A356 homojenizasyon 1

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Effect of a short solution treatment time on microstructureand mechanical properties of modified

Al–7wt.%Si–0.3wt.%Mg alloy

D.L. Zhang a,*, L.H. Zheng b, D.H. StJohn b

a Department of Materials and Process Engineering, University of Waikato, Private Bag 3105, Hamilton, New Zealand b CRC for Cast Metals Manufacturing (CAST), Department of Mining, Minerals and Materials Engineering,

University of Queensland, Queensland 4072, Australia

Abstract

Microstructural change caused by a short solution treatment and the corresponding change in tensile properties and impact

energy of a strontium modified Al–7wt.%Si–0.3%Mg cast alloy were studied. It was found that a solution treatment of 10 min at 540

or 550 °C is sufficient for the a-aluminium phase to homogenise and achieve the maximum level of magnesium and silicon as

predicted by the solubility and alloy composition limits. A solution treatment of 30 min causes spheroidisation, coarsening and an

increase in inter-particle spacing of the eutectic silicon particles leading to a significant improvement in ductility and impact re-

sistance. Compared with a standard 6 h solution treatment, solution treatment of 30 min at 540 or 550 °C is sufficient to achieve

more than 90% of the maximum yield strength and more than 95% of the maximum UTS and the maximum average elongation to

fracture. However, only 80% of the maximum impact energy can be attained by the short solution treatment. The values of the

ductility and impact energy pass through a minimum between 1.5 and 10 min of solution treatment time indicating that solution

treatments of less than 10 min should be avoided.

Ó 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Aluminium casting alloys; Heat treatment; Mechanical properties; Microstructural change; Castings

1. Introduction

When cast components for structural applications

such as alloy wheels are manufactured using Al–Si–Mg

based casting alloys (typically A356 and A357), T6 heat

treatment is in most cases an essential step in the man-

ufacturing process. The T6 heat treatment provides two

beneficial effects: an improved ductility and fracture

toughness through spheroidisation of the eutectic silicon

particles in the microstructure and a higher alloy yieldstrength (YS) through the formation of a large number

of fine b00 precipitates which strengthen the soft alu-

minium matrix. The first benefit is realized through the

solution treatment (normally at a temperature around

540 °C) while the second benefit is achieved through

the combination of solution treatment, quenching and

artificial ageing (at a temperature in the range of 140–

170 °C) [1–3]. In the casting industry, it is often specified

that a cast component should be solution treated for 6 h

at 540 °C.

While the benefit of T6 heat treatment is accepted, the

additional cost and production time associated with

such a treatment is also substantial. Taking a cast

component made by a low pressure die casting process

as an example, the casting process normally takes less

than 10 min, while a typical T6 heat treatment cycle may

take more than 10 h. This means that shortening thetotal time of the T6 heat treatment cycle has a major

impact on productivity and manufacturing cost. For this

reason, there is a strong interest in establishing the

feasibility of shortening the solution time. Shivkumar

et al. [4] demonstrated that for permanent mould cast

test bars of a modified A356 alloy, a solution treatment

of 50 min at 540 °C is sufficient to attain more than 90%

of the maximum YS, more than 95% of the ultimate

tensile strength (UTS) and nearly 90% of the maximum

elongation for a given ageing condition. In agreement

with the observation on the tensile property change,

Journal of Light Metals 2 (2002) 27–36

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* Corresponding author. Tel.: +64-7-838-4783; fax: +64-7-838-4835.

E-mail address: [email protected] (D.L. Zhang).

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they also observed that the magnesium and silicon

contents in the a-aluminium dendrites reached the max-

imum equilibrium level according to the alloy compo-

sition and the distribution of magnesium and silicon

became homogeneous within 50 min of solution treat-

ment at 540 °C. It is therefore clear that for permanent

mould castings of a modified A356 alloy, a solution

treatment of 50 min is sufficient to attain at least 90% of

the maximum tensile properties of the alloy. Although

the maximum tensile properties are not achieved, the

dramatic reduction of the solution time may offer an

opportunity to increase productivity and reduce cost

while maintaining the performance of the components.

Full modification of the as-cast structure appears to

be an essential precondition for the feasibility of a short

solution treatment. It has been well established that the

fibrous eutectic silicon phase in the modified structure is

fragmented and spheroidised much more rapidly than

the plate shaped silicon particles in the unmodified

structure [1,4–8]. With a completely unmodified micro-structure, solution treatment of 1–2 h at 540 or 550 °C

has little effect on the morphology of the eutectic silicon

particles, while with a fully modified microstructure the

effect of short solution time of 1–2 h on the eutectic

silicon particles is much more significant [4,5,7,8].

In this study, the effect of even shorter (<30 min)

solution treatment time at 540 or 550 °C on the micro-

structure and properties of a fully modified Al–7wt.%Si–

0.3wt.%Mg alloy was investigated. This study focuses

on examining the changes in microstructure and me-

chanical properties during the very early stages of so-

lution treatment (0–30 min).

2. Experimental method

The sample bars used in all the solution treatment

experiments except those for producing impact test sam-

ples, were cut from the rim region of an Al–7wt.%Si–

0.3wt.%Mg alloy wheel cast by low pressure die casting.

The alloy was modified by using 0.015 wt.% Sr.

The actual composition of the alloy was determined to

be Al–6.7wt.%Si–0.26wt.%Mg–0.12wt.%Fe–0.02wt.%Ti–

0.015%Sr by using the inductively coupled plasma

technique. The approximate dimensions of the bars were12 Â 15 Â 55 mm3. The bars were solution treated in a

high temperature salt bath held at 540 or 550 °C. The

advantage of using a salt bath was that the heating rate

was high, and thus the isothermal holding time formed

the major part of the solution treatment, which ranged

from 2 to 30 min. The temperature of the bars was

monitored during solution treatment by using a ther-

mocouple embedded in one of the bars and a comput-

erised data logging system. Fig. 1 shows a typical

heating curve of the samples during solution treat-

ment. From the heating curve, it is observed that it took

approximately 30 s to heat the samples from room

temperature to 505 °C. Due to fast heating and fairly

low temperature, the solution treatment effect experi-enced by the samples during this heating stage is negli-

gible, so this heating time was discounted. The solution

treatment time quoted was the net time that the sample

temperature is above 505 °C. Fig. 1 shows that shortly

after reaching 505 °C the samples reach 540 or 550 °C.

After each solution treatment, the samples were

quenched in a hot water bath held at 60 °C. The quen-

ched samples were then immediately aged in an air cir-

culated furnace at 140 °C for 4 h. This is a typical

underageing treatment used in the manufacture of

wheels. To prepare for tensile testing, round specimens

were machined from the heat treated samples. The ten-

sile tests were performed using a screw driven Instron

tensile testing machine. The cross-head speed used was 1

mm/min. The strain was measured by using an extenso-

meter attached to the sample and with a measuring

length of 10 mm. The 0.2% proof stress was used as the

measure of yield stress. Five samples were tested for

each heat treatment condition.

The microstructure of the as-cast and heat treated

samples was examined using an optical microscope and

quantitatively analysed using an image analyser (QM

750). To quantify the microstructural change during

solution treatment, the emphasis of the image analysis

was placed on the aspect ratio, equivalent circle diam-eter of the eutectic silicon particles and the inter-particle

spacing. Each measurement included 800–1200 particles

obtained from several areas inside the eutectic regions.

The iron-rich intermetallic and Mg2Si phases were de-

liberately excluded from the measurements. In order to

easily observe changes to the silicon morphology the

data were grouped around the selected values of 2 for

the aspect ratio and 1 lm for the particle size. The

magnesium and silicon contents in the a-aluminium

matrix were measured using an electron probe micro-

analyser (JEOL JXA-8800L) with an operating voltage

Fig. 1. Sample temperature as a function of time during solution

treatment in a salt bath held at 540 °C.

28 D.L. Zhang et al. / Journal of Light Metals 2 (2002) 27–36



than 2 as a function of solution time at 540 °C. By in-

creasing solution time from 1.5 to 19.5 min, the fraction

of the silicon particles with an aspect ratio of less than 2

increased slightly from 0.63 to 0.71. A further increase

of the solution time did not result in any further increase

in the number fraction of the silicon particles with a low

aspect ratio of less than 2. This shows that prolonged

solution treatment beyond 20 min at 540 °C had little

effect on the extent of spheroidisation. For a given short

solution treatment time, increasing the solution tem-

perature from 540 to 550 °C clearly increased the

number fraction of the silicon particles with low aspect

ratio. As shown in Fig. 4(b), for the same solution

treatment time of 9.5 min, the number fraction of thesilicon particles with an aspect ratio of less than 2 in-

creased by approximately 10% when the solution tem-

perature increased from 540 to 550 °C. With a longer

solution time of 19.5 min, this effect was much less sig-

nificant.

Fig. 5(a) shows the number fraction of the silicon

particles with an equivalent circle diameter of less than 1

lm and greater than 1 lm as a function of solution time.

By increasing the solution treatment time at 540 °C from

1.5 to 19.5 min, the number fraction of the silicon par-

ticles with a diameter of greater than 1 lm almost

doubled. With a further increase of the solution time,

the number fraction of the silicon particles with a dia-

meter greater than 1 lm did not increase any further.

Instead, it clearly decreased. This unusual change in the

number fraction of silicon particles with a diameter of

greater than 1 lm might be caused by the relatively large

number of small residual particles produced by the

dissolution process of many silicon particles which is an

essential process of coarsening [10]. As shown in Fig.

5(b), with a short solution treatment time of 9.5 min,

increasing the temperature from 540 to 550 °C increased

the number fraction of silicon particles with a diameter

of greater than 1 lm by more than 10%. This shows that

the rate of coarsening at the beginning of solution

treatment increases with temperature.Fig. 6(a) and (b) show the change in the average dia-

meter of the silicon particles and the average inter-

particle spacing as a function of solution time at 540 and

550 °C. The average diameter of silicon particles and the

average inter-particle spacing increased rapidly within

the first 10 min at the solution temperature. Then they

increased slowly with further increase in solution time. It

should be noted that the slight decrease in average dia-

meter and inter-particle spacing by increasing the so-

lution time from 19.5 to 29.5 min was due more to the

uncertainty of the image analysis technique and incon-

Fig. 4. The number fraction of silicon particles with an aspect ratio of

<2 and >2 (a) as a function of solution time at 540 °C and (b) cor-

responding to two solution treatment times at 540 and 550 °C.

Fig. 5. The number fraction of silicon particles with an equivalent

circle diameter of <1 lm or >1 lm (a) as a function of solution time at

540 °C and (b) corresponding to two solution treatment times at 540

and 550 °C.




sistency of the samples than reflecting an actual trend.

After a prolonged solution treatment of 6 h at 540 °C,

the average diameter of the silicon particles increased

to 2.6 lm, while the average inter-particle spacing in-

creased to 7.8 lm.

3.1.2. Homogenisation

Fig. 7(a) and (b) show the typical distribution of

magnesium and silicon content across the width of an

a-aluminium dendrite in as-cast and solution treatedsamples. In the as-cast condition, the magnesium con-

tent is distributed fairly homogeneously with a compo-

sition of approximately 0.15 wt.% which is substantially

lower than the equilibrium level of 0.3 wt.%. After the

samples were solution treated for 9.5 min, the magne-

sium content increased to 0.3 wt.% and its distribution

became very homogeneous. In the as-cast condition, the

silicon content was significantly higher near the centre of

the dendrite arms than at their edge, as observed pre-

viously by Shivkumar et al. [4] and Closset et al. [11].

After 1.5 min at 540 °C, the distribution of silicon was

still inhomogeneous but after 9.5 min the distribution of

silicon was observed to be homogeneous. With the

higher solution temperature of 550 °C, it took a shorter

time for the distribution of silicon to become homoge-

neous (Fig. 7(b)).

Fig. 8 shows the change of average magnesium andsilicon contents across a-aluminium dendrites as a

function of solution treatment time at 540 and 550 °C.

The average magnesium content in the a-aluminium

phase increased significantly in the first 1.5 min of so-

lution treatment at 540 or 550 °C. However, the distri-

bution of magnesium was still fairly inhomogeneous as

reflected by the large variation of its value from one

dendrite to another. By increasing the solution time to

9.5 min, the average magnesium content reached the

equilibrium level of 0.3 wt.%, and the distribution be-

came very homogeneous. The trend was the same for

Fig. 6. (a) Average diameter and (b) inter-particle spacing of the silicon

particles as a function of solution treatment time at 540 and 550 °C.

Fig. 7. The distribution of magnesium and silicon across the width of

an a-aluminium dendrite arm in the as-cast condition and after 1.5 and

9.5 min of solution treatment at (a) 540 and (b) 550 °C.

D.L. Zhang et al. / Journal of Light Metals 2 (2002) 27–36 31



the solution time beyond 9.5 min and up to 29.5 min, the

average elongation again fluctuated around 10%. The

average elongation corresponding to a solution treat-

ment time of 6 h at 540 °C was 10.5%. This shows that

with a short solution treatment time of 30 min, 95% of

the maximum average value of the elongation to frac-

ture can be achieved.

Fig. 11 shows the impact energy obtained from the

Charpy impact tests as a function of solution treatment

time at 540 °C. The impact energy decreased with in-

creasing solution treatment time from 0 to 5.5 min, and

then gradually increased up to 11.5 min. By further in-creasing the solution treatment time beyond 11.5 min

and up to 29.5 min, the impact energy fluctuated around

4 J. The average impact energy of the samples solution

treated for 6 h at 540 °C, was 4.9 J. This shows that with

a short solution treatment time of 30 min, the alloy can

achieve approximately 80% of the maximum impact

energy. The difference in impact energy between the

samples that were solution treated for a short time and

those that were solution treated for the standard long

time of 6 h, is much more substantial than the differ-

ences measured for strength and ductility.

4. Discussion

To determine whether a short solution treatment time

is feasible, it is essential to be clear on whether the two

functions of the solution treatment can be achieved

within a short solution treatment time. As mentioned

above, the two intended functions are: (1) to raise the

magnesium and silicon solute contents to the maximum

level and to homogenize their distribution; and (2) to

sufficiently reduce the aspect ratio and increase the size

and spacing of the eutectic silicon particles. The first

function is essential for achieving the maximum level of

YS corresponding to the alloy composition and the

ageing condition used through precipitation hardening.

The second function is necessary for improving the

ductility of the alloy from the as-cast state. These two

functions are largely independent of each other. The

UTS is improved when both the yield and ductility are

improved, as has been confirmed by Taylor et al. [14]

through an empirical analysis of trends in mechanical

properties of T6 heat treated Al–Si–Mg casting alloys.

4.1. Yield strength

By consideration of both tensile properties and solute

content and distribution, this study establishes that, at

least for an Al–7wt.%Si–0.3wt.%Mg alloy, the majority

(>90%) of the potential YS is achieved after a short

solution treatment of 10 min at 540 °C. This is princi-

pally due to achieving homogenisation of the a-alu-minium dendrites with equilibrium levels of magnesium

and silicon within 10 min of solution treatment (Fig. 8).

Shivkumar et al. [4] reported that 25 min solution

treatment at 540 °C is sufficient for achieving a similar

result. However, it is important to note that the ‘‘10

min’’ referred to in this study is the net time at 540 °C,

while the ‘‘25 min’’ quoted in Shivkumar et al.’s paper

may be the total time the sample was in the furnace (not

clearly specified in their paper). In establishing our ex-

perimental method, it was found that in an air circulated

furnace, a small sample of 12 Â 12 Â 60 mm3 took 5–6

min to heat to a temperature above 520°

C. In this sense,the results of this study are probably in reasonable

agreement with those reported by Shivkumar et al. [4].

Although the majority of the YS is realised by hold-

ing the samples at 540 °C for 10 min, there still exists a

5–10% difference in strength between a short solution

treatment of 10 min and the standard long solution

treatment (6 h in this study). Yield strength is very

sensitive to the magnesium content in the matrix [15],

indicating that the actual average magnesium content

over the whole sample is possibly a little less than that

calculated from the microprobe measurements that were

taken on a relatively small number of scans.

The conclusion that only a short solution treatmenttime of less than 10 min is needed for realising sufficient

YS may only be applicable to alloys with low magne-

sium contents (e.g. 0.3–0.4 wt.%) that have been cast at

relatively high cooling rates, typical of those obtained in

the rim of low pressure die cast wheels. If the level of

magnesium is higher (up to 0.7 wt.%), a longer solution

treatment time will be required, as predicted by the so-

lution and homogenisation model for A365 and A375

alloys developed by Rometstch et al. [16]. Since solidi-

fication rate has a dramatic effect on the size and dis-

tribution of Mg2Si particles and the size of the other

Fig. 11. The impact energy of the modified Al–7wt.%Si–0.3wt.%Mg

alloy as a function of solution treatment time at 540 °C.




possible magnesium containing phases, it is important to

be aware of the influence of the solidification rate on the

required minimum solution time for realizing the re-

quired YS. A separate study [17] indicates that a short

solution treatment of 10 min at 540 °C was also suffi-

cient for attaining greater than 90% of the strength for

the samples cut from the hub region of the wheels of Al–

7wt.%Si–0.3wt.%Mg alloy. The solidification rate in the

hub region was substantially lower than the rim region.

Therefore, a short solution treatment of 10 min may be

sufficient for most low pressure die castings or perma-

nent mould castings with a composition of magnesium

not greater than 0.4 wt.%.

Although it appears that the time required for

achieving sufficient YS is very short, it is important that

this time requirement is met at the solution treatment

temperature. This study and the study undertaken by

Shivkumar et al. [4] both show that the average mag-

nesium content in the as-cast a-aluminium dendrites is

substantially lower than the equilibrium level (i.e. 0.15compared with 0.3 wt.%), even though the distribution

of magnesium appears to be homogeneous. This would

make the strength achieved by the so-called simplified

solution treatment [18], where a casting is quenched

directly after the casting process and followed by arti-

ficial aging, to be substantially lower than that allowed

by the alloy composition. In addition, the strength ob-

tained by the simplified solution treatment may be very

sensitive to the casting condition which influences the

magnesium content and its distribution in the a-alu-

minium. This is a critical issue, since a manufacturer

generally needs consistency of strength and hardness

throughout their castings.

Increasing the solution treatment temperature to 550

°C accelerates the solution and homogenisation process,

so 10 min at 550 °C is more than sufficient to achieve

optimum YS for the Al–7wt.%Si–0.3wt.%Mg alloy. This

was confirmed by microstructural examination and

tensile property measurement.

4.2. Ductility

This study shows that a solution treatment of 30 min

at 540 °C causes substantial spheroidisation and coars-ening of the silicon particles in a fully modified Al–

7wt.%Si–0.3wt.%Mg alloy. This microstructural change

has an effect on the ductility of the alloy, resulting in a

substantial increase in the elongation to fracture. It is

interesting and important to note that the ductility of

the alloy achieved by a solution treatment of 30 min is

almost the same as that obtained by the standard solu-

tion treatment time of 6 h. Previous work [4,7,9] estab-

lished that with a fully modified as-cast microstructure,

a solution treatment of 1 h at 540 or 550 °C leads to a

significant degree of spheroidisation and coarsening of

the eutectic silicon particles. This study further confirms

the previous observations and shows that the spheroidi-

sation and coarsening achieved with a much shorter

time at 540 or 550 °C is also significant.

It is striking to observe that the average fracture

strain of the tensile test samples solution treated for

a short time of less than 8 min is substantially lower

than that corresponding to a solution treatment time of

longer than 10 min. This feature is shown for both so-

lution temperatures, indicating it is unlikely to be caused

by experimental uncertainty or microstructural incon-

sistency of the test samples. This observation suggests

that there exists a region during the early stage of so-

lution treatment (0–8 min), where the ductility of the

alloy reaches a minimum level. The cause of this region

is likely to be a timing mismatch between an increase in

strength and the improvement of the features of the

silicon particles brought about by the solution treat-

ment. With very short solution treatment time, the

strength rapidly increases to its nearly maximum leveldue to the fast solution and homogenisation kinetics.

However, over this short time the silicon particles have

only begun to spheroidise and thus the elongated silicon

fibres are more likely to fracture when the YS increases

[19]. Therefore it is possible that the ductility does not

begin to increase until most of the silicon particles begin

to approach a spheroidal morphology. Spheroidisation

and coarsening (to some extent) of the eutectic silicon

particles in the cast alloy increases the fracture strain, as

they make it more difficult for the silicon particles to

fracture [19–21]. To achieve a high confidence in the

improvement of ductility brought by solution treatment,

this region should be avoided, and therefore the solution

time used should be more than 10 min. There exists a

similar and slightly wider region of low impact energy as

a function of solution time at 540 °C (Fig. 11). Again to

avoid this region, a solution treatment time of greater

than 10 min at 540 or 550 °C should be used. It is noted

that the recently published work by Pederen and

Arnberg [22] also showed a similar phenomenon.

The results of this study show that after 30 min of

solution treatment at 540 °C, about 80% of the maxi-

mum impact energy value is achieved. The difference is

much larger than that for tensile ductility. The spheroi-

disation and coarsening of the silicon particles and theincrease in inter-particle spacing corresponding to a

short solution treatment time of 20 min appear to be

insufficient to achieve the maximum value of the impact

energy. Increasing the solution time to 30 min results in

no further improvement. The impact energy reflects the

ease of crack nucleation and growth at high strain rate.

It is likely that the inter-particle spacing plays a domi-

nant role in determining how easily cracks nucleate and

grow at a very high strain rate. The nucleation of the

cracks is likely to start with cracking of the brittle silicon

particles during impact, as has been shown by previous




observations on the more slowly deformed tensile sam-

ples [18]. Once a large fraction of silicon particles are

cracked, cracks grow by linking microvoids formed by

the cracking of the silicon particles [23]. With a smaller

inter-particle spacing, it is easier for the microvoids to

link and grow a crack. This substantial difference in the

impact energy value between short and long solution

treatment times is a concern if the short solution treated

cast components are used in a service environment

where resistance to mechanical impact is a basic expec-

tation. The tolerance of a casting to a low impact re-

sistance and the minimum solution time required to

reduce the difference to 5–10% could be important fac-

tors in determining the practical feasibility of using a

short solution treatment time for some applications.

Another possible concern is the difference in fatigue re-

sistance between a short and a long solution treatment

time. This is a subject for further investigation.

4.3. Practical implications

The practical implications of the studies on short

solution treatment times are clear. As long as the as-cast

microstructure is appropriate, a short solution treatment

of less than 30 min at 540 or 550 °C can be used in many

cases where impact resistance is not a prime concern.

The essential precondition is that the as-cast micro-

structure of the Al–7wt.%Si–(0.25–0.45)wt.%Mg alloy

must be fully modified. This ensures fast spheroidisation

and coarsening, allowing a short solution treatment to

be implemented.

While this may not be very important for a long so-

lution treatment, it is important to know the exact time

at the intended solution temperature when a short so-

lution treatment of 30 min or less is used. To avoid the

low ductility region, longer than 10 min at temperature

must be experienced by every critical region of the cast

component. While it is not difficult to ensure this in

laboratories where small and regular samples are heat

treated, it may be a challenging task for practitioners in

manufacturing plants where castings of complex shape,

and large numbers have to be dealt with every day.

5. Conclusions

For a low pressure die cast Al–7wt.%Si–0.3wt.%Mg

Sr-modified alloy, a solution treatment of 10 min at 540

or 550 °C is sufficient to obtain the maximum level of

magnesium and silicon in the a-aluminium phase. Si-

multaneously, homogenisation of the magnesium and

silicon in the a-aluminium phase is also achieved. A

solution treatment of 30 min causes spheroidisation,

coarsening and an increase in inter-particle spacing of

the eutectic silicon particles leading to a significant im-

provement in ductility and impact resistance.

It is established that for the alloy, casting and ageing

conditions studied, a short solution treatment of 30 min

at 540 or 550 °C is sufficient to achieve more than 90%

of the maximum YS and more than 95% of the maxi-

mum UTS and the maximum average elongation to

fracture compared with a solution treatment time of 6 h.

However, only 80% of the maximum impact energy can

be attained by the short solution treatment.

There exists a region where the elongation to fracture

and the impact energy decrease to a minimum before

increasing. This region corresponds to 1.5–10 min of

solution time. The cause of this region is probably due

to a mismatch between the negative effect of solution

treatment on ductility and impact resistance associated

with a rapid increase in YS and the more slowly devel-

oping positive effect associated with the spheroidisation

and coarsening of silicon particles.

Acknowledgements

The authors thank Southern Aluminium Pty Ltd,

Comalco Aluminium and David Farnsworth for their

support for this project and Dr. Carlos Caceres for

useful discussion. The CRC for Cast Metals Manufac-

turing (CAST) was established under and is supported

by the Australian Government’s Cooperative Research

Centre Scheme.

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A356 homojenizasyon 1

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