The Treatment of Liquid Aluminium-Silicon Alloys. Topics to discuss

Transcription

1 MME 6203; Lecture 15 The Treatment of Liquid Aluminium-Silicon Alloys 3. Modification in Hypereutectic Alloys and Grain Refining AKMB Rashid Department of MME BUET, Dhaka Topics to discuss 1. Refinement and Modification of Hypereutectic Alloys Structure of hypereutectic alloys Phosphorous refinement The effect of refinement on properties Strontium treatment Sodium treatment Refinement by other element 2. Grain Refining Principles of grain refinement Grain refinement by chilling and effect on dendrite arm spacing Chemical grain refinement Effect of grain refinement on properties Interrelationship between chemical grain refinement, cooling rate and modification 1

2 1. Refinement and Modification of Hypereutectic Alloys 1.1 Structure of hypereutectic alloys The eutectic in the equilibrium AI-Si alloy system occurs at 12% Silicon. all normal hypereutectic alloys should have primary silicon phase in binary eutectic matrix. more complex microstructures are frequently observed, which are difficult to explain simply in terms of the equilibrium phase diagram. Depending on the circumstance, three seemingly odd types of microstructures may occur in cast hypereutectic alloys: primary aluminum dendrites in slightly hypereutectic alloys (12% < Si < 14%); a completely eutectic microstructure in hypereutectic alloys containing up to 0.l% Sr; primary aluminum dendrites in hypereutectic alloys in addition to primary silicon and eutectic. 2

3 primary aluminium dendrites in slightly hypereutectic alloys These are found in alloys to which a modifier, such as Sr, has been added, or in alloys which are rapidly solidified. Both eutectic modification and/or rapid freezing displace the eutectic concentration to higher Si values, and depress the eutectic temperature. eutectic composition 12% 14% Si eutectic temperature up to 10 C Consequently, alloys in the 12% to 14% Si range hypereutectic slow freezing and no modification hypoeutectic treated with Na or Sr, or if rapidly frozen 3

4 hypereutectic alloys with a completely eutectic microstructure The 3HA alloy which contains up to 15% Si, 0.1% Sr and substantial amounts of Cu, Ni, Mg and Fe solidifies with a completely eutectic microstructure, despite its relatively high silicon concentration. The origin of this structure is believed to be related to the eutectic shift associated with strontium, coupled with displacements due to the other alloying elements. Thus, the alloy composition corresponds to the eutectic in a complex multi-component alloy system. hypereutectic alloys with primary Al, primary Si and eutectic These structures are commonly found in alloys with more than about 15% Si, which freeze relatively quickly as in permanent mold or die casting. The microstructure arises from the fact that the freezing process is only quasi-equilibrium, and follows the phase diagram in only an approximate way. 4

5 Primary Si crystals nucleate and begin to grow at a temperature below the equilibrium liquidus temperature (point A in Fig. 7.3). As the Si grows, the liquid around it becomes highly enriched in aluminum, and follows the depressed liquidus line to point B where solid Al nucleates and freezes as a halo around the primary silicon. As this halo grows, it may become dendritic in form, or remain as a layer encapsulating the silicon. With growth of the aluminum phase, the neighboring liquid is enriched in silicon, and finally the eutectic is nucleated at C. When solidification of the eutectic is completed, the alloy is entirely solid and will contain two primary phases as well as the eutectic. primary silicon morphology In hypereutectic alloys, primary silicon may appear in several different forms, and it is not uncommon for many of these to be found in the same casting. The silicon morphology is highly dependent on the solidification parameters freezing rate, temperature gradient in the liquid, and local liquid composition. All of these change continuously during the freezing of a casting, and so it is not surprising that different silicon forms should occur almost side by side. 5

6 Figure 7.4. Some common morphologies of primary silicon (scanning electron micrographs) Star-shaped primary silicon which forms along five axes originating from a single nucleus (several variations of this shape can be found). Polyhedral primary silicon which appears hexagonal or octagonal on a polished section (sodium treatment results in a so called spherical primary silicon, but this is simply a multifaceted form of the basic polyhedral shape). Dendritic primary silicon which resembles the dendritic form of simple metals, such as the primary aluminum dendrites in hypoeutectic alloys. eutectic silicon morphology The eutectic silicon in hypereutectic alloys can occur in either an acicular (plate-like) or fibrous (modified) form exactly as in hypoeutectic alloys. In addition, local regions of a very geometric lamellar eutectic can sometimes be found. feathery This is known by a variety of names such as feathery, skeleton, web, etc., and is confined to hypereutectic alloys. acicular / fibrous Figure 7.5. Feathery eutectic structure found in hypereutectic alloys. 6

7 1.2 Phosphorous Refinement The primary silicon phase of hypereutectic alloys is not readily nucleated by the usual impurities which are present in these alloys. considerable undercooling is necessary to crystalize primary silicon particles The scarcity of nuclei causes to form only a relatively few but extremely large primary silicon crystals Figure 7.6a. Typical primary silicon size. Unrefined Al-17Si alloy chill cast at a rate of 29 C/s. In addition, primary silicon is slightly lighter than the liquid alloy and tends to float. As a result, castings which solidify slowly, such as sand castings, can exhibit pronounced gravity segregation of silicon. Here, the upper portions of the casting contain a high concentration of primary silicon while the lower part is of essentially eutectic composition. 7

8 Each of these rather unique features of the solidification of cast hypereutectic alloys are unfortunately detrimental. Hypereutectic alloys are used mainly for their wear resistance, which is imparted by the hard particles of primary silicon. In any cast part, uniform and predictable wear resistance is essential, and therefore macrosegregation of the wear resistant phase is intolerable. The machining of hypereutectic alloy castings is more difficult than their hypoeutectic counterparts because of the presence of the hard primary phase. Large unevenly distributed particles, cause greater tool wear than does a smaller, more uniformly distributed primary phase. Thus, fine, evenly distributed primary silicon, is an essential feature of cast hypereutectic alloys. Fine distribution of primary silicon particles, Figure 7.6(b), can be promoted by facilitating the nucleation of the primary silicon through the addition of an appropriate nucleating or refining agent. Figure 7.6b. Typical primary silicon size of refined Al-17Si alloy chill cast at a rate of 29 C/s. There are several ways in which silicon nucleation can be promoted. But the only technique commonly used in foundry is the addition of phosphorus to the melt. Phosphorus reacts with the liquid aluminuim to form aluminum phosphide, AlP, which has a crystal structure very similar to that of silicon, and acts as an effective heterogeneous nucleant. 8

9 types of additives Many different phosphorus salts and phosphorus containing compounds have been shown to be effective. Some are easier to work with than the others. Cu-8%P alloy Very convenient to use; available as a brazing alloy in rod or shot form. Adds considerable copper to the melt. Can be a strengthener, but detrimental to the corrosion properties of alloy. Specially designed commercial nucleants, or flux Most fluxes contain red phosphorus as the active agent and have additions of other salts to clean and degas the melt. Recoveries of phosphorus vary somewhat according to the type of additive used, and the exact addition technique, but they are always low, and range from 5% to 20%. An increasingly popular form of phosphorus addition is by means of Ni 3 P. This offers excellent phosphorus recoveries. optimum phosphorous concentration Depend on many variables, the most important being the silicon concentration of the alloy and the metal freezing rate. It is difficult to obtain precise values for the best phosphorus level. However, it does lie in the range 0.003% to 0.015% depending on the casting conditions. A quick metallographic check which is often used is to look for star-shaped primary silicon (Fig. 7.4a). This is usually found in alloys that are deficient in phosphorus. 9

10 silicon concentration of hypereutectic alloys Primary silicon particles have a natural tendency to be larger with silicon content. For example, under identical freezing conditions, and in the absence of phosphorus, the silicon size can increase from 40 to 180 mm when the alloy silicon concentration changes from 12% to 20%. Larger amounts of phosphorus will, therefore, be required for higher silicon alloys. Complete refinement of very high silicon alloys, e.g., 30% Si, will be difficult to achieve. solidification rate A rapid solidification rate can exert a significant refining effect even in the absence of phosphorus. A 390 alloy processed by conventional die casting does not require a phosphorus treatment due to the rapid cooling rates associated with die casting. On the other hand, hypereutectic alloys such as 390 are very difficult, if not impossible, to sand cast. Even with phosphorus treatment, solidification rates are so slow that unacceptably large primary phase particles form and float to the upper surfaces of the casting. 10

11 Figure 7.8. The effect of cooling rate through the solidification range on the size of primary silicon in an Al-23%Si alloy refined with phosphorous. Optimum size of silicon mm over-refinement Caused by the addition of too much phosphorus. This is manifested as a slight coarsening of the silicon particles and a broadening of their size distribution. The untreated alloy contains a broad spectrum of silicon sizes (varying by a factor of 6) from less than 20 mm to about 120 mm. The best refinement occurs with % P, and this causes the silicon particle size distribution to become much narrower and to peak at about 20 mm. Over-refinement takes place if the phosphorus level is raised to 0.009%. A broadening of the size distribution occurs, and the most frequently occurring particle is now about 30 mm in size % 0.009% 0.005% Analysed Phosphorous, wt.% No P added % No Phosphorous Mean Particle Size, mm

12 holding time and re-melting Like many melt additives, phosphorus is subject to fading, although at a much slower rate than sodium, for example. Due to the agglomeration of AlP nuclei, as evidenced by a gradual increase in the silicon particle size. Reduces the number of possible nucleation sites and results in a coarsening of the silicon. P treated melts can be held up to 3 to 5 hours before fading becomes a problem and re-refinement becomes necessary. Re-melting of P treated ingot has only a small effect on phosphorous refiner. effect of degassing The higher melting and casting temperatures used with hypereutectic alloys encourage hydrogen dissolution. Degassing is, therefore, an important part of the technology of these alloys. Some loss of refinement is to be expected due to the floating out of the melt with degassing and so the degassing treatment time should be kept to the minimum to achieve the desired hydrogen concentration. A brief degassing is, however found to be beneficial. A more uniform distribution of AlP nuclei is resulted due to the mild stirring of the melt. 12

13 treatment temperature and casting temperature More and better distributed nuclei are formed at higher rather than lower temperatures. Therefore, treatment and casting temperatures should be as high as are feasible. Much finer silicon is obtained in Al-20%Si alloy treated with 0.02%P if the melt is treated and cast from 900 C than from 700 C. Re-melting a coarse material which was previously cast at a lower temperature will reverse the earlier structure, provided the second casting operation is done at a high temperature. During die cast of alloy without phosphorus refining, the alloy temperature in the holding furnace should not be allowed to fall below the liquidus temperature. If it does, large silicon crystals will grow and be injected into the die. The resultant microstructure will then contain a mixture of coarse and fine silicon. Even if the alloy is phosphorus refined, casting temperature should be high enough to allow complete filling of the mould cavity before precipitation of the primary silicon begins. If this is not done, silicon will form during the mould-filling process, and the forced convection which accompanies pouring will result in clustering of the primary silicon. Some areas of the microstructure will therefore appear extremely rich in silicon while others will be depleted. 13

14 1.3 Effect of Refinement on Properties fluidity and feeding Hypereutectic alloys possess excellent fluidity due to the high latent heat of fusion of the primary silicon phase. In the unrefined state, however, the large primary silicon crystals interlock during the final stages of solidification and act to impair feeding. Phosphorus refinement improves this situation somewhat by creating much finer silicon which has less tendency to interlock. Nevertheless, this family of alloys remains prone to feeding difficulties. It has the widest freezing range of any of the aluminum casting alloys, and should be used with generous feeding and chilling. tensile properties Refinement improves tensile properties since it produces a finer and more even distribution of the brittle silicon phase. The improvement in tensile strength can vary from 10% to almost 100% depending on the silicon content of the alloy. Compositions near the eutectic yield relatively little primary phase. Refinement has a lesser effect than on alloys whose composition is far from the eutectic, and which contain very large quantities of primary silicon. Of course, the tensile strength generally decreases as the silicon concentration is raised. 14

15 machinability Improvements in machinability are the chief reason for utilizing phosphorus refinement. When large silicon particles are present in the microstructure, machining may be almost impossible. Excessive tool wear and uneven heating of the work piece result in poor dimensional control on machining. The extent of improvement in tool life will depend on the type of casting process used. Machinability of sand castings is improved the most by refinement, since these contain the largest and most non-uniformly distributed silicon in the unrefined state. The improvements in die casting are less, due to the finer as-cast structure produced by this process. Improvements in tool life of 50% can be realized in permanent mould castings. Surface roughness can also be improved by refining as indicated by the data in Table 7.2. A lower value indicates a smoother machined surface. 15

16 wear properties Hypereutectic alloys are most often used in applications which exploit their wear resistance. Surprisingly, it is not clear that refinement has any effect on wear properties. One school of thought says that it is the total amount of silicon rather than its morphology which determines wear properties. Thus, higher silicon alloys wear better than lower silicon alloys. Another point of view is that refinement does indeed maximize wear properties. Perhaps the conflicting information simply illustrates that silicon shape and size is only a minor factor in determining wear, and that other variables such as lubrication and applied load are of much greater importance. Related to wear is the observation that in some cases, hardness is reduced by refinement, while in others it is increased. The differences are never very large, and the erratic behavior reflects the influence of other variables on the properties. In automotive applications, wear properties are enhanced by a special etching process which partially removes some of the eutectic matrix. The primary silicon particles then stand out on the surface to provide wear resistance. 16

17 1.4 Strontium Treatment P leads to a coarse acicular eutectic structure, and so its deliberate addition to hypereutectic alloys guarantees an unmodified eutectic silicon surrounding the refined primary phase. The benefits of eutectic modification could be found in hypereutectic alloys, if primary phase refinement and eutectic modification could be caused to occur simultaneously. Unfortunately, modifying elements are chemically incompatible with the phosphorus added to refine the primary silicon. Both Sr and Na react in the melt to form either strontium phosphide or sodium phosphide. These compounds are more stable than AlP and hence a coarsening of the primary silicon occurs. However, with sufficient additions, the eutectic can be completely modified, and morphological changes can be brought about in the primary silicon. In many cases the resultant tensile properties are as good as those in phosphorus refined alloys. (a) No strontium (b) 0.01% Sr (b) 0.03% Sr Figure The effect of strontium additions to a phosphorous refined A390 alloy chill cast at 29 C/s. In Fig. 7.13, we see the result of adding various amounts of Sr modifier to a chill cast phosphorous refined A390 alloy. Levels typical of those required to cause modification in hypoeutectic alloys coarsen the primary silicon, but allow it to retain its blocky shape (Fig. 7.13b). Very much higher concentrations cause a change in the primary silicon morphology itself, resulting in a dendritic shape (Fig. 7.13c). 17

18 In Fig. 7.13, sufficient Sr is present at even 0.03% to modify the eutectic. This modification has, however, been achieved at the expense of a three-fold increase in the primary silicon size. Such effects are very cooling rate dependent. For example, the same alloy sand cast solidifies with very coarse dendritic silicon and almost no modification of the eutectic (Fig. 7.14). (a) No strontium (b) 0.03% Sr Figure The effect of strontium additions to a phosphorous refined A390 alloy sand cast at 1.4 C/s. Despite the coarsening of primary silicon, tensile properties do not change, and may even show a slight improvement. 1.5 Sodium Treatment Sodium, like strontium, can be added in sufficient quantity to a phosphorus treated alloy to neutralize the effect of phosphorus and to modify the eutectic. Two forms of primary silicon seem possible: 1. Spheroidal silicon 2. Dendritics silicon 18

19 spheroidal silicon The most interesting is the almost spheroidal shape silicon. Close examination will show that it is actually a multi-faceted form. With increasing sodium concentration, these primary silicon grains decrease in size and become more spheroidal. Require very high concentrations of sodium to produce this structure and the level probably depends on the cooling rate. At this high concentration of Na (over 0.1%), the eutectic is always well modified. Figure Primary silicon shape found in sodium treated A390 alloy. dendritic silicon More readily produced with sodium treatment than is the spheroidal variety. The exact conditions required to form either type are not at all defined, but dendritic silicon is likely favored at lower freezing rates and lower sodium concentrations. Again, a well modified eutectic structure forms. Figure Primary silicon shape found in sodium treated A390 alloy. 19

20 A390 alloy treated with 0.045% Sr, and then 0.08% Na is added and allowed to fade. [a] dendritic silicon formed first - typical of strontium treatment. [b] transforms to a multifaceted, almost spheroidal shape after initial Na addition. [c-d] degenerates to a dendritic morphology as the Na fades to 0.02%, 145 minutes after addition. (a) 0.045% Sr, no sodium (b) 0.045% Sr plus 0.08% Na; 10 minutes after Na addition A combined Na-Sr treatments can result in a greater number of fine spheroidal silicon particles than is obtained by a single sodium treatment. (c) 0.039% Sr plus 0.03% Na; 100 minutes after Na fading (d) 0.037% Sr plus 0.02% Na; 145 minutes after Na fading Figure Sodium-strontium treatments of an A390 alloy chill cast at 29 C/s. In summary, Current melt technology does not allow the coexistence of a modified eutectic with a primary silicon phase having the fineness and uniformity of distribution made possible by phosphorus treatment. The eutectic can, however, be modified by using larger quantities of classical modifiers than are needed in hypoeutectic alloys. This results in a primary silicon which is somewhat different in shape and coarser than that found in phosphorus refined alloys, but which is still finer than in untreated alloys. Because of the large amount of eutectic present, these treatments can have a beneficial effect on properties. The tensile properties obtainable by modifier treatment are as good, and in some cases better, than those which result from phosphorus treatment alone. 20

21 1.6 Refinement by Other Elements Several others element have been identified to modify in a similar way like phosphorus. None of these has been shown to produce the high level of refinement possible with phosphorus, and at the same time to modify the eutectic. Therefore, none of these elements present any advantages over phosphorus, which is relatively easy to add to the melt, acts in low quantities, is long lasting and is inexpensive. Sulphur is the additive which has received the most attention. Elemental sulphur vaporizes at 445 C and so is best added as a sulphide, and will produce excellent refinement once it is dissolved in the melt. Optimum levels have not been identified due to analytical problems, but if recoveries of 10%- 20% are assumed, it appears that good refinement can be obtained in alloys containing 0.05% to 0.1% S. Like phosphorus, sulphur forms stable compounds with sodium and strontium, precluding refinement and modification at the same time. Another similarity with phosphorus is that higher melt temperatures result in better refinement. Selenium very stable in the melt; yields finer primary silicon if the melt is heated at 900 C or even higher; toxic. Arsenic additions of up to 0.6% As provides refinement in sand cast alloys and even somewhat better in die casting; probably very stable in aluminum melts; toxic and its effect is cancelled by sodium treatment. Rare earth elements such as cerium, lanthanum and neodymium have also been said to refine primary silicon. An intermittent electromagnetic stirring of the melt during freezing in every 30 seconds has been shown to be beneficial to refinement of both primary and eutectic silicon and to reduce gravity segregation tendency. 21

22 2. Grain Refinement 2.1 The Definition of a Grain Grain size in single phase metals has long been known to be of significance in determining, for example, the tensile properties. The general rule is that finer grains are, for most applications, preferable to coarse grains. This rule is true only if the alloy is mainly single phase and applies to the 200 series of aluminum foundry alloys based on the Al-Cu system. The properties of cast alloys containing a large fraction of eutectic, such as the AI-Si alloys, are much less dependent on grain sizes, and grain refinement of these alloys is of lesser value. It is the brittle eutectic silicon phase which determines their properties, and hence processes of eutectic modification become much more important than grain refinement. In addition to the eutectic silicon size and shape, the dendrite arm spacing (DAS) determined by the cooling rate through the mushy zone is also of prime importance. 22

23 DAS Grain Size Eutectic Silicon 1. Grain Size Each grain contains a family of aluminum dendrites which all originate from the same nucleus. 2. Dendrite Arm Spacing (DAS) It is determined by the cooling rate through the mushy zone, with slower cooling resulting in larger values of DAS. 3. Eutectic Silicon Between the dendrite arms is found the eutectic silicon phase which may or may not be modified. These three features of the microstructure are all, more or less, independent of each other and, very different in size. Grains in aluminum foundry alloys are found in the range 1-10 mm; DAS values vary from 10 to 150 mm, Eutectic silicon may be found in plates up to 2 mm in length or spheres of less than 1 mm in diameter. 2.2 Principles of Grain Refinement The size of grain of a cast material is inversely proportional to the number of nuclei present in the liquid which are able to act during the solidification process. If each grain is nucleated by one foreign particle or nucleus (during heterogeneous nucleation) then a greater number of nuclei will allow more grains to form, resulting in a smaller grain size. Time also enters the process of grain nucleation since at least some nuclei require a certain time in the liquid phase before they become active. Thus, the best combination of conditions to promote extensive nucleation, and hence a fine grain size, would seem to be the presence of a large number of nuclei coupled with a slow rate of freezing to provide the required time span for them to act. 23

24 It is well known that not all foreign particles present in a metallic melt are equally effective as nuclei. the interfacial energy between the nucleant and the nucleus (the material which is solidifying), is of prime importance. Condition (c) is usually regarded as the optimum. Here, the interfacial energy between nucleus and nucleant is minimal and the nucleus is able to envelop the nucleant. In effect, it forms a film of high radius of curvature with little expenditure of energy. The minimal surface energy between nucleant and nucleus is usually achieved if a similarity of crystal structure exists between at least one atomic plane in the nucleus and one in the nucleant. Then very close atomic matching can occur across the interface separating the two, as seen in Figure 8.3. Since a large part of surface energy can be related to atomic mismatch across the surface, any minimizing of this mismatch will promote a low surface energy. This very simple view of heterogeneous nucleation suggests that good nuclei should possess atomic planes in their crystal lattice similar to atomic planes in the lattice of the material to be nucleated, which in our case is aluminum. Thus a definite crystallographic relationship should exist between aluminium and its nucleant. 24

25 Liquid aluminum foundry alloys contain a multitude of foreign particles and substrates ranging from oxides and spinels to the wall of the mould itself. At a given temperature, or undercooling below the melting point of the alloy, any foreign particle may or may not be effective as a nucleant. Particles which offer the best crystallographic similarity to aluminum will become effective nucleants at temperatures very close to the alloy liquidus temperature, i.e., at a small undercooling; particles which offer poorer crystallographic similarity will require higher undercoolings to become effective nuclei. 25

26 There exists, therefore, a spectrum of possible nuclei in any liquid foundry alloy which can become effective nucleants over a range of temperatures. If it is possible to undercool the liquid by a large amount, say several tens of degrees, then many different particles will act as nucleants, and a fine grain size casting should result. Most often it is simply not possible to chill the liquid to a sufficient degree that all nuclei can act, and hence very often, nucleating agents are added to the liquid alloy in a process called chemical grain refinement. 2.3 Grain Refinement by Chilling and Effect on Dendrite Arm Spacing If a melt is rapidly cooled (chilled), the rate of heat extraction can greatly exceed the rate of heat generated by the freezing process (latent heat of solidification). As a result, the liquid undercools as its temperature falls below the liquidus temperature. If this undercooling is sufficient, the full range of heterogeneous nuclei present in the liquid can become active. This multiple nucleation results in a fine as cast grain size. 26

27 The range of grain sizes ( mm) achievable by chilling is greater than with any other grain refinement technique. The finest sizes are, of course, achieved by the most rapid cooling which is possible in only very thin sections and using rapid solidification techniques. In any event, the rapid chilling of large castings produced by any method is usually impractical due to the shear amount of latent heat which must be removed. At best, some local control of grain size can be achieved by the insertion of chills in critical locations. The DAS is far more important in determining mechanical properties of multiphase alloys such as the family of AI-Si foundry alloys. Smaller DAS values are caused by faster solidification rates which are usually associated with finer as cast grain sizes, but it is the DAS and not the grain size which is the determining factor in the mechanical properties. Finally, in the Al-Si alloys, chilling can also cause partial modification of the silicon phase and this too can contribute to the mechanical properties. Again, the grain size itself is not a major factor. 27

28 2.4 Chemical Grain Refinement Chemical grain refinement is the most widely practiced, and most foolproof method of grain refinement. Here additions of effective nuclei are made to the melt through either master alloys or fluxes. A fine grain size is promoted by the presence of an enhanced number of nuclei, and solidification proceeds at very small undercoolings. Chemical grain refinement does not influence the freezing rate, so it has no effect on the DAS which remains relatively large due to the slow solidification associated with the low undercooling. In this regard, chemical grain refinement exerts a less beneficial effect on the mechanical properties than does grain refinement by chilling. Nevertheless, it improves mechanical properties, reduces the hot tearing tendency and leads to a finer distribution of porosity. 28

29 titanium and titanium-boron grain refining Aluminum alloys are grain refined by the addition of typically from 0.02 to 0.15% Ti or Ti-B mixtures in the range of % Ti and 0.01 % B. The titanium and boron are added via master alloys available in ingot or waffle form or as salt mixtures. Grain refinement by the addition of Ti alone is reasonably well understood; however, the role of boron which acts to make the titanium refinement more effective is still the subject of great controversy. Al-Ti master alloys usually contain up to 10% Ti. The intermetallic phase TiAl 3 present in pure aluminium matrix acts as an effective nucleant for aluminium. energetically favoured have preferred crystallographic relation with aluminium 29

30 When cooled below 665 C, TiAI 3 particles reacts with the liquid phase according to the following peritectic reaction: L + TiAl 3 The a phase envelops the TiAI 3 particle and acts as the site for further growth of the aluminum grain. The number of nuclei and the final as-cast grain size is related to the microstructure of the original master alloy. a (solid solution) AI-Ti alloy containing many small TiAI 3 particles will be a better grain refiner than one which contains fewer but larger TiAl 3 particles. This, in fact, is one of the difficulties associated with the use of Al-Ti master alloys; the effectiveness depends on the microstructure of the alloy and can vary from batch to batch and from supplier to supplier. TiAl 3 particles dissolve and fade away with time. If Ti and B are used together, not only is the grain size finer, but the fading time is very significantly prolonged. Melt pouring rates of lb/hr are common with furnace capacities of up to 10,000 lb. Thus, most commercial grain refiners contain a mixture of both titanium and boron. 30

31 Grain refining ability seems to depend on the morphology of the intermetallic phases which are present in the alloy. (a) Figure Morphologies of some intermetallic phases found in Al-Ti-B grain refiners: (a) blocky type TiAl 3, (b) duplex intermetallics consisting of TiAl 3 studded with borides. (b) Poor grain refiners contain blocky type TiAl 3 crystals, which are similar to those found in binary AI-Ti grain refiners. Here the boron has apparently exerted no influence on the intermetallic phase during manufacture of the grain refiner. Good grain refiners contain duplex intermetallic phases, which consists of TiAl 3 particles covered with small boride particles, perhaps TiB 2 or (Ti, Al)B 2. Primary alloys contain little silicon (< 0.05%), and in these alloys, titanium or titanium-boron mixtures provide powerful grain refinement. Boron, by itself, will not grain refine at all. In foundry alloys, the picture is quite different, since silicon, copper and zinc actually hinder titanium grain refinement. Surprisingly, boron acting on its own is a much better grain refiner. 31

32 Figure Comparison of grain refinement with Al-5%Ti-1%B master alloy (curve a) and Al-2.5%Ti-2.5%B (curve b) alloy. The casting alloy is 725 C and the contact time is 5 minutes. Although high-boron grain refiners appear to offer many advantages to the aluminium foundry industry, one of their negative aspects appears to be a tendency to contribute to hard spots in castings. 2.5 The Effect of Grain Refinement on Properties Grain refinement exerts a positive influence on several properties of cast alloys; notably, hot tearing tendency and porosity and shrinkage distribution. Only very limited data is available for alloys of the Al-Si type. The more popular research alloy seems to have been AI-4%Cu. The AI-4%Cu alloys are largely single phase materials containing a minimum of eutectic. They are therefore quite different from Al-Si casting alloys which are used because of their high eutectic content. Much of what is presented in the remainder of this chapter is taken from literature on Al-Cu alloys, and it is not necessarily applicable to Al-Si alloys. 32

33 hot tearing tendency The temperature at which general contraction of the solid begins is called the semi-solidus temperature. There is a considerable evidence to indicate that grain refined castings are less prone to hot tearing. Grain refinement results in a more general nucleation, but a less continuous solid as films of liquid remain to surround each individual grain. This has the effect of lowering the temperature at which contraction of the whole solid begins, and consequently the temperature range over which hot tearing can occur is narrowed. The lowering of the semi-solidus temperature reduces the hot tearing tendency in two ways: 1. The temperature at which the semi-solid is able to sustain a measurable load is reduced (in this example from 650 C to 630 C), and hence the time interval over which cracking can occur is shortened. 2. Grain refined castings gain mechanical strength at a faster rate than non-grain refined ones. This can be seen by comparing the slopes of the lines in Figure Grain refined castings, therefore, have a shorter temperature range over which they can hot tear, and they develop strength faster in this range. 33

34 porosity distribution The effect of grain refinement is to smooth out porosity into a more even distribution of small pores. Non grain-refined alloys tend to solidify with pipes or regions of highly concentrated internal porosity. With grain refinement the structure is much more uniform and this result in more pressure tight castings. intermetallic distribution In alloys which contain a high volume fraction of eutectic, such as the Al-Si alloys, it is not expected that grain refinement would influence the distribution of intermetallics. These are found in the interdendritic spaces and hence the DAS rather than the grain size is of more importance. On the other hand, the AI-Cu type alloys do contain some brittle intermetallic which can be distributed in a more favorable manner by grain refinement. For these alloys, grain refinement provides definite advantages. surface appearance Castings which are to be anodized should be grain refined, since large grains are evident on the surface, appearing as areas of different reflectivity. fluidity A 10-15% decrease in fluidity of an AI-4.5% Cu alloy is seen on grain refinement. The early nucleation caused by grain refinement results in slurry flow (solid plus liquid) from virtually the moment of pouring, and since slurries flow with more difficulty than simple liquids, fluidity should be reduced. 34

35 mechanical properties Properties such as tensile strength and elongation are usually improved by grain refinement. In AI-Si alloys, this is mainly due to improvements in porosity distribution rather than to any decrease in grain size. the properties of these alloys are controlled by the eutectic and any influence of grain size, per se, is secondary. Figure The variation of tensile properties of a 201 alloy with grain size. In AI-Cu alloys, however, grain size is particularly important in determining the distribution of intermetallics and in reducing hot tearing tendency. It is these factors, coupled with improved porosity distribution, which lead to enhanced mechanical properties, 2.6 The Interrelation between Chemical Grain Refinement, Cooling Rate and Modification In this lecture we have discussed how chemical grain refinement and cooling rate (chilling) can be used to change the metallurgical structure of a casting. In the previous lecture, we have explored the process of modification which can influence the structure of the eutectic silicon in casting alloys which contain silicon. The final as-cast structure is determined by a complex interplay between the cooling rate of the casting and the effect of additives such as grain refiners and/ or modifying elements. Often, the same structure can be achieved by different processing routes. 35

36 36