functions and mechanism of modification elements in ... · si eutectic solidification is the...

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Vol.11 No.4 July 2014Special Report CHINA FOUNDRYCelebrating the 10th Anniversray 2004-2014

Functions and mechanism of modification elements in eutectic solidification of Al-Si alloys: A brief review

*Zu FangqiuBorn in 1956, Ph D., Professor. He is currently Director of Liquid/Solid Metal Processing Institute in HFUT; a member of the Scientific Committee of HFUT; Outstanding Expert granted by The State Council of China; Excellent Key Teachers of Chinese Universities granted by Chinese Ministry of Education; Principal Teacher of the Course of Materials Processing Foundations ranged in Chinese Open Online Courses. Research interests are mainly focused on metal solidification theory & technology, cast alloys and processing methods, liquid metal structures & properties, bulk metallic glasses and thermoelectric materials. So far over 160 papers have been published in Chinese & international journals and most of which have been indexed by SCI and/or EI. He is the author of the Chinese national higher education text books “Principles of Casting” and “Materials Processing Foundations”.

E-mail: [email protected]

Received: 2014-04-30Accepted: 2014-05-20

*Zu Fangqiu and Li XiaoyunLiquid/Solid Metal Processing Institute, School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China

Abstract: Being used more and more widely in engineering, Al-Si alloys comprise about 80% of all kinds of aluminum alloys, which are the most widely utilized nonferrous alloys. Although most Al-Si alloys consist of multiple components, the eutectics in the structure accounts for 50%-90% of the sum volume of such alloys. Therefore, understanding the modification mechanism and function rules of the Al-Si eutectic solidification is the technical key in controlling the structures and properties of such casting alloys. The present paper chiefly reviews recent investigation developments and important conclusions along the lines of the functions of modification elements and their modification mechanism in the eutectic solidification of Al-Si alloys.

Key words: Al-Si alloys; eutectic solidification; modification; silicon

morphology; growth kineticsCLC numbers: TG146.21 Document code: A Article ID: 1672-6421(2014)04-287-09

Al-Si eutectics accord with general characteristics of nonfaceted-faceted irregular eutectics, and the eutectic Si phase grows with an alternant divergent and convergent

pattern [1]. Under common casting conditions, the morphology of Si crystals is shown in Fig. 1 (a) and (c) when solidified from Al-Si alloy melts without the modification treatment. Although eutectic Si crystals look like needles or strips on a polished surface of a metallographic specimen, they are spatially connected thick plate structures, with no directional irregular distribution. There is often a small amount of primary Si in eutectic and near-eutectic composition Al-Si alloys. Such a metallographic structure would result in lower mechanical properties of Al-Si alloys, so Al-Si alloy liquid is usually modified with an alterant containing Na or Sr, etc. in casting practice. The modification purpose is to achieve "flake to fibrous" morphological transformation for eutectic silicon and to eliminate the primary Si crystals, finally changing into a tiny fibrous shape, as shown in Fig. 1(b) and (d). But the treatment sometimes accompanies with a small amount of fine flake shape. The microstructure changes have great significance for improving the properties of Al-Si cast alloys. For example, the tensile strength can be increased by about 50%, and ductility even increases to about three times.

Why is it then that modification treatment can change the Al-Si alloy microstructures? Understanding its effect rule and modification mechanism would undoubtedly be of great significance for the improvement of casting quality. Since the invention of modification technology for Al-Si alloys, various theories have been put forward to explain the modification mechanism of alterant elements to eutectic Si based on production practices and experimental studies. These theories can broadly come down to two aspects: affecting

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Vol.11 No.4 July 2014Special ReportCHINA FOUNDRY Celebrating the 10th Anniversray 2004-2014

Fig. 1: Microstructures of eutectic silicon of unmodified and modified eutectic Al-Si alloys: (a) and (c) [2] optical and SEM pictures of unmodified alloy; (b) and (d) [3] optical and SEM pictures of modified alloy

Fig. 2: Crystal structure of silicon (a) and growth manner of twin re-entrant groove (b)

the growth of eutectic silicon (restricted growth theory), and inhibiting the nucleation of silicon crystal (restricted nucleation theory). Although the modification mechanism is not fully unified so far, some significant related phenomena and laws have gradually become consensus, and some meaningful and important progresses have been made in recent years.

1 Eutectic growth of un-modified Al - Si alloy

In the eutectic growth of Al-Si alloy, Si crystal is the leading phase, and α -Al nucleates and grows attaching eutectic Si.

Furthermore, α-Al would form constantly along with the growth of eutectic Si in development process of eutectic group [4]. So understanding the growth of eutectic Si is crucial to grasp the eutectic crystallization rule of Al-Si alloys. Si crystal is the diamond face-centered cubic structure composed of several tetrahedrons, as shown in Fig. 2(a). For eutectic growth of un-modified Al-Si alloys, the outer surface of Si crystals is constituted of specific crystal plane (111). Un-modified Si crystal would grow along sole <112> crystal orientation because of the faceted phase property. This anisotropic characteristic determines that it is difficult to change the characteristic direction in the growth process, and the branching pattern

(a) (b)

becomes the basic way that maintains the average lamella spacing broadly constant during the eutectic stage. In addition, the (111) plane of Si crystal may appear concave angle twin, as shown in Fig. 2(b), and twin plane forms a groove of 141° in front of a growth interface. Here it is easy to form two-dimensional growth steps which can offer convenience for Si atoms landing to maintain its growth and a certain degree of branching. This is called the twin plane re-entrant edge mechanism — TPRE [5]. However, the twin density of un-

(a) (b)

(c) (d)

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modified eutectic Si is quite low, and the branching course is relatively limited, thus the straight, thick plate-like morphology forms combined with the specific growth orientation.

The irregular eutectic cells appear as a radial growth with a loose symbiosis pattern. They can grow from the mold wall inward toward the middle of the ingot or there are some isolated eutectic colonies in the endogenous way, as shown in Fig. 3 [6]. Because the irregular eutectic growth interface is uneven, and it is a non-isothermal surface, each Si piece top would go deep into the liquid ahead of α-Al phase. The final formed eutectic cells are a two-phase mixture of disordered arrangement with α -A1 and plate-like Si. The eutectic Si is derived from the limited branches of the same core. This is similar to the eutectic cells of grey cast iron. The only difference is that the graphite and silicon form from a rotating twin and concave angle twin respectively, and the branching of eutectic Si plates is much less than that of graphite flakes and not easy to bend. They seem like separated acicular eutectic silicon [Fig.1 (a)] under the optical microscope, but actually they are interconnected coarse silicon plates in eutectic cells [Fig.1 (c)].

2 Effect of modification elements on growth manner of eutectic Si — IIT mechanism

The growth manner of the eutectic silicon would alter remarkably when Al-Si melt is modified by Sr, Na or other elements. It is most widely accepted that, because modification elements adsorb and gather in front of the silicon growth interface, the original twinning steps of eutectic Si are ceaselessly blocked and substantial entrant concave angle twins are continually triggered, which causes many more branches of the growing eutectic silicon crystals than in un-modified cases. Furthermore, the significant increase of twin density results in a remarkable change of the growth pattern of eutectic Si, i.e. from original anisotropy to isotropy. Then, the growth manner of eutectic Si sharply changes from the model of limited branch and thick-plate development before modification to the model of fibrous growth with a large number of frequent branches. Eventually, both morphology and size of eutectic Si crystals have substantial changes. The theory is developed based on TPRE mechanism, and it is called impurity-induced twinning mechanism — IIT [7,8]. This mechanism is clearly demonstrated by coral-like morphology consisting of fibrous eutectic Si with frequent branching, as shown in Fig. 4 (a). Studies show that twin density of un-modified eutectic silicon is very low, with the typical twin spacing about 0.4-1.0 mm, while the twin density of the silicon section after modification is increased significantly, with the spacing about 0.005 - 0.1 μm [10]. Figure 4(b) shows the twin density status of fiber eutectic Si after modification. The phenomenon that modification promotes significantly the eutectic Si twinning provides an important basis for IIT mechanism.

In addition, the detection of modified hypoeutectic Al-Si alloys by μ-XRF technology (micro X-ray fluorescence) has disclosed that most of the modification element atoms are in eutectic Si and distribute relatively uniformly, while the distribution of the modification elements in primary and eutectic

Fig. 3: Eutectic with flake Si advanced radially inward (quenched solidifying sample of unmodified Al-12.6wt.%Si, ×21)

Fig. 4: Fibrous eutectic Si crystals in modified Al-Si alloy: (a) with frequent branching [9], (b) Si fibre with concentrated twins [10]

(a) (b)

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α-Al phases is negligible. This phenomenon is also deemed to be an important basic proof of the IIT mechanism[11,12], although it is not logically exclusive. Moreover, it is found that, after modification, the growth direction <110> is in the majority [13,

14] for eutectic silicon fibers although there is still the growth direction of <211>. The phenomenon of the growth direction <001> [15] is also found. Despite the fact that the observed results from different researchers are not entirely consistent, the changed growth orientations from the original specific single direction <211> provide consequentially the conditions for frequent branching and changing growth orientation. The phenomenon of the growth orientation <001> for eutectic Si is particularly of significance after modification, because it indicates an “out-of-plane growth” model, which is different from the “in-plane growth” model (both <211> and <110> orientations are in the (111) plane).

According to the IIT mechanism of Al-Si alloy modification, the ideal atomic radius ratio (r/rSi) of modification elements to silicon is 1.646 by calculating from crystal geometry. Other elements close to the ratio, including Ca, Ba, Sb, as well as Y, Eu, Yb and some other rare earth elements are used to investigate the modification effect on the microstructures of Al-Si alloys. The results show that these elements really have different modification effects. For example, modification elements, such as Na, Sr, Ba, Ca, Eu, etc., transform the morphology of eutectic silicon into fibrous configuration, while elements Y, Sb, Yb etc. transform eutectic Si to short flakes or blocks, as described in Fig. 5 [12]. But in practical

application, Sr and Na are universally used because of many other factors and constraints of the other elements. In view of technological stability and environmental factors, Na modification is gradually becoming unused in some countries.

Although the impurity-induced twinning mechanism (IIT) has been widely recognized, it still can not explain some phenomena. For example, in the case of high-speed solidification, fine fibrous eutectic silicon can also be obtained in Al-Si alloys even without any modification elements. As shown in Fig. 6, with enhancing the solidification rate, the eutectic Si is significantly refined, and more significantly,

Fig. 5: Ratio of atomic radii vs atomic number and their modified effects [12]

Fig. 6: Silicon morphology dependence of directional solidification rate R in Al-Si eutectic alloys [2]

(SEM of sample cross section: R changes from low to high, thermal gradient GSL remains at 7 - 14 k·mm-1 )

10 μm 5 μm

10 μm·s-1

20 μm

10 μm

100 μm·s-1

10 μm

5 μm

250 μm·s-1 950 μm·s-1

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Fig. 7: Heterogeneous nucleation of AlP for silicon crystals: (a) Phosphorus-rich particle inside silicon crystal in an Al-10% Si alloy [16] , (b) High resolution FEG-TEM micrograph[17]: there is no lattice mismatch between the (111) lattice planes of Si (A) and AlP (B)

(a) (b)

the ideal fibrous morphology of eutectic Si can be acquired without any modification elements [2]. According to the analysis of the experimental results, the researchers believe silicon crystal morphology of Al-Si eutectics changes from flake to fibrous with the increase of solidification rate. They boil this phenomenon down to the relationship of “aspect ratio and solidification rate” of silicon crystal growth. For a thermal gradient of 7–14 K·mm-1, the transition was found to occur in two stages, appearing over velocity regimes from 100 to 500 μm·s-1 and from 500 to 950 μm·s-1. The results also show that the "in-plane growth" of (111) plane in low rate gradually transforms to the "out-plane growth" when the solidification rate is enhanced to 500–950 μm·s-1.

On the other hand, even without Sr and Na modification or high rapid cooling conditions, the fibrous eutectic structure can also be formed in high purity Al-Si alloys. Moreover, twin density in unmodified fibrous eutectic silicon is not high. Although these phenomena cannot deny the role of modification elements on inducing high twin density of eutectic silicon at the industrial purity, at least they can

show that high density twin is not the sole condition for the formation of fine fibrous eutectic silicon.

3 The role of modification on Al-Si eutectic nucleation —Restricted Nucleation Theory

There are usually some quantity of phosphor and its compound AlP in industrial Al-Si alloy melts. Many researchers have confirmed that AlP can function as the substrate of heterogeneous nucleation for silicon, as shown in Fig. 7. It can not only cause the nucleation of primary crystal of silicon, resulting in appearance of hypereutectic eutectic structures in hypoeutectic or near eutectic Al-Si alloys, but also promote the nucleation of the leading silicon phase in eutectic structures. Based on this fact, the “Restricted Nucleation Theory” was proposed as a modification mechanism of Al-Si alloys. The theory considers that modification elements can inhibit the role of AlP on the heterogeneous nucleation of silicon, and reduce

the diffusion coefficient of silicon atoms in the melt. So the eutectic growth supercooling of Al-Si alloys after modification is significantly increased, and eutectic Si is refined so that the modification effect is reached.

Figure 8 shows the effects of P and Sr on the supercooling of Al-Si eutectic growth. In order to understand the phenomenon of Fig. 8, Fig. 9 is introduced to explain several parameters of Al-Si eutectic crystallization and their meanings. The right figure is the amplification of the dotted box in the left figure. In Fig. 9, Tc is the cooling curve (temperature-time curve, T-t)of the sample center, dT/dt is the first derivative of Tc, which expresses the instantaneous cooling rate of the corresponding time t.

It can be seen that cooling rate dT/dt remains almost unchanged at about 0.6 °C·s-1 before eutectic crystallization,

A

B

Fig. 8: Influence of P and Sr on eutectic undercooling [16]

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Fig. 9: Method used to analyze cooling curves [10]: (a) A cooling curve with corresponding derivative (A 356 alloy modified with Sr) (b) magnified eutectic arrest with salient points

while dT/dt begins to rise (the decrease rate of Tc is slowed down) when Tc dropped to below 570 °C, because the eutectic reaction of Al-Si alloy begins with its latent heat released. Correspondingly, TN is the eutectic nucleation temperature. More latent heat is released along with the formation and growth of eutectic grains until Tc drops to its minimum value TMin (dT/dt = 0), and the temperature falling stops. Then the temperature starts to rise (recalescence) until stable eutectic growth temperature TG. In the stable stage of eutectic growth, there are different lengths of growth platform temperature under different conditions (for instance, the proportion of eutectic, growth speed and so on). Looking once again at Fig. 8, we find that Sr additions produce the following effects: (1) The eutectic nucleation temperature TN is reduced and TN≤TG; furthermore, the recalescence degree increases significantly. (2) Eutectic platform temperature TG

reduces, and eutectic growth supercooling ΔTG increases (ΔTG is the difference of TG and equilibrium eutectic temperature ). (3) Eutectic solidification time is shortened. The added P produces the opposite effect compared with Sr.

It is recognized as a common phenomenon that Na and Sr modification treatment of Al-Si alloys can induce the increase of supercooling degree for industrial alloys. But it is still unclear how to change the morphology of eutectic Si. In particular, an elaborate study [18] demonstrated that the modification effect transforming eutectic Si from thick plate to fibrous and refined is not the role of increasing supercooling of eutectic growth by restraining nucleation. This study adopted Al-10%Si alloy of both industrial purity and high purity to carry out solidification experiments without modification and with Sr modification, respectively. Through solidification curves precisely measured and corresponding structures observed, it could be concluded that, whether with or without Sr modification, the structures and thermal analysis results are the same as the above laws for industrial purity Al-Si alloys. On the other hand, TN and TMin of the un-modified high purity alloy are significantly reduced as comparison with the un-modified industrial purity alloy, but its

recalescence degree has a small change and the eutectic growth temperature TG is only slightly lower. However, the structure of the un-modified high purity alloy is similar to that of the modified industrial alloy, i.e. the primary Si crystals disappear, and the eutectic Si crystals also change from coarse flakes to refined fibers. Compared with the un-modified industrial purity alloy, the temperature TMin of the Sr-modified high purity alloy decreases, and its recalescence degree is more obvious, but the eutectic growth supercooling ΔTG was not significantly different. Nevertheless, all the eutectic Si crystals of the modified high purity alloys are very refined fibers. So it could be deduced that, in essence, the modification effect on eutectic Si is not induced by the increase of eutectic growth supercooling.

Though “Restricted Nucleation Theory” is not quite perfect, it is widely considered as very important in industrial productions of Al-Si alloys to restrict phosphorus content and to add modification elements according to the phosphorus level. For example, it is generally accepted that A356 alloy (7% Si) can obtain the best silicon modification effect when 0.012%Sr (120 ppm) is added [16]. With Sr content as low as 60-70 ppm, good modification can be completely acquired if the phosphorus level is not more than 10 ppm and the melt doesn’t contain other harmful elements (e.g. Sb with “toxic” effects on Sr). Generally, when 10 ppm phosphorus is subjoined in an alloy, 30 ppm Sr should be added to offset the toxicity of P. Moreover, the change rules of eutectic solidification characteristic parameters of industrial Al-Si alloys resulting from modification do provide significant technical references for quality control in practice.

4 Modification and eutectic crystallization kinetics of Al-Si alloys

Recent studies have confirmed that, whether industrial purity or high-purity Al-Si alloys, modification treatment reduces

(a) (b)

°C°C °C·s-1°C·s-1

TG

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Fig. 10: Influence of Sr modification on size of eutectic cells and growth rate with time evolvement [20]

Fig. 11: Modification effects at different zones within same eutectic cells of Al-10%Si alloy

eutectic nucleation rate, eventually evidently increases the size of eutectic cells, and meanwhile remarkably reduces the number of modification eutectic cells in orders of magnitude [18-

20]. Interestingly, the growth rate R (radial advance speed) of Al-Si eutectic cells is significantly increased after modification. This is in line with the phenomenon of eutectic solidification time shortened. Some predicted R-ΔT relationships have been established. Typically:

In the above formula[20], ΔT is the difference between the actual temperature and the equilibrium eutectic temperature. Although the exponential n is 4 and 2 respectively, the eutectic growth rate R is much higher as a whole after modification because ΔT is obviously increased and the coefficient μ is

almost an order of magnitude larger due to modification. Additionally, from the thermal analysis curves shown in Fig. 8, the growth rate R of eutectic group in growth process fluctuates because the supercooling of eutectic stage changes with time.

Dahle et al. measured supercooling ΔT in real time in an Al-10%Si alloy solidification experiment, and then calculated growth rates R from eutectic beginning tN to ending tE for unmodified and Sr-modified Al-Si alloys, the results are shown in Fig. 10 [20]

. From Fig. 10(a) and (b) it is obvious that the growth rate fluctuates with time, the size of un-modified eutectic cells is only tens of microns, while the size of Sr-modified eutectic cells can reach millimeter level. Eutectic grain radius in the figure is obtained by the integral of r = ∫μ(ΔT)ndt from the time tN to tE. The calculation results are consistent with eutectic group size of a corresponding metallographic observation. The dashed line in Fig. 10 (b) denotes the critical growth rate which means that the Al-Si alloys could be fully modified when the R is above 7 μm·s-1.

On the other hand, the modification effect (the morphology of eutectic Si) differs obviously with the variation of solidification time and position even within the same eutectic cells because of the fluctuation of the growth rate with time. Figures 11 and 12 clearly illustrate this important phenomenon. Symbols ①②③ in Fig. 11 show three distinct regions traversing from inside to outside at the same eutectic cells. Figure 12(a),(b),(c) are the magnified structures corresponding to the regions of ①②③ in Fig. 11 in which opt ical micrographs are on the right, and SEM micrographs are on the left whose samples were deeply etched and the eutectic α-Al was partially removed. As shown, in region ① [Fig. 12(a)], the eutectic Si is well modified and it is difficult to resolve any connectivity between individual silicon fibers in the deep-etched sample (in the actual space they are connected). In region ② [Fig. 12(b)], the eutectic Si has a coarse flake-like morphology with typical unmodified eutectic. In region ③ [Fig. 12(c)], the eutectic Si is once again refined and well-modified with even better effect, and the silicon in the deep-etched samples appears more connected than that of region ①.

Based on the above mentioned results, it can be concluded that the modification effect is closely related to the eutectic growth rate. And it has been confirmed repeatedly in industrial production practice that the faster the cooling rate of castings

(un-modified Al-Si alloy)

(modified Al-Si alloy)

(b) Sr 100 ppm modified Al-Si alloy(a) Un-modified Al-Si alloy

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or of their regions, the better the effect of modification. This rule is more obvious in the case of Sr modification. It should be noted that significant increasing of Al-Si eutectic grains by modification tends to promote negatively the tendency of casting shrinkage and gas holes, which increases the porosity of castings [16, 21].

5 Surface tension and modification effect of Al-Si melt

For Al-Si alloy melt, modification elements are surface-active agents. Therefore, modification could reduce the surface tension of Al-Si alloy melt. This phenomenon has been confirmed by many experimental studies.

Based on the surface active properties of modification elements, there have been many theories to explain the modifying effect of eutectic Si. Research confirms that the modification elements are adsorbed on the solid-liquid interface in Si crystal growth. It is suggested that Si crystal growth is restrained and the growth rate slows down because of interface adsorption of modification elements. Therefore, α-Al turns into leading eutectic phase in the eutectic crystallization process; eutectic Si growth is limited and turns to fibrous. Someone else

Fig.13: Relationship between modification level and surface tension [22]

as the surface tension of the Al-Si alloys is measured rapidly, the modification level can be predicted instantly by the surface tension.

Until now, the cognition of the modification mechanism of

Fig. 12: Detailed microstructures of (a)(b)(c) corresponding to the regions ①②③ of Fig 11

(a)

(b)

(c)

has a view [22] that modification not only reduces the surface tension, but also decreases the interfacial tension between Si and α-Al. This enlarges the wetting angle between Si and α-Al at solidification interface, and eventually changes the morphology of eutectic Si.

On the other hand, many modification theories based on the surface tension have been discovered not to be perfect. Even so, the relationship between the decreased surface tension by modification elements and modification effect has been proved by many research examples. Shi et al. [22] carried out a large number of experiments to explore the relationship between the surface tension and modification effect by Na salt modifying near eutectic composition of Al-Si alloys. As shown in Fig. 13, the surface tension is divided into three sectors according to different modification levels. With the decrease in the surface tension, the modification level will change from partial modification to perfect modification. When the surface tension is above 530 mN·m-

1, the modification level is partial; when the surface tension is between 400 mN·m-1 and 530 mN·m-1, the modification level is moderate; and when the surface tension is below 400 mN·m-1, the modification level is perfect. According to these criteria, as long

Sur

face

tens

ion

(mN

· m-1)

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Al-Si alloys has been not perfect. Recent views point out that the improvement of modification theories should consider not only the growth process, but also the nucleation process [23]. The good news is that some important phenomenon, conclusions and laws obtained through the efforts of many years have played important guiding roles in the production and quality control of Al-Si alloys.

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