Elsevier

Scripta Materialia

Volume 54, Issue 5, March 2006, Pages 943-947
Scripta Materialia

Effect of the addition of refiners and/or modifiers on the microstructure of die cast Al–12Si alloys

https://doi.org/10.1016/j.scriptamat.2005.10.067Get rights and content

Abstract

The modification of eutectic die castings of Al–Si alloys using strontium and/or titanium refinement was studied. The results show that the most adequate microstructure is obtained from the alloy modified with the highest percentage of Ti.

Introduction

The most widely employed aluminium–silicon alloys are often hypoeutectic or eutectic. The microstructure of the Al–12 wt.%Si commercial alloy presents dendrites of α-Al, as well as silicon in the form of cuboids, and is eutectic with silicon of an acicular and/or laminar morphology.

In those structures adequately modified by the precise addition of Sr, the eutectic silicon is found in the form of fine fibers [1], [2], [3], [4]. The benefits of modification with strontium are more important in sand castings, which are cooled at slower rates, although certain benefits are also observed when the cooling rate increases, for instance in a metallic mould [1], [2], [3], [5].

Refining of the α-Al phase grains in Al–Si alloys by means of the addition of elements such as titanium or boron is a common industrial practice. The weight ratio required to form the nucleating TiB2 particles is Ti/B = 2.2/1, although all the commercially manufactured grain refiners present higher ratios of Ti/B, such as 3Ti–1B, 5Ti–1B, 5Ti–0.2B, etc. [6]. The Al–5Ti–1B master alloy is an efficient grain refiner in pure aluminium and in alloys with a low Si content. However, when the content in silicon is higher than or equal to 7 wt.%, the refining power of this master alloy is lower than that of Al–3B and Al–3Ti–3B alloys [7]. In their studies of Al–7Si, Kori et al. [8] used levels of addition of the Al–5Ti–1B master alloy ranging between 0.4 wt.% and 0.6 wt.%. In the tests conducted in the present study, the effects of refining with the refiner 3Ti–1B (3 wt.%Ti–1 wt.%B–balance Al) were determined with levels of addition of higher than 1 wt.%.

Section snippets

Experimental procedure

The nominal composition of the Al–12Si commercial alloy employed in the present study is listed in Table 1.

The industrial practice followed was: after melting the Al–12Si alloy ingots in the WMHOR-T-5000/2000 furnace that the firm Thyssen–Krupp Guss possesses in Mieres, Asturias, Spain, amounts of around 700 kg of alloy were poured into a treatment ladle at a temperature of around 714 °C. Subsequently, the corresponding amounts of titanium were added to the mix in the form of the Al–3Ti–1B master

Results

The microstructures observed after the different additions are shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5. The variations in the volume fraction and in the grain size of the α-aluminium phases and of the primary silicon cuboids are shown in Table 3.

Discussion

In the present research, the addition of titanium in the form of the Al–3Ti–1B master alloy was studied. The use of different master alloys for the purposes of refining Al–Si alloys is the subject of numerous research publications. Murty et al. [14] pointed out that Al–Ti–B master alloys are more efficient for refining than Al–Ti master alloys. He showed that the presence of intermetallic particles is responsible for this refinement. In binary alloys only TiAl3 particles act as heterogeneous

Conclusions

  • The addition of the Al–3Ti–1B master alloy in near-eutectic Al–Si alloys has a markedly positive influence on the refinement of the dendritic α-Al phase. Furthermore, the addition of Ti leads to an increase in the volume fraction of the α-Al phase and in the primary silicon cuboids and a decrease in the size of the latter.

  • The addition of the Al–10Sr master alloy was found to increase the volume fraction of the primary α-Al dendrites. However, it should be noted that the results show an

Acknowledgements

The authors wish to express their thanks to Dr. Juan Jose del Campo, Professor of Materials Science at The University of Oviedo, and former manager of the Thyssen–Krupp Guss foundry for his help in designing the chemical compositions at a full-size scale, and for having granted access from the start of the research project to the present day to all the existing facilities at the foundry labs and elsewhere in the factory. We likewise wish to express our gratitude to the Vice-rectorate for

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