M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 5 5 9 – 1 5 6 3

Refinement performance and mechanism of an Al-50Si alloy

H.S. Dai, X.F. Liu⁎

Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, 73 Jingshi Road,Jinan 250061, PR China

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +86 531 8839541E-mail address: xfliu@sdu.edu.cn (X.F. Liu).

1044-5803/$ – see front matter © 2008 Elsevidoi:10.1016/j.matchar.2008.01.020

A B S T R A C T

Article history:Received 25 August 2007Received in revised form11 January 2008Accepted 25 January 2008

The microstructure and melt structure of primary silicon particles in an Al-50%Si (wt.%)alloy have been investigated by optical microscopy, scanning electron microscopy, electronprobe micro-analysis and a high temperature X-ray diffractometer. The results show thatthe Al-50Si alloy can be effectively refined by a newly developed Si-20Pmaster alloy, and themelting temperature is crucial to the refinement process. The minimal overheating degreeΔTmin (ΔTmin is the difference between the minimal overheating temperature Tmin and theliquidus temperature TL) for good refinement is about 260 °C. Primary silicon particles can berefined after adding 0.2 wt.% phosphorus amount at sufficient temperature, and theiraverage size transforms from 2–4 mm to about 30 μm. The X-ray diffraction data of the Al-50Si melt demonstrate that structural change occurs when the melting temperature variesfrom 1100 °C to 1300 °C. Additionally, the relationship between the refinement mechanismand the melt structure is discussed.

© 2008 Elsevier Inc. All rights reserved.

Keywords:Al–Si alloyRefinementPrimary silicon

1. Introduction

Al–Si alloys have been of intense interest due to theirexcellent mechanical properties and good castability [1–5].Hypereutectic Al–Si alloys are becoming increasingly impor-tant in the foundry, especially in the piston industry, owing totheir excellent properties, such as good abrasion and corro-sion resistance, low coefficient of thermal expansion (CEP)and high strength-to-weight ratio. Their properties mainlydepend on the size and morphology of primary siliconparticles. The increasing silicon content will reduce CEP ofthe alloys. Coarse primary silicon particles result in poorproperties, especially low ductility and limited machineabil-ity. Extensive research has been carried out on the control ofprimary silicon morphology. In addition, hypereutectic Al–Sialloys possess a combination of physical properties that makethem attractive for thermal management applications,including the required relatively high thermal conductivity,low density and low coefficient of thermal expansion.Therefore, hypereutectic Al–Si alloys with high Si con-tent (Si–Al alloys) can be used in electronic packaging

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applications. Spray method and other rapid solidificationtechniques have been used to refine primary silicon particlesin the alloys [6–9].

Adding phosphorous is an effective way to refine primarysilicon for hypereutectic Al–Si alloys, which is commonly usedin factories. But little information is available on the refine-ment of hypereutectic Al–Si alloys with high Si content (suchas Al-50Si alloy). Additionally, the solid microstructure is in-fluenced by the liquid structure before solidification. Hitherto,little work has been done on liquid structure and properties ofAl–Si alloys with high Si contents. The aim of this work is toinvestigate the refinement of the primary silicon in Al-50Sialloy by a newly developed Si-20P master alloy. The refine-ment mechanism is discussed on the basis of the liquid struc-ture of the Al-50Si alloy.

2. Experimental Procedures

The Al-50Si alloy used in this work was produced by a 25 kWmedium frequency induction furnace using commercially

.

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pure aluminum (99.85%) (all compositions quoted in this workare in wt.% unless otherwise stated) and commercially puresilicon (99.5%). The Al-50%Si alloy melt was poured into achilled permanent mold with dimensions of 75×35×20 mm at1100 °C, 1300 °C and 1500 °C, respectively. The Al-50Si meltwas poured into the samemold at 1100 °C, 1300 °C and 1500 °Cafter adding 1% Si-20%P master alloy (supported by ShandongShanda Al & Mg Melt Technology Co., Ltd,). All ingots wereallowed to cool in air.

Metallographic samples were cut from the lower partof the ingots (10 mm apart from the bottom). The crosssections were mechanically ground and polished usingstandard routines. Microstructures were observed by anoptical microscope (Hi-Scope Video Microscope) and aHitachi-430 field emission gun scanning electron micro-scope (FEG-SEM). In order to avoid the reaction betweenAlP and air, some specimens were prepared under an inertargon atmosphere for the electron probe micro-analysis(JXA-8840-EPMA).

The melt structure of the Al-50Si alloy was investigatedusing a high temperature X-ray diffractometer, the experi-ments were carried out in a high purity helium atmosphereof 1.3×105 Pa before the chamber was evacuated to thevacuum of 10−6 Pa. The specimen was placed in an alumina

Fig. 1 –Microstructure of Al-50Si alloy: (a) unrefined;(b) refined by adding 1 wt.% Si-20P master alloy at 1300 °C.

crucible of 24×20×8 mm in size. The Ta sheet was used asthe heating resource. The parameters used in this diffractionexperiment were as follows: scanning voltage 40 kV, elec-trical current 30 mA, exposition time half minute, measuredangle 2θ from 5 to 90°. The X-ray scattering intensity wasmeasured in arbitrary units, and it was normalized into thetotal structure factor S(Q). Q is diffraction vector, equal to4πsinθ/λ.

3. Results and Discussion

Figs. 1(a) and 2(a) show the typical microstructure of theunrefined Al-50Si alloy, which contains coarse plate-like pri-mary silicon particles. The average size of the unrefined pri-mary silicon is about 2–4 mm while it is about 30 μm in thewell-refined Al-50Si alloy (as shown in Figs. 1b and 2c). Themicrostructure observation of the unrefined alloys poured atdifferent temperatures demonstrates that the superheatingtreatment cannot refine the primary silicon particles underthis cooling rate. Fig. 2b–d shows that the refinement effectvaries with increasing superheating temperature after addi-tion of the 1% Si-20P master alloy. The Al-50Si alloy cannot berefined by the Si-20P alloy at 1100 °C, as shown in Fig. 2(b).Good refinement of primary silicon is achieved at 1300 °C, asshown in Fig. 2(c). Many nucleation substrates can be found inthe centre of the refined primary silicon particles. The EPMAresults show that the aluminum phosphide (AlP) is thenucleation substrate, as shown in Fig. 3. The total structurefactor S(Q) of the Al-50Si alloy in the temperature ranges from1100 °C to 1300 °C is shown in Fig. 4. The liquidus temperatureof the Si-20P alloy is about 1300 °C as shown in the Si–P phasediagram (Fig. 5).

Silicon atoms diffused into the melt when the Si-20Pmaster alloy was added to the Al-50Si melt. Some phosphorusatoms dissolved and reacted with aluminum. The solventphosphorus atom separated from the melt due to its limitedsolubility at a low melting temperature. It can be concludedthat the size and morphology of primary silicon are greatlyaffected by the phosphorous addition and melting tempera-ture. Nucleation and growth play an important role in theprimary silicon refinement process. The liquidus temperatureof the Al-50Si alloy is approximately 1040 °C, but the well-refined temperature is about 1300 °C. The minimal over-heating degree ΔTmin (ΔTmin is defined as the difference be-tween the overheating temperature Tmin and the liquidustemperature TL) for the refinement of the Al-50Si alloy is about260 °C. Bian and Wang [10] have investigated the influence ofsuperheating on the Al–Si alloy. The liquid structure changeswhen the melt is overheated to a certain temperature.Sufficiently overheating temperature is necessary for goodrefinement. The massive homogeneous nucleation tempera-ture is 1.2Tm (Tm is the equilibrium melting point) in thesuperheating crystal according to the literature [11]. The Al–Si–P ternary melt is constituted by aluminum atoms, siliconatoms, phosphorus atoms, aluminum atomic clusters, siliconatomic clusters and aluminum–silicon cluster et al. Theamount of solvent phosphorus atoms in the melt increaseswith increasing melting temperature. Some solvent phos-phorus atoms that separate out from the melt after the

Fig. 2 –SEM micrographs of Al-50Si alloy: (a) unrefined; (b–d) refined by 1% Si-20P master alloy at 1100 °C, 1300 °C and 1500 °C,respectively.

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formation of primary silicon have no effect on the nucleation,as shown in Fig. 3. Therefore, sufficient phosphorus must beadded to the high temperature Al–Si melt for the nucleation ofprimary silicon particles.

The peaks of the total structure factor S(Q) implied thestructure of the Al-50Si alloy melt to some extent. The shortrange order of Al and Si in S(Q) curves was investigated byQin Jingyu et al and Gu Tingkun et al, respectively [12,13].Aluminum and silicon have some particular characteristicsin their S(Q) curves. The height of the first peak and theshape of the second peak about S(Q) curve are relation to theshort range order in the melt. The first peak of S(Q) at 1100 °Cis higher than that at 1300 °C. So the short range order of themicrostructure is strong in the melt at 1100 °C, and it is weakin the melt at 1300 °C. The second peak of the S(Q) impliesthe short range order of Al in the melt. The shoulder of thesecond peak implies the short range order of Si, which be-comes weak in the melt at 1300 °C, as shown in Fig. 4.Accordingly, silicon atoms have intense interaction force atthe low overheating degree (60 °C) in the Al-50Si melt. Alu-minum phosphide particles and solvent phosphorus atomshave a limited refinement effect on the strong short range

order of silicon, which is the reason for the formation ofthe plate-like primary silicon particle at a low overheatingtemperature.

4. Conclusions

A refinement technology about primary silicon particles of Al-50Si alloy using a newly developed Si-20P master alloy ispresented. It has been found that both the melting tempera-ture and the adding phosphorous amount play importantroles in the refinement process, and the minimal overheatingdegree ΔTmin for the refinement of the Al-50Si alloy usingphosphorus is about 260 °C. The primary silicon particles canbe well-refined when adding 0.2 wt.% phosphorus at 1300 °C.The mean size of the unrefined primary silicon particles isabout 2–4 mm while that of the well-refined particle is about30 μm. The structural variation of the Al-50Si melt, which isrelated to the refinement, occurs with increasingly meltingtemperature. The short range order of silicon becomes weakwhen themelting temperature ranges from 1100 °C to 1300 °C,which is the main reason for the necessary of high

Fig. 3 –EPMA analysis of the nuclei of primary Si particles: (a) SEI of the primary Si; (b–d) X-ray images for elements Al, Si and P.

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overheating temperature to refine the primary silicon parti-cles in the Al-50Si alloy.

Acknowledgements

This work was supported by National Science Fund forDistinguished Young Scholars (No.50625101) Key Project of

Fig. 4 –The total structure factor S(Q) of the Al-50Si alloy.

Science and Technology Research of Ministry of Education ofChina (No. 106103) and Shandong Natural Science Foundation(No. Z2004F03). The authors are grateful to Brad Patterson forthe useful discussions.

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