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
Solid State Phenomena Vols. 141-143 (2008) pp 355-360online at http://www.scientific.net© (2008) Trans Tech Publications, SwitzerlandOnline available since 2008/Jul/07
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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
358 Semi-Solid Processing of Alloys and Composites X
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.
Solid State Phenomena Vols. 141-143 359
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.
References
[1] M.A. Easton and D.H. StJohn: Metall. Mater. Trans. A, Vol. 30A, (1999), p. 1613.
[2] M.A. Easton and D.H. StJohn: Metall. Mater. Trans. A, Vol. 30A, (1999), p. 1625.
[3] M.A. Easton and D.H. StJohn: Acta Mater., Vol. 49, (2001), p. 1867.
[4] D.H. StJohn, P. Cao, M. Qian, and M.A. Easton: Advanced Engineering Materials, Vol. 9,
(2007), p. 739.
[5] D.H. StJohn, M.A. Easton, P. Cao, and M. Qian: Inter. J. Cast Metals Res., Vol. 20, (2007),
p. 131.
[6] M.A. Easton and D.H. StJohn: Metall. Mater. Trans. A, Vol. 36A, (2005), p. 1911.
[7] D.H. StJohn, Q. Ma, M.A. Easton, P. Cao, and Z. Hildebrand: Metall. Mater. Trans. A, Vol.
36A, (2005), p. 1669.
[8] M.J. Bermingham, S.D. McDonald, M.S. Dargusch, and D.H. StJohn: Scripta Mater., Vol.
58, (2008), p. 1050.
[9] M.A. Easton, H. Kaufmann, and W. Fragner: Mangesium Technology 2005. (The Minerals,
Metals and Materials Society, 2005), p. 345.
[10] B.S. Murty, S.A. Kori, and M. Chakraborty: Inter. Mater. Rev., Vol. 47, (2002), p. 3.
[11] M.A. Easton and D.H. StJohn: Mater. Sci. Engng. A, Vol. 486, (2008), p. 8.
[12] J.E.C. Hutt and D.H. StJohn: Inter. J. Cast Metals Res., Vol. 11, (1998), p. 13.
[13] T. Motegi, K. Kondou, R. Chi, and S. Aoyuma: 4th Decennial International Conference on
Solidification Processing. (Department of Engineering Materials, University of Sheffield,
1997), p. 14.
[14] D.J. Browne, M.J. Hussey, A.J. Carr, and D. Brabazon: Inter. J. Cast Metals Res., Vol. 16,
(2003), p. 418.
[15] H. Kaufmann, H. Wabusseg, and P.J. Uggowitzer: Aluminium, Vol. 76, (2000), p. 69.
[16] H. Wang, C.J. Davidson, J.A. Taylor, and D.H. StJohn: Mater. Sci. Forum, Vol. 396-402,
(2002), p. 143.
[17] G.I. Eskin, Ultrasonic treatment of light alloy melts. (Gordon & Breach, 1998).
[18] B. Chalmers, Principles of solidification. (John Wiley & Sons, 1964), 86.
[19] Q. Han, X. Jian, H. Xu, and T.T. Meek, Method and apparatus for semi-solid materials
processing. 2007: USA. 7,216,690 B2
[20] X. Jian, T.T. Meek, T. Greer, and Q. Han: Magnesium Technology 2006. (The Metals,
Minerals and Materials Society, Warrendale PA, 2006), p. 103.
[21] X. Jian, T.T. Meek, and B.Q. Han: Scripta Mater., Vol. 54, (2006), p. 893.
[22] A. Ramirez and M. Qian: Magnesium Technology 2007. (The Metals, Minerals and
Materials Society, Warrendale PA, 2007), p. 127.
360 Semi-Solid Processing of Alloys and Composites X
Semi-Solid Processing of Alloys and Composites X doi:10.4028/3-908451-59-0Controlling the Semisolid Grain Size during Solidification doi:10.4028/3-908451-59-0.355
Solid State Phenomena Vols. 141-143 361
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