Controlling the semisolid grain size during solidification

Controlling the Semisolid Grain Size during Solidification

David H. StJohn1,a, Mark A. Easton2,b and Ma Qian3,c 1CAST Cooperative Research Centre, School of Engineering, The University of Queensland,

Brisbane, 4072, Australia

2CAST Cooperative Research Centre, Department of Materials Engineering, Monash University, Melbourne, 3800, Australia

3Department of Mechanical and Design Engineering, University of Portsmouth, PO1 3DJ, UK

[email protected], [email protected], [email protected]

Keywords: Grain refinement, Solidification, Semisolid processing, Ultrasonic treatment

Abstract. This paper will use a new method for predicting grain size and then apply it to various

solidification environments to reveal which factors are dominant in determining the final grain size.

This study will only focus on methods where the grain size is set during a solidification process.

These processes include grain refinement by inoculation of the melt with grain refining particles,

increasing the cooling rate, low superheat casting, ultrasonic treatment and the use of chill moulds.

Each of these methods can control the grain size to some extent but in order to predict the outcome

it needs to be understood how the alloy constitution, inoculant particle characteristics and the

casting conditions affect the prime nucleation event for the formation of new grains. These

methodologies are currently being used, or have potential to be used, for the production of fine

spherical grained semi-solid slurries.

Introduction

The most efficient and economic methods to achieve fine and near-spherical primary grains for

semisolid forming involve processes that are part of the first solidification event removing the need

for subsequent processing before undertaking semisolid forming. This paper focuses on methods

where the grain size is set during the prime solidification event. These processes include grain

refinement by inoculation of the melt with nucleants, increasing the cooling rate, low superheat

casting, ultrasonic treatment and the use of chill moulds. Each of these methods can control the

grain size to an extent but in order to predict the outcome it needs to be understood how the alloy

constitution, inoculant particle characteristics and the casting conditions affect the prime nucleation

event for the formation of new grains.

Research [1-3] has shown that the grain size is determined by the potency of the nucleant

particles and the rate of development of constitutional supercooling which is determined by the

growth restriction factor Q of the particular alloy. Recent work [4, 5] has demonstrated the benefits

of a new model in revealing important information about the factors controlling the development of

grain size. The advantage of this model is that the relationship between grain size and nucleant

potency and alloy constitution is represented by a simple linear relationship of the form

d = a + b/Q (1)

where ‘a’ is related to the number of particles that actively nucleate grains and ‘b’ is a constant

related to the potency of the inoculant particles.

It has been found [4, 5] that the model can be used to reveal information about the potency of

nucleating particles, their refining effectiveness and the role of alloy constitution even when little is

known about these factors. Our research has shown that the approach is applicable to grain

refinement of Al [5, 6], Mg [7] and Ti [8] alloys. It is likely to apply to other alloy systems as well.

Although Eq. 1 assumes a simple form there are a number of complicating factors that

researchers need to be aware of when using it to interpret experimental results [4, 5]. For example,

the added particles may partly dissolve in the melt changing the Q value and/or reducing the

number of particles available for nucleation, or they may react with the melt changing their

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crystallography and thus their potency. In addition, differences in, or lack of control of, casting

conditions can lead to less reliable values of ‘a’ and ‘b’. An example is where there is slow cooling

after grain nucleation leading to coarsening [9].

Since grain refinement is an important technology for producing or assisting the production of a

suitable grain size for semisolid forming, this paper examines different solidification methods from

the perspective of Eq. 1, where the influence of alloy composition, cooling rate, melt superheat and

ultrasonic treatment on grain size control is assessed.

Grain Refinement by the Addition of Grain Refining Additions

The common grain refiners used for wrought Al alloys such as Al5Ti1B contain both nucleant

particles of TiB2 and an excess of Ti solute. Fig. 1 shows the relative effects of adding solute Ti and

TiB2 particles by plotting the grain size against 1/Q (measurement of solute) according to Eq. 1. The

first addition of TiB2 particles has the largest effect on reducing the grain size. Once a certain

amount of TiB2 has been added, further addition increases the grain density by approximately a

similar proportion [6]. On the other hand, the addition of solute, Ti, is very effective in refining low

solute (lean) alloys, but has little effect on refining high solute content alloys, particularly foundry

alloys [3].

However, there is a limit to how much grain refinement can be achieved due to the diminishing

effect of further additions of solute at higher Q values as shown in Fig. 1. Also, the effectiveness of

reducing the grain size by the addition of nucleant particles diminishes as more particles are added.

Furthermore, the number of effective nuclei is typically around 1% or less of the number added to

the melt for the case of TiB2, so the efficiency of particle addition is limited. Hence, there is a limit

to the amount of grain refinement that can be practically achieved by inoculation.

0

200

400

600

800

1000

0 0.1 0.2 0.3 0.41/Q (K

-1)

Gra

in s

ize (

μm

)

0%TiB2

0.005%TiB2

0.02%TiB2

0.01%TiB2

Fig. 1. Lines of best fit for the grain size data obtained from a range of aluminium alloys with TiB2

additions varying from 0 to 0.02% TiB2 and Ti additions from 0 to 0.05%. After Ref. [6]. Note that

the value of ‘b’ significantly decreases when the first addition of TiB2 is made. ‘b’ then remains

constant.

Grain Refinement by Increasing the Cooling Rate

Casting conditions normally affect the grain size [10]. Using pre-heated graphite crucibles, as was

done to produce the data in Fig. 1, the grain size was investigated for a range of alloys with a

constant TiB2 addition rate of 0.005% under cooling rates between 0.3 and 15oC/s [11]. It was

found that both the values of ‘a’ and ‘b’ decreased with increasing cooling rate, which means that

both the proportion of active nucleants involved in grain refinement and the rate of development of

constitutional undercooling increased. However, the change in the ‘b’ value was more pronounced.

This study [11] has led to the development of Eq. 2, on the basis of Eq. (1), for the prediction of

grain size as a function of Q, nucleant particle type and number, and/or cooling rate as follows

2

13

.1

TQ

Tb

NTfd n

v

(2)

356 Semi-Solid Processing of Alloys and Composites X



where d is the grain size, Nv is the number of nucleant particles added, f is the fraction of nucleant

particles that are able to nucleate grains at the cooling rate, T (f is a function of T ), nT is the

nucleation undercooling and ''b is a fitting constant. Fig. 2 indicates that the grain size can be

approximately halved by increasing the cooling rate from 0.3 oC/s to 15

oC/s.

Grain Refinement using Low Superheat Methods

Another means of changing the casting conditions is to pour the melt into a cold mould [12] or onto

an inclined plate [13] to generate thermal undercooling. This is particularly effective at low

superheats. There are a number of semisolid casting techniques that use this as a method of

producing slurries for rheocasting or billet for thixocasting [14-16]. A way of mimicking this

process in the laboratory is by pouring the melt into a cold die at different melt temperatures. The

same set of alloys that were used to produce Fig. 2 were also cast into a steel die at two different

pouring temperatures. Fig. 3 shows that reducing the superheat has a different effect to increasing

the cooling rate. Whilst the effect of increasing the cooling rate is to reduce both the ‘b’ value and

the ‘a’ value (Fig. 2), reducing the superheat was found to reduce the ‘a’ value whilst the ‘b’ value

remained constant. In other words, reducing the superheat led to a substantial increase in the

proportion of nucleant particles that are able to nucleate grains.

0

100

200

300

400

500

600

0 0.1 0.2 0.3 0.4 0.5

1/Q (K-1

)

Gra

in S

ize

(μ

m)

0.3C/s

1C/s

4C/s

15C/s

Fig. 2. Grain size plotted against 1/Q for four

alloys (1050, 5083, 6060 and 6082) for a TiB2

addition of 0.005% and Ti contents varying from

0 to 0.05% at cooling rates ranging from 0.3 to

15°C/s. After [11].

0

50

100

150

200

250

300

350

0 0.1 0.2 0.3 0.4

1/Q (K-1

)

Gra

in S

ize (

μm

) 65oC

35oC

Fig. 3. Grain size measurements for the alloys

used to develop Fig. 2, cast into a steel mould

preheated at 300°C. The alloys were cast at

superheats of 35 and 65°C

Grain Refinement by Ultrasonic Treatment

The positive effects of dynamic conditions on the solidification of metals have been recognised and

pursued for many years [17, 18]. As a result, a wide variety of techniques and devices have been

developed to facilitate dynamic nucleation for grain refinement or microstructural modification.

Ultrasonic treatment is one means that has been shown to be effective for various alloys [17, 19-22].

Acoustic cavitation is essential to ultrasonic grain refinement [17]. As such, the ultrasonic

intensity applied must exceed a threshold. Fig. 4 shows the influence of ultrasonic intensity,

measured by the displacement amplitude (A), on grain refinement of Mg-Al alloys [17]. The

cavitation threshold varied from A = 1 m for the Mg-8%Al alloy to approximately A = 6.5 m for

the other three alloys. Fig. 5 re-plots the grain size data versus 1/Q for A 6.5 m, where the Mg-

8%Al alloy was excluded from the case when A = 6.5 m, which is the approximate cavitation

threshold for the other three alloys.

Solid State Phenomena Vols. 141-143 357



The slopes of the lines of best fit corresponding to A = 15 - 25 m in Fig. 5, are essentially the

same beyond the cavitation threshold while the intercept decreases. This reveals that increasing the

ultrasonic intensity beyond the threshold refines the grain structure mainly by activating more

nucleant particles but the potency of the particles is little changed. These predictions agree well

with Eskin’s observation that increasing the ultrasonic intensity consistently led to better refinement

of pure aluminum inoculated with an addition of 0.01%Al2O3 than with 0.005% Al2O3 [17],

because more Al2O3 particles were activated in the former case. On the other hand, the noticeable

change in the slope of the line of best fit from A = 6.5 m to A = 15 m and beyond signifies the

transition from underdeveloped cavitation to developed cavitation and thus Fig. 5 can be used to

help determine the threshold.

Fig. 4. Refinement of the structure of

magnesium alloys (Mg-Al) versus ultrasonic

intensity, where A is the ultrasound amplitude

(m). Reproduced from Ref. [17].

Fig. 5. Grain size data re-plotted as a function of

1/Q for the four Mg-Al alloys shown in Fig. 4

with respect to different amplitudes (A) in the

range from 6.5 m to 25 m.

Comparison of Methods

With constant casting conditions Eq. 1 is a reliable method for comparing the effect on grain size

due to changes in alloy composition, nucleant particle density and changes in nucleant particle type

[4, 6]. When the cooling rate is increased both the values of ‘a’ and ‘b’ decrease but by a decreasing

amount with each incremental increase in cooling rate to the point of being negligible above 4oC/s

(Fig. 2 and Eq. 2). From this observation it appears that an increase in cooling rate increases the rate

of development of constitutional undercooling [3] leading to the next nucleation event occurring

sooner with a concomitant increase in grain density (i.e. a decrease in the value of ‘a’). Currently,

Eq. 2 has been shown to apply to aluminium alloys [11] for particular casting conditions. If Eq. 2

applies to other alloy systems then increased cooling rate provides an avenue to approximately

halve the grain size if the addition of grain refiner particles alone results in an insufficient reduction

in grain size.

Although low superheat casting also reduces the grain size, it appears that the mechanism is

different. Fig. 6 shows the amount of grain refinement achieved by increasing the cooling rate

(dashed lines). The cooling rate for the alloy cast into the cast iron moulds at 65°C is also about

15°C/s and the superheat is enough to minimise or prevent the generation and/or survival of wall

crystals, hence the grain sizes are very similar to the highest cooling rate. But when the superheat is

0

100

200

300

400

500

600

0 0.1 0.2 0.3 0.4 0.5

Gra

in s

ize d

(m

)

1/Q

A= 6.5 m

A = 15 m

▲ A = 20 m

A = 25 m




reduced to 35°C, the grain size reduces dramatically. By considering the ‘a’ values in Fig. 6 the

increase in cooling rate led to an increase in the proportion of active nucleant particles from

approximately 0.4% to 1.4%. The reduction in superheat to 35°C led to approximately 5% of the

added nucleant particles actively nucleating grains. Since reducing the superheat and pouring into

chill moulds is much more effective than increasing cooling rate it is no surprise that many

semisolid casting processes involve a reduction in superheat as part of their process [14-16]. The

increase in the number of active particles is probably due to the fact that the origin of the extra

grains is from on or near the mould walls due to thermal undercooling rather than constitutional

undercooling [12]. This mechanism has a similar effect to adding new potent particles into the melt.

0

100

200

300

400

500

600

0 0.05 0.1 0.15 0.2 0.25 0.3 0.351/Q (K

-1)

Gra

in s

ize

(μ

m)

0.3C/s

15C/s

65C

35C

Fig. 6. A comparison of the effect of increasing the cooling rate (dashed lines) with reducing the

superheat (continuous lines). Data obtained from Fig. 2 and Fig. 3.

When ultrasonic treatment is used its intensity is the variable rather than superheat or cooling

rate. However, for intensities beyond the cavitation threshold, ‘a’ decreases with ultrasonic

intensity while the slope ‘b’ remains constant which is also observed when lowering superheat. In

this situation the nucleant particles are already present in the melt but the increased thermal

fluctuations effectively increase the thermal undercooling leading to more nucleation events. Thus

‘a’ decreases but ‘b’ remains constant. This implies that for ultrasonic and low superheat conditions

the cooling rate term should be removed from the second part of Eq. 2 and the function of cooling

rate in the first part of Eq. 2 should be replaced with a function of superheat or ultrasonic intensity.

This will be the subject of future work.

Summary

A number of approaches to controlling grain size and thus the morphology for semisolid processing

have been examined using a new and simple analytical model. The mechanisms for controlling

grain size were revealed for each method. These results show a very important point, that in order to

achieve a very fine grain size, the use of inoculants is only one of several factors that can

significantly control grain size. These other factors include controlling the cooling conditions

(cooling rate, melt superheat) and ultrasonic intensity. Decreasing melt superheat and increasing

ultrasonic intensity both effectively increase the amount of thermal undercooling which leads to the

nucleation of grains on many more nucleants already present in the melt. It should be noted that the

growth restriction factor Q as defined by the alloy chemistry, is an important factor controlling

grain size in all the solidification processes examined in this study. Although the results presented

here provide useful but qualitative comparative results, this study was based on data produced by a

number of separate studies. More reliable conclusions and better predictive capability will be

generated by a more complete and coordinated study in the future.




Acknowledgements

The CAST Cooperative Research Centre was established under and is supported in part by the

Australian Government’s Cooperative Research Centre’s Programme. MQ thanks the UK

government’s Engineering and Physical Sciences Research Council (EPSRC) for financial support.

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Controlling the semisolid grain size during solidification

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