Solidification structure
5.4 Aluminium alloys
5.4.4 Modification of the AI-Si eutectic by Na and Sr
(2000) discovered that pFe plates (A15FeSi intermetallic) in A1-Si alloys precipitated on the wetted outside surfaces of bifilms. Initially, the pFe precipitate is sufficiently thin that it can follow the folds of the bifilm. On a fracture surface the iron- rich phase can be clearly seen through the thin oxide film that represents one half of the bifilm (Figure 5.39). At this early stage it is faithfully following the undulations of the oxide film.
However, as the pFe particle thickens, the particle becomes increasingly rigid, taking on its preferred crystalline form, and so forces the film to straighten.
Finally, the bifilm is often seen as a crack aligned along the centre of the pFe particle, or along the matrix/particle interface if the pFe happened to nucleate only on one side of the bifilm (Figure 5.40).
In Figure 5.41a the bifilm on one side of the particle has been pulled away by gas precipitation
or by shrinkage forces, opening a pore on one side of the pFe plate. Because it has been common to observe an association between pores and pFe particles, it has in the past been assumed that the pFe particles blocked the movement of feed liquid along interdendritic channels, and so caused shrinkage porosity. However, this seems most unlikely, in view of the three-dimensional access routes for feed liquid, and in view of the strong probability that pores probably cannot be formed without bifilms.
5.4.4 Modification of the AI-Si eutectic by
Solidification structure IS3
(b)
Figure 5.41 Platelets of PFe in an Al-Si alloy showing pores opened by shrinkage or gas, initiated by the bi$lm ( a ) courtesy Cao (2000); ( b ) courtesy Samuel et al. (2001), Internat. J. Cast Metals Research.
being assumed to act as crack initiators. For this reason, for alloys containing above about 5 to 7 per cent Si, the addition of sodium or strontium to the melt has been favoured to refine the eutectic silicon phase.
(As an aside, it seems highly probable that the apparent initiation of cracks by large silicon particles was the result of the unsuspected presence of bifilms on which the particles had formed. Thus the whole ruison d’2tre of modification may have been misguided. If so, it represents a massive and continuing loss of effort and resources in the aluminium casting industry. Research is required to confirm or negate this important point. However, we do not have any real facts to justify further speculation on this sobering possibility at this stage.) The action of modification itself was first explained by Flood and Hunt in 1981. They
interrupted the solidification of unmodified and Na- modified AI-Si eutectic alloy to study the form of the growth front in a slice of a cylindrical casting.
Their results are summarized and included as part of Figure 5.42. Added into Figure 5.42 is an illustration of the effect of Sr modification. In the case of the unmodified alloy the growth form appeared dendritic, with some nucleation apparently ahead of the freezing front. In the case of the addition of sodium, planar growth of the eutectic front was stabilized, with an effective length of solidification front a factor of 17 times shorter than the dendritic front. Thus for the same given quantity of heat extracted by the mould over a given area, the interface in the case of the planar front would advance at 17 times the rate, and therefore have a much finer structure than the unmodified alloy.
Interestingly, the form of the freezing front for
Castings
(a) Unmodified (b) Sr modified (c) Na modified
Figure 5.42 Portions of the growth front of the AI-Si eutectic in ( a ) the unmodified condition, and modified with (b) strontium and ( c ) sodium (based partly on Flood and Hunt 1981).
the Sr-modified eutectic alloy is cellular in the freezing conditions commonly found in sand castings. This intermediate condition explains the intermediate performance of Sr. The cellular pattern, resembling a honeycomb, is seen occasionally on the surface of Sr-modified castings that have suffered some degree of poor feeding, encouraging the loss of some residual liquid from around the cell, and so giving slight depressions on a cast surface that outlines the cell boundaries (Figure 5.43). The cellular structure of the front does lead on other occasions to some problem to feed Sr-modified castings, as contrasted with those cast from Na- modified alloy. Surface-initiated porosity is also favoured by modification with Sr whereas it is discouraged by modification with Na.
The choice between the use of sodium or strontium depends on the circumstances. Sodium is usually lost from a melt within a time of the
order of 15 to 30 minutes, as a result of evaporation.
Strontium, on the other hand, is a normal, stable, alloy in liquid A1 alloys. Although it is slowly lost by oxidation at the surface, and sometimes can soak away into refractory furnace linings until they saturate, the alloy will usually survive several remeltings.
The addition of strontium to the melt is an expensive option, but is taken in an effort to improve the ductility of the alloy. However, the results are not always straightforward to understand, and are accompanied by a number of problematical factors.
Much confusion has existed over whether strontium can be successfully added without the deleterious effect of hydrogen pick-up. Amid the confusion, the hydrogen already in solution in the strontium master alloy addition has often been blamed for this problem. However, we can, with
Figure 5.43 Thin unfed casting of Al-7Si alloy mod$ed with Sr, showing the cellular growth form.
Solidification structurc I55
contain strontium oxide particles as islands, resembling icebergs o r boulders, so that on entrainment, the film will be unable to close so easily, thus assisting the easier opening and easier initiation of pores. These thoughts indicate the complexity of the subject. It will take years to sort out the real answers.
Turning now from the question of gas pick-up and aided nucleation to other factors. It is possible that the finer structure of the silicon phase in the AI-Si eutectic may have better properties than the coarse unmodified structure. There are many results of published work that cite the benefits to strength and ductility following modification. However, some researchers have reported only mediocre improvements. Some have reported no benefit. A few have reported reductions in properties. This confusion of experience requires some examination.
Recently, the Cosworth casting foundry operated by Ford/Nemak in Windsor, Ontario, has reported that mechanical properties of A3 19 alloy are actually reduced by Sr modification (Byczynski and Cusinato 2001). This is an important result. It is often not easy to control small batches of A1 alloys so as to carry out reproducible experiments in laboratory conditions. However, in a continuously operating plant handling tons of material each hour the melt quality can be assessed accurately, and is closely reproducible because of the process controls that are in place. Furthermore, most experimenters will have cast under gravity, mostly using relatively poor filling system designs, and thus their material will be expected to be impaired with new oxide films.
On the other hand, the Cosworth process will have very few new films because of the relatively quiescent handling, and the counter-gravity filling under the control of a pump. (However, it is known that the metal delivered by the process contains old films, mostly spinels.)
A reasonable, but unproven, interpretation of these observations is that strontium acts to speed up the deactivation of newly entrained films. Any benefit from the refinement of the eutectic may also be present, but seems practically negligible.
However, the problem from the increase of hydrogen leading t o additional porosity is a severe disadvantage.
Strontium will be expected to aid the rebonding of bifilms by the conversion of the newly formed alumina films into spinels. The accompanying reorganization of the crystal lattice will encourage the diffusion bonding between the two sides of the bifilm. This may be sufficiently rapid that it may occur in the short time available between pouring and solidification of the casting.
Alternatively, the apparent pairing of pFe platelets, like chromosomes, on a polished section of an A1-Si alloy modified with Sr has been observed occasionally in the author’s laboratory. but not yet some certainty, dismiss this minute source as
negligible. What alternative possibilities remain?
The enhanced rate of oxidation of the strontium means that any moisture in the environment is quickly converted to surface oxide, and the hydrogen is released into the melt (Equation 1.2). In furnaces open to the air therefore the addition of strontium is usually accompanied by an increase of hydrogen porosity. The porosity naturally increases with increasing strontium content, temperature, time and environmental water. This makes it practically impossible to add strontium without an increase in porosity in most foundry melting systems. In the experience of the author a single 0.05 weight per cent addition to a 1000 kg holding furnace caused the gas level to rise to such a high level that gas porosity caused the castings to swell, solidifying oversize, and had to be scrapped. It took three days of waiting for the melt to return to a castable quality.
(The arrival of rotary degassing has, thankfully, eliminated such difficulties nowadays.) T h e absorption of hydrogen can, of course, be reduced by reducing the time available for absorption, for instance, by the casting of the whole melt immediately after the strontium treatment. Treating the strontium as a late addition in this way has been adopted successfully (Valtierra 2001).
Strontium is, however, generally used with success in low-pressure casting furnaces where the melt is transferred immediately after treatment into an enclosed furnace that excludes any environmental moisture. It may then be held indefinitely, provided that the pressurizing gas that is introduced into the furnace from time to time is dry or inert, and that the furnace lining is already saturated with strontium from previous additions.
A factor that may be important is the amount of disturbance of the melt. Gruzleski has carried out careful laboratory measurements on A1-7Si-0.36Mg alloy (Dimayuga et al. 1988; Mulazimoglu et al.
1989) and found no increase in the rate of absorption of hydrogen after additions of strontium. Thus when undisturbed, the melt may continue to be protected from the environment by its oxide. The stabilization of the oxide by moisture as seen in Figure 5.34 is a further factor to strengthen this conclusion. In an industrial furnace the stirring and ladling actions may fracture the protective oxide and so allow further reaction. encouraging the ingress of hydrogen in a furnace open to the atmosphere.
A recent observation by Liu and co-workers (2002) of polished sections of an A1-Si alloy modified with Sr has discovered SrOz particles in pores. One side of the pores is seen to be formed by a beta-iron platelet, indicating the pore was almost certainly initiated on a bifilm. Thus ordinary alumina or spinel films are probably present in addition to the SrO? particles. A little reckless speculation may be forgiven. Perhaps therefore the alumina films
been investigated. Perhaps these are indications that Sr aids the liquid metal to wet the dry inner sides of the bifilm, and thus creeps between the films, separating the two halves. The separation is only apparent in those cases where iron-rich particles are attached. If there is any truth in this suggestion, such twin intermetallic phases should not, of course, contain a central crack, and both sides on the intermetallic would be expected to be well bonded to the matrix. The question also arises, what happens to the entrained air between the films? This would be pushed ahead of the advancing liquid, and might be exuded as a bubble. Alternatively it may react and be consumed by reaction with the advancing fresh interface.
In summary, the most likely explanation of the action of Sr is that most operators using poorly designed gravity filling systems will benefit because the new bifilm defects introduced by pouring are partially healed. Some will enjoy a useful net benefit if the increase of hydrogen can be controlled. In contrast, those operators using a process such as the Cosworth process will have few new films and so cannot benefit from any healing process, and will only suffer from the extra porosity as a result of any increase in hydrogen. The refinement of the eutectic, much sought-after by metallurgists and assumed to be the main mechanism of property enhancement, appears to have little effect either way.
In passing, it is noted that even with the Cosworth process, where gas content of the continuously processed liquid metal is nicely controlled, the hydrogen level will rise during its passage into the mould because of the reaction with the organic sand binder. In heavy sections, there will be several minutes for the hydrogen to diffuse into the casting sections. From Figure 1.6 the coefficient of diffusion of hydrogen is seen to be close to 10" m's-'. From Equation 5.21, for a typical time around 100 s the average diffusion distance f o r hydrogen in aluminium is close to 10 mm. Thus the gas will easily penetrate the thickest sections of castings such as automotive cylinder blocks.
This interesting observation calls into question the self-imposed task that the foundry adopts to reduce hydrogen levels. Clearly, if sand casting, or even semi-permanent mould casting (i.e. a metal mould with sand cores), the degradation of the sand binder will always raise the hydrogen level at the worst moment, as the melt enters the mould.
Extremely low hydrogen content of the melt therefore is not feasible.
It seems inescapable therefore that the really important quality requirement should perhaps be the absence of nuclei for pores, i.e. absence of bifilms. This means really clean metal and excellent designs of melt handling. In such a case, the gas content probably need not be controlled.