Applications of Phase Equilibria in Aluminium Alloy Products and ...

tsc

Applications of Phase Equilibriain Aluminium Alloy

Products and Processes

P.V. Evans and R.A. RicksTechnology Strategy Consultants

Commercial-in-Confidence© Technology Strategy Consultants

tsc Introduction

Aluminium alloys are generally not used in a state of thermodynamic equilibrium� many contain metastable precipitates or intermetallic phases

� with process routes designed to avoid attainment of equilibrium

Even alloys relying on solid solution strengthening are usually in a metastable state at their operating temperatures

However equilibrium can be of relevance during earlier stages of production, and at high temperature stages

G.A. Edwards et al. Acta Mat. 46, p3893 (1998)

Bright field TEM image of metastable β’’ precipitates in AA6061 (Al-Mg-Si alloy)

High resolution image down β’’ needle


tsc Equilibrium during processing aluminium

In some cases the thermodynamic understanding is embedded in the practices…

…in other cases it comes back to haunt us!

We will consider a few of examples drawn from the aluminium process stream1. virtually all aluminium undergoes some form of treatment whilst molten

– measurement and removal of dissolved hydrogen can be problematic

2. extrusion: a process stream designed around attainment of equilibrium in situ

3. rolled product development: a case of being wise after the event

melttreatment

casting

rolling(plate, sheet)

extrusion

othersemi-fab

fabrication

smeltermetal

secondarymetal

(recycled)

re-melting

shapecasting


tsc 1) Hydrogen in aluminium

Aluminium melt treatment generally focuses on improving 3 quality attributes� dissolved hydrogen, inclusions, alkali metals

Hydrogen is the major contaminant controlled by equilibrium considerations

Hydrogen is far more soluble in liquid aluminium cf. solid aluminium� solidification of a melt containing too much hydrogen results in porosity

example of internal porosity visible on machined engine face

surface blistering after homogenisation


tsc Effects of porosity

Fatigue life is also reduced as porosity increases

dendrite 47 µm

dendrite 22 µm

fatigue life variation with pore size

Y.X. Gao et al., Fatigue Fract. Eng. 27, p559 (2004)

fatigue failure surfaceand pore initiation


tsc

50µm

Porosity in wrought alloys

Porosity also causes problems during subsequent fabrication in wrought alloys, e.g. hot mill edge cracking

hot mill edge cracking

original ingot porosity (AA5182)

20 mm

reroll (gauge ~10mm)

slab

25mm


tsc Hydrogen solubility

The solubility of hydrogen in pure liquid aluminium is now well established� dating back to pioneering work of Ransley, Talbot and others

� note hydrogen concentration usually expressed as ml. H2 per 100g aluminium

Data shown for solubility in contact with an atmosphere of pure hydrogen

liquid

solid

D.E.J. Talbot“The Effects of Hydrogenin aluminium and its alloys”Maney (2004).

T(°C)

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

7 9 11 13 15

104/T (K-1)

log

10(S

) (m

l/100

g)

5006608001000 400

0.1

10

1.0

hydrogen(ml/100g)T

SL

270072.2log10 −=

0.01

TSS

332022.2log10 −=


tsc Expected relationship

Equilibrium constant for the reaction between gaseous hydrogen and hydrogen dissolved in molten aluminium

So we expect the following form for hydrogen solubility:

)(2)(2 metal in solutiongas AlHH ⇔

2

2

Heq p

SK =

22 HoHeq pSpKS ==

where S is Hydrogen concentration in ml / 100g, partial pressure is in atmospheres

S = activity ~ concentration of hydrogen (ml/100g)= partial pressure hydrogen2Hp

equilibriumconstant

So is the equilibrium solubility in contact with pure hydrogen

(Sieverts’ Law)


tsc What about alloying additions?

Some elements influence the solubility of hydrogen in liquid aluminium� relatively sparse data available, and even fewer for the effect in solid aluminium

Magnesium, titanium and lithium increase hydrogen solubility in liquid aluminium...

...whilst iron, copper, silicon and zinc reduce the solubility levels

pure aluminium

data from P.N. Anyalebechi,Scripta Met and Mater., 33 (8), p1209 (1995)

So Mg containing alloyspick up hydrogen more readily……and harder to degas

0.0

0.5

1.0

1.5

2.0

2.5

0 5 10 15 20

Element wt.%

Hyd

roge

n so

lubi

lity

(ml/1

00g)

magnesium

titanium

zinc

silicon

copper

iron

pure aluminium


tsc Alloy correction factors

The simple solubility equation for hydrogen in liquid aluminium needs to be corrected for the alloy composition� generally use interaction coefficients for excess solution free energy

Published experimental data extensively reviewed � some first order coefficients reliable, second order more variable

– some concern over Mg data

� temperature dependence not available for all elements� P.N. Anyalabechi, Light Metals, p827 (1998)

Alloy hydrogen solubility treated with a correction factor Calloy = 1/fH

2Hoalloyalloy pSCS =

...

...)log(

2222ZnZnSiSiCuCuMgMg

ZnZnSiSiCuCuMgMgH

WkWkWkWk

WkWkWkWkf

′′+′′+′′+′′+

′+′+′+′= fH = hydrogen activity coefficientWx = wt.% of xk’ = 1st order interactionk” = 2nd order interaction

C.H.P. Lupis, in “Liquid Metals Chemistry and Physics”,ed. S.Z. Beer (Marcel Dekker, 1972) G.K. Sigworth & T.A. Engh,

Met Trans B, 13B, p447 (1982)


tsc But where does hydrogen come from?

The solubility of hydrogen is expressed on the basis of equilibrium between the melt in contact with H2 gas� but the partial pressure of hydrogen in the atmosphere is vanishingly small

Water vapour provides the source of hydrogen� aluminium can react with water vapour to form alumina and dissolved hydrogen

0.000.05

0.100.15

0.200.25

0.300.35

0.400.45

0.50

660 680 700 720 740 760 780 800

Temperature (C)

Hyd

roge

n (m

l/100

g)

0.075

0.050

0.045

0.040

0.035

0.030

0.025

0.020

0.015

0.010

0.005

water vapour pressure (atm.)

5ºC, 50%RH

35ºC, 90%RHincreasingwater vapour

example conditions

25ºC, 80%RH

45ºC, 80%RH


tsc Molten aluminium will pick up hydrogen from atmospheric humidity

But melting furnaces are usually gas-fired

Combustion products comprise ~15% water vapour H2O (allowing for excess air)

OHCONOCHN 222242 25.725.7 ++→++

furnace : combustion water vapour

summer : 45°C, 80% humidity

winter : 20ºC, 50% humidity

It gets worse…

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

650 700 750 800

Furnace temperature (C)

Hyd

roge

n le

vel (

ml/1

00g)

furnace

summer

winter

holding furnacetemperatures


tsc Typical hydrogen levels exiting a furnace

Example shows measured hydrogen levels in melt (ml/100g) exiting furnace, sampled over many hundreds of casts� commercial purity aluminium, furnace running at 725-750ºC

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 100 200 300 400

cast #

H (m

l/100

g)

Generally found that hydrogen levels in melting and holding furnaces are dominated by equilibrium with furnace atmospheres


tsc How much hydrogen can we live with?

Furnaces are quite turbulent environments

The melt rapidly achieves equilibrium with the local atmosphere

For nearly all applications, the level of dissolved H2 exiting furnace is too high…

…and must be reduced before casting

H2(ml/100g)

demanding products

typical commodity wrought products

aerospace products

holding furnace hydrogen levels

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

melting furnace hydrogen levels

The acceptable level of hydrogen varies � depends on product and application

“Degassing” is the process by which the melt is treated to lower H2 levels


tsc Hydrogen removal: batch degassing

The degassing process bubbles a dry inert gas through the melt� the melt tries to reach equilibrium with the gas by mass transfer of hydrogen

The kinetics are controlled by local equilibrium at the bubble-melt interface

H atom in liquid AlH2 molecule gas bubbleAr atom in gas bubbleextent of boundary layerFoseco batch degassing unit


tsc Mass transfer between the melt and the bubbles

Hydrogen is transferred into gas bubbles, recombining as H2 molecules� partial pressure of hydrogen in the bubble stays in local equilibrium…

� …with concentration of hydrogen in the boundary layer around the bubble, Hi

This establishes a difference in hydrogen concentration between

� the bulk liquid, Hb and the boundary layer, Hi

which then drives mass transfer from the bulk melt to the bubbles

hydrogen partial pressurein bubble (atm.)

hydrogen concentrationin boundary layer

hydrogen concentrationin bulk liquid

)( ibii HHAk −=flux hydrogen

ki = hydrogen mass transfer coefficient (m/s)Ai = surface area of bubble

2Hp2Hoi PSH =

bH


tsc Batch degassing kinetics

The mass transfer and associated local equilibrium boundary conditions, along with re-absorption from the atmosphere, is built into process models� quantitative understanding of local equilibrium at the bubble-melt interface is crucial

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0 200 400 600

Fluxing Time (seconds)

H (m

l/100

g)

model

#764

Comparison of kinetic modelwith trial degassing Al-9% Si

Web base degassing model developed for Foseco


tsc Large furnaces… the need for in-line degassing

Batch degassers work well for small melts (e.g. < 1 tonne)

But modern wrought plants cast from 30-130 tonne furnaces� casts can take 1-2 hours duration

� ample time for the melt to up-gas once more in the furnace

� furnace degassing has become ineffectual and largely abandoned

M. B. Taylor, Aluminium 79, (1/2), p44, (2003)

Degassing and up-gassing kinetics in a holding furnace


tsc In–line degassing

Wrought aluminium plants generally use in-line degassers� just before casting, allows no time to up-gas before solidification

Continuous process where metal flows through a reaction chamber� characterised by an average residence time (chamber volume/metal flow rate)

In line SNIF degasserwww.pyrotek.com


tsc Just how do we measure hydrogen?

The levels of hydrogen we are dealing with in solution are rather low� 0.1 ml/100g ~ 0.09 ppm or ~ 0.1 gram of hydrogen per tonne of aluminium

And yet these levels are measured on line, in real time: all rely on equilibrium

A small volume of carrier gas re-circulates through a porous probe in the melt � hydrogen diffuses to the gas and recombines as H2 reaching equilibrium

www.abb.com

AlSCAN hydrogen measurement

Original Telegas system (Alcoa)


tsc Data processing

All on-line hydrogen measurement methods entail measuring the partial pressure of hydrogen in equilibrium with the melt� we are not measuring hydrogen in the melt directly

In order to estimate the concentration of hydrogen dissolved in the melt:1. temperature used to calculate the equilibrium solubility for H2 in pure Al

2. measured used to calculate the H level in the melt (assuming pure Al)

3. calculated concentration corrected for effect of alloying elements on H solubility2Hp

0.05 atm. H2 pressure converts to three different H levels,depending on the melt composition

…so measurement devices relyentirely on knowledge of equilibrium H solubility, and its variation with alloy0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.05 0.1 0.15 0.2

PH2 (atmospheres)

H m

l/100

g

2Hp

Al

Al-5Mg

Al-10Si


tsc Current issues: aluminium and hydrogen

Uncertainty over interaction parameters affects our ability to both measure gas levels on line reliably, and control degassing processes� the effect of magnesium is probably the most worrying

There are a reasonable amount of data concerning hydrogen solubility in pure liquid aluminium, and some data for a selection of binary alloys…� but hardly any for fully commercial alloys

Binary interaction coefficient contributions are assumed additive…� very limited evidence suggests this may not always be the case:

0.00

0.50

1.00

1.50

2.00

2.50

700 750 800 850 900

Temperature (C)

Hyd

roge

n so

lubi

lity

(ml/1

00g)

7050 exp

7050 predicted

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

700 750 800 850 900

Temperature (C)

Hyd

roge

n so

lubi

lity

(ml/1

00g)

Al-4Cu-0.9Mg-0.5Siexp

Al-4Cu-0.9Mg-0.5Sipredicted

P.N. Anyalebechi,Scripta Met and Mater., 33 (8), p1209 (1995)

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tsc 2) Extrusion processing


tsc Extrusion alloys (6xxx)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4

wt% Si

wt%

Mg

150MPa 200MPa 250MPa300MPa

350MPa

AA6082

AA6005

AA6063

AA6060

“balanced” alloys

AA6061

Most common extrusion alloys are based on the Al-Mg-Si system (AA6xxx)� often with additions of Cu, Mn

Precipitation hardening system, based on the Al-Mg2Si pseudo-binary


tsc Al-Mg-Si phase diagrams

The area of interest is the aluminium rich corner of the Al-Mg-Si system

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0Al

wt.% silicon

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

550ºC500ºC

400ºC

“balanced”alloys

J. Zhang et al, Mat. Sci. Tech. 17, p494 (2001)

Pseudo-binary sectionAluminium solid solution isotherms


tsc

0

100

200

300

400

500

600

700

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Tem

pera

ture

(°C

)

wt% Mg2Si

AA6060 AA6063

AA6061

Al solidsolution

Extrusion process route: homogenisation

Extrusion logs are homogenised after casting, typically >570ºC� dissolution of soluble Mg2Si eutectic (quick)

� formation of Mn dispersoids, and phase transformation of cast intermetallics (slow)

H. Feufel et al, J. Alloys & Comp. 247, p31 (1997)

melttreatment

casting extrusionmeltinghomogenise

& coolpreheating agequench


tsc Cooling after homogenisation

Note: no formal solution heat treatment stage in the process route

It might be thought quenching rapidly after homogenisation would be desirable� preserving Mg and Si in solid solution

� retaining it during extrusion and quenching, maximising the ageing response

Perhaps surprisingly, a slower cooling rate is usually deliberately applied� resulting in extensive fine re-precipitation of β’-Mg2Si needles

� (note these would be very over-aged in terms of final mechanical properties)

Quenched after homogenisation:Mg and Si retained in solid solution

Controlled cool after homogenisation (350°C/hr): nucleation of β’-Mg2Si needles

R.A. Ricks et al, Extrusion Technology 1992, pages 57 - 69

1µm


tsc Preheating and extrusion

So what is the point of encouraging precipitation prior to extrusion?

The extruders have come up with a cunning plan:

Having the solute out of solution before extrusion lowers the flow stress� so they can extrude faster on a given extrusion press

But a correct selection of billet preheat temperature, combined with the heat of deformation, will carry the extrudate above the solvus� re-dissolving the precipitates, just before the extrudate is quenched

deformation heating occurs here

billetram

container

die

quench


tsc Effect of cooling rate from homogenisation on flow stress

controlled cooling

waterquench

S. Zajac et al.,Mat. Sci. For. 396-402, p399 (2002)


tsc In situ re-solutionising

This process allows maximum press productivity� whilst still ensuring maximum strengthening potential during subsequent ageing

Hence precipitates formed after homogenisation must not be too coarse� otherwise they won’t re-dissolve in short time available above the solvus

temperature rise due to deformation takes billet above solvus temperature for the alloy

0

100

200

300

400

500

600630

0 5 10 15 20 25 30

melting starts (solidus)

Mg and Si in solid solution (solvus)

induction furnace 100°C/min

criticalquench range

preheat

extr

usio

n

quench

Tem

pera

ture

(°C

)

Time (mins)


tsc Potential problems (1)

The amount of deformation heating depends on the details of the extrusion� mainly the extrusion ratio (strain)

� and the strain rate (extrusion speed)

So operating practices need to be fine tuned for each product…� to ensure the extrudate achieves a temperature above the solvus…

� but below the solidus: otherwise incipient melting is observed

…and more concentrated alloys have a diminishing operating window:

0

100

200

300

400

500

600

700

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Tem

pera

ture

(°C

)

wt% Mg2Si

AA6060 AA6063

AA6061

Al solidsolution 50ºC

150ºC

surface melting(“speed cracks”)

N. C. Parson et al. Extrusion Technology 1992, p13


tsc Potential problems (2)

Example of speed cracking, showing underlying re-melted eutectic colonies

O. Reiso,Mat. Forum 28, p32 (2004)


tsc 3) Practical alloy development

Brazing sheet is a clad aluminium rolled product widely used heat exchangers � core typically AA3xxx, cladding typically Al-10%Si

A very elegant product, with one small problem…

www.aluminium-brazing.com


tsc Scrap

What to do with the inevitable process scrap (ingot crops, edge trims, etc.) ?

The core and clad are metallurgically bonded: can’t separate� re-melted average composition typically : Al -1.4%Si -0.8%Mn-0.1%Mg

� this is x10 more Si than can be tolerated in most rolled products

The more successful the sales of brazing sheet, the more scrap created

Solution: register a new alloy designed to absorb the scrap: AA4015� typical application: low cost (painted) building sheet

Only problem: mechanical properties rather poor cf. normal building sheet


tsc (Blind?) Alloy development

This is a cold rolled product, deriving its strength from a combination of work and solute hardening� not feasible to increase cold rolling reductions (start and end gauges fixed)

� only option: increase level of solid solution

Our client proposed increasing magnesium (and/or manganese) in the alloy� (wrought metallurgists rule of thumb: if in doubt, increase Mg for work hardening)

0

50

100

150

200

250

300

350

0 0.5 1 1.5 2

cold rolling strain

yiel

d st

reng

th (

MP

a)

AA3004

AA4015

commercialreduction

The base alloy contains 0.1%Mgoff line process used to investigate:0.3% and 0.5%Mghoping to close the gap


tsc Mechanical properties

Experimental alloys were cast up, homogenised, then hot and cold rolled

The results were surprisingly disappointing

Rolled to a common final gauge(strain), all A4015 variants showed~identical strength

And exactly the same behaviourseen on making Mn additions

We were invited to consider whether the client’s strategy made sense……and could we explain why it was all going wrong?

0

50

100

150

200

250

300

AA3004 AA40150.1Mg

AA4015,0.3Mg

AA4015,0.5Mg

yiel

d st

reng

th (M

Pa)


tsc The thermodynamic equilibrator

After homogenisation, slabs are hot rolled to ~ 3mm intermediate gauge

For this client, the final 3 passes are coil to coil� ~50% reductions, with many minutes between passes at ~350ºC

On reflection, you would be hard pressed to find a process better designed to drive an alloy to equilibrium in the solid state!

melttreatment

casting cold rollmelting homogenise hot roll


tsc Al-Mg-Si again

The diagram shows the limit of solid solubility at 350ºC� data taken from Phillips (1961) and Feufel et al. (1997)

Increasing Mg in this alloy simply advanced into the 3 phase region at 350ºC� with a Mg solid solution level pegged to the corner of the triangle

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0Al

wt.% silicon

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

350ºC

Al-Mg 2Si-Si

H. Feufel et al, J. Alloys & Comp. 247, p31 (1997)


tsc What else can we add?

If Mn and Mg are ruled out because of the high Si in the alloy, what could we add (cheaply) that would be retained in solution?� Cu was a candidate, provided the Mg level was also kept low (avoiding Q phase)

� potentially could retain e.g. 0.4-0.8 %Cu in solution

calculations undertaken by Chart Associates (2005)

Al-0.1Mg-0.8Mn-1.4Si-0.8Cu

400 450 500 550 600 650 700

0.000

0.002

0.004

0.006

0.008

0.010

0.012

T/C

Cu Cu

Cu

Mg MgMn

Si

Si Si

mas

s fr

actio

n in

FC

C

Al-0.2Mg-0.8Mn-1.4Si-0.8Cu

400 450 500 550 600 650 700

0.000

0.002

0.004

0.006

0.008

0.010

0.012

T/C

CuCu

Cu

MgMg Mg Mn

Si

SiSi

mas

s fr

actio

n in

FC

C


tsc Success?

Measured

Predicted

0

50

100

150

200

250

300

0 0.3 0.5 0.8Cu (wt %)

Yie

ld S

tren

gth

(MP

a)

predictedmeasured

A series of alloys were cast based on AA4015 with Cu additions� containing 0.3, 0.5 and 0.8 wt.% Cu

For a fixed strain, 0.8% Cu alloy achieved ~75MPa strength increase� in reasonable agreement with strain hardening prediction


tsc Strength ok, but corrosion?

Not surprisingly, although Cu additions addressed the strength issue…

…they also degraded the corrosion resistance of the product

Not as much of an issue as might be expected: it is a coated product� but would limit its application as a bare product

0% Cu 0.3% Cu 0.5% Cu 0.8% Cu

corrosion test samples, 500 hours acidified salt spray (ASTM B287)(overlaid with image analysis of pitting corrosion)


tsc Conclusions

� These application examples have been chosen to try and demonstrate the importance of setting phase equilibria predictions in a larger context

� There are times when thermodynamic equilibrium is achieved, either unavoidably or by design…

�…and other times when the process route has evolved to avoid equilibrium at all costs

� One of the most important application areas of phase equilibria relate to the provision of realistic local equilibrium boundary conditions� increasingly required by microstructural kinetic models during processing

� Understandably there are gaps in the availability of reliable experimental data concerning metastable phase equilibria…

�…but some equilibrium data also very sparse for molten aluminium alloys� e.g. relating to hydrogen in ternary or higher alloys

� e.g. interactions of molten salts with molten aluminium alloys (alkalis)

� e.g. solubility of carbon in molten aluminium, and precipitation of carbides

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