Direct-Chill Casting of Light Alloys: Science and Technology

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An important and quite persistent feature of non-equilibrium eutectic distribution in the billet is the minimum in the central portion of the billet cross-section [33,48]. This means that the amount of last available liquid is always lower in the centre of the billet than at its periphery as shown in Fig. We can suggest the following line of logic for the case of direct-chill casting. According to Diepers et al. On the other hand, structure coarsening may facilitate liquid penetration into the mushy zone by increasing the permeability of the solid network. Interestingly enough, at very low casting temperatures, the amount of non-equilibrium eutectic decreases with increasing the casting speed [33].

More eutectic is concentrated in the centre of the billet upon casting from high melt tempera- tures. Macrosegregation in direct-chill casting of aluminium alloys Macrosegregation is an irreparable defect and is bound to occur in large castings. The question then remains: how far we can exert a control over this in order to minimise mac- rosegregation, if not preventing it altogether. Modelling the macrosegregation is normally aimed at semi quantitative predicting the occurrence and severity of macrosegregation by considering the basic mechanisms involved.

Most of the alloying elements and impurities e. The compositional variation of these elements in a commercial scale DC cast Al alloy exhibits a pattern as shown in Fig. There is a negative solute-depleted segregation in the centre adjoined by positive solute-rich segregation approximately at mid-radius or mid-thickness with a solute depletion at subsurface followed by strong positive segre- gation at the surface. In the absence of macrosegregation, the deviation should follow the horizontal straight line at zero. If K is close to 1, it implies close spacing of liquidus and solidus and hence little tendency for segregation.

On the other hand, values with K much smaller than 1 result in strong partitioning of an alloying element. Table 3 provides the data of K for various ele- ments [9,67,80]. For example, in a Fig. Centreline segregation of alloying elements vs. It is worth mentioning here that the extent of segregation of a particular alloying element depends more on the base alloy itself rather than on its absolute content in this alloy [80].

Macrosegregation can be represented in the following ways: 1. Mechanisms of macrosegregation We have already mentioned on several occasions that the relative movement between solid and liquid phases and the solute rejection by the solid phase are two essential condi- tions to form macrosegregation. The possible mechanisms behind this relative movement and the enrichment will be discussed in the following sections. Let us analyse how can the macrosegregation form upon these two conditions.

At the upper surface of the representative volume, since solid fraction is lower than at the bottom surface, the liquid is less enriched. As a result, the representative volume is diluted. Hence the representative volume element is enriched. Although the idea behind macrosegregation mechanisms seems to be clear, it took quite a time to get to the current stage of understanding. Representative volume of the mushy zone. Historical overview The fact that large-scale castings and ingots are not homogeneous with respect to their chemical composition has been known for centuries.

It is widely cited that Italian metal- lurgist and foundryman V. In Austro-Hungarian chemist L. Ercker published his observations of liquation in precious alloys [82]. According to a bril- liant review by Pell-Wallpole [83], most observations and studies of macrosegregation dur- ing the XIXth century were done on precious metals, including works by W. Roberts- Austin and E. Matthey in Great Britain. We can mention the pioneering works of T. Turner, M. Murray, E. Smith, O. Bauer, H. Arndt, R. Reader, R. Rowe in copper alloys and those of G. Masing, W. Claus, S. Voronov, and W.

Roth in aluminium alloys citation information is avail- able in Ref. It is a direct consequence of microsegregation and can be easily predicted and estimated using the phase diagram data. However, the fre- quently observed macrosegregation pattern in DC cast ingots and billets is just opposite — the periphery of the casting is enriched in the solute while the centre remains solute-lean see Fig.

These theories attempted to explain the numerous observations. It was experimentally found that the inverse segregation occurred in alloys with a considerable freezing range, e. The presence of hydrogen was shown to promote exudations, while melt overheating increased the degree of segregation.

The cooling rate was noted to be a determining factor in segregation already in early accounts, e. Smith in Bauer and Arndt emphasized a steep temperature gradient in the ingot as an essential condition for macrosegregation. While Voronov showed that any change in casting conditions which increased the cooling rate, i.

The properties and grain structure of an alloy were also under scrutiny in relation to macrosegregation [83]. As early as in , Mas- ing et al. Some of the proposed theories were short-lived; some were developed into the current theory of macrosegregation. First theories suggested in the —s wrongly assumed that segregation occurred in the liquid state. W Smith who suggested in — the theory of mobile equilibrium and Benedicks with the theory of thermo-solutal segre- gation. Within the same timeframe, the theory of contraction pressure was suggested and supported by a num- ber of scientists, e.

The idea was that a solid shell formed at the top of a casting exerted pressure onto the liquid and forced it sideways to the surface. This theory explained well the for- mation of surface exudations but failed to explain the occurrence of inverse segregation in castings with the open liquid surface. One of the most popular theories of inverse segregation was proposed by Genders in From his viewpoint, gas dissolved in the melt concen- trated in residual, solute-rich liquid pools and then evolved, forcing this liquid along grain boundaries towards the cooling surface.

An interesting story happened with the theory of crystal migration proposed by Voronov in [84] and supported by Watson in This theory was rejected in the s because it contradicted numer- ous experimental observations on inverse segregation happening in castings without stray crystals.


In the s this theory was revived in application to DC casting by Yu and Gran- ger [8] and Chu and Jacoby [9], and will be discussed in more detail in Section 3. The foundation for our current understanding of macrosegregation mechanisms, espe- cially with regard to inverse segregation during DC casting has been laid down by a num- ber of prominent scientists.

It was assumed that normal segregation occurred in equiaxed structures, and inverse — in colum- nar structure, because the latter provided interdendritic channels. It was explicitly stated that the segregation was a phe- nomenon occurring in the transition region of the casting. This theory could be equally applied to columnar and equiaxed grain structures.

The concept of interdendritic feeding was supplemented with the idea of intradendritic feeding by Dobatkin in [1]. The role of air gap for the formation of surface exudations was highlighted by Pell-Wal- pole in [83]. Let us now look at these mechanisms in more detail. It contributes to the negative centreline segregation and positive surface segregation [10,87,88]. Recently a simple analytical model [44] was suggested to estimate the magnitude of the shrinkage-induced macrosegregation during DC casting.

The horizontal component along the billet radius is directed towards the billet surface and the vertical component goes along the casting direction. Step by step, however, an overall solute transfer takes place from the centre of the billet to its surface. This analytical approach can be used to estimate roughly the extent of the shrinkage- induced macrosegregation by running heat transfer calculations only. Only half of a billet is shown, the centre being on the left and the surface on the right [44].

It brings the less enriched liquid into the deeper part of the two-phase region. Now the relative movement is directed from the deeper colder part of the mushy zone to the less enriched hotter slurry centre. An important feature of solutal convection is the solutal buoyancy. In multicompo- nent alloys the contribution of every solute to the buoyancy force may complement or counteract each other. As a result, Fig. The analysis above is also valid for forced convection. One can see that the controlled forced Fig. Note the change in the segregation of copper.

Recently similar results were reported by Zhang et al. As we have already mentioned in Section 3. It is inferred that the origin of these coarse-cell structure are the detached dendrites, which have been initially formed at the time of ingot shell formation [8,9].

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It is explained that these broken dendrites are carried by the convection currents see Section 2. A larger DAS which is often 3—3. Electron probe microanalysis EPMA mea- surements across a coarse dendrite reveals a uniform plateau of copper depletion away from the cell boundary [8]. Note that the nominal composition of this alloy was 3. The liquid also exerts a resistance for this sedimentation. In other words, they have the same trajectories.

As a result, there will be no macrosegregation. Another approach could be the two-phase model with one phase for the grains and the other for the liquid, in which the interaction between these two phases is modelled by a drag force present in the momentum equations of each phase. This model was applied to model the macrosegregation of steel ingots [98]. Deformation can be externally induced in the case of continuous casting of steels or thermally induced.

The latter applies to DC casting of Al alloys. This mechanism is important for the subsurface and surface segregation [99] when exudation results when the coherent mush is not in direct contact with the mould Fig. With reference to Fig. After the discussion of various mechanisms of macrosegregation in DC cast Al alloys, it is clear that the important question that remains open for now is the extent of contribution of each of the mechanisms to the overall segregation picture. The computer simulations that take into account this or that mechanism of segregation could help in fundamental understanding.

These parameters will be discussed in this section. Macrosegregation depends on the alloy in question. Generally, as we have discussed in Section 3. However, studies pertaining to various alloys under identical DC casting conditions are rare. From modelling perspective, this is very important [91]. Technologically, the casting speed determines the productivity. It is well known from the early and following works [1,8,18,21,28,33,58,69,80] that the severity of macrosegregation increases with the casting speed.

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This change is fundamentally corre- lated with the sharp variation in the sump depth and the extent of transition region see Section 2. Good correlation was obtained between the measured radial distribution of segregation and the vertical distance between liquidus and solidus isotherms Fig. But closer to the periphery, the role of the transition region thickness becomes less pronounced, obviously due to the decisive role of high temperature gradients and exudation.

Similar results were reported elsewhere [28]. Although it seems to be important, little consideration has been given to the melt tem- perature melt superheat in the DC casting literature on the defect formation. Our exper- iments upon DC casting of an Al—2. As we have already mentioned in Section 2. This is explained by the disproportionate increase in the sump depth with the ingot thickness as compared to the sump deepening with the casting speed []. Livanov et al. At lower casting speeds and smaller diameters the macro- segregation is normal, otherwise — inverse.

The dependence Eq. Optimum melt level metal head is essential for the formation of a stable and strong solid shell as the liquid metal starts to freeze onto the mould surface. The hypothesis suggested to explain the reduced macrosegregation includes a lim- ited presence and growth of isothermal dendrites due to the general displacement of Fig.

The result can be dramatic as illustrated in Fig. Despite the additional equipment and the costs involved, this method provides a way to minimise macrosegrega- tion both in the billet centre and at the periphery. In addition, the absence of air gap upon EMC eliminates the exudations.

The correlation between structure and macrosegregation is not straightforward. From the above discussion, it is certain that macrosegregation is an irreparable defect. But sometimes these solutions are technolog- ically impracticable. And in reality, it may not be possible to manipulate the alloy composition.

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The former is widely used in Europe while the latter is common in the United States [24]. The remaining Ti is combined with aluminium to form Al3Ti. For example, in the Al3Ti1B master alloy, 2. In this category, Al3Ti0. During industrial DC casting, this rod is continuously fed into the molten metal stream at a constant rate as the melt passes through the launder. The average TiB2 particle size is around 1—2 lm [].

Hence, it is considered that upon dissolution of a master alloy rod, Ti is evenly distributed in the liquid []. Several mechanisms have been postulated in the lit- erature and have been extensively reviewed [56,57,,]. A discussion on the theories is out of the scope of this review. Recently, it was experimentally observed that a thin layer of a metastable Al3Ti phase was formed at the surface of TiB2 particles prior to the nucleation of Al solid solution [].

This layer then disappeared while the Al phase grew. The other issue concerns the permeability of the mushy zone. Alloy design in wrought commercial Al alloys is based on the requirements with regard to formability, corrosion resistance, mechanical properties, etc. So any changes in the alloy chemistry beyond the grade limits are generally not admissible as a means to control alloy castability, as-cast microstructure, etc. We shall discuss these factors below. The smaller the undercooling required for an Al grain to form on the substrate particle, the more potent the particle.

The undercooling necessary for free growth increases with decreasing the nucleant particle size Fig. In addition to this, both nucleation and growth of the grain greatly depend on alloy composition. The review by Easton and St John [56] drew important con- clusions on the role of alloying elements in the melt. When this rate exceeds the rate of external heat extraction, the temperature of melt starts to increase thus limiting its undercooling []. The amount of constitutional solute undercooling see Eq. Table 5 provides the data for GRF given by Fig.

The size distribution of TiB2 particles in an Al3Ti1B master alloy along with the undercooling necessary to initiate a free growth of Al []. For ternary and higher-component systems e. Table 6 lists estimated values of GRF for some commercial alloys. The proportion of nucleant particles that become active is very sensitive to the GRF of an alloy [].

When Q is increased, the increase in fraction solid at a given undercooling is slowed and this allows less-potent TiB2 particles to be active. Due to cost considerations the practice over the years tends towards using a lower Ti:B-ratio alloys. Data from model experiments [24] and DC cast trials [58,74] justify this point. Furthermore, as it is discussed later Section 4.

Permeability is also important in under- standing of feedability and porosity in shaped castings. We have already discussed the basics of permeability in Section 2. Permeabilities for globular structures non-dendritic are approximately one order of magnitude greater than permeabilities for dendritic-globular structures at the same fraction of solid [38]. On the other hand, for spherical grains a decrease in grain size will lead to a reduction in permeability [66]. With reference to DC casting, it is often assumed that grains are globular for GR con- dition hence grain size is the deciding factor.

The variation of permeability with solid fraction is important see Eqs. For Al alloys, coherency isotherm is around 0. When compiling data from various reports, care should be exercised in drawing conclu- sions as the experimental conditions vary greatly, e.

The reported variable parameters included feeding system, ingot thickness Variation of Mg centreline segregation in DC cast billet with Ti concentration and melt-feeding system. Data obtained from Ref. Variation of Mg centreline segregation in DC cast billet with the Ti concentration and melt-feeding system. The arrows indicate the direction of segregation variation with increasing casting speed. Data obtained from [80] and replotted. Similar experimental data for the alloy Fig.

On the other hand, Glenn et al. They argued that in all cases small showering crystals were responsible for the negative centreline segregation.

Direct-Chill Casting of Light Alloys: Science and Technology

Lesoult et al. Data from their work is plotted along with the data obtained by Glenn et al. It was pointed out that grain morphology was an important factor to be considered [70], which in their case was more dendritic in NGR ingot and more globular in the GR ingot. Both the DC cast billets were produced at a casting speed of 3. It is worth noting that the casting parameters speed and size in the above experiments were close to the threshold condition for the transition from positive to negative centreline segregation described by Eq.

Considering the nature of microstructures and the amount of coarse-cell grains, it is inferred that mushy zone permeability might be high enough so as to compensate the depletion of alloying elements caused by solute-lean grains. For the same feed- ing conditions, the increased potency of an Al5Ti0. The current understanding of the macrosegregation mechanisms is based on two essen- tial conditions: relative movement between solid and liquid phase and the enrichment of the liquid phase.

While some successes have been reported in predicting measured macrosegrega- tion patterns in industrially relevant casting processes, there are still numerous areas where further development is required. Macrosegregation phenomena cannot be pre- dicted without detailed consideration of the evolving microstructure. References [1] Dobatkin VI. Continuous casting and casting properties of alloys. Moscow: Oborongiz; The principles of continuous casting of metals. Metallurgical reviews, vol. London: The Institute of Metals; Ingots of aluminium alloys.

Sverdlovsk: Metallurgizdat; Int Met Rev 6 — Direct-chill casting of aluminium alloys.

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Moscow: Metallur- giya; Ingot casting in the aluminium industry. Aluminum alloys — contemporary research and applications. Treatise on materials science and technology, vol. London: Academic Press; International conference on aluminum alloys — their physical and mechanical, properties. In: Bickert CM, editor. Light metals. Materials processing in the computer age II. J Heat Transfer ;— Progr Mater Sci ;— Mater Forum ;— Metall Trans ;—4. Continuous casting in electromagnetic mould. Moscow: Metallurgiya; Dendrite coherency, vol.

Aluminium ;— Mater Sci Eng A ;A— In: Crepeau PN, editor. Light Metals Metall Trans A ;23A—5. Light metals Theory of special casting methods. Moscow: Mashgiz; Lausanne: EPFL; In: Schneider W, editor. Mater Sci Eng A ;A—— In: Tabereaux AT, editor. In: Anjier JI, editor. In: Campbell PG, editor.

Metall Mater Trans A ;28B—9. Metall Mater Trans A ;36A— Metall Mater Trans A ;38A—9. Int J Heat Mass Transfer ;— The theory of ground-water motion and related papers. New York: Hefner Publishing; Mc-Graw Hill; Mater Sci Eng A ;— Trans Inst Chem Eng London ;—6.

Sitzungsberg Akad Wiss Wien ;— Metall Trans A ;24A— Metall Mater Trans B ;25B— Scripta Mater ;—8. Scand J Metall ;— Metall Mater Trans A ;29A— Metall Mater Trans A ;35A— In: Eighth international light metals congress; Light Metal Age 10 —6. J Met ;— Sci Technol Adv Mater ;— In: Starke Jr. EA, Sanders Jr. TH, editors. Im: International conference on aluminum alloys, their physical and mechanical properties; To aid in this process, this book provides comprehensive coverage on topics such as the history of process development in this field, industrial applications, including vertical and horizontal casting, melt preparation, fundamentals of solidification in DC casting, and more.

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