Post on 15-Jul-2020
11 AluReport 01.2014
PRODUCTION
In the aluminium-processing industry,
several heat treatment process steps
that are part of a complex production
route are usually necessary to obtain the
desired properties of a material. Although
more expensive and highly complex, heat
treatment processes are absolutely essen-
tial in most applications and, if sufficiently
understood and properly used, definitely
are a key step towards producing the op-
timum material.
In general, heat treatment is a process of
changing the material properties by ther-
mal, thermochemical or thermomechan-
ical means [1]. Heat treatment processes
are aimed at optimizing the performance
characteristics of an aluminium product.
During casting, purely thermal treatment
is usually performed, whereas wrought
alloys experience an additional forming
process to heat treatment; this is what is
called „thermomechanical“ treatment of
material. Processes such as stress-relief
annealing, soft annealing, solution anneal-
ing, diffusion annealing, recrystallization
annealing, aging and hardening are aimed
at deliberately changing the microstruc-
ture, bringing certain elements and phases
into solution or transforming precipitates
of defined types, quantities and sizes [2].
#is article describes the processes occur-
ring during these steps and their effects on
the resulting material properties, illustrat-
ed by carefully selected examples. #e un-
derlying physical processes and the great
variety of treatment processes are too
complex to explain here in full, but we will
provide a useful overview below.
It is important to internalize the met-
al structure in order to understand heat
treatment: #e aluminium atoms are ar-
ranged in a face-centered cubic crystal lat-
tice, which means that they are located at
exactly defined sites. #at crystal is never
perfect, though. #e crystal lattice is dis-
torted by defects, which increases strength.
Crystal lattice defects may be caused, for
example, by foreign atoms with a radius
different from that of aluminium. In gen-
eral, there are zero-dimensional (point),
one-dimensional (line), two-dimensional
(surface) and three-dimensional (volume)
defects. Zero-dimensional defects are,
for example, lattice vacancies and foreign
atoms located at lattice sites, one-dimen-
sional defects are dislocations, two-dimen-
sional defects are grain and phase bound-
aries, and three-dimensional defects are
precipitates and pores. Such defects lead
to a significant increase in lattice energy,
and the material is not in thermodynam-
ic equilibrium. To compensate for that
imperfection by mass transfer (migration
of atoms), the atoms must be mobile and
have room to move. Lattice vacancies play
a decisive role [3], the number of vacancies
existing in the lattice being highly depend-
ent on temperature. In equilibrium, lattice
vacancies hardly occur at room tempera-
ture. #e probability of finding a specific
person on earth is 500 times higher than
finding a lattice vacancy in aluminium.
#e higher the temperature, the larger the
number of lattice vacancies. Shortly before
the metal melts, the probability of finding a
lattice vacancy is the same as finding a spe-
cific person in Kitzbühel. As the temper-
ature increases, the mobility of the atoms
also increases, and the atoms can migrate
through lattice vacancies by lattice ex-
change. #is process is referred to as diffu-
sion. Almost all heat treatment processes
are based on these two processes, and heat
treatment is aimed at exerting a systematic
influence on these two processes.
Each finished product that leaves AMAG
has a complex thermal history. In order
for a rolled product to have the property
profile specified by the customer, the al-
loy composition, forming parameters and
all heat treatment parameters must be
matched with one another along the en-
tire process chain.
As an example, Figure 1 shows the pro-
duction route for an AlCuMg-type sheet:
Alloys of the Al-Cu-Mg (2xxx), Al-Mg-Si
(6xxx) and Al-Cu-Mg-Zn (7xxx) families
are referred to as heat-treatable wrought
aluminium alloys. At AMAG, these alloys
are continuously cast into slabs. Particularly
in high-strength alloys, such as those of the
7xxx family, segregation (inhomogeneous
distribution of alloying elements) occurs
during metal solidification in addition to
extremely high residual stresses as a result
of non-uniform cooling due to physical rea-
sons. For this reason, such rolling slabs can
burst at room temperature even without ex-
ternal influence. To ensure that the employ-
ees can safely handle the slabs, high residual
stresses must be reduced by the so-called
stress-relief annealing process, in which the
material is heated to elevated temperature,
where the yield strength and the limiting
creep stress are significantly reduced and
stresses in the material can be reduced by
plastic deformation. #e subsequent cooling
process must be slow and uniform to pre-
vent new stresses from being introduced [4].
At AMAG, this process is often combined
with homogenization to eliminate any lo-
cal differences in chemical composition,
i.e., microsegregation, resulting from so-
lidification without any remelting of the
material. Concentration equalization of
alloying elements in the grain by diffusion
does not occur until very high annealing
temperatures are reached that approxi-
mate the temperature at which a fused
area may begin to form on the material, for
which long annealing times are required.
Accordingly, the homogenization of
wrought alloys is aimed at
producing fine dispersoids that are ho-
mogeneously dispersed (particles that
are often smaller than 100 nm), dissolving or spheroidizing coarse pri-
mary precipitates formed during so-
lidification (depending on whether the
phase is soluble or insoluble), eliminating inhomogeneous distribution
of alloying elements (inhomogeneous
and extremely saturated structure re-
sults in poor properties), dissolving phases with low melting
point (avoiding local melting during hot
forming),
Different phases with low melting point
(MgZn2, Al
2Mg
3Zn
3, Al
6CuMg
4) occur, for
example, in the well-known 7075 aero-
space alloy. #is alloy also shows a distinct
segregation of elements in the alumin-
ium mixed crystal. Additionally, a large
number of intermetallic phases occur as
a result of the large proportion of alloying
elements (primarily Cu and Zn). It is es-
sential to homogenize the microstructure
and dissolve particularly the intermetallic
phases with low melting point to prevent
fused areas from occurring during subse-
quent hot rolling.
10 AluReport 03.2013
Heat Treatment – A Brief IntroductionHeat treatment of heat-treatable aluminium alloys.
What actually is it and why is it so important for product quality?
Fig. 1: Simplified thermal history of an AlCuMg-type sheet material. Cold rolling is not required for plate products.
Melting
Casting
and
Solidification
Homogenization
Hot Rolling
Cold Rolling
Solution Annealing
Quenching
Stretching
Natural Aging
50 µm 50 µm 50 µm
Tem
pera
ture
Time
13 AluReport 01.201412 AluReport 01.2014
Figure 2 shows the differences in micro-
structure (a) prior to and (b) after homo-
genization. Homogeneously dispersed fine
dispersoids are also clearly visible.
After homogenization, the slabs are sawn
and milled, and heated to hot rolling tem-
perature in a slab heating furnace. #is
preheating process varies according to al-
loy and the required true strain and some-
times can replace separate homogeniza-
tion. #e hot rolling temperature must be
high enough to enable the material to be
appropriately formed (degree of rolling re-
duction) but in no case may be so high that
fused areas occur. It is absolutely necessary
to bear in mind that the energy input dur-
ing rolling is actually converted into ther-
mal energy and the material may heat up to
even higher temperatures during rolling.
#e key material characteristics are highly
dependent on correct temperature control
during rolling. Since variations of ± 5 °C
may have a clearly measurable impact on
the properties of the finished product, pro-
cess control as a function of the alloy and
continuous process control are a must. De-
pending on temperature control and com-
position, dynamic recrystallization may
occur as early as in the rolling process.
During dynamic recrystallization, both
solidification and softening processes
occur at the same time, resulting in sub-
grain formation in the microstructure.
Subgrains can be prevented from forming
or coarsening by selective addition of al-
loying elements. Here, too, it is essential
to have in-depth knowledge of the mech-
anisms of thermomechanical action and
of how to manage them. Any process an-
neals that may be required will soften the
material again and allow it to continue to
be formed [5].
Prior to solution annealing, sheet materials
are cold-formed, by which their strength is
significantly increased because new dis-
locations are formed, whereas elongation
after fracture decreases.
Subsequently, heat-treatable materials
such as the alloy families mentioned above
are solution-annealed. During solution
annealing, specific alloying constituents
(magnesium, silicon, zinc, copper) in the
aluminium mixed crystal are to be brought
into solution and insoluble portions are to
be made coalesce in a favorable manner
(e.g., silicon). #is is performed as near
the alloy solidus temperature as possible
to achieve fast kinetics during diffusion
and avoid long annealing times. At the
same time, however, the temperature must
be lower than the melting temperature of
phases with low melting point of the alloy
because fused areas will make the product
useless. AMAG has extensive expertise in
that area and state-of-the-art heat treat-
ment equipment.
Close cooperation with customers in
the aerospace industry has raised our
awareness of the issue of accurate tem-
perature control. For example, TUS
(Temperature Uniformity Survey) and
SAT (System Accuracy Test) certifi-
cations ensure the reproducibility of
heat treatment in our heat treatment
furnaces, which our customers in the
aerospace and automotive industries
and all customers whose products are
treated in our furnaces can benefit
from. !is technology provides precise
measurement to guarantee that the set
temperature is correct and maintained
throughout the furnace, even in cor-
ners and at the furnace entry section.
After solution annealing, the material
is rapidly cooled from red heat to room
temperature. In this quenching process,
the solution annealing condition is „fro-
zen in“, the proportion of dissolved, hard-
ening alloying elements being as large as
possible, with a high concentration of lat-
tice vacancies to ensure that the atoms are
mobile enough during the subsequent ag-
ing process and, consequently, maximum
kinetics of the subsequent precipitation
processes. Aging is a temperature- and
time-dependent process performed at
elevated temperature (artificial aging) or
at room temperature (natural aging), de-
pending on the material. During aging,
the foreign atoms that are dissolved in a
supersaturated state migrate through the
excess lattice vacancies by diffusion and
can form coherent (little difference from
the crystal lattice of the alloy), partly co-
herent and non-coherent precipitates.
Most of these particles are metastable,
and some of them can change the me-
chanical characteristics of the product
by evolution at room temperature, which
must be avoided with a view to the stor-
age life of forming materials by taking
special process steps.
Metastable phases affect the storage life
and also the behavior of the material
when reexposed to elevated tempera-
ture after storage: #e so-called T6 heat
treatment is the classical heat treatment
for the highest possible strength at good
elongation. In this process, particles of
optimum size are precipitated. #is pro-
cess is shown in Figure 3. If the material
should have maximum formability, it is
not artificially aged prior to processing
but delivered to the customer in the soft
T4 temper, which means that it is solu-
tion-annealed only (Figure 3).
If other characteristic values have to be
specifically adjusted in addition to the
classical mechanical properties, such as
strength and formability, heat treatment
may be required prior to or after solution
annealing treatment.
For example, fracture toughness and the
crack growth rate can be controlled by
performing an adapted heat treatment
process prior to solution annealing. Fig-
ure 4 shows an example of optimized
heat treatment of a 2xxx-series aerospace
alloy with improved damage tolerance
properties. #e sheets were produced in
two different rolling and heat treatment
cycles [6].
Homogenizing Furnace Horizontal Heat Treatment Furnace Continuous Heat Treatment Line
Fig. 3: Classical T4- and/or T6-heat treatment
process steps.Fig. 2a: 7075 microstructure prior to homogenization Fig. 2b: 7075 microstructure after homogenization
Heat Treatment Furnace
Solution Annealing
QuenchingTem
pera
ture
Time
Artificial Aging
T4 T6
14 AluReport 01.2014
Rp0
,2 [M
Pa]
Tensi
le s
trength
Rm
[M
Pa]
0
50
100
150
200
250
300
Rp0,2
Rp0,2
(T4/T4*)
Rm
Rm
(T4/T4*) (2% + 185C/20min) (2% + 185C/20min)
AA6016-T4/T6 AA6016-T4*/T6* with additional heat treatment
AluReport 01.2014 15
Condition „A“ material was solution-an-
nealed in a continuous heat treatment line
at a temperature of just under 500 °C and
a holding time of a few minutes, and then
quenched to room temperature using cold
water. A complete coil of condition „B“ ma-
terial was additionally annealed in a coil
furnace at a temperature between 300 °C
and 400 °C for two to five hours. After this
annealing process, the material was solu-
tion-annealed in the continuous heat treat-
ment line in the same way as condition „A“
material.
Figure 5 shows a three-dimensional rep-
resentation of the different grain sizes and
shapes of material in the conditions „A“
and „B.“ Condition „A“ material has a fine-
grained, equiaxed microstructure, where-
as condition „B“ material has a coarse
grain structure that is stretched in rolling
direction.
At the measured fracture toughness val-
ues, the coarse-grained „B“ material has a
lower crack growth rate. #e better perfor-
mance of the „B“ material is due to its ob-
long, coarser grain structure. A crack will
follow the path of least resistance, which
is normally found along the grain bound-
aries. In fine-grained microstructures, the
crack can therefore grow along a relatively
straight line. In a stretched, coarse grain,
the crack is deflected and must cover a
considerably longer distance, resulting in
a significantly reduced crack growth rate
[6]. Accordingly, fracture toughness and
crack growth can be optimized by modify-
ing the grain structure through heat treat-
ment, while at the same time substantially
improving the general resistance to corro-
sion. #is is possible by just changing tem-
perature control during the process.
An exceptionally complex sequence of
heat treatment processes is performed
when manufacturing automotive sheets
from 6xxx-series alloys.
Natural aging after solution annealing re-
sults in the formation of dispersed clusters
of foreign atoms and so-called GP zones
in the T4 temper. #e GP zones act as nu-
clei for the β“ phase („beta double prime
phase“) formed during artificial aging,
which increases strength. During artifi-
cial aging, however, small GP zones partly
dissolve and will no longer be available as
nuclei. It turned out that it was possible to
noticeably improve the size and thermal
stability of the GP zones by an addition-
al heat treatment shortly after quench-
ing from solution annealing temperature
(Figure 6). #is T4* temper describes
fast-hardening variants of 6xxx-series al-
loys, which, after another heat treatment
process (normally performed by the cus-
tomer), are in the T6* temper.
#e increased concentration of nucle-
ating agents leads to a faster response
to heat treatment. #e heat treatment
process can be integrated, for example,
into the cathodic dip coating process as
is common in the automotive sector, to
avoid separate artificial aging. #e ma-
terial must rapidly harden to its final
strength because paint drying times are
very short and low temperatures are used.
#is property is referred to as paint bake
response.
Additionally, significantly higher final
strength values can be achieved if appro-
priate material in the T4* temper is used
and properly processed (Figure 7).
If sheet for automotive applications is
surface-passivated in a strip passivation
line, the time-temperature curve of the
drying process after T4* heat treatment
must also be taken into account.
AMAG‘s high-strength 7xxx-series alloy,
TopForm UHS, which is used, for exam-
ple, in automotive structural components,
is delivered in the T6 temper and „warm-
formed“ by the customer at temperatures
of approximately 200 °C. In this thermody-
namic process, the high-strength material
is highly formable and is specifically over-
aged (Figure 8). In its T7 temper, the com-
ponent has excellent elongation and corro-
sion properties in addition to high strength.
Summary:
The examples shown are just a fraction
of the heat treatment processes used
by AMAG. A separate heat treatment
process tailored to the specific alloy
is required for each material and each
combination of properties requested by
the customer. With its abundance of
products (from products for aerospace
and automotive applications to sports
applications and general mechanical
engineering, sheets and plates made of
different alloys, with different property
levels), AMAG, unsurprisingly, sees the
greatest potential for developing and
improving aluminium for even more ef-
ficient use and additional applications
in an optimized heat treatment of the
material.
AMAG – Your competent develop-
ment partner for optimized heat
treatment
It is important to understand that any
thermal, thermomechanical and/or
thermochemical process step along
the production route has a significant
influence on the microstructure and,
consequently, on the properties of
the material. Hence, heat treatment
is an exceptionally complex issue.
AMAG benefits from the fact that all
families of wrought alloys (1xxx–8xxx)
in addition to cast alloys are produced
and processed mostly via recycling at
the Ranshofen location and thus has
know-how and equipment for all alloy
systems.
AMAG has well trained personnel
and its own comprehensive pi-
lot-plant equipment to optimize heat
treatment processes along the pro-
cess chain (laboratory furnaces for
homogenization, solution annealing
and artificial aging, experimental
casthouse and rolling mill, etc.).
Moreover, a broad range of simula-
tion tools are available; we can per-
form in-house thermodynamic (e.g.,
Pandat, Thermocalc, FactSage) and
kinetic calculations (MatCalc) in ad-
dition to process simulations (hot and
cold rolling), including fluid dynamic
processes (Nougat, Ansys, etc.). We
evaluate the formability of sheets with
their specific characteristics on the
component, through simulation and
field trials.
AMAG is continuously working on
developing and improving materials,
processes and simulation tools in co-
operation with research partners and
customers.
Fig. 8: Thermomechanical treatment
process steps, represented by a
time-temperature curve: Material
overaging by forming after T6 heat
treatment at elevated temperature for
improved forming behavior.
Fig. 5: (a) Three-dimensional representation of the
grain structure of condition „A“ (fine-grained, equi-
axed); (b) condition „B“ (coarse-grained, stretched
in rolling direction)
Fig. 6: Thermomechanical treatment process steps, including short-time
heat treatment after quenching (referred to as T4*), represented by a
time-temperature curve.
a)
a) b)
b)
Fig. 4: (a) Schematic of the rolling cycle and heat treatment cycle for (a) condition „A“ and/or (b) condition „B“
Solution Annealing
Tem
pera
ture
Time
Artificial Aging
Warm Forming
T7
T4*
Hot Rolling
Cold Rolling
Solution Annealing
Quenching
Artificial Aging or
Paint Bake Process
Tem
pera
ture
Time
T6*
Fig. 7: Increase in strength by additional heat treatment to T6*.
Thermomechanical T7-Heat treatment
LITERATURE:
[1] H. Kugler, „Umformtechnik, Umformen metallischer Kon-
struktionswerkstoffe“, Fachbuchverlag Leipzig im Carl Hanser
Verlag, München, 2009, ISBN 978-3-446-40672-8
[2] C. Kammer, „Aluminium Taschenbuch 1, Grundlagen und
Werkstoffe“, 16. überarbeitete Auflage, Aluminium Verlag Mar-
keting & Kommunikation GmbH, Düsseldorf, 2009, ISBN 978-
3-87017-292-3
[3] H.J. Bargel, G. Schulze (Hrsg.), „Werkstoffkunde“, 8. über-
arbeitete Auflage, Springer Verlag, Berlin, 2004, ISBN 3-540-
40114-8
[4] D. Altenpohl, „Aluminium von Innen“, 5. Auflage, Alumi-
nium Verlag Marketing & Kommunikation GmbH, Düsseldorf,
2005, ISBN 3-87017-235-5
[5] E. Hornbogen, H. Warlimont, „Metallkunde, Aufbau und
Eigenschaften von Metallen und Legierungen“, 2. überar-
beitete Auflage, Springer Verlag, Berlin, 1991, ISBN 3-540-
52890-3
[6] J. Berneder, R. Rachlitz, C. Melzer, H. Antrekowitsch, P.
J. Uggowitzer, „Influence of the grain size on the IGC, crack
propagation and fracture toughness behaviour of AA2024-T3
sheet material“, TMS 2010, 139th Annual Meeting & Exhibiti-
on, 14. – 18. Februar 2010,
Hot Rolling
Cold Rolling
Solution Annealing
Tem
pera
ture
Time
Hot Rolling
Cold Rolling
Solution Annealing
Process Annealing
Tem
pera
ture
Time