J. Cent. South Univ. Technol. (2011) 18: 1789-1794

DOI: 10.1007/s11771-011-0903-3

Microstructure evolution of Al-Si semi-solid slurry by

gas bubble stirring method

ZHANG Lei(张磊)¹, DONG Xuan-pu(董选普)¹, LI Ji-qiang(李继强)², LI Kan(李侃)¹,

ZHANG Zong-kui(张宗奎)¹, WANG Wen-jun(王文俊)¹, FAN Zi-tian(樊自田)¹

1. State Key Laboratory of Material Forming and Mould and Die Designing,

Huazhong University of Science and Technology, Wuhan 430074, China;

2. School of Mechanical and Energy, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: A novel technique of introducing gas bubble stirring during solidi?cation was studied to prepare Al-Si semi-solid slurry. The microstructure evolution of the slurry during slow cooling process after stirring was investigated. The effects of the solidification rate on the microstructure of the semi-solid slurry were investigated under three different solidification conditions. The results show that fine non-dendritic slurry can be obtained using the gas bubble stirring method. Ripening and coarsening of primary Al grains are observed during the slow cooling process, and at last coarsened eutectic Si appears. Primary Al grains with different sizes and eutectic Si are obtained, corresponding to three different solidification rates.

Key words: gas bubble stirring; semi-solid slurry; solidification rate; microstructure

1 Introduction

Semi-solid metal processing technology was ?rst proposed by SPENCER et al [1]. Compared with traditional casting and forging process, semi-solid forming has advantages such as lower processing temperature, smaller deformation resistance, less macro-segregation and solidi?cation shrinkage [2-4]. Various techniques of semi-solid metal processing have been developed, such as mechanical stirring [5], electromagnetic stirring [6-7], strain-induced melt activation [8], ultrasonic vibrations [9], cooling slope casting [10], liquidus casting [X1] [11] and melt conditioner direct chill casting [12]. However, each of these techniques has its disadvantages such as high cost and energy consumption, inhomogeneous or impure slurry, or the process is too complicated and difficult to control. Therefore, novel methods of preparing semi-solid slurry still need to be explored, in order to reduce the production cost and simplify the process.

For rheoforming, two processes are involved, the first process is the slurry making, and the second process is the forming of the slurry in the mold. Several studies have shown that both the slurry making and the slurry forming processes are important because solidi?cation of the slurry happens, which directly relates to the microstructure and corresponding mechanical properties of the ?nal rheoformed parts [13]. ATKINSON and LIU [14] had investigated microstructural coarsening of semi-solid aluminum alloys during slurry making. GUO et al [15] had studied pressurized solidi?cation of A356 semi-solid aluminum in die casting, their results showed that higher density and mechanical properties of the final parts could be obtained with the increase of filling pressure. However, there are not much researches on the microstructure evolution of the slurry during holding process after stirring, particularly the morphology evolution of eutectic silicon. Also few studies have focused on the effects of different solidification conditions on the microstructure of the solidi?ed slurry, including the morphologies of primary Al and eutectic silicon.

In the present work, the gas bubble stirring technique was applied to achieving semi-solid slurry. This process used gas bubbles as the medium to stir the melt during the initial stage of solidi?cation. A rotating tube-like graphite agitator was applied, which is different from the static graphite diffuser used by WANNASIN et al [16]. Three different solidification conditions were obtained by pouring the semi-solid slurry into an iron mold, a copper mold cooled in water, and directly quenching in water. The microstructure evolution of the slurry during holding process after stirring and the morphologies of primary Al and eutectic Si of the final solidified slurry were reported.

2 Experimental

The alloy used in this study was an Al-Si alloy, with the following chemical composition (in mass fraction): 8.72% Si, 0.12% Mg, 0.09% Fe, and Al is the balance. The solidus and liquidus temperatures were detected by differential scanning calorimetry to be 565 °C and    614 °C, respectively. The alloy was melted in an electric furnace. A thermocouple was inserted near the center of the melt to record the temperatures during the experiments.

Figure 1 shows a schematic diagram of the apparatus employed. A tube-like graphite agitator was used, which had small holes with the diameter of 1 mm on its wall. The graphite agitator was connected to an argon gas cylinder through a hollow stirring rod. The agitator and the rod were rotated by an electric motor via a transmission belt. So, argon gas could be introduced into the melt by the rotating graphite agitator. Under the buoyant force of the melt and the centrifugal force induced by the rotating graphite agitator, the gas bubbles climbed up and took a spiral-like motion curve in the melt, which generated a strong convection and a weak stirring effect to the melt.

Fig.1 Schematic diagram of apparatus

The alloy was first melted and superheated to   740 °C, then cooled down. The graphite agitator was lowered to just above surface of the melt to be preheated when the temperature was about 40 °C above the liquidus temperature. The agitator was immersed when the melt cooled to 0-15 °C above the liquidus temperature. At the same time, the argon gas was imported and the electric motor was started. Pure argon gas with a flow rate of about 5 L/min was introduced into the melt through the rotating graphite agitator. The rotation speed of the agitator was about 130 r/min. The stirring process was carried out for about 3 min before the agitator was quickly removed, and then the melt was holding isothermally. Samples of the melt were removed and quenched immediately in water at different holding time. In another experiment, the semi-solid slurry was poured immediately into different molds after stirring, and thermocouples were placed near the center of the molds to collect temperature readings, which were used to plot temperature curves of the melt. Metallographic examinations of the samples were performed using an optical microscope and a scanning electron microscope (JSM-5610LV). The constituent phases of the semi-solid slurry were identi?ed by X-ray diffraction (D8 X-ray diffractometer).

3 Results and discussion

3.1 Micrographs corresponding to different holding time and slurry temperatures

Figure 2 shows representative microstructures of Al-Si slurry with and without stirring quenching at   610 °C. The microstructure of the melt without stirring is given in Fig.2(a) as a reference, showing typical dendritic structure, and most of the primary Al particles are dendrites. However, with gas bubble stirring, non-dendritic grains are clearly observed, as shown in Fig.2(b). This indicates that gas bubble stirring has an important in?uence on the formation of spheroidal primary particles. This is consistent with the work of WANNASIN et al [16]. Semi-solid slurry with well spheroidal primary particles can be obtained with suitable processing parameters.

Fig.2 Representative micrographs of Al-Si slurry with and without stirring quenching at 610 °C: (a) Solidi?ed under normal conditions; (b) Stirred for 3 min, and then holding for 60 s

The temperature curve of the semi-solid slurry in holding process after stirring is shown in Fig.3. A cooling rate of about 0.09 °C/s can be roughly estimated from the curve. Figure 4 shows representative micrographs of the slurry in holding process after stirring at 625 °C for    3 min, corresponding to different holding time and  slurry temperatures. It can be seen that ripening and coarsening of the primary Al particles happen during this process, and coarsened eutectic Si appears after longer holding time. Numbers of fragmented grains are formed after gas bubble stirring, as shown in Fig.4(a). This can be explained by the dendrite multiplication mechanism. Numerous nuclei will be formed around the graphite agitator due to the cooling effect of the agitator and argon gas, and these nuclei will grow into small mother dendrites. The dendrites are fragmented and transported to every part of the melt by the strong convection and weak stirring effect, resulting in dendrite multiplication. Small fragmented particles (Fig.4(a)) grow up and ripen into near globular particles after holding for a while, as shown in Figs.4(b), (c), and (d). A fully non-dendritic structure is achieved when holding for 60 s, as shown in Fig.4(c). It has been investigated by NAFISI and GHOMASHCHI [17] that solid particles with any morphology in the semi-solid state will become more spheroidal and coarse under the driving force for reduction of interfacial free energy, if they are held isothermally for an extended time. Primary Al particles are coarsened, as shown in Figs.4(e), (f), (g), and (h). It is interesting to note that nucleation still occurs in the bulk liquid during the holding process, as shown in Figs.4(e), (f), (g), and (h). When the slurry was held for 240 s to the temperature of 588 °C, coarse eutectic Si appears in the remaining liquid, as shown in Fig.4(i), and this will deteriorate the properties (such as fluidity) of the semi-solid slurry. The phase composition of the Al-Si semi-solid slurry determined by XRD analysis is shown in Fig.5, which mainly consists of primary α-Al and eutectic Al+Si+Al_3.21Si_0.47.

Fig.3 Temperature curve of semi-solid slurry in holding process
 

Fig.4 Representative micrographs of slurry in holding process after stirring at 625 °C for 3 min: (a) 0 s, 609 °C; (b) 30 s, 608 °C;   (c) 60 s, 607 °; (d) 90 s, 605 °C; (e) 120 s, 602 °C; (f) 150 s, 600 ?C; (g) 180 s, 597 ?C; (h) 210 s, 592 ?C; (i) 240 s, 588 ?C
 

Fig.5 X-ray diffraction pattern of Al-Si semi-solid slurry (quenched in water)

3.2 Micrographs corresponding to different solidifica- tion rates

Figure 6 shows a schematic diagram of the molds used in this study. Thermocouples were installed near the center of the molds to record the temperatures to plot solidification curves of the slurry. Figure 7 represents the solidification curves of the slurry in three different molds. Solidification rates of about 0.55, 1.5 and 5.25 °C/s can be roughly estimated from the curves, corresponding to Figs.7(a), (b), and (c), respectively. The morphologies of the slurry solidified under these rates are shown in Fig.8. It is obvious that both the morphologies of the primary Al and the eutectic Si are different. The eutectic Si coarsens like laths and disorderly distributes in the Al matrix when the solidification rate is slow, as shown in Fig.8(a). The eutectic Si is refined like needles, and distributes along the primary Al grain boundaries, with the increase of the solidification rate, as shown in Fig.8(b). With the solidification rate of 5.25 °C/s, as shown in Fig.8(c), the eutectic Si is extremely refined. Figure 8(d) shows the morphology of the eutectic Si in Fig.8(c), and small dendritic Si is observed. At the same time, the primary Al grains in Fig.8(c) are smaller than those in Figs.8(a) and (b). The above results indicate that a mold with fast enough solidification rate is needed, in order to obtain final parts with semi-solid microstructure.

The different microstructures under different solidification conditions can be explained by the growth of primary Al particles and the concentration of solute element Si in the bulk liquid. Studies have shown that Si will be rejected into the remaining liquid with the growth of primary Al particles, so the liquid becomes more and more saturated in alloying elements, and has a lower liquidus temperature [18]. Large number of eutectic Si nucleation would appear in the remaining liquid, and globular Al particles without much growth are captured, if the solidification rate is fast enough, as shown in Fig.8(c). On the other hand, globular Al particles would grow bigger and bigger until they connect with each other, if the solidification rate is slow (Fig.8(a)). At the same time, the solute Si is extremely concentrated in the remaining liquid and at last coarse eutectic Si is formed.

Fig.6 Schematic diagrams of molds (not to scale): (a) Iron mold;   (b) Copper mold cooled in water; (c) Water quenching

Fig.7 Solidification curves of slurry in three different molds

Fig.8 Morphologies of semi-solid slurry solidified under different rates: (a) 0.55 ?C/s; (b) 1.5 ?C/s; (c) 5.25 ?C/s; (d) Morphology of eutectic silicon in (c)
 

4 Conclusions

1) It is feasible to use the gas bubble stirring method to prepare semi-solid slurry with suitable processing parameters, as strong convection and weak stirring effect are introduced in the melt by the argon gas.

2) Ripening and coarsening of primary Al grains happen during the holding process after stirring, and at last coarsened eutectic Si appears, which detracts the properties of the slurry, reminding us that the rheoforming process must be quick enough in order to maintain good properties of the slurry.

3) Primary Al grains with different sizes and eutectic Si are obtained, corresponding to three solidification rates, indicating that the solidification rate of the rheoforming mold must fast enough in order to get final parts with semi-solid microstructure.

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(Edited by HE Yun-bin)

Foundation item: Project(50775085) supported by the National Natural Science Foundation of China; Project(M2009061) supported by Special Fund for Basic Research and Operating Expenses of Central College, China; Project(2008A610049) supported by the Natural Science Foundation of Ningbo City, China

Received date: 2010-09-07; Accepted date: 2011-02-05

Corresponding author: DONG Xuan-pu, Professor, PhD; Tel: +86-27-87558252; E-mail: xpdonghust@yahoo.cn