Synergistic effects of Gd and Zr on grain refinement and eutectic Si modification of Al-Si cast alloy

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Abstract

The effects of Gd and/or Zr additions on the grain refinement and eutectic Si modification of A356 cast alloys have been systematically investigated. The results showed that a separate addition of Gd or Zr had almost no effect on eutectic Si modification, while except for an appreciable grain refinement, a combined addition of Gd and Zr significantly refined the eutectic Si and changed its morphology from plate-like to fibrous. As compared with Gd, Zr and Sr separate modification, the Gd and Zr combined addition produced fewer twins but more nanosized precipitates within eutectic Si. The excellent modification effect was attributed to the increase of constitutional undercooling and nanosized precipitates inhibited the growth of eutectic Si during solidification. The Gd and Zr combined addition obviously improved the mechanical properties of A356 cast alloy due to grain refinement and eutectic Si modification. Optimal tensile properties were achieved in the 0.4Gd+0.5Zr modified alloy. A high amount of Zr addition resulted in the formation of coarse Al5Si2Zr3 phase and the decrease of ductility.

Introduction

It has been recognized that the coarse structures such as coarse grains and plate-like eutectic Si, will heavily deteriorate the mechanical properties of Al-Si cast alloys [1]. During the past decades, many efforts have been dedicated to refine the grain size by addition of grain refiners such as Al-Ti-B, Al-Ti-C [2], [3], and modify the plate-like eutectic Si into fibrous structure by adding modifiers, such as Na, Sr, etc. [4], [5], [6]. Nevertheless, several problems associated with grain refiners and eutectic Si modifiers still exist. For example, when Al-Ti-B or Al-Ti-C is added into Al-Si alloys, Si will react with Ti, resulting in the formation of Ti-Si intermetallic compounds, which degrades the grain refinement efficiency [7], [8]. Furthermore, a mutual poisoning effect exists between B and eutectic Si modifier Sr [9]. Therefore, there are growing interests in developing novel grain refiners and eutectic Si modifiers.

In recent years, many studies have focused on eutectic Si modification by rare earths (REs), such as La, Ce and Eu [10], [11], [12], [13], [14]. Among the REs, Gd was expected to have a good modification effect on eutectic Si because of its suitable atomic radius [15] and a high negative mixing enthalpy with Si [16]. Shi et al. [17] reported that although Gd could refine primary α-Al grains and modify eutectic Si phase in a certain extent, the modification efficiency is fairly lower than that of Sr modifier commonly used in current industry. On the other hand, Zr could produce an acceptable grain refinement in Al alloys due to peritectic reaction with Al, and it has been used as an alloying element in wrought Al alloys [18], [19], [20]. Baradarani et al. [21] reported that the Zr addition also shows a grain refinement effect on cast Al-Si alloys, but the investigation on the effect of Zr on eutectic Si modification has been rarely known until now.

Recently, Zr coupled with Sc addition was found to exhibit better fluidity in cast Al-Si alloys [22]. It has been suggested that as compared with Zr separate addition, the microstructure, i.e. grain size and eutectic Si, can be further refined by Zr and Sc combined addition [23]. Due to Sc has high cost, adding Sc into Al-Si cast alloys is not commonly acceptable to industry. The synergistic effects of alternate less expensive RE element Gd and Zr on the grain refinement and eutectic Si modification of A356 cast Al-Si alloy were evaluated on this study. We found that Gd and Zr simultaneous additions not only refined the grain size, but also led to obvious modification effect on eutectic Si. The microstructural evolution by different Gd and Zr combinations is reported in this paper. For comparison, some results of Gd and Zr separate addition are also included.

Section snippets

Experimental procedures

The A356 base Al alloys with various combinations of Gd and Zr were prepared from commercial pure Al, Al-9Si-0.9Mg, Al-5Gd and Al-5Zr (wt%, hereafter in weight percentage) master alloys. The nominal composition of A356 alloys is Al-7Si-0.45Mg. The raw materials were melted down in a DD25 I high-frequency induction melting furnace at 750 ℃ and held for 10 min after completely melted. The melt was degassed by pure Ar with a constant flow rate of 0.1 L/min for 30 min. Then modifiers were added into

Grain refinement

The effect of Gd separate addition and Gd+Zr combined addition on the macrostructure of as-cast A356 alloy is presented in Fig. 1. The unmodified A356 alloy displays coarse equiaxed grains (Fig. 1a). The grains were found to be refined by increasing Gd addition. Fig. 1(b) presents the grain structure of the A356 alloy with 0.4Gd addition. The alloy also exhibits an equiaxed grain structure with an apparent smaller size compared to the unmodified alloy. In order to avoid the excessive formation

Discussion

The present results indicate that Gd and Zr combined additions can lead to better grain refinement than the Gd or Zr separate addition. It is well-known that the grain size of casting alloy is highly dependent on the potent nucleation sites and undercooling during solidification. The addition of solute will cause constitutional undercooling and may introduce heterogeneous nucleation sites. The role of solute concentration on the grain size (d) of casting alloy can be expressed by a

Conclusions

The synergistic effects of Gd and Zr additions on grain refinement and eutectic Si modification of A356 cast alloy have been investigated. The following conclusions can be drawn:

  • (1)

    A separate addition of Gd or Zr caused grain refinement of A356 alloy, while the Gd and Zr combined additions had a more appreciable grain refinement due to the introduction of Al3Zr heterogeneous nucleation sites and the increase of constitutional undercooling.

  • (2)

    In comparison with Gd, Zr and Sr separate modification, the

Acknowledgment

The authors are grateful to the financial support by National Natural Science Foundation of China (NSFC, Nos. 51671007 and 51401010), the National 863 Project (No. 2013AA031001) and International Science and Technology Cooperation Program of China (No. 2015DFA51430) to carry out this work.

References (32)

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