s kumar intermetallic phase formation in wrought al-mg-si alloy 2012.pdf

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T E C H N I C A L P A PE R TP 2589

Fe Bearing Intermetallic Phase Formation in a WroughtAlMgSi Alloy

S. Kumar P. S. Grant K. A. Q. OReilly

Received: 6 July 2012/ Accepted: 16 September 2012 / Published online: 10 October 2012

Indian Institute of Metals 2012

Abstract This paper investigates the two dimensional

(2D) and three dimensional (3D) morphologies of the Febearing intermetallics that forms during the direct chill

casting of an AA6063 Al alloy. An intermetallic phase

extraction technique was used to facilitate 3D intercon-

nectivity, morphology and fraction of intermetallics.

Metallographic 2D analyses suggest the presence of Chi-

nese-script-type and needle-type morphology Fe bearing

intermetallics typically at the primary Al grain boundaries,

whereas 3D analyse of the extracted intermetallics suggests

those particles have dendrite-type and platelet-type mor-

phologies. In-addition, globular shaped intermetallics

which were observed in 2D within the primary Al grains

where observed to have sphere shaped rosette-type mor-

phology in 3D. ac-AlFeSi and b-AlFeSi were the two

dominant intermetallic phases observed in the as-cast bil-

let. Clusters of Ti rich particles were observed at the point

from which growth appears to have started suggesting a

possible nucleating site for the Fe intermetallics to form

during solidification.

Keywords AA6063 Al alloy DC casting

Intermetallics Extraction

1 Introduction

Due to superior strength, good formability and heat-treat-

ability, AlMgSi (6xxx series) alloys have found potential

applications as structural materials in the automotive and

building industries. Most commercial wrought Al alloysare cast using the direct chill (DC) casting process [1].

During solidification, the solid solubility of the minor

alloying elements (Fe, Si, Mn and Mg) in the primary Al

decreases, as a result they tend to segregate to the liquid

that is the lost to solidify. Hence, this results in the for-

mation of complex intermetallics at the cell and grain

boundaries. Since the solidification conditions in DC

casting are non-equilibrium, the type of intermetallic that

forms may vary [2]. In-order to modify the as-cast Fe

bearing intermetallics into more favourable forms for

downstream processing, the cast billet is usually subjected

to heat treatment. The necessity to increase the use of

recycled aluminium pushes casting technologies to toler-

ate higher impurity content, particularly Fe [3]. Therefore,

in-order to develop advanced casting technologies and

heat treatment processing routes for recycled alloys it is

essential to understand the nature of the intermetallics that

form during DC casting. There is considerable literature

available to understand the type of Fe bearing interme-

tallic in DC cast 6xxx series Al alloys billet specifically

focused on two dimensional (2D) metallographic analysis

[4, 5], but very few investigations are available concern-

ing 3D morphological analysis [6]. Therefore in this

study, the 3D morphological nature of the Fe bearing

intermetallic particles that form during DC casting is

investigated. A phase extraction technique is used to

facilitate observation of the 3D nature of the intermetallic

particles by dissolving the Al matrix [7]. In-addition

inclusions in liquid metals are inevitable. They either

enter through inoculant additions or form in situ during

liquid metal handling. This paper also highlights the

possible role of inclusions on the intermetallic phase

formation during solidification.

S. Kumar (&) P. S. Grant K. A. Q. OReilly

Department of Materials, The EPSRC Centre for Innovative

Manufacturing in Liquid Metal Engineering,

University of Oxford, Oxford OX1 3PH, UK

e-mail: [email protected]

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DOI 10.1007/s12666-012-0221-y


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2 Experimental Details

The elemental composition of the AA6063 Al alloy used in

this study is 0.45Mg, 0.41Si, 0.19Fe, 0.07Mn, 0.01Cu,

0.01Ti and balance Al (all in wt%). The alloy was DC cast

into a 190 mm billet. The billet was analysed across its

cross-section. The mounted samples were grounded and

mechanically polished with a colloidal silica suspensionhaving a grain size of 0.04 lm for 2D microstructural

analysis. For grain size analysis the samples were anodised

using Barkers reagent (7 ml HBF4 [48 %], 93 ml H2O) at

20 V for 60 s. Grain size and dendrite arm spacing (DAS)

were measured using the mean liner intercept method on

the polarised images taken using a Zeiss Axiophot2 optical

microscope. Intermetallic particles were extracted by dis-

solving the Al matrix using anhydrous boiling butan-1-ol

(butanol) while keeping the intermetallics intact. The

intermetallics were then collected on a polytetrafluoroeth-

ylene filter paper with pore size of 2 lm. A Philips 1700

X-ray diffractometer (XRD) was used for the phase iden-tification. A JEOL 840A scanning electron microscope

(SEM) equipped with secondary electron and back-scat-

tered electron (BSE) detectors and an energy-dispersive

X-ray spectrometer (EDS) was used to analyse the inter-

metallic phases. A JEOL 840F field emission gun (FEG)

SEM was used for high resolution images.

3 Results

3.1 2D Analysis

Anodised microstructures of the sample from the centre

(Fig. 1) of the cast billet suggest fine equiaxed primary Al

grains. The average grain size and DAS of the primary Al

at the centre of the cast billet is 102 13 and 30 7 lm,

respectively. It is interesting to note that the large aspect

ratio containing discrete particles are decorated along the

primary Al grain boundaries (Fig. 1). In-addition to these

grain boundary particles, fine spherical shaped particles

were observed within the primary Al grains. SEM 2D

metallographic analysis reveals that those large aspect ratiointermetallic particles were either in the form of Chinese-

script-type (Fig. 2a) or needle-type (Fig. 2b) morphologies.

Further, the spherical shaped particles were observed to

have rosette-type (Fig. 2c) morphology. The EDS analysis

of the metallographic samples revealed a narrow range of

Fe:Si ratio within these intermetallics. Among these

intermetallics, the Chinese-script-type particles have higher

(4.3) Fe:Si ratios than the needle-type (2.4) ones.

3.2 3D Analysis

In-order to understand the 3D nature of these intermetallics

it is essential to remove the Al matrix and analyses the

extracted particles. It is very interesting to note that these

extracted intermetallics were well interconnected (Fig. 3)

and they are typically more than two hundred of microns in

length, which could not be ascertained from the 2D

observations made of the metallographic samples. There-

fore it may not be appropriate to characterise the Fe

bearing intermetallic using only 2D metallographic sam-

ples. Such lengthy and well interconnected intermetallics

have been observed to usually form along the grain

boundaries. It is also interesting to note that the interme-tallics which were observed with the Chinese-script-type

and needle-type morphologies seen in 2D were observed to

have dendrite-type (Fig. 4a) and platelet-type (Fig. 4b)

morphologies in 3D analysis, respectively. In-addition, the

intermetallics which were observed within the primary Al

grains have a very different rosette-type 3D morphology

(Fig. 4c). XRD analysis of the extracted particles revealed

that ac-AlFeSi and b-AlFeSi are the two dominant as-cast

Fe bearing intermetallic phases observed in this alloy

(Fig. 5).

4 Discussion

4.1 Solidification and Phase Selection

Due to non-equilibrium solidification conditions in DC

casting and the low solid solubility of solute elements (Fe,

Si) in Al, excess solute tends to segregate into the liquid at

solidliquid interface. This solute rich liquid will be the

last to solidify and results in the formation of various

Fig. 1 Optical micrograph shows the primary Al grains, where the

label A denotes the location of large aspect ratio particles and label

S denotes the location of fine spherical particles

554 Trans Indian Inst Met (December 2012) 65(6):553557

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intermetallic phases. The anodised microstructure clearly

reveals that these intermetallic phases mostly form at the

grain and cell boundaries. Thermodynamic calculation

using PandaT simulation [8] suggest that the primary Al is

the first phase to form during solidification and followed by

Al13Fe4, ac-AlFeSi, b-AlFeSi and Mg2Si as temperature

lowers. These intermetallic phases form as the result of

various eutectic and peritecitc solidification reactions. The

absence of Al13Fe4 in the present billet suggests that the

corresponding reaction might be suppressed or allowed to

transform to different phase. The absence of Mg2Si in the

XRD pattern is not unusual for as-cast alloy and is likely to

be due to Mg2Si being present below the detection limit of

the XRD.

Fora given local composition, intermetallic phase selection

is determined by two criteria: competitive nucleation and

competitive growth [2]. Competitive nucleation favours the

intermetallics which havehigh nucleation temperaturesor low

under-cooling for nucleation, whereas competitive growth

favours the phase having the higher growth temperature orhigher growth rate. The nucleation of Fe bearing intermetallic

phases may be enhanced by the presence of potent nucleating

substrates such as preformed primary Al or pre-solidified

reaction products or inclusions. Interestingly in the present

study, Ti rich particle and oxide particle clusters were fre-

quently observed to be associated with ac-AlFeSi and

b-AlFeSi. Figure 6a shows a cluster of TiB particles asso-

ciated with a b-AlFeSi platelet, suggesting TiB acted as a

nucleating substrate. The higher magnification of this cluster

shows (Fig. 6b) typical hexagonal morphology of TiB2 par-

ticles. TEM analysis [9] showed the existence of good lattice

matching between TiB2 and b-AlFeSi. Such Fe bearingintermetallics nucleated in this way have been observed to

have distinct points of origin. Figure 4a shows a petal-like

dendrite-type ac-AlFeSi particle growing from a central point.

EDS analysis revealed the presence of Ti rich particles at this

location and suggests it as the point of origin for the petals.

Inclusions such as TiB2 and oxides may nucleate the Fe

bearing intermetallics either directly or through forming an

intermediate compound. Thus inclusions can trigger inter-

metallic phase selection.

Fig. 2 BSE-SEM images showing 2D morphology of the different Fe

bearing intermetallics observed in the metallographic sample. Where

a Chinese-script-type, b needle-type and c rosette-type particles,

respectively

Fig. 3 FEG-SEM image of an extracted intermetallic showing a well

interconnected intermetallic network

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split and engulf small pockets of solute rich liquid or such

pockets also can form between secondary dendrite arms.

These pockets of entrapped solute rich liquid may not

contain active nuclei and therefore may under cool well

below the ac-AlFeSi and Al eutectic reaction temperature.

Thus there may be copious nucleation in this super cooled

liquid resulting in rapid freezing and the formation of

clusters of fine nano-sized ac-AlFeSi particles, as observed.

It is important to note that most of these spherical type

particles were found separate without interconnectivity,

this further supports that these intermetallics form from

isolated liquid.

Intermetallics morphology influences the physical

properties of the final product. It has been reported that

needle/platelet-type morphology containing intermetallic is

detrimental to mechanical properties [12].

5 Conclusions

In this AA6063 alloy DC cast billet, ac-AlFeSi and

b-AlFeSi were the two dominant Fe bearing intermetallic

phases. Grain boundary ac-AlFeSi has a dendrite-type

morphology whereas b-AlFeSi has a platelet-type mor-

phology. ac-AlFeSi which was observed within the Al

grains can have a sphere like rosette-type morphology.

Other than this spherical type particle, most of the Fe

bearing intermetallic that were observed at the grainboundaries and cell boundaries of the primary Al where

well interconnected. Interconnectivity plays a crucial role

in the downstream, secondary deformation processing of

wrought Al alloys. Therefore it is essential to develop

processing routes which can reduce this intermetallic

interconnectivity.

Acknowledgments The authors would like to acknowledge the

financial support of SAPA and EPSRC Centre for Innovative Man-

ufacturing in Liquid Metal Engineering and Feng Yan, of the Brunel

University, UK, for providing the PandaT data.

References

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(1999) 1205.

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8. http://www.computherm.com/pandat.html. Accessed 5 July 2012

9. Sha G, Intermetallic Phase Selection in 6xxx Series Al Alloys,

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10. Langsrud Y, Key Eng Mater 44&45 (1990) 95.

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Eng A 304306 (2001) 119.

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Fig. 6 FEG-SEM images

showing Ti rich particles

associated with platelet-type

b-AlFeSi. Where b is the higher

magnification image of the

arrow make in a indicates the

location of Ti rich particles

cluster

Trans Indian Inst Met (December 2012) 65(6):553557 557

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http://www.ijmr.de/MK110760http://www.computherm.com/pandat.htmlhttp://www.computherm.com/pandat.htmlhttp://www.ijmr.de/MK110760

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