3 ,',& ,
Chapter 3 Flow
3.3 Fluidity (maximum fluidity length) Lf
3.3.1 Mode of solidification
Flemings (1 974) demonstrated that the fluidity of pure metals and eutectics that freeze at a single temperature is different to that of alloys that freeze over a range of temperatures. These two different solidification types we shall call skin freezing and pasty freezing for short.
For a skin freezing material the mode of solidification of the stream in a fluidity test appears to be, a s one might expect, by planar front solidification from the walls of the mould towards the centre (Figure 3.3). The freezing occurs at some point along the length of the channel, after the
(a)
I--
- - + L(b)
Figure 3.3 Flow arrest: ( a ) in pure metals by complete solidification; and (b) in long-freezing-range alloys hx partial solidtfication.
metal has lost its initial superheat (the excess temperature above its melting point). The solidified region actually migrates downstream somewhat as the leading edge of the material is remelted by the incoming hot metal, and refreezing occurs further downstream. We shall return to this phenomenon in a later section. However, neglecting these details just for the moment, it is clear that the stream can continue to flow until the moment at which the freezing fronts meet, closing the flow channel. Note that this choking of flow happens far back from the flow front. In addition, solidification needs to be 100 per cent complete at this location for flow to stop. Assuming the liquid has an approximately constant velocity V, then if its freezing time in that section is tf, we have the flow distance:
Lf =
v .
tf (3.1)This is an approximate equation first proposed by Flemings and co-workers (1963). It overestimates Lf because the velocity does reduce somewhat as the channel becomes constricted, giving a smaller measured fluidity. Nevertheless Equation 3.1 is good enough for many purposes, and we shall assume its applicability here.
The pattern of solidification of this short freezing range material can be confirmed from the evidence of polished cross-sections of the castings. Columnar crystals are seen to grow from the sides of the mould, angled inwards against the flow, and meeting in the middle of the section. Confirmation of the presence of liquid cut off by the meeting of the solidification fronts is clear from the shrinkage pipe that forms at the liquid front, and grows back towards the point at which the flow is blocked.
The pattern of solidification of longer freezing range materials can be inferred from metallography in a similar way. It turns out to be quite different to that of the skin freezing material described above.
The dendrites growing from the mould wall at an early stage of freezing are fragmented by the flow.
The stream therefore develops as a slurry of tumbling dendritic crystals. They are carried in the central flow too close to the liquid front, where, when the amount of solid in suspension exceeds a critical percentage, the dendrites start to interlock, making the mixture unflowable. There is some evidence that this critical concentration depends somewhat on the applied head of metal (Flemings et al. 1961).
However, for most practical situations this appears to lie between approximately 20 and 50 per cent.
The arrest of flow occurs therefore when there is only approximately 20 to 50 per cent solidification.
Thus for long freezing range alloys Equation 3.1 becomes
Lf = 0.2V. tf to 0.5V. t f (3.2) This factor of roughly 2 to 5 between the fluidities
of short and long freezing range alloys seems to be a common feature of all alloy systems, and yet does not seem to have been widely recognized. For instance, Figure 3.4 illustrates the profound effect of a small amount of Sn on pure Al. A few mass per cent of Sn reduces the fluidity of A1 by a factor of 3 or 4.
A similar effect is seen in Figure 3.5 illustrating the leadtin system at 50°C above its liquidus. The curve joining the data points is based on the fluidity of the long freezing range alloys being approximately one-fifth of the short freezing range alloys when lead-rich, from the available data points.
There are no equivalent data points on the tin-rich side of the diagram, so here we have guessed that the longer freezing range alloys might have only approximately half fluidity, because the freezing range is narrower, and the one data point is consistent with this. More experimental points would have removed this uncertainty.
The fluidity of the eutectic in Figure 3.5 appears to be over 50 per cent higher than the straightforward method of mixtures of its components (i.e. the straight line joining the fluidities of the two pure metals). This may be understandable in terms of the sum of two effects. (i) The pure metals may be exhibiting some dendritic growth, probably due to the presence of some impurity. This would suppress the fluidity of the pure constituents, but not necessarily the eutectic (especially if the 'impurities' consisted of its pure constituents). (ii) The determination of fluidity of alloy at a constant
70Q r
._ 2:
0 2 300
L L
200
100 600
500
-
E 400v E
Superheat
"C 100 75 50 25 0
0 5 10 15
Tin (wt per cent)
Figure 3.4 Variation offluidity with composition of Al-Sn alloys. Data from Feliu et al. (1960).
Flow I 1 700
600
500
400
300
E E
.= 200
n
.
2.- LL
100
O + - - - - - -
100 I I I I I I I I I I
0 Pb
50 Tin/weight per cent
temperature automatically increases the fluidity of the eutectic because of its low melting point. The effect is clearly seen in the Al-Zn system (Figure 3.6). This issue will be considered in more detail later.
Portevin and Bastien (1934) suggest from their results that the shape of precipitating solid crystals affects the flow of the remaining liquid; smooth crystals of solidifying intermetallic compounds creating less friction than dendritic crystals. Their famous result for the Sb-Cd system poured at a constant superheat of 100°C as shown in Figure 3.7 illustrates this convincingly. The greater fluidity of intermetallic compounds and eutectics with
Figure 3.5 Fluiditv of lead-tin alloys at 50°C 100
Sn
superheat determined in the glass tube Jluidity test. Original data from Ragone et al. (1956).
respect to their pure constituent elements may be a universal fact. (Once again, as for the Al-Zn system, we shall see more evidence that effective superheat enhances this advantage of eutectics further, although reduces the advantage to intermetallic compounds.)
Incidentally, it is worth dwelling on other details of this classic work. For instance, the peritectic reaction shown in Figure 3.7 at approximately 63 per cent Cd was found not to exist in later work on this binary phase diagram (Brandes and Brook 1992). Thus the small step in fluidity, carefully depicted at this point, seems likely to be attributable to experimental error. Furthermore, at the Sb-rich
Figure 3.6 Al-Zn alloys poured into a straight cast iron channel showing jluidity at constant superheat, and the
enhancedfluidity of the eutectic when cast at constant temperature (Lann 400 -
100
Zn 1972).
0 AI
50 Zinc/weight per cent
end of the figure, the fall in fluidity will probably be steeper than that shown, so that the plateau minimum will be reached sooner than the limit of solid solution as assumed by the authors. This will be as a result of non-equilibrium freezing. However, these are minor quibbles of an otherwise monumental and enduring piece of work, years ahead of its time.
We can usefully generalize these results in the following way. Figure 3.8 shows a schematic illustration of a simple binary eutectic system. The fluidity at zero superheat is linear with composition for the skin freezing metals and alloys (the pure metals A and B and the eutectic), whereas it is assumed to be about half of this for the long freezing alloys. The resulting relationship of fluidity with composition is therefore seen to be the cusped line denoted (i). This would be the expected form of the fluiditykomposition relation for a constant superheat.
If, however, the alloys are cast at a constant temperature, T,, there is an additional contribution to fluidity from the superheat that now varies with composition as indicated in Figure 3.8 (ii). To find the total fluidity as a function of composition curves (i) and (ii) are added (the addition in these illustrative examples neglects any differences of scale between the two contributions). The effect is to greatly
enhance the fluidity at the eutectic composition.
The same exercise has been carried out for a high melting point intermetallic compound, AB, in Figure 3.9. The disappointingly low fluidity predicted for the intermetallic explains some of the problems that are found in practice with attempts to make shaped castings in such alloys. It explains why intermetallic compounds are rarely used as natural casting alloys.
The expected poor fluidity of intermetallics underlines the suggestion by Portevin and Bastien that the high fluidity of intermetallics that they observe, although at constant superheat as seen in Figure 3.7, is nevertheless in fact strongly influenced by some other factor, such as the shape of the solidifying crystals.
Another interesting and important lesson is to be gained from the predictions illustrated in Figures 3.8 and 3.9. The peaks in fluidity are impressively narrow. Thus for certain eutectic alloys the fluidity is awesomely sensitive to small changes to composition, particularly when it is realized that Figures 3.8 and 3.9 are simple relationships. Many alloy systems are far more complicated, and much steeper cusps of fluidity with compositional changes are to be expected. Dramatic changes of fluidity performance are to be expected with only minute changes in composition. Perhaps this is the reason
Flow 79 0°C
Sb
h v E
0.4 a
2.
-
E
0.3E - 3
L
E’
0.2=
0.1._
x m
0
0 10 20 30 40 50 60 70 80 90 100
Lead (wt per cent)
0 1 I 1 I 1 I 1 I l l
0 10 20 30 40 50 60 70 80 90 1 Sb Cadmium (weight per cent)
Castability of antimony-cadmiurn alloys
why some castings can sometimes be filled, and at other times not; some batches of alloy cast well, but some nominally identical alloys cast poorly.
Clearly, to maximize reproducibility we must improve the targeting of peak performance. The unattractive alternative is to forsake the peaks and devise more easily reproducible alloys in the troughs of mediocre fluidity. A good compromise would be to target less narrow peaks, where the penalties of missing the peak are less severe.
The approach illustrated in Figures 3.8 and 3.9 can be used on more complex ternary alloy systems.
In more complex multi-component alloy systems the understanding that pure metals and eutectics exhibit a fluidity at least twice that of their long freezing range intermediate alloys was the key that allowed even sparse and apparently scattered data on alloy systems to be rationalized. This was the
0°C 700°C 600°C 500°C 400°C 300°C
Figure 3.7 Fluidity of Sb-Cd alloys showing the high fluidity of the intermetallic phase SbCd determined b y castinn at a constant 1
Cd superheat into a cast iron spiral mould (Portevin and Bastien 1934).
background of Figure 3.10 (a, b and c) that showed results for the Al-Cu-Si systems that could not be drawn by its investigators, but was only unravelled in a subsequent independent effort (Campbell 1991).
However, the author learned only later, while the 1991 paper was in the process of publication, that Portevin and Bastien had already accomplished a similar evaluation of the Sn-Pb-Bi ternary system as early as 1934. In this prophetic work they had constructed a three-dimensional wax model of their fluidity response as a function of composition, showing ridges, peaks and valleys.
In practice, it is important to realize that the improved fluidity of short versus long freezing range materials forms the basis of much foundry technology.
The enormous use of cast iron compared to cast steel is in part a reflection of the fact that cast iron
Tc
Temp
A Composition
(i) Fluidity at zero
superheat ,,-,r
(ii) Additional fluidity due to superheat at tempe- rature Tc
Total fluidity ((i) + 00) at casting temperature
Tc
I
A A0 0
(ii) Additional fluidity due to superheat at tempe- rature T,
Total fluidity at casting temperature ((i) + (io) T,
Figure 3.8 Schematic illustration of the different behaviours of eutectics when tested at constant superheat or constant temperature.
A A0 0
Figure 3.9 Schematic illustration of the different behaviours of intermetallics when tested at constant superheat or constant temperature.
Flow X I
C D :
0 4 8 1 2 1 6 2 0 2 4 2 8 3 2 3 6 4 0 4 4 4 8 5 2 5 6 6 0 Copper/weight per cent
(a)
I
<
12.5x 14.5 CUAI,
--+ Maxima
n 4 ,
“ 0 10 20 30 40 50 60
Copper/weight per cent (b)
400
E 300 E
5
.
Ln
2 200
-
x ._0 -
LL
100
v 5 15 0 21 ---a Maxima
0 5 10 15
Silicon/weight per cent (c)
Figure 3.10 Phase diagram and fluidiq of the AI-Si-Cu system. Data from Garbellini et a/. ( I 990); interpretation Campbell (1991).
is a eutectic or near-eutectic alloy and so has excellent fluidity. Steel, in general, has a longer freezing range and relatively poor fluidity.
Additionally, of course, the higher casting temperatures of steel greatly increase the practical problems of melting and casting, and cause the liquid metal to lose its heat at a faster rate than iron, further reducing its fluidity by lowering its effective fluid life t f .
Fluidity can be affected by changes in composition of the alloy in other ways. For instance, the effect of phosphorus additions to grey iron are well known: the wonderful artistic castings of statues, fountains, railing and gates produced in the nineteenth and early twenthieth centuries were made in high-phosphorus iron because of its excellent fluidity. The effect is quantified in Figure 3.1 la. The powerful effect of phosphorus on cast iron is solely the result of its action to reduce its freezing point. This is proved by Evans (1951), who found that when plotted as a function of superheat (the casting temperature minus the liquidus), the phosphorus addition hardly affects the fluidity (Figure 3.1 1 b).
.g x 0.5 3 U
.- - 1.5
-
E 1.0n
v c x .- -
LL 0.5
C
-
#.
a -*.
m a - a a 100-
a. - -
50I I I I
Casting temperature/%
1300
I I I I
I 1 2 3 4
Phosphorus (wt per cent) (a)
1.0
r
SuperheatPCFlemings has taken up this point, suggesting that the good fluidity of cast irons compared to steels is only a function of the higher superheat which can be used for cast iron. However, this is only part of the truth. Figure 3.11 shows data replotted from early work by Evans (195 1) on grey iron and Andrew et al. (1936) on steels. The reasonably linear plots of fluidity against casting temperature are important and interesting in themselves. However, they can be redrawn as in Figure 3.12. Here the horizontal flat portions of the curves show that the long freezing range alloys have constant fluidity at a given superheat, in agreement with Flemings and confirming the similar effect of phosphorus that we have already witnessed;
any increase in fluidity is only the result of increases in superheat as the composition changes. However, as either the pure metal, or the eutectic at 4.3 per cent carbon, is approached there is clear evidence of enhanced flowability, showing that there is an additional effect at work here, almost certainly relating to the mode of solidification as we have discussed above. This effect was found as long ago as 1932 by Berger. The Al-Zn alloy system investigated by Lang in 1972 shows a similar special enhancement of fluidity when cast at constant superheat as seen in Figure 3.6.
The effect of freezing range on fluidity is not confined to metals. Bastien et al. (1962) have shown that the effect is also clearly present in molten nitrate mixtures and in mixtures of organic compounds. It is to be expected that the effect will be significant in other solidifying systems such as water-based solutions and molten ceramics, etc.