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