Grain refinement of hypoeutectic Al-Si alloy prepared
with ELTA by Al-4B master alloy

WANG Ming-xing(王明星), MENG Xiang-yong(孟祥永), LIU Zhi-yong(刘志勇),
LIU Zhong-xia(刘忠侠), WENG Yong-gang(翁永刚), SONG Tian-fu(宋天福), YANG Sheng(杨 升)

Department of Physics, Laboratory of Materials Physics of Ministry of Education,

Zhengzhou University, Zhengzhou 450052, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Electrolytic low-titanium aluminum (ELTA) was produced by adding TiO₂ powder to an industrial aluminum electrolyzer. The grain refining effect of Al-4B master alloy in the hypoeutectic Al-Si alloy prepared by using ELTA was investigated, and compared with those of Al-5Ti, Al-5Ti-1B and Al-4B master alloys in the similar alloy prepared by using pure Al. The results indicate that when Al-4B is added to the melt of the alloy prepared by using ELTA in terms of the Ti/B mass ratio of 5?1, the grain refining effect is better than those of Al-5Ti, Al-5Ti-1B and Al-4B master alloys. Thus, using Al-4B to refine the grain of Al-Si alloys prepared by using ELTA will possibly become a feasible way of obtaining Al-Si alloy with homogeneous and fine microstructure.

Key words: electrolytic low-titanium aluminum (ELTA); grain refining; hypoeutectic Al-Si alloy

1 Introduction

Al-Si alloy is a type of very important cast aluminum alloys. Due to their good cast performance and high mechanical properties, Al-Si cast alloys have a wide applications in the casting industry [1]. Grain refinement is an effective means to improve the comprehensive mechanical properties of aluminum and its alloys [2]. Grain refining of pure Al and wrought Al alloys is usually achieved by adding Al-Ti or Al-Ti-B master alloy to the melts of the metals before casting [2]. However, grain refining of Al-Si cast alloys containing high level of Si (＞5%) by Al-Ti or Al-Ti-B is difficult [1, 3]. It is well-known that the grain refining effect of Al-B master alloy in pure Al and wrought Al alloys is very poor, but is excellent in Al-Si alloys with high level of Si [4]. Therefore, Al-B master alloy gradually becomes a kind of effective grain refiner for Al-Si alloy [1, 4].

Electrolytic low-titanium aluminum(ELTA) can be produced by adding a little of TiO₂ powder to an industrial aluminum electrolyzer [5-6], and the Ti content is about 0.10%-0.30%. If using ELTA instead of pure Al to produce Al alloy, the addition of Ti will be finished in advance. This can save a lot of troubles of producing the Ti-containing master alloys and adding them to the melts of Al alloys. Previous works have proved that such method of Ti addition exhibited an excellent grain refining effect in pure Al and wrought Al alloys [5, 7]. And if the addition of Al-B master alloy is made during preparing Al alloys, the grain refining effect could be improved obviously. In order to understand the grain refining effect of Al-B master alloy in Al-Si cast alloy prepared by using ELTA, the grain refining effect of Al-4B master alloy in Al-9%Si alloy is experimentally studied, and compared with those of Al-5Ti, Al-5Ti-1B and Al-4B for the similar alloys prepared by using pure Al. The optimal addition amount of B is also determined when using Al-4B to refine the grains of Al-Si cast alloy prepared by using ELTA.

2 Experimental

The raw materials used in this study included pure Al, pure Si, Al-5Ti-1B, Al-5Ti and Al-4B master alloys and four kinds of ELTA with different Ti contents. The chemical compositions of ELTA are listed in Table 1. The tested alloys were prepared in a 5 kW electric resistance furnace. The Si contents of all the tested alloys were 9%. After the ELTA or pure Al was melted in a graphite crucible, the temperature was raised up to 780 ℃, and the pure Si was added to the melt. When the added Si was melted completely, Al-5Ti, Al-5Ti-1B or A-4B was added to the melt, the melt was then poured into a steel mould (30 mm in inner diameter and 70 mm in depth) at about 720 ℃. The specimens were sectioned at the height of 35 mm from the bottom of the casting. They were polished and etched using 0.5%HF to reveal their microstructures. At least 25 images were taken from each specimen for analyzing the grain sizes. Grain size analysis was carried out by using the standard linear intercept method.

Table 1 Chemical compositions of ELTA (mass fraction, %)

3 Results

The variations of the grain sizes of the tested alloys with Ti contents are shown in Fig.1. For the alloys prepared by using ELTA, the addition amount of Al-4B is determined in terms of Ti/B mass ratios of 5?1, which is the same as that of Al-5Ti-1B. It can be seen that the grain refining effect of Al-5Ti in Al-9%Si alloys prepared by using pure Al is the worst. Al-5Ti-1B is obviously better than Al-5Ti in the grain refining effect because of containing B, while the grain refining effect of Al-4B in the alloys prepared by using ELTA is the best. Fig.2 shows that the grain sizes of the alloys prepared by using pure Al or the ELTA change with B contents. The Ti contents of the alloys prepared by using ELTA are 0.15%. It can be seen from Fig.2 that the grain refining effects of Al-4B in the both alloys are all very good. For the alloys prepared by using pure Al, the grain sizes decrease rapidly with the B content increasing. When the B content exceeds 0.06%, the grain size does not decrease significantly with the B content further increasing. For the alloys prepared by using ELTA, the grain sizes also decrease obviously with the B content increasing. In the case of without Al-4B, the grain sizes are somehow refined. This indicates that the Ti, which is brought into the alloy by ELTA, still has the grain refining ability. It also can be seen from Fig.2 that when the B content is low, the grain sizes of the alloys prepared by using ELTA are smaller than those of the alloys prepared by using pure Al. And when B content is 0.03%, that is, the Ti/B mass ratio is just 5?1, the grain size is the smallest. With the B content further increasing, the grain size becomes large. And when the B content is 0.067%, that is, the Ti/B mass ratio is 2.22?1(TiB₂ stoichiometry), the grain size is the biggest.

Fig.1 Variations of grain size of alloys with Ti content

Fig.2 Variations of grain size of alloys with B content

When the B content is increased once again, the grain size yet begins to decrease. When the B content is 0.15%, the grain size is commensurate with that of the alloy prepared by using pure Al, which is grain refined by Al-4B. Fig.3 shows that the grain sizes of the alloys, whose Ti contents brought by ELTA are 0.06% and 0.22% respectively, change with B content. The patterns of two curves are similar to Fig.2. The grain sizes at the Ti/B mass ratio of 5?1 are all the finest, and the grain sizes are the maximum at the Ti/B mass ratio of 2.22?1.

4 Discussion

As seen from the experimental results mentioned above, Al-5Ti mater alloy exhibits some grain refining ability in the Al-9%Si alloy prepared by using pure Al, but the grain refining effect is not very ideal. We all know that there are many TiAl₃ phases with different sizes in the Al-5Ti. When it is added to the melt of Al alloy, the small TiAl₃ phases will be fast melted, but the big TiAl₃ phases will serve as the heterogeneous nucleation sites for liquid aluminum during solidification. The more solute Ti atoms will present in the melt with the fusion of the TiAl₃ phases, and the Ti atoms may react with Al and Si atoms to form a ternary aluminide, Ti(Al_1-x, Si_x)₃ (x＜0.15) [8], which subsequently nucleates α(Al) phase via a peritectic reaction with liquid aluminum above the liquidus temperature of the alloy, resulting in the grain refining of the alloys [8]. In the present study, the Si content of the alloys is 9%. Such high Si content can cause the temperature of the peritectic reaction to decrease. That may weaken the nucleation ability of the ternary aluminide phases [9]. In addition, a thin layer of Ti-containing silicate may also form on the surface of the TiAl₃ phases, which will poison the nucleating ability of TiAl₃ phases [10]. Therefore, the grain refining ability of Al-5Ti is very limited. Only by adding more Al-5Ti to the melt of the alloy can the grain refining effect be improved (Fig.1). Our experimental results show that the grain refining effect of Al-5Ti-1B in the alloy is obviously better than that of Al-5Ti (Fig.1). This indicates that the addition of B is beneficial to the improvement of the grain refining ability of Ti element.

Fig.3 Variations of grain size of alloys prepared by using ELTA with B contents

The grain refining ability of Al-4B in the Al-Si alloys is very strong. According to the analysis of MOHANTY and GRUZIESKI [8], there is an eutectic reaction, L→α(Al)+AlB₂, at about 0.022%B and 659.7 ℃ as seen from the Al-B phase diagram [11]. The Si content of the tested alloys in this study is 9%, the liquidus temperature of the alloy is surely lower than 659.7 ℃. Therefore, when adding Al-4B to the melt of the alloys, the B released by Al-4B will probably nucleate numerous α(Al) phases via the eutectic reaction at above liquidus temperature. The solid will grow readily from the pre-existing α(Al) phases without undercooling, resulting in the grain refining of the alloys. In addition, the eutectic composition may shift towards low B content because of the presence of Si [7]. Therefore, even if the B content is lower than 0.022%, the grain refining effect is still very good (Fig.3). If the B content is higher than the eutectic composition, AlB₂ particles will be firstly produced. However, AlB₂ particles do not refine the grain of the alloys, so the addition of more Al-4B can not further improve the grain refining effect. The AlB₂ particles may well agglomerate and settle down at the bottom of the melt.

When using Al-4B to refine the alloys prepared by using ELTA, the B atoms that come from the Al-4B will react with pre-existing Ti atoms to form TiB₂ particles [6]. At low addition level of B, some excessive free Ti atoms will still remain in the melt after the reaction. Both the free Ti atoms and Si atoms will segregate towards the TiB₂ particles, forming ternary aluminide, Ti(Al_1-x,Si_x)₃, on the surface [8]. As a result, the TiB₂ particles are turned into the effective nucleation sites, which subsequently nucleate α(Al) phase. When the Ti/B mass ratio is 5?1, there will be not only a large number of TiB₂ particles formed by the reaction, but also enough free Ti atoms in the melt of the alloy. Compared with the TiB₂ particles that come from Al-5Ti-1B, the sizes of the TiB₂ particles formed by the reaction will surely be finer. So, there will be much more TiB₂ particles in the melt at the same contents of Ti and B, and there may be more heterogeneous nucleation sites in the melt. Thus, the grain refining effect will be improved (Fig.1). When increasing B content, more Ti atoms will react with the B atoms to form TiB₂ particles, resulting in the decrease of the free Ti atoms. If Al-4B is added to the melt at the Ti/B mass ratio of 2.22?1 (TiB₂ stoichiometry), all of the free Ti atoms in the melt will be consumed in forming TiB₂ particles. As a result, the TiB₂ particles will be turned into effective heterogeneous nucleation sites, and will be pushed to the grain boundaries during solidification [8]. In this process, TiB₂ particles may hinder the growth of grain. Thus, the alloys are still grain refined to some extent (Fig.3). When the added B content exceeds the TiB₂ stoichiometry, the excessive solute B atoms will be present in the melt of the alloy. The solute B atoms may also segregate towards the TiB₂ particles [8]. When the B contents in the vicinity of the TiB₂ particles go up to a certain value, the nucleation of α(Al) phases will take place via the eutectic reaction. So, with the B content increasing, the grain refining effect is improved, and the grain sizes of the alloy decrease obviously (Fig.3).

5 Conclusions

1) Al-5Ti, Al-5Ti-1B and Al-4B master alloys all exhibit grain refining ability in the Al-9%Si alloy prepared by using pure Al. The grain refining effect of Al-5Ti-1B is better than that of Al-5Ti. The grain refining effect of Al-4B is very good. When the addition level of B is  0.06%, a good grain refining effect can be achieved.

2) When Al-4B master alloy is used to refine grain of  the Al-9%Si prepared by using ELTA, the grain refining effect is superior to Al-5Ti-1B, and also better than that of Al-4B at the low addition level of B.

3) Using Al-4B to refine grain of the Al-Si alloys prepared by using ELTA should be a feasible way of obtaining Al-Si alloy with fine and homogeneous microstructure. In order to achieve the optimal grain refining effect, the addition amount of Al-4B should be determined in terms of the Ti/B mass ratio of 5?1.

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(Edited by YANG Bing)

Foundation item: Project (0322020600) supported by the Science and Technology Foundation of Henan Province, China

Corresponding author: WANG Ming-xing; Tel: +86-371-67767776; E-mail: wangmx@zzu.edu.cn