aluminium technologies 1.12.2015

# Name Date Hours Presentation title

1 Cenk Eken 15 Aralık 10:50-11:20 Ultrasonic processing of aluminium melts

2 Murat Teke 11:30-12:00 Assesment of melt quality via K-mold method

3 Alper Güneren 22 Aralık 09:30-10:00 Friction stir processing of aluminium alloys

4 Haluk Erdemir 10:10-10:40 Additive manufacturing of aluminium alloys

5 Gazican Özkan 10:50-11:20 Squeeze casting of aluminium alloys

6 Mehmet Yasak 11:30-12:00 High integrity casting processes

7 Erkut Özer 29 Aralık 09:30-10:00 Casting of hypereutectic Al-Si alloys and their applications

8 Serhan Köktaş 10:10-10:40 Rheocasting of aluminium alloys and their applications

9 Kürşat Kambur 10:50-11:20 Severe plastic deformation processing of aluminium alloys

10 Engin Kılınç 11:30-12:00

11 Mehmet Ağılkaya 5 Ocak 09:30-10:00 Steel vs aluminium for automotive applications

12 Emre Baran 10:10-10:40 Production of aluminium foams

13 Berkay Oral 10:50-11:20 FSW of automotive aluminium

14 İzzet N. Demir 11:30-12:00 Casting of Al-Mg based foundry alloys and their applications

production routes of wrought

aluminium alloys

(semi) continuous casting direct chill (DC) casting

round section – billet extrusion profiles

slab – ingot hot/cold rolling sheet/foil

semi-continuous casting

Twin roll casting (TRC) cold rolling sheet/foil

Twin belt casting (TBC) hot/cold rolling sheet

continuous casting

● DC casting was invented in the early 1930s (VAW-

Germany and Alcoa-USA).

● Today it is the premier process for producing

aluminium shapes suitable for subsequent

processing in extrusion, hot/cold rolling operations.

● Around ten million tons/year of aluminium is DC

cast worldwide.

● The process is also used to cast copper, zinc and

magnesium.)

Direct Chill (DC) casting

● DC casting of aluminium provides the link between

liquid metal, as obtained from reduction cells or

from scrap melting, and the semi-fabricator.

Products include:

● large rectangular sections known as rolling blocks,

rolling ingots or slab (typically ≈ 500 x 1500-2000

mm in section) which are used by rolling mills for

plate, sheet or foil production.

● Large circular section ingots (typically 150-400mm

dia) known as billets which are used by extruders

for various extrudates-profiles

DC casting

DC casting/circular sections

DC casting/rectangular sections

DC casting/shaped sections

DC casting/rectangular sections

DC casting/rectangular sections

● A furnace, or more often multiple furnaces, contain

the liquid metal allowing it to be alloyed.

● The furnace tilts, or a drain or plug hole is opened,

initiating a flow of metal along the launder.

● In-line treatment is usually employed to remove

dissolved hydrogen, alkali metals and solid

impurities prior to entering the casting station.

DC casting

Basic layout of a typical vertical direct chill casting installation

Alloyed and refined liquid metal is supplied to the

caster. Distribution can be by separate launders to

each mould with individual flow control or by a

flooded table,

where the moulds

are mounted in a

common water

jacket and the

metal flows

through a

refractory pan

mounted on top.

DC casting

DC casting/circular sections

It is typically in the 1-3

mm/s range.

The casting

speed

depends on

alloy and size

● DC casting produces ingots of uniform cross

section, initially by containment of the liquid

metal in a cooled mould and then by direct cooling

of the casting.

● the cooling medium is water, both for the mould

cooling (primary cooling) and the direct or

secondary cooling.

● The solid moves out of the mould, liquid is fed

into the mould opening and is contained by the

water cooled mould.

DC casting

Surface formation during DC casting

The direct chill

water spray

cools the ingot

as it emerges,

extracting

enough heat for the solid

shell to form above the

spray inside the mould and

contain the liquid.

● At the start of casting, the open ended metal

mould has to be plugged with a starting head or

“dummy” block to contain the liquid and allow the

cast to proceed.

● Metal is poured into the water cooled mould,

freezing onto the starting head.

● After a delay, the starting head is lowered into a

pit.

● casting stops when the bottom of the pit is

reached.

DC casting

Horizontal DC casting

Molten

aluminium

Water

chilled

mould

Water spray

Secondary cooling

primary cooling

Extrusion

log

In the horizontal version, a flying saw

cuts the ingot to length as it emerges and casting can be fully continuous.

● All wrought alloys may be DC cast.

● Circular sections, known as rounds or billets, up to

1.1 m diameter, are usually sent to extrusion

operations but can be used to supply forging

presses.

● 6000 alloys are used for most extrusions, although

high Si foundry alloys, high conductivity 1000 series

(3000 and 5000) and high strength 2000 and 7000

series alloys can also be DC cast.

● Remelt ingots, both pure and alloyed, in small

rectangular sections (eg. 150 x 50 mm) or large T

sections (500 x 1200 mm), can be produced.

DC casting alloys

Surface temperature of DC cast ingots

impact point

dis

tance

Effect of Casting Variables ● if one increases casting speed, heat input increases,

leading to increased temperature gradients and

greater diffusive heat flow, which balances the

increased heat input.

● Similarly, a consequence of larger ingot size is higher

heat input and a longer diffusion path. In practice,

casting speed is reduced as diameter increases.

● The casting temperature has a small effect on the

heat flow as the specific heat of the liquid is only

~4-5% of the total heat input.

● Latent heat accounts for about 35% of the heat input

and specific heat of the solid the remainder.

Effect of Casting Variables

● Since the specific and latent heats for the various

aluminium alloys are very similar, variation in

temperature distribution from alloy to alloy is

due to changes in thermal conductivity.

● As alloy content increases, thermal conductivity

decreases, the pool depth increases and

temperature gradients increase.

● Alloy content also determines the liquidus and

solidus temperatures, ie. the freezing range.

Solidification ● Under the influence of the temperature distribution,

solid forms where the liquid temperature goes below

the liquidus for a given alloy.

● TiB2 grain refiner is added to provide nucleation sites

for the formation of solid alpha aluminium crystals,

giving a very fine equiaxed structure.

● If casting is performed without grain refiner, the

classical grain structure of a columnar exterior and

equiaxed centre results.

● The smaller equiaxed grain size prevents cracking at

normal casting speeds.

● In addition to the level of grain refiner, cooling rate

also determines the fineness of microstructural

features such as dendrite arm spacing (DAS), grain

size and intermetallic particle size.

● The cooling rate decreases from surface to centre as

the distance from the water spray increases.

● This gives rise to a variation in microstructure from

the surface to the centre which is particularly

apparent with larger castings.

● In many cases, this variation is not important.

● However, for some AlFe alloys it causes a change in

intermetallic particle phase producing a fir tree

structure.

Solidification

● This in turn causes anodising streak defects.

● The volume fraction, shape and size of

intermetallics will also vary due to cooling rate

variation from edge to centre.

● Very little can be done about this variation as it is

a function of the size and thermal diffusivity of the

ingot.

Solidification

Composition is the controlling factor for microstructure. Apart from composition, ingot dimension thermal conductivity are the main parameters affecting cooling rate and refinement of microstructure.

Solidification

Fine equiaxed structure

of a grain refined DC casting

Grain structure of a non grain refined 178 mm diameter billet

Measured grain size showing coarse microstructural zone in DC cast 3004 alloy.

Solidification

Macrosegregation

● In larger DC castings, significant compositional

variation is found.

● Elements that form eutectics with aluminium rise

in concentration away from the edge of the

casting, with the centre being depleted.

● Elements that form peritectics with aluminium

show a reverse trend.

● This composition difference is likely to give rise to

a variation in properties across the section when

extruded or rolled.

Macrosegregation

Hot-Top Casting ● Insulation placed at the top of the mould was

introduced to control the location of the initial

contact point of the liquid against the mould.

● In this way, the liquid metal contact point was

fixed at a point below the insulation and was

independent of the metal level used.

● This minimizes reheating and improves surface

quality.

● Modern hot top moulds are similar in principle,

with a refractory header inserted into the top of

the mould.

Liquid metal

mould

Primary cooling

Water jets

Secondary cooling

Casting speed

solid ingot

liquid+solid

sıvı

Hot top

DC casting/circular sections

Simple hot top mould formed by addition of thin insulating paper to an open top mould

Air Assisted Hot-Top

Gas pressurised

hot top process

DC casting

Electromagnetic Casting (EMC) ● The electromagnetic casting mould uses an

inductor coil through which a high frequency

(typically a few kHz) current is passed.

● The currents induced in the liquid metal interact

with the magnetic field of the coil.

● This produces a restraining force on the liquid

which acts against the metallostatic head pushing

the metal away from the mould.

● Lack of contact between liquid and mould

eliminates mould cooling and the problem of

reheating, producing very good surface

microstructures.

Electromagnetic casting

extrusion

● The extrusion process involves a preheated billet

being squeezed through an opening in a steel die

forming the cross-section of the extrusion or

profile.

● This may be a simple or an intricate form and the

cross-section may be solid or hollow.

● The most widely-used extrusion alloys are 6xxx

series, being the easiest to extrude in very

complex cross-sectional shapes.

● Materials with a higher alloy content are more

difficult to extrude and are therefore not

available in complex shapes.

extrusion

● The extrusion process gives designers and users

unique opportunities:

● Cross-sections of profiles with all kinds of special

functions – such as grooves, joining details,

stiffeners – which reduce fabrication and

installation costs as compared to rolled steel

products

● Extrusion costs are relatively low.

● Extrusion is therefore already beneficial for small

production series

extrusion

extrusion

● aluminium takes full advantage of the extrusion

process.

● Other metals can be extruded but few with the ease

of aluminium and its alloys.

● Aluminium’s high strength-to-weight ratio, and its

ability to be extruded into any shape – no matter

how complex, with tight tolerances, make it an ideal

material for design applications which require

maximum versatility from a cross-sectional area.

● Aluminium extrusions are also highly cost effective

since they need virtually no machining or

maintenance.

Aluminium extrusion

● Aluminium extrusions are produced by heating

aluminium billets to approximately 500°C and

forcing the hot metal through a steel die.

● As the extruded section emerges it is cooled and

cut to the desired length.

● Heat treatment is then used to optimise the

material’s inherent mechanical properties.

● Computer aided design and manufacture is used to

reduce design time, improve tolerances, and

control each stage of the process more accurately.

Aluminium extrusion

Less fabrication: there is greater design potential

because intricate shapes may be extruded, reducing the

need to manufacture component parts for future

assembly. Some shapes achievable by extrusion are

unattainable by any other process.

Low prototype costs: the cost of tooling aluminium

extrusions is low compared to that of roll forming,

casting, forging and moulding of competing materials.

Great structural efficiency: optimum structural

efficiency is possible, as, by using extrusions, metal can

be placed where it adds strength and omitted where it is

not needed.

Benefits of extrusion

Less weight: extruded aluminium is lightweight yet strong

and durable. Differences in properties between aluminium

and competing materials can result in aluminium structures

being up to half the weight of equivalent structures made

from other heavy materials that are not so readily

fabricated.

A wide range of attractive, corrosion-resistant finishes:

can be achieved through powder or electrophoretic coating;

designers can specify virtually any colour they wish. Natural

silver or colour anodized films are also available.

Less maintenance: aluminium is a naturally durable metal

and the surface finishing described above further enhances

durability.

Benefits of extrusion

Extrusion alloys

6xxx extrusion alloys Role of Mg and Si

● 6XXX series alloys are typical heat-treatable

aluminium alloys which gain their strength from

thermal processing rather than mechanical

deformation.

● The key elements of these alloys are Mg and Si

which combine to form the Mg2Si precipitates.

● These precipitates occur in several forms which

may be divided into the following 3 categories.

6xxx extrusion alloys Role of Mg and Si

● (beta double prime) Mg2Si, the smallest type of

Mg2Si precipitate that is rod-shaped and

contributes most to mechanical properties when

densely dispersed.

● (beta prime) Mg2Si: a larger version of rod-

shaped precipitate that grows from category.

● The precipitates have a negligible contribution

to mechanical properties.

● (beta) Mg2Si: the largest Mg2Si precipitate that is

cube-like in shape and due to its size, contributes

nothing to mechanical properties.

● Most 6XXX alloys are designed to have either

balanced Mg and Si levels or an excess of Si.

● The diagonal line in the next Figure is for a balanced

composition, assuming that the Fe content in these

alloys is %0.16.

● Excess Si alloys are preferred over excess Mg alloys,

because, excess Mg does not add to the final

mechanical properties of the product.

● Excess Mg increases the flow stress of the alloy and

makes it difficult to extrude.

● Excess Si aids the artificial ageing response and so

increases the strength of the final product.

6xxx extrusion alloys

Extrusion alloys ● Hardening by Mg2Si precipitation

● Hardness increases with the increase in the

amount of Mg2Si!

● Fe content must be limited to improve the

surface quality (particularly in the case of

high extrusion speeds!)

● Mn and/or Cr is also added to the alloy in

the relatively higher strength 6061 and

6082 alloys to control recrystallization!

Extrusion alloys

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

AA 6082

AA 6005 A AA 6060

Si %

Mg %

AA 6063

Low strength

Medium strength

AA 6061

AA 6063

Excess Si Excess Mg

Role of alloying elements Iron ● Iron is also present in 6XXX alloys and combines with

Si and Al to form AlFeSi intermetallics.

● These intermetallics do not contribute to the

strength of the alloy but, if they are not correctly

processed, they will have a detrimental effect on

the extrudability of the alloy.

● Accurate control of Fe contents in 6XXX series alloys

is important for surface finishing applications.

● Different levels of Fe will cause variations in colour

response during anodising. Fe will also reduce

conductivity.

Role of alloying elements Manganese ● Mn is used in a number of ways in 6XXX series alloys. Mn

helps to reduce the homogenization times by promoting the

transformation of -AlFeSi to -AlFeSi and

● Helps to prevent coarse grain growth during post extrusion

heat treatments of medium strength alloys such as 6061 and

6082.

● Another benefit is that the fracture toughness can be

improved by additions of Mn as it helps to prevent the

nucleation of Si at the grain boundaries which embrittles

the material.

● In higher strength 6XXX seris alloys with significant Mn

additions (e.g. > %0.10), the detrimental effects of Mn are

an increase in flow stress of the billet which decreases

extrudability, as well as increasing quench sensitivity.

Role of alloying elements Copper

● Cu additions may improve conductivity and

machinability of the extruded alloy

● counteracts the detrimental effect of room

temperature storage on the mechanical properties

of artificially aged high strength alloys,

particularly 6061.

● The corrosion resistance of 6XXX series alloys is

lowered by the presence of Cu once the level

exceeds %0.2.

Role of alloying elements Chromium

● Cr acts in much the same way as Mn but its effect

on quench sensitivity is more pronounced than

Mn.

Zinc

● Zn is not known to have any detrimental effect on

the mechanical properties of 6XXX alloys.

● However, in amounts greater tha %0.03 Zn may

cause a differential effect known as “spangle”

during the anodising process.

Role of alloying elements

Extrusion processing cycle

melting DC casting homogenization preheating

Extrusion cooling stretching

cutting

storing

forming

ageing

anodising

painting

fabrication

Thermomechanical process

casting

Homogenization

soaking

cool-down : cooling rate is critical

Pre heating

Extrusion

quenching ageing (T5)

solutionizing

ageing (T6)

Extrusion processing cycle

DC casting

Quality characteristics of cast

billet ● Minimum surface segregation;

● Shell zone as shallow as possible (<200 m)

● Section must be free of inclusions and porosity!

● Uniform structure across the section:

● Minimum segregation

● Fine and equiaxed grain structure, uniform

across the section (<200 m)

● Uniform distribution of intermetallic

compound particles

Cast billet

Thermal cycle of 6XXX series alloys

● 6XXX alloys obtain the maximum strength from the

following heat treatment cycles:

● Solutionizing during extrusion or solution heat

treatment

● Followed by quenching at a rate that is alloy

dependent and

● Then precipitation age hardening

● Maxiumum mechanical properties cannot be

obtained without accurate temperature control

which regulates the Mg2Si characteristics

throughout the process.

Extrusion processing cycle

200 -500 C/h

200 C/h

440-490 C

185 C

580 C

5-45 min

induction/

gas

0.5-1 min

(5 s deformation

zone

2-3 h

600 C

Forced air

/water

quenching

5 h

DC casting homogenization preheating extrusion cooling ageing

700 C

Cast 6XXX alloy billet

Extrusion process cycle

Cast ingot time

heating soak cool down pre extrusion rapid

Heating cooling

tem

pera

ture

gas

quenching

solvus

Heat treatments are not independent! The process

must be designed with a consideration of their effect

on the subsequent steps!

Homogenization anneal

300 660 900

600

300

tem

pera

ture

(C

)

time (min)

heating soak cool down

soak: segregations must be eliminated/

transformation must be completed

Cool down: neither too fast nor too slow/ optimum

400C/st

homogenization Temperature

Annealing t

ime

550 C

8 h

580 C

2 h

homogenization ● Marked improvements must be achieved in the as-cast

structure:

● Detrimental effects of Fe must be reduced by encouraging

transformation

● To minimize dendritic segregation

● To modify Fe-bearing compound particles

● To solutionize Mg2Si phase during soaking and precipitation

of Mg2Si phase in a suitable form during the cool down.

● Precise T control to avoid incipient melting (<570C).

Homogenization anneal

Cooling of billets to room temperature after homogenization

is a critical operation!

Cooling is performed with forced air or with water mist in

batch operations. It is difficult to maintain rapid cooling and

control cooling rates!

New technology cool down

homogenization The as-cast billet consists of a solid Al matrix

containing dissolved Mg and Si as well as a network

of -AlFeSi intermetallics located at the grain

boundaries

For 6XXX alloys not containing additions of Mn, longer holding

times in the homogenization cycle would be necessary to

transform -AlFeSi to -AlFeSi.

Typically, a cycle would be 4 to 8 hours at 575 C, followed by

similar cooling for Mn containing alloys.

homogenization

● Homogenization cycle is specifically designed to modify the

as-cast structure to one which can be extruded at high

speed while achieving excellent surface finish and optimum

mechanical properties.

● For low strength 6XXX alloys containing Mn, a typical

homogenization cycle consists of heating the billet to 575C,

holding for 2 to 4 hours and then cooling the billet at a

controlled rate through the critical range of 450C to 200C.

● This cooling practice avoids the formation of -Mg2Si and -

Mg2Si precipitates in the microstructure which are

detrimental to the final mechanical properties of the

extrusion.

● The actual cooling rate used is dependent on the type of the

alloy and the billet diameter.

homogenization

Homogenization cycle:

● Transforms hard, needle-like -AlFeSi

intermetallics (detrimental to extruded surface

finish) to more globular -AlFeSi intermetallics.

● Ensures an even distribution of chemical

elements in the alloy.

● Nucleates a large number of fine, evenly

dispersed and -Mg2Si precipitates that are

readily dissolved during extrusion.

homogenization

homogenization

Cool down

200C/h 350C/h

600C/h

preheating ● Preheating must be carried out below the solvus

temperature!

● Preheating must be quick and be synchronized

with the extrusion press operation.

● Induction heating must be preferred as it is quick

and as it offers precise temperature control.

● In the case of gas preheating, the process must be

performed so as to minimize the time elapsed

above 360 C.

● Mg2Si precipitates rapidly above 360 C. It may be

difficult to solutionize these coarse precipitates

solely with the frictional heating inside the press.

preheating

Pre-heating ● The purpose of pre-heating billets for extrusion is to

lower the flow stress of the alloy to allow extrusion at

maximum speed while maintaining an excellent

surface finish and mechanical properties.

● The preheat temperature used must be sufficient to

dissolve -Mg2Si precipitates during extrusion.

● This will ensure that optimum final mechanical

properties are obtained subsequently with proper

press quenching and artificial ageing.

● A typical preheat temperature for dilute 6XXX alloys is

450 C but this may vary depending on how difficult a

section is to extrude, the alloy type and the particular

mechanical properties/surface finish requirements.

Pre-heating ● Two common type of billet preheaters are gas fired,

tunnel type furnaces and electrical induction heaters.

● The latter type of preheating is very rapid with billets

reaching temperature for extrusion in a matter of

minutes.

● The rapid heat up rates in induction heating allow for

littletime for the post

homogenized microstructure

of the billets to change.

● The homogenized

microstructure

mainly contains -Mg2Si

precipitates and some Mg

and Si in solution.

Graph of preheat rates and zones of and -Mg2Si formation.

Rapid induction heating: 200C/hr

Gas preheating: 20C/hr

Delayed gas preheating: 10C/hr

Pre-heating

● Mg and Si retained in solution will increase the

hot flow stress of the billet thus decreasing

extrusion speeds.

● However, the rapid heat up through the range

200C-450C is beneficial because it avoids the

formation of coarse -Mg2Si precipitates.

● Coarse -Mg2Si precipitates are not readily

dissolved during extrusion and avoiding this

phase enables optimum mechanical properties to

be reached.

● When using gas preheating the situation is almost

reversed.

Pre-heating

The slower heat-up allows the homogenized

microstructure to change so that Mg and Si may be

precipitated from

solution as Mg2Si

which lowers the

hot flow stress of

the billet allowing

greater extrusion

speeds to be used.

Pre-heating

Billet microstructure showing

coarse Mg2Si formed during a

delay in gas preheating

● The detrimental aspect of gas heating is the

possible reduction in final mechanical properties.

● Slower delayed preheating can coarsen the Mg2Si

as the temperature approaches 400 C.

● Mg2Si begins to form at around 400 C.

● The coarse Mg2Si requires higher exit

temperatures for dissolution and the Mg2Si will

not dissolve during extrusion.

● The reduced amount of Mg and Si in solution will

result in less than optimum mechanical properties

after artificial ageing.

Pre-heating

Direct extrusion process

extrusion

Manufacturing tubes via extrusion

● Aluminium billets are heated to 450-500 C,

depending on alloy, extrusion shape and

extrusion ratio) and loaded into a pre-heated

container (420-470 C).

● A hydraulic ram pushes the billet through the die

aperture at pressures up to 680 MPa.

● The hot metal flows through the die to produce a

continuous extrudate with a cross-section

identical to the die aperture.

● The shapes may vary from complex hollows to

simple solids.

Direct extrusion process

Factors affecting extrusion process

The solid line is the boundary between sufficient and

İnsufficient pressure to extrude through a given die over a

range of extrusion speeds and billet temperatures.

Press capacity

For maximum press productivity,

an extrusion press is operated

close to the maximum

running pressure which

usually corresponds to

maximum speed.

Factors affecting extrusion process ● This boundary may be moved to the left by using

soft (dilute) alloys or by utilising more available

pressure in order to maximise productivity.

● Harder alloys, higher extrusion ratios, or more

complex shapes will move the boundary to the

right.

● This will expand field A and reduce the extrusion

speeds (and productivity) obtained for a

particular press capacity (pressure).

● Using higher billet temperatures may compensate

for the reduction in extrusion speed but other

variables may limit the temperature increase.

Factors affecting extrusion process

Surface finish

Soft alloys

poor

surface

Factors affecting extrusion process Surface finish

● Complex shapes, high extrusion ratios and hard alloys

move this boundary to the left reducing the extent of

field B (extrusion possible).

● This shift may also correspond to the surface

deterioration such as surface tearing, pick-up and

melting in some extrusions.

● These factors reduce the extrusion speeds and billet

temperatures that can be used before the surface

finish breaks down.

● Coarse Mg2Si particles formed either due to slow post

homogenization cooling or slow preheat, also move

the surface finish boundary to the left (large field C).

Factors affecting extrusion process Mechanical properties

● Mechanical properties also impose some limits on the

extrusion process.

● The new boundary moves to the right by using billet

with coarse -Mg2Si precipitates, extrusion of thick

cross-sections and where higher mechanical property

levels are required.

● Extrusion speeds and billet temperatures must

therefore be increased for the extrusion to satisfy final

mechanical property requirements.

● This is to say the energy supplied to the billet to

dissolve Mg2Si must be increased.

Factors affecting extrusion process Billet containing and fine Mg2Si, thin cross-section

extrusions and lower mechanical property requirements all

move the boundary to the left. Constraints on extrusion

speed and

billet

temperature

requirements

are reduced

and final

mechanical

properties are

more easily

attained.

Extrusion operation window The combination of the press capacity (available

pressure) surface finish and final mechanical

properties

act as the

boundaries of

the operating

window

for a quality

product.

Extrusion operation window

● Where these boundaries are located depends on

actual product requirements for surface finish

and specific mechanical property limits.

● These boundaries also highlight where maximum

press productivity can be achieved.

● Extrusion speed and billet temperatures are

limited to within these boundaries.

● Other variables, such as alloy type, extrusion

ratio/shape and final extrusion properties, may

enlarge or reduce the size of the operating

window.

Press quenching/cooling

● Post-extrusion cooling rates must be fast enough

to retain Mg and Si in solid solution so that

mechanical properties are maximized by their

precipitation during subsequent age hardening.

● Cooling rates are determined by section size and

cooling type such as still air, fans, water mists or

water baths.

Schematic illustration of various cooling rates and

areas for precipitation of coarse Mg2Si.

Press quenching/cooling

Log TIME

Extr

usi

on T

Typical cooling rates required

for some 6XXX alloys

Heat treatment of 6XXX alloys

Mg2Si wt%

● The material is heated above the solvus line in

the phase diagram and is held inside the single

phase region until all secondary phases are

solutionized.

● Mg2Si in 6XXX alloys (and Al2Cu preipitates in

2XXX alloys) is completely solutionized by the

end of solution soak.

● Solution heat treatment temperature is between

500-550°C so as to avoid both the melting and

eutectic temperatures.

solutionizing

● Once the billet is solutionized, it is cooled to room

temperature rapidly enough to retain the Mg and Si in

solution.

● Hence, a supersaturated matrix is obtained.

● Quenching may be employed after a solution heat

treatment as well as after a high temperature forming

operation.

● For instance, the press exit temperature is often

higher than the solvus temperature and the extrudate

may be quenched right after extrusion.

● This quenching may be performed in a water tank as

well as with a fan.

Cooling-quenching

precipitation

● There must be a fine distribution of Mg2Si

precipitates (actually ) to achieve high

hardness values!

● Formation of coarse Mg2Si precipitates before or

during extrusion will lead to low hardness levels.

● Formation of such coarse precipitates is

inevitable if the material is held between 230 C

and the solvus temperature, 500 C, too long.

Precipitation in Al-Mg-Si system soft hardness peak hardness overaged

solid coherent partially incoherent

solution zones coherent precipitates

precipitates

(Mg2Si)

zones rods plates

Al atoms Mg/Si atoms

● The solutionized and quenched material is not

stable and is anxious to undergo precipitation.

● Mg and Si in solution is precipitated in forms so as

to maximize hardness.

● Room temperature natural ageing: hardness

increases in a marked fashion shortly after the

solution heat treatment and quenching with time

spent at room temperature.

● This hardness increase is prominent in the first

couple of hours but is markedly lower than the

hardness that can be achieved with an artificial

ageing treatment.

Artificial ageing

● The material undergoes ageing-precipitation if

held at room temperature after the solution heat

treatment.

● The precipitation hardening that occurs under

these circumstances is prominent within the first

couple of hours after quenching. It slows down

with time and stops completely.

● The hardening thus obtained is much lower than

that obtained with artificial ageing.

Natural ageing

● Precipitates that form during artificial ageing are

coherent with the matrix for a maximum hardness.

● In other words, the crystal structure of the

precipitates is the same weith that of the matrix.

● This leads to coherency stresses at the precipitate-

matrix interface and limits the mobility of the

dislocations producing high hardness.

● The coherency deterorates as the precipitates

grow and they finally form another phase.

● The coherency stresses are gradually reduced and

the matrix starts to soften.

Artificial ageing

Precipitation in Al-Mg-Si system

Equilibrium structure:

coarse -Mg2Si

precipitates at

grain boundaries

tem

pera

ture

time

Solution

heat

treatment

%100 solid solution

retained upon cooling

Fine dispersion of

precipitates within grain

(retained upon cooling Precipitation

heat treatment

Precipitation in Al-Cu system

tem

pera

ture

time

Solution

heat

treatment

%100 katı eriyiği

Hızlı soğutma ile oda

sıcaklığında korunmuş

Tane içlerinde

küçük çökeltiler

Artificial

ageing:/precipit

ation heat

treatment

Fine dispersion of

precipitates within grain

(retained upon cooling

%100 solid solution

retained upon cooling

Equilibrium structure:

coarse -CuAl2

precipitates at

grain boundaries

Peak hardness/optimum

precipitate size and

disitribution

Over aged/

Coarse precipitates Under aged/

Fine not developed

precipitates

Ageing time

Hard

ness

/st

rength

t

Supersaturated

solid solution

Precipitation in Al-Mg-Si/Al-Cu

systems