRO-developed T6I4 and
T6I6 tempers also serve as the basis
for a series of other interrupted-aging
tempers. They are shown schemati150
210
130
170
150
T6I4
110
90
90
7050
70
10–2
100
102
104
130
(b)
150
110
130
90
T6I4
70
357
101
102
Time, h
100
101
Time, h
102
103
104
103
T6
110
90
T6I4
70
T6
100
2001
10–1
Hardness, HV
Time, h
Hardness, HV
(a)
(c)
T6I4
110
130
50
10–1
T6
Hardness, HV
Hardness, HV
190
T6
50
10–2
(d)
6061
100
102
Time, h
104
Fig. 2 — Secondary aging (T6I4) curves compared with T6 curves for aluminum alloys (a)
7050, (b) 2001, (c) 357, and (d) 6061. The solid line is the T6 curve, while the broken lines
show the secondary aging response following different times of preliminary artificial aging.
24
cally in Figure 1 and defined in Table
1. The purpose of this article is to report some of the effects of these interrupted-aging treatments on the
mechanical properties of selected
aluminum alloys.
T6I4: Cost Reduction Focus
As already indicated, the T6I4
temper utilizes secondary precipitation after a short period of underaging at an elevated temperature to
achieve, or sometimes to surpass, the
properties of the conventional T6
temper, but with notably reduced
times of artificial aging and hence
processing costs. Figure 2 shows examples of this behavior for the commercial wrought aluminum alloys
7050, 2001, and 6061, and cast aluminum alloy 357. Alloys were underaged at normal T6 aging temperatures, followed by rapidly quenching
from the aging temperature and
holding at lower temperature for a
dwell period. In each case, the hardness of the T6I4 material (dashed
lines) begins to approach, or sometimes to exceed, that of the corresponding T6 temper (solid line).
Alloys aged to the T6I4 temper
also display tensile properties close
to, and sometimes exceeding those
of the T6 temper. In most instances,
the fracture toughness is also significantly improved, as shown in Table
2 for cast alloy 357 and wrought alloys 7050 and 6061. For other alloys
in Table 2, such as the Al-Cu-Mg alloys 2001 and 2214, the fracture
toughness is similar to that measured
for T6-temper material.
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Table 2 — Mechanical properties of T6- and T6I4-temper alloys
Alloy
Temper
Yield strength,1
MPa
Tensile strength,
MPa
Elongation at
failure, %
Fracture
√m
toughness,2 MPa√
7050
T6
T6I4
546
527
621
626
14
16
37.6
52
2214
T6
T6I4
386
371
446
453
14
13
26.9
27.1
2001
T6
T6I4
265
260
376
420
14
23
56.3
56.9
Al-Cu-Mg-Ag
T6
T6I4
442
443
481
503
12
8
23.4
28.1
6111
T6
T6I4
339
330
406
411
13
14
—
—
6061
T6
T6I4
267
302
318
341
13
16
36.8
43.2
357
T6
T6I4
287
280
340
347
7
8
25.5
35.9
1. 0.2% offset. 2. S-L specimen orientation.
150
Hardness, HV
tentially suitable for increasing the
resistance of 7000-series wrought alloys to stress-corrosion cracking (currently under investigation). It has
also been found to be effective in improving the hardness and strength
properties of the heat-affected zone
(HAZ) in welded structures.
The alloy is first artificially aged
to peak hardness and then heated to
a higher temperature, so that retrogression (dissolution of precipitates)
occurs before it is quenched to room
temperature to promote secondary
aging. This produces a microstructure that has previously been assumed to be overaged, through loss
of solute to grain boundaries and
other solute sinks. Nevertheless, results in Fig. 4 show that alloy 7075
then responds well to secondary
hardening at the low temperature.
This complex aging cycle produces
a microstructure containing two
dominant precipitate phases: large
particles of the eta (η) phase that
were stable enough to be retained
during high-temperature aging, interspersed with fine Guinier-Preston
(G-P) zones formed during secondary aging. In contrast to the T7X
tempers commonly used for 7000-series alloys, invoking secondary precipitation does not produce a decrease in mechanical properties as
tensile properties for the T77I4
temper are equivalent to those for the
T6 temper.
T8 at 177°C
130
T9I4
T8I4
110
10% cold work
90
10–1
100
101
Time, h
102
103
Fig. 3 —T8I4 and T9I4 hardening curves
for aluminum alloy 2001. The conventional
T8 curve is also shown for comparison. Peak
hardness values: T8, 140 HV; T8I4, 135 HV;
T9I4, 143 HV.
200
Hardness, HV
Several other aging schedules
have been derived from the more
basic T6I4 temper. Their descriptions
follow.
T8I4 and T9I4: These two tempers
incorporate cold work into their processing schedules. For T8I4, an alloy
is cold worked after solution treatment and quenching but before commencing aging, whereas the T9I4
schedule involves cold work after
preliminary underaging and before
secondary aging. Both exhibit continuing secondary precipitation after
each of the treatments.
An example of this behavior is
shown in Fig. 3 for the wrought aluminum alloy 2001. This alloy had a
hardness of 83 HV after quenching
from the solution treatment temperature. The application of 10% cold
work by rolling raised the hardness
to 98 HV. The peak value rose to 140
HV on artificial aging at 177°C for
the T8 temper.
For the T8I4 temper, the alloy was
cold worked 10% after quenching
from the solution treatment temperature, and then aged at 177°C for 0.5
h and again quenched. The hardness
was then 105 HV, which subsequently increased to 135 HV after 400
h at 65°C.
For the T9I4 temper, the solution
treated and quenched alloy was aged
1.5 h at 177°C and again quenched,
the hardness then being 114 HV. Cold
working 10% raised this value to 123
HV, which subsequently increased
to 143 HV after 400 h at 65°C, a value
slightly higher than for the T8I4
temper.
T77I4 temper: This T6I4-based
temper has been developed as po-
Peak hardness, as-received
T6 material
Retrogression heating
190
at 177°C
Secondary
hardening
occuring at
low
temperature
180
Quench
170
10–3
10–2
10–1
100
101
Time, h
102
103
Fig. 4 — Example of the T77I4 temper for
aluminum alloy 7075. T6-aged material is
put through a retrogression treatment that
causes dissolution of precipitates (solute “redissolves”) so that secondary precipitation
can occur.
T6I6 Temper: Better Properties
The T6I6 temper is completed
when an alloy that has undergone a
complete or partial T6I4 temper is
then aged again at an elevated tem-
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200
T6I6
Hardness, HV
180
160
140
150
120
140
T6
100
130
100
200 300
80
10–1
100
101
102
Time, h
Fig. 5 — Interrupted-aging hardness curves and microstructures for the Al-Cu-Mg alloy
2014. The dashed (vertical) line in the main diagram is represented by the inset plot, and shows
secondary hardening (vertical axis, HV) as a function of dwell time at 65°C (horizontal
axis, h). In addition to its refined microstructure, T6I6-temper material contains 75% more
S(S’)-phase (Al 2CuMg) rods than are present after a T6 temper. (The S(S’)-phase rods are
viewed end-on and appear as “dots” in the photomicrographs.)
Table 3 — Tensile properties of T6 and T6I6-temper alloys
Temper
Yield strength,1
MPa
Tensile strength,
MPa
Elongation, %
Al-4Cu
T6
T6I6
236
256
325
358
5
7
2014
T6
T6I6
414
436
488
526
10
10
Al-Cu-Mg-Ag
T6
T6I6
442
502
481
518
12
7
6061
T6
T6I6
267
299
318
340
13
13
6013
T6
T6I6
339
380
404
416
17
15
7050
T6
T6I6
546
574
621
639
14
14
7075
T6
T6I6
505
535
570
633
10
13
8090
T6
T6I6
349
391
449
512
4
5
357
T6
T6I6
287
341
340
375
7
5
Alloy
1. 0.2% offset.
Table 4 — Fracture toughness
of T6- and T6I6-temper alloys1
√m
Fracture toughness, MPa√
Alloy
T6
T6I6
Al-Cu-Mg-Ag
2014
60612
7050
8090
357
23.4
26.9
36.8
37.6
24.2
25.5
30.3
36.2
58.4
41.1
31.0
26.0
1. Average results for three or more tests of each alloy. S-L specimen orientation. 2. Plane strain conditions not possible for alloy 6061.
26
perature for a time to reach peak
strength. Typically, average improvements of 10 to 15% in
hardness, yield strength, and
tensile strength occur during
this aging procedure, with simultaneous improvements to
fracture toughness in most alloys. Changes that may occur in
microstructure are shown for
the Al-Cu-Mg wrought alloy
2014 in Fig. 5, together with the
respective aging curves for the
T6I6 and T6 conditions. Both the
sizes and dispersion of precipitate phases (θ’ and S(S’)) have
been modified by the T6I6
temper. Moreover, quantitative
analysis of these phases has
shown that the number of S(S’)
precipitates is increased by
some 75% as a result of the T6I6
temper. In other words, the mix of
precipitate species has been changed.
Tensile property comparisons of a
range of alloys aged to the T6I6 and
T6 tempers are shown in Table 3, and
fracture toughness values are shown
in Table 4. For wrought aluminum
alloy 8090, significant improvements
in fracture toughness are produced
by a T6I6 temper, whereas this did
not occur for the T6I4 treatment.
Several other aging schedules have
also been derived from the basic T6I6
temper. Descriptions follow.
T8I6 and T9I6: As with the T8I4
and T9I4 tempers, these tempers incorporate cold work in their aging cycles, again either before or after preliminary artificial aging (see Table 1).
Tensile properties of alloys 8090-T8I6
and Al-4Cu-T8I6 are shown in Table
5, and are compared with values for
their conventional T8 tempers.
The tensile properties of alloy 6056
in five tempers, three of which include a stage involving 3% cold
work, are given in Table 6. The application of cold work during interrupted-aging tempers may produce
additional benefits to the mechanical
properties of some alloys. This is particularly true for the T9I6 temper,
where improvements may be greater
than those observed for T6I6-temper
material. The effect of cold work in
the T8I6 temper, however, may
slightly reduce the magnitude of improvements noted for the T6I6
temper.
T6I76 temper: This T6I6-derived
temper involves a relatively high
final aging temperature (Table 1). As
for the T77I4 temper, the T6I76 treat-
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ment was also designed to promote
a higher resistance to stress-corrosion cracking. Experiments with
alloy 7050 have shown that the T6I76
treatment modifies the microstructure while retaining tensile properties equivalent to those for the T6
temper. Work is ongoing to characterize the microstructure-property
relationships in both the T77I4 and
T6I76 tempers, and preliminary results indicate that resistance to corrosion and stress-corrosion may be
improved in both cases.
Table 5 — Effect of cold work (CW) on tensile properties
Exploiting the Paint-Bake Cycle
The automotive industry is increasing its use of aluminum alloys
for applications that include internal
and external body panels. Prior to
final vehicle assembly, much of the
body sheet is painted and then
dried/cured in an oven at 150 to
180°C. Because this “paint-bake” temperature is similar to that used for artificial aging, it is common practice in
the automotive industry to simultaneously perform some of the required
age hardening during paint baking.
However, the relatively short paintbake time (typically less than an hour)
T6
T6I6
T651/T8 (3%CW)
T8I6 (3%CW)
T9I6 (3%CW)
Temper
Yield strength,1
MPa
Tensile strength,
MPa
Elongation, %
Al-4Cu (5% CW)
T8
T8I6
242
265
339
358
7
7
8090 (5% CW)
T8
T8I6
414
441
495
518
5
5
Alloy
1. 0.2% offset.
Table 6 — Effect of cold work (CW) on tensile properties
of aluminum alloy 6056
Temper
Yield strength,1 MPa
Tensile strength, MPa
Elongation, %
350
381
342
363
398
417
416
386
389
433
17
16
16
16
19
1. 0.2% offset.
is insufficient to achieve alloy
strengths close to T6-temper values.
The generic CSIRO T6IX processes
lend themselves well to paint-bake
processing, as the component can receive its solution treatment, quench,
and short primary elevated temperature aging step prior to leaving the
supplier’s plant. It can then undergo
secondary precipitation during
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transportation, storage, machining,
and forming operations, before receiving further strengthening during
the paint-bake cycle, which represents the final stage of the T6I6 treatment. In this way, strengths much
closer to normal T6 values can be
achieved, or occasionally exceeded,
in the finished component. Alternatively, the paint-bake cycle can be
27
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2
3
4
Position
5
Hardness, HBS 5/250
(a)
100
90
80
1
2
70
Hardness, HBS 5/250
(b)
(c)
28
100
3
Position
Standard T6
T6I4-25°C
T6I4-65°C
90
80
1
2
Standard T6
T6I6 PB1
T6I6 PB2
4
5
70
3
Position
4
5
used as the first stage of a T6I4
temper. No additional primary artificial aging step is required.
Examples: The graphs in Fig. 6
show the changes in hardness that
occur in sections of automobile
wheels, Fig. 6(a), cast from alloy 356
and given the T6I6 (b) and T6I4 (c)
tempers. Hardness data also are
plotted for material given the standard T6 heat treatment, followed by
a normal paint-bake thermal cycle.
In the T6I6 case, Fig. 6(b), preliminary underaging was conducted for
a time substantially shorter than that
normally used for full T6 processing.
The alloy was then held at room temperature to simulate the time required for normal machining and
finishing operations, and then subjected to a typical paint-bake cycle.
Fig. 6 — T6IX technologies compared with
T6 as applied to wheel sections where a paintbake cycle is used. (a) Positions within the
wheel at which hardness was measured. (b)
Hardness plots of two T6I6 variants incorporating a paint bake (PB) cycle as the final
stage of heat treatment. (c) Hardness plots of
two T6I4 variants following secondary aging
for 1 month at either 25°C (black squares) or
65°C (red squares).
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The T6I6 PB1 and PB2 curves are for
two different plant schedules.
In the T6I4 case, Fig. 6(c), the only
artificial aging conducted was a simulated paint-bake cycle for alloy 356,
followed by a period of secondary
aging at either 25 or 65°C.
The data show that hardness in the
wheel’s critical spoke and rim regions is improved over conventional
T6 practice by both T6I4 and T6I6
treatments. In addition, the shorter
elevated-temperature heat treatment
cycles made both T6IX tempers more
cost effective than the conventional
T6 temper as hardness increases
were achieved with cost savings.
Other examples of the effectiveness of the T6I4 process for paintbake schedules can be seen in Table
2, where, for example, the time of artificial aging for a T6 temper in alloy
357 can be reduced from 8 hours to
about 1 hour (followed by secondary
aging), to achieve similar tensile
properties together with enhanced
fracture toughness.
Implications for Heat Treaters
Application of T6I4 technology, in
particular, facilitates a significant re-
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duction in the duration of artificial
aging and thus cost and energy of
heat treating, when compared with
a conventional T6 temper. For some
applications, where a paint-bake
cycle is involved, a T6I6 temper may
also be used to reduce the artificial
aging time. Apart from the energy
and the work-in-progress savings
this represents, there is a significant
opportunity to take further advantage of the process through the design of continuous systems that can
use small furnaces that take up less
space in the plant.
Conclusions: By taking advantage
of the positive effects of secondary
precipitation, CSIRO has been able to
develop two competitive heat treatments (T6IX) that are applicable to all
age-hardenable aluminum alloys and
offer the potential to improve alloy
properties and/or reduce processing
costs. The T6I6 heat treatment can be
tailored for improvements in strength,
ductility, toughness, and fatigue resistance, while the T6I4 process focuses on reducing the cost of heat
treating. The ability to incorporate
product-specific thermal treatments
(such as a paint-bake cycle) into these
processes further demonstrates
their flexibility.
11. R.N. Lumley, I.J Polmear, and A.J.
Morton: International Patent Application
PCT/AU02/00234, 2002.
12. R.N. Lumley, I.J. Polmear, and A.J.
Morton: Mat. Sci. Forum, Vol.426–432,
303–308, 2003.
13. R.N. Lumley, I.J. Polmear, and A.J.
Morton: Conf. Proc. ICAA9, Brisbane, Australia, 85–95, 2004.
14. R.G. O’Donnell, R.N. Lumley, and I.J.
Polmear: Conf. Proc. ICAA9, Brisbane,
Australia, 975–980, 2004.
For more information: Dr. Roger N. Lumley is metallurgist/materials scientist, Dr.
Allan J. Morton is honorary fellow, and Dr. Robert G. “Rob” O’Donnell is senior research scientist, CSIRO Manufacturing & Infrastructure Technology, Private Bag 33,
Clayton South, VIC 3169, Australia; tel: 61 3 9545 2894 (Lumley), 2860 (Morton), or
2733 (O’Donnell); e-mail: roger.lumley, allan.morton, or robert.o’[email protected].
Prof. Ian J. Polmear is emeritus professor, School of Physics & Materials Engineering,
Monash University, VIC 3800, Australia; tel: 61 3 9905 3506; e-mail: ian.polmear@
spme.monash.edu.au.
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