melts, then batch additions of master alloys were made, whilst now fast ... master alloy additions are made using a rod master alloy in direct ..... 1998: Trans Tech.
MATERIALS FORUM VOLUME 31 - 2007 Edited by J.M. Cairney and S.P. Ringer © Institute of Materials Engineering Australasia Ltd
THEORETICAL ADVANCES LEADING TO IMPROVEMENT IN COMMERCIAL GRAIN REFINEMENT PRACTICES IN AL ALLOYS M. Easton1 and D. StJohn2 CAST Co-operative Research Centre Department of Materials Engineering, Monash University, Melbourne, 3800, Australia 2 School of Engineering, The University of Queensland, Brisbane, 4072, Australia
1
ABSTRACT This paper traces how advances in the fundamental understanding of initial solidification during grain refinement has led to practical changes in grain refinement practice leading to substantial cost savings for alloy producers. A theoretical study of grain refinement showed that the achievement of a fine grained microstructure is dependent upon the addition of potent and numerous nucleating substrates and solute elements that provide constitutional undercooling to facilitate further nucleation in the melt. In Al alloys, Ti is the most effective solute element for grain refining as its ability to generate constitutional undercooling far exceeds all other elements. It is particularly effective in low solute wrought alloys, where it has been shown that it can be used to partially substitute for more expensive grain refiners substantially reducing the cost of the grain refining process. In contrast, because of the high solute levels in Al-Si foundry alloys, Ti additions are not effective grain refiners despite being regularly added to these alloys for this purpose and are therefore an unnecessary addition. Finally, a new methodology of assessing grain refinement is applied to the assessment of different grain refiners, poisoning of grain refinement in Al alloys and grain refinement mechanisms in Mg alloys. believed that AlB2 14or the mixed boride, (Al,Ti)B2 the effective nucleant in these alloys.
1. INTRODUCTION It was over 50 years ago when the discovery was made that additions of Ti to Al alloys reduces the grain size substantially 1. Relatively soon after, it was found that adding B in addition to Ti leads to an even further reduction in the grain size of cast Al alloys 2. In the intervening years many hypotheses have been proposed attempting to explain how grain refinement works in these alloys. It was apparent that elements that were peritectic in nature with Al were generally found to be good grain refiners 3 and this was generally postulated to be due to a peritectic reaction occurring on the pro-peritectic phase (Al3Ti in the Al-Ti system) causing a low energy barrier to nucleation 4. However, the peritectic theory was not able to explain the grain refinement by Ti at Ti concentrations less than the peritectic composition (~0.15wt% Ti), especially when small additions of B were made 5-7. There has been a lot of debate about whether TiB2 particles themselves are effective nucleant sites for α-Al 8 or whether they may just provide substrates for Al3Ti to be stable and thus responsible for the nucleation of α-Al at concentrations below the peritectic composition 9, 10.
11
is
It was recognized that there was an effect of alloy chemistry on the grain size 15 but it was not until the 1990’s that it was realized how important the role of alloy composition actually is 16-18. A factor was developed, called the growth restriction factor (denoted as GRF16, 19 or Q 20 in various papers). It was defined as
Q = ∑i mi c0,i (k i − 1) ,
(1)
where for each element i, m is the liquidus gradient, c0 is the composition and k is the binary partition coefficient. It was denoted as such as it is inversely proportional to the dendrite growth rate in steady-state dendrite growth theory 21. Originally it was applied to binary systems 15 and then to multi-component alloys 16. Generally it was found that an increase in Q leads to a decrease in the grain size, however, in Al-Si systems it was found that at Si contents above 2-3wt% the grain size increased again 16. Meanwhile, grain refinement became a standard practice in the casting industry as a fine grain size was associated with improved castability, mechanical properties and productivity for most alloys in most casting operations. Originally Ti- and B- containing salts were added to melts, then batch additions of master alloys were made, whilst now fast acting, continuous, in-line grain refiner master alloy additions are made using a rod master alloy in direct chill casting operations. The master alloys contain fixed Ti:B ratios and the most commonly used one is Al-5Ti-1B which contains 3.2wt% TiB2 and 2.8wt%Ti as Al3Ti particles which dissolve when diluted in Al melts. Common addition rates are between 0.003 to 0.01wt% Ti, well below the peritectic composition. Al-Si foundry alloys, however, generally contain a Ti
In the 1980’s, there was a significant amount of work on the grain refinement of Al-Si foundry alloys. It was found that, unlike, pure Al, these alloys are poorly grain refined by Ti additions and a combination of Ti and B additions is more effective, whilst the most effective is the addition of B without any Ti addition at all 11, 12. B additions lead to little, if any, grain refinement in pure Al 13 . This led to the development of grain refiners with high B content either alone, or with Ti levels below the stoichiometric weight ratio of 2.2 for TiB2. It was 57
Furthermore this relationship can be used with data from thermodynamic databases to calculate the Q values for different alloys. It has been shown that the additive relationship for Q given in equation 1 is reasonably good for most wrought alloys especially the leaner alloys, however, in Al-Si foundry alloys, the partitioning of Ti reduces dramatically and it has a much reduced effect compared with low solute alloys (Table 2). It can be seen that the relative increase in the Q value for a solute poor alloy such as 1050 is far greater than in a solute rich alloy such as the Al-Si foundry alloy 601 (also designated alloy 356). A detailed description of the underlying thermodynamics is found elsewhere 28.
specification of up to 0.2wt%, which seems to be an artifact of the belief that Ti was a potent grain refiner in all Al alloy systems 22. It was not until after the development of the new grain refiners for Al-Si foundry alloys in the 1980’s and 1990’s that the grain refinement of these alloys was re-considered. This paper describes the advances in the understanding of grain refinement over the last 10 years or so by the current authors and others that has led to improved grain refining practices that can improve the quality and reduce the cost of grain refining operations. Finally, the same theoretical understanding is shown to provide a framework for revealing the mechanisms of grain refinement and for assessing potential grain refiners.
Table 1. Phase diagram data for determining Q values for common elements in binary Al alloys 29. The data for Al-Ti was obtained from 30.
2. HOW GRAIN REFINEMENT WORKS Classical nucleation theory indicates that a fine grain size is achieved when there are plentiful heterogeneous nucleant substrates available which are effective nucleation sites 21. For many years, determining the most effective nucleant particles appeared to be the focus of most grain refinement research 23. Whilst important, it did not take into account the effect of solute on grain refinement and led researchers into the mistake of adding compounds to pure Al to assess their grain refining ability 24. Most of these compounds, except those that dissolved on addition, e.g. Al3Ti, were found not to grain refine, because the availability of potent nucleant substrates is only one important factor for effective grain refinement. To achieve a fine grain size the addition of solute is also critically important 23, 25.
d∆Tc df s
where
k
m
Ti Zr Si Cr Ni Mg Fe Cu Mn
7-8 2.5 0.11 2.0 0.007 0.51 0.02 0.17 0.94
33.3 4.5 -6.6 3.5 -3.3 -6.2 -3.0 -3.4 -1.6
max. conc. (wt%) 0.15 0.11 ~12.6 ~0.4 ~6 ~3.4 ~1.8 33.2 1.9
m(k-1) ~220 6.8 5.9 3.5 3.3 3.0 2.9 2.8 0.1
Table 2. A comparison of the Q values determined for typical compositions for various alloys using equation 1 and thermodynamic modelling using Thermocalc based on equation 2.
The addition of solute leads to the development of constitutional undercooling during solidification 26. This means that there is a region of maximum undercooling in front of the growing grains in which nucleation can occur if the undercooling required for nucleation on the available sites is reached. If that undercooling is achieved more rapidly then the there will be less grain growth leading to a finer grain size 27. One of the important discoveries has been that, as well as Q being inversely proportional to the dendrite growth rate which in turn reduces the rate of evolution of latent heat, it is also the rate at which the constitutional undercooling develops at the beginning of solidification20, 27, i.e.
Q=
Element
Alloy 1050 1050-0.05Ti 2014 3003 5083 6061 601-0.02Ti 601-0.2Ti
(2)
Equation 1 1.9 12.96 22.69 5.41 16.35 7.83 47.1 91.4
Equation 2 1.86 17.43 21.86 6.60 20.60 7.55 43.1 52.7
Ref 31 31 31 31 31 31 27 27
The other important factor (other than the casting conditions) in grain refinement is the nucleant particles, where both the potency, which is the inverse of the undercooling required for nucleation, and the density of the particles are important.
fs →0
∆Tc is the rate of development of constitutional
undercooling and fs is the fraction solid. Hence it would be expected that the final grain size would be inversely proportional to Q. This means that the Q values for each of the individual elements is important and Table 1 shows that Ti as a solute has an exceptionally high Q value per unit composition and hence is especially effective at producing constitutional undercooling and grain refinement.
By considering, equation 2, if it is assumed that the final grain size is proportional to the amount of growth that occurs to generate the amount of undercooling required for nucleation to occur, i.e. , ∆Tn = ∆Tc , then
d∝ 58
∆Tn
Q
(3)
allows the expensive grain refiner rod addition to be reduced whilst still achieving the same grain size. There are casthouses that have added Ti solute in this way to reduce grain refiner additions 36, 37. However, once the relationships between the grain size, alloy composition and nucleant particle addition rate is known, a simple algorithm can be set up to determine the optimum additions of Ti solute and grain refiner rod to minimize the grain refiner cost.
Hence if the grain size is plotted against against 1/Q then the gradient of the line will be proportional to the nucleant potency. Figure 1 shows that the gradient decreases substantially when nucleant particles are added, but remains the same as a greater number of nucleant particles are added, which brings us to the second point about the nucleant particles. It would be expected that, at least at low nucleant particle densities that the grain size would be proportional to the density of the nucleant particles. It is found that the intercepts of the lines for the different TiB2 contents in Figure 1 show exactly this behaviour. A very simple analytical equation based on the data in Figure 1 can be proposed 32, i.e.
d=
1 3
fρ
+
b′.∆Tn Q
Based on grain refiner costs (Table 3), estimates of savings were determined for alloys of various Q values compared to a 0.005wt% Ti as Al-5Ti-1B master alloy whilst still achieving the same grain size (Figure 2). As expected, it was found that the greatest savings can be obtained in the alloys with the lowest Q. In fact for alloys with Q values of about 3, such as 1050 about $1.20/ tonne Al can be saved by the addition of Ti to the melt and reducing the addition of rod grain refiner. Soft 6000 series alloys such as 6060 and 6063 have Q values of 4-6 and a significant saving of approximately $0.60/tonne Al can be achieved. For alloys with Q values around 10, such as 6082 the potential saving is minimal and for alloys with Q values greater than 15, such as 5083 and 2014, no savings can be achieved through this approach.
(4)
where f is the proportion of particles added that actually nucleate grains, ρ is the density of added particles and b’ is a fitting factor. In the particular case of Figure 1, the best fitting equation 32 is
d=
32
3
[TiB2 ]
+
650 . Q
(5)
Table 3. Indicative costs of grain refiner additions in $USD in 2004. The Ti tablet additions are assumed to contain 75% titanium38. Grain refiner cost Cost /tonne Ti
Al-3Ti-1B
Al-5Ti-1B
Ti tablet
$3,200/tonne
$3,250/tonne
$4,200/tonne
$107,000
$65,000
$5,600
2
Saving ($/tonne Al)
Figure 1. A plot of grain size versus 1/Q for a range of aluminium alloys, including 1050, 2014, 3003, 5083, 6060, 6061 and 6082 at different TiB2 contents added via a Al-3Ti-1B master alloy, different Ti solute contents up to 0.05% cast at a pre-solidification cooling rate of 1°C/s into graphite cups 32. Factors affecting the nucleation potency such as wetting of the heterogeneous sites 33, lattice mis-match 34 and nucleant particle size 35 have also been considered recently, but will not be discussed here. The effect of cooling conditions is also important and will be discussed elsewhere.
1.5
1
0.5
0 0
5
10
15
20
Q (K-1)
Figure 2. Calculated savings plotted against the Q value of an alloy 38.
3 OPTIMISATION OF GRAIN REFINER ADDITIONS
This approach to reducing the cost of grain refinement can be implemented using the Opticast technology 39. It should be noted that the savings that can be achieved differ depending upon the current grain refinement practice. Furthermore, since 2004, the price of Ti has increased substantially so the savings will be lower now than they were then.
3.1 Wrought Alloys Most wrought alloys have low Q values and hence the addition of solute Ti is quite effective at reducing the grain size. In a DC casting operation Ti solute can be added to the furnace to assist grain refinement which then 59
3.2 Al-Si Casting Alloys Since doing this calculation an assessment of the effect of reducing the Ti content on the mechanical properties has been undertaken which has shown that reducing the Ti content leads to a mild increase in ductility and a slight increase in the age hardening response of the alloys 41.
The difference between the effect of Ti in Al-Si casting alloys and low solute alloys can be explained by considering the effect of solute Ti on the Q value. Solute Ti has a substantial effect on the grain size in pure Al and alloys with low Q values because the addition of Ti leads to a substantial relative effect on the Q value of these alloys. For example, an addition of 0.005% solute Ti to alloy 1050 will double its Q value.
Hence it appears that reducing the Ti content has added benefits other than just the cost saving. Therefore, if grain refinement is required then TiB2 particles need to be added, either as an Al-5Ti-1B master alloy or one of the master alloys developed for Al-Si foundry alloys.
In Al-Si foundry alloys the Q value is already very high, i.e. >40. This means that an addition of 0.2Ti is required to double Q, if Ti is assumed to have the same partition coefficient and liquidus gradient as in the binary systems, but both of these reduce as well (Table 2) 40. Therefore even for a 0.2Ti addition as is currently found in many Al-Si foundry alloys there is little effect on grain size, particularly when nucleant particles have been added (Figure 3).
4. EVALUATING GRAIN REFINEMENT One of the most interesting results given the theoretical approach given in section 2 and the experimental results given in Figure 1, is that a very simple methodology can be used to evaluate the effectiveness of added nuclei and to assist with the identification of grain refinement mechanisms.
800
Assuming constant casting conditions and that the same density of a particular type of nucleant particle is added to alloys with a range of Q values, equation 4 can be simplified to
700
0% TiB2
Grain Size ( m)
600 500 400
d =a+ b
300 200
0.01% TiB2
100
0.05
0.1
0.15
0.2
0.25
w t % Ti
(a) 0.025 ∆Tn
RGS (fs)
0.02
(6)
The gradient b is proportional to the nucleant potency and the intercept a is related to the maximum number of particles that are active nucleants of grains. Hence, adding a more powerful nucleant with the same maximum number of active particles will have the same intercept a but a lower gradient (Figure 4(a)). However, an increase in the nucleant particle density will decrease the intercept a but the gradient b will remain constant (Figure 4(b)). To quantify the factors in equation 2 grain size data for a range of Q values are required.
0 0
Q
0.2
In Al-alloys one of the most revealing uses of this technique has been to re-analyse data related to the poisoning of grain refinement, particularly by Zr. Zr has long been known to poison grain refinement by Al-Ti-B grain refiners 42-44, probably due to the reaction of Zr with TiB2 particles to form ZrB2, which is a less potent nucleant.
0.5
0.015
0.7 1
0.01 0.005 0 0
0.05
0.1
0.15
0.2
A re-analysis of data from Kearns and Couper 18 has been able to elucidate the mechanism of poisoning across a range of Q values for alloys (Figure 5). If the addition of Zr was to reduce the potency of all the TiB2 particles then it would be expected that a single steeper line C (i.e. particles of lower potency) might pass through the same value of the intercept a. This is true for alloys with high Q values, i.e. up to a 1/Q value of 0.1. After this point a limit is reached and it can be observed that line B can be drawn through the four limiting points that is parallel to the Zr-free line A. This implies that not all of the TiB2 particles are poisoned (i.e. line A shifts upward to line B) and that the Zr-poisoned particles (line C) only become
0.25
w t % Ti
(b) Figure 3. A comparison of the grain size reduction by the addition of Ti solute and TiB2 particles for alloy 601. (a) Experimental results. (b) Theoretical predictions, where RGS is a relative grain size27. Since Ti is not an effective means of grain refinement in Al-Si foundry alloys, it appears that it can be removed and given the cost of Ti in Table 3, about $11.20/tonne Al can be saved. 60
compositions. Figure 6 shows that the Al-3Ti-1B grain refiner contains a greater density of nuclei but these are not as potent as the Al-5Ti-1B master alloy. Hence, the Al-5Ti-1B master alloy is more effective at grain refining low Q alloys, whilst the Al-3Ti-1B alloy is more effective at grain refining the high Q alloys. Hence to do a grain refinement test on one particular alloy may not provide enough information to assess a grain refiner for a product range. It should be noted that the performance of grain refiners from different producers are different and it should not be assumed that all Al-5Ti-1B or Al-3Ti-1B grain refiners behave in the same way.
active nucleants when line C crosses below line B. Thus at high Q values the Zr-poisoned particles provide a finer grain size than the remaining more potent TiB2 particles because the constitutional effect is greater and therefore many more of the poisoned particles become active nucleants. This observation also implies that the poisoning effect of Zr decreases in very high solute content alloys.
b Grain
b’ b’’ a
Particles giving b’’ have
1/Q
(a)
Grain
Figure 6. Grain refinement performance of an Al-3Ti-1B alloy 32 and an Al-5Ti-1B alloy (data originally sourced from Lee et al 45). Minimu m grain
a
/
Whilst some success has been achieved in revealing the mechanisms of poisoning in Al alloys, it is in Mg alloys where this method of analysis has been able to be of the most benefit.
(b)
The most effective grain refiner of pure Mg and many non-Al containing Mg alloys is Zr which is a good nucleant 46 and has a high relative Q value 47. Changes in grain size on Zr additions, holding time and stirring was also able to be traced and explained by the loss of nucleants or changes in solute levels 48
Figure 4. (a) The effect of changing the nucleant particle potency on the gradient b while keeping the maximum number of active particles a constant and (b) the effect of adding more of the same type of particles on the value of a while the potency b remains constant 32.
The greatest technical problem in Mg alloys is to explain the grain refining effects in Mg-Al alloys and to obtain a commercially viable grain refinement addition. The addition of SiC particles to Mg-Al alloys has been found to also produce parallel lines for two different SiC addition rates. Whilst some refinement was obtained it was shown that Mn poisons nucleation 49 to some extent, which makes it difficult for it to be used as a commercial grain refiner without further investigation into the mechanisms of poisoning. High purity Mg-Al alloys have been found to have a fine grain size compared with lower purity alloys 50. This grain refiner assessment technique has been able to show that this is because the nuclei are poisoned 51. By comparing the gradients of the lines of a range of Mg-Al alloys with and without superheating it was found that grain refinement achieved by superheating also changes the nucleant potency 52. However, the particles in the high purity alloy and the superheated alloy both have the same gradient and hence could be the same phase.
Figure 5. Data for alloys with and without Zr additions, obtained originally from Kearns and Couper 18 and re-plotted by the current authors 32. Grain refining master alloys can also be compared as in Figure 6. This shows two master alloys, one being Al-5Ti-1B and the other Al-3Ti-1B and shows that each of the grain refiners may be better for different alloy 61
Therefore, it is likely that both superheating and increasing the alloys purity removes a lower potency impurity layer from the surface of the nucleant particles 51 .
15. 16.
5. CONCLUSIONS 17. Advances in understanding of the importance of solute in grain refinement has been able to open avenues for reducing the cost of grain refining practices for both wrought Al alloys in direct chill casting by optimizing the Ti content in the alloy and reducing the grain refiner additions and for removing Ti from Al-Si foundry alloys. Furthermore, a new methodology for assessing grain refiners and identifying grain refinement mechanisms has been developed. It has been able to shed light on the poisoning mechanism of Zr when Al-Ti-B grain refiners are used, and to compare the effectiveness of grain refiners across a range of alloys. Furthermore, it has been able to elucidate the mechanisms of grain refinement in magnesium alloys, especially high purity alloys and during superheating.
18. 19. 20. 21. 22.
23. Acknowledgements
24.
The CAST Co-operative Research Centre was established under, and is supported in part by, the Australian Government’s Cooperative Research Centre’s Programme.
25. 26. 27.
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