J. Cent. South Univ. (2012) 19: 1503-1507

DOI: 10.1007/s11771-012-1168-1

Dispersion of SiC particles in mechanical stirring of A356-SiC_p liquid

ZHANG Jun(张君), ZHANG Peng(张鹏), DU Yun-hui(杜云慧), YAO Sha-sha(姚莎莎)

School of Mechanical and Control Engineering, Beijing Jiaotong University, Beijing 100044, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: In order to clarify the dispersion of SiC particles in straight-blade mechanical stirring of Al-SiC_p liquid, the dispersion of SiC particles in A356-3.5% SiC_p (volume fraction) liquid in a cylindrical crucible was studied. The relationship between rotating speed of stirrer and radial relative deviation of SiC_p content in A356 liquid between the center and the periphery of crucible was established in the conditions of 35° for the gradient angle α of blade and 10 mm/s for the speed of moving up and down of stirrer. The results show that the radial relative deviation of SiC_p content increases gradually with increasing the rotating speed of stirrer. When the rotating speed of stirrer is 200 r/min, the vertical dispersion of SiC particles in A356 liquid is even, but the radial relative deviation of SiC_p content is 0.24. Consequently, the nonhomogeneous dispersion of SiC particles in A356 liquid is mainly resulted from the nonhomogeneous radial dispersion of SiC particles.

Key words: dispersion; straight-blade mechanical stirring; A356-SiC_p liquid

1 Introduction

Metal matrix composites possessing excellent integrated performance are attractive in numerous fields. Al-SiC_p composites own high specific strength, high specific stiffness, small coefficient of thermal expansion and good wear resistance [1-5], and are widely used, especially in automotive, aerospace and sports fields [6-10].

The most economical technique to process Al-SiC_p composites is stir casting technique which uses Al-SiC_p liquid stirred by straight-blade mechanical stirrer to cast Al-SiC_p composites [11]. However, for stir casted Al-SiC_p composites, SiC particles always disperse nonhomogeneously in aluminum alloy matrix. Thus, other further techniques such as extrusion processing and squeeze casting have to be adopted to ameliorate the dispersion of SiC particles in aluminum alloy matrix [12-13]. The structure of Al-SiC_p composite is improved by these further techniques, but the processing of Al-SiC_p composite becomes complicated.

SiC particles are dispersed into the aluminum alloy liquid under the shearing action induced by straight- blade stirrer [14]. This shearing action can be enlarged with increasing the rotating speed of stirrer. The faster the rotating speed of stirrer is, the larger the shearing action is and the more homogeneous the dispersion of SiC particles should be. Nevertheless, no homogeneous dispersion of SiC particles can be achieved in straight-blade mechanical stirring, no matter how fast the rotation of stirrer is.

The nonhomogeneous dispersion of SiC particles in Al-SiC_p liquid will remain and result in the nonhomogeneous structure of cast Al-SiC_p composite. So far, the reason for nonhomogeneous dispersion of SiC particles in Al-SiC_p liquid is not clear. In this work, several new means were taken to study the dispersion of SiC particles in A356-3.5%SiC_p (volume fraction) liquid stirred by a straight-blade stirrer. The influence of straight-blade mechanical stirring on the nonhomogeneous dispersion of SiC particles was determined. The reason for nonhomogeneous dispersion of SiC particles in Al-SiC_p liquid was revealed.

2 Experimental

The materials used in this work are 625-mesh black SiC powder (about 20 μm) and commercially available casting A356 alloy (containing 6.3%Si, 0.2%Mg, 0.04%Cu, 0.015%Zn, 0.037%Ni, 0.15%Fe, 0.19%Ti and 93.068%Al, mass fraction).

The experimental equipment is shown in Fig. 1. Stirrer, driven by rotating motor with 3 kW rating power via rotating gearing, is used to stir A356-SiC_p liquid in a 200 mm-inner diameter stainless steel crucible which is

Fig. 1 Drawing of experimental equipment: (a) Whole setup configuration; (b) Cross section of A-A in (a); (c) Cross section of B-B in (a); 1-Stirrer; 2-Crucible; 3-Cooling-pipes; 4-Heating-rods; 5-Cover; 6-Ar gas pipe; 7-Thermocouple; 8-Al-SiC_p liquid; 9- Center stopple; 10-Lower side stopple; 11-Upper side stopple; 12-Lower bracket; 13-Water tank; 14-Rotating motor; 15-Rotating gearing; 16-Moving board; 17-Moving screw; 18- Moving gearing; 19-Moving motor; 20-Upper switch; 21-Lower switch; 22- Upper bracket

fixed on lower bracket. The vertical movement of stirrer, at a distance of 200 mm between the surface and the bottom of A356-SiC_p liquid in crucible, is accomplished by moving board via a moving screw driven by moving motor and moving gearing under the controlling of upper switch and lower switch on the upper bracket. Thermocouple, cooling-pipes and heating-rods in the wall of crucible are used to control the temperature of A356-SiC_p liquid. Ar gas pipe inserted in the cover on crucible is employed to avoid the oxidation of A356-SiC_p liquid. Center stopple, lower side stopple and upper side stopple are utilized for pouring the resulting A356-SiC_p liquid at the center and the periphery of crucible into the water tank to prepare the water quenching ingots.

The straight-blade stirrer made of heat-resistant ceramic is displayed in Fig. 2. Four 8 mm-thick flat blades B₁, B₂, B₃ and B₄ are normal to the axis of stirring rod (R) with a 40 mm diameter at the square lower end  (S) whose four sides are the tangents of the stirring rod. Each blade allocates along the diagonal of side of square lower end, making an angle α of 35° with the horizontal plane. D direction is the rotating direction of stirrer. The length between the center of rotation (O) and the ultimate outer point (T) (as shown in Fig. 2(b)) is 95 mm.

The experimental procedure is as follows:

1) Preparing A356 liquid. The temperature of A356 liquid was held at 700 °C after degassing.

2) Mixing SiC particles and A356 liquid. Firstly, SiC particles were poured into the crucible which was preheated to 500 °C by the heating-rods in the wall of crucible. Secondly, the SiC particles at the bottom of crucible were flatted and a 0.5 mm-thick 200 mm- diameter A356 cushion was put on the SiC particles. Then, the above A356 liquid was poured in crucible. Lastly, the cover on crucible was fastened and Ar gas was poured via the Ar gas pipe to avoid the oxidation of A356-SiC_p liquid.

3) Stirring A356-SiC_p liquid. The stirrer was switched on at a required rotating speed to stir the A356-SiC_p liquid whose temperature was held at 650 °C using the cooling-pipes and the heating-rods in the wall of crucible under the controlling of thermocouple. The temperature precision was ±1 °C. The speed of moving up and down of stirrer was 10 mm/s.

Fig. 2 Illustration of straight-blade stirrer: (a) Side view;     (b) Bottom view

4) Casting A356-SiC_p ingot. 8 min later, the stopples were pulled out and A356-SiC_p liquid was poured into the water tank to prepare the water quenching ingots for determining the dispersion of SiC particles in A356 liquid. The water quenching ingots of the A356-SiC_p liquid at the center of crucible top were prepared using a ladle.

5) Conducting microstructural analysis. The resulted A356-SiC_p ingots were sectioned with a wire cut machine into metallographic samples with dimension of 10 mm ×10 mm × 5 mm. A grinding machine was employed for the grinding of samples using a grinding disc with 30 μm diamond suspended in water lubricant and, the polishing, with 9, 6, 3 and 1 μm diamond suspended in water as a lubricant. Keller’s reagent was used to etch the polished sections. The metallographic samples were investigated by an OLYMPUS BX61 optical microscope attached with Clemex Vision Image analyzer to study the dispersion of SiC particles in A356 liquid.

3 Results and discussion

3.1 Relationship between rotating speed of stirrer and radial relative deviation

According to the experimental data (as shown in Table 1), the relationship between rotating speed v of stirrer and radial relative deviation d of SiC_p content in A356 liquid between the center and the periphery of crucible along the radial of crucible is obtained (as shown in Fig. 3). The radial relative deviation d of SiC_p content in A356 liquid between the center and the periphery of crucible along the radial of crucible, used for describing the radial dispersion of SiC particles in A356 liquid quantitatively, is calculated according to the follow equation:

                               (1)

where c_p is the SiC_p content in A356 liquid at the periphery of crucible, c_c is the SiC_p content in A356 liquid at the center of crucible. If SiC particles disperse homogeneously along the radial of crucible, c_p will equal c_c, and d will be 0. If SiC particles disperse nonhomogeneously along the radial of crucible, c_p will not equal c_c and d will be larger than 0. The more nonhomogeneous the radial dispersion of SiC particles becomes, the larger the d value will be. Thus, the radial relative deviation of SiC_p content in A356 liquid can describe the radial dispersion of SiC particles in A356 liquid quantitatively. From Fig. 3, it can be seen that the radial relative deviation of SiC_p content in A356 liquid at crucible bottom is very close to that at crucible top. After regressive analysis of the average d values of those at crucible bottom and top using nonlinear theory, the regressive equation is

d=0.051+0.001 2v-8.65×10^-7v²                  (2)

Table 1 Rotating speed v of stirrer and SiC_p content

Fig. 3 Rotating speed v of stirrer and radial relative deviation d of SiC_p content in A356 liquid at center and periphery of crucible

The regression coefficient R_l is 0.994 66. This indicates that regressive Eq. (2) has built a correct relationship between v and d.

From Fig. 3, the radial relative deviation of SiC_p content in A356 liquid increases with increasing the rotating speed of stirrer. The greater the rotating speed of stirrer is, the larger the radial relative deviation of SiC_p content becomes, i.e., the more nonhomogeneous the radial dispersion of SiC particles is. In addition, from Table 1, the SiC_p content in A356 liquid at the periphery of crucible top increases with increasing the rotating speed of stirrer and when the rotating speed of stirrer is 200 r/min, the SiC_p contents at the center and the periphery of crucible bottom are equal to those at the center and the periphery of crucible top correspondingly, that is, SiC particles can disperse homogeneously along the vertical direction although the radial dispersion of SiC particles is nonhomogeneous. Therefore, for straight-blade mechanical stirring A356-SiC_p liquid, when the stirring conditions are 35° for gradient angle α of blade, 200 r/min for rotating speed of stirrer and     10 mm/s for speed of moving up and down of stirrer, the nonhomogeneous dispersion of SiC particles in A356 liquid is primarily resulted from the nonhomogeneous radial dispersion of SiC particles.

3.2 Discussion

In straight-blade mechanical stirring of A356-SiC_p liquid, SiC particles are dispersed in A356 liquid gradually under the shearing action of stirrer. The surface of blade, which inclines of 35°, can generate a suitable rising movement of SiC particles and the SiC particles at the bottom of crucible can rise up into the whole A356 liquid following the vertical movement of stirrer. However, a centrifugal movement of SiC particle will also happen when the resistance of A356 liquid to SiC particle is smaller than the following centrifugal force of SiC particle [15]:

F=m×ω²×r                                  (3)

where m is the mass of SiC particle; ω is the rotating angular speed of SiC particle with A356 liquid and r is the distance of SiC particle from the central axis of crucible. It can be seen that the centrifugal movement of SiC particles, which may generate the nonhomogeneous radial dispersion of SiC particles in A356 liquid, can always exist in the straight-blade mechanical stirring of A356-SiC_p liquid under high rotating speed of stirrer.

In this work, the 0.5 mm-thick 200 mm-diameter A356 cushion on the SiC particles at the bottom of crucible could eliminate the impact of A356 liquid turbulence on the SiC particles at the bottom of crucible and ensure a uniform dispersion of SiC particles at the bottom of crucible before stirring, thus the experimental radial relative deviation of SiC_p content in A356 liquid at the center and the periphery of crucible could quantitatively characterize the radial dispersion of SiC particles in straight-blade mechanical stirring.

In this work, when the rotating speed of stirrer is small, such as 60 r/min, the shearing action of stirrer to A356-SiC_p liquid is weak and the rising movement of SiC particles generated by the inclining blade is slight, also the centrifugal movement of SiC particles is tiny. Thus, lots of SiC particles remain at the bottom of crucible and the radial relative deviation of SiC_p content is small. With increasing the rotating speed of stirrer, the shearing action of stirrer, the rising and the centrifugal movement of SiC particles all increase. Thus, the SiC_p content enlarges at the periphery of crucible top gradually, and the radial relative deviation of SiC_p content also increases. The greater the rotating speed of stirrer is, the larger the SiC_p content becomes at the crucible top and periphery, hence the larger the radial relative deviation of SiC_p content is and the more nonhomogeneous the radial dispersion of SiC particles becomes. Specially, when the rotating speed of stirrer is 200 r/min, the SiC contents at the center and the periphery of crucible bottom are equal to those at the center and the periphery of crucible top correspondingly, that is to say, SiC particles disperse homogeneously along the vertical direction of crucible under this condition. But the nonhomogeneous radial dispersion of SiC particles, whose radial relative deviation of SiC_p content is about 0.24 (as shown in Fig. 3), still exists. Figure 4 shows the micrographs of A356-3.5%SiC_p liquid stirred by straight-blade mechanical stirrer in the conditions of 35° for gradient angle α of blade, 200 r/min for rotating speed of stirrer and 10 mm/s for speed of moving up and down of stirrer. The dark parts are SiC particles, and the other is A356 matrix. It can be seen that the nonhomogeneous radial dispersion of SiC particles is remarkable. Thus, the nonhomogeneous dispersion of SiC particles in A356 liquid is resulted from the nonhomogeneous radial dispersion of SiC particles in straight-blade mechanical stirring ultimately. If the nonhomogeneous radial dispersion of SiC particles can be eliminated, the dispersion of SiC particles in A356 liquid will be uniform.

Fig. 4 Micrographs of A356-3.5%SiC_p liquid stirred by straight- blade stirring: (a) Center of crucible bottom; (b) Periphery of crucible bottom

4 Conclusions

1) Under the conditions of 35° for gradient angle α of blade and 10 mm/s for speed of moving up and down of stirrer, the relationship between rotating speed v of stirrer and radial relative deviation d of SiC_p content in A356 liquid at the center and the periphery of crucible along the radial of crucible in straight-blade mechanical stirring of A356-3.5%SiC_p liquid is d=0.051+0.001 2v- 8.65×10^-7v².

2) In straight-blade mechanical stirring, the nonhomogeneous dispersion of SiC particles in A356 liquid is resulted from the nonhomogeneous radial dispersion of SiC particles ultimately. If the nonhomogeneous radial dispersion of SiC particles can be eliminated, the dispersion of SiC particles in A356 liquid will be uniform.

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(Edited by HE Yun-bin)

Foundation item: Project(50974010) supported by the National Natural Science Foundation of China; Project(3093023) supported by the Natural Science Foundation of Beijing, China; Project(2009JBM091) supported by the Fundamental Research Funds for the Central Universities of China

Received date: 2011-09-18; Accepted date: 2011-10-28

Corresponding author: DU Yun-hui, Associate Professor, PhD; Tel: +86-10-51682226; E-mail: pzhang1@bjtu.edu.cn