Distribution of Si particles in hypereutectic aluminum alloy tubes prepared under electromagnetic field
ZHANG Zhi-qing(张志清)1, 2, LI Qiu-lin(李丘林)2, LIU Wei(刘 伟)1, LIU Qing(刘 庆)1
1. Laboratory of Advanced Materials, Department of Materials Science and Engineering,
Tsinghua University, Beijing 100084, China;
2. Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
Received 28 July 2006; accepted 15 September 2006
Abstract: Hypereutectic Al-Si alloy tubes were produced by centrifugal casting process using an electromagnetic field (EMF). A gradient distribution of the primary Si particles was formed along the tube thickness direction. In the absence of EMF the primary Si moves to inner periphery with increasing rotation speed. The distribution of primary Si can be controlled by the EMF. With increasing electromagnetic field intensity, the primary Si moves from the inner periphery to the outer periphery of the tubes. Most of the primary Si can be driven to the outer if the electromagnetic field intensity is increased to a certain value. It is found that the particle distribution and local volume fraction vary with both the rotation speed and the electromagnetic field intensity.
Key words: Al-Si alloy; centrifugal casting; electromagnetic field (EMF); gradient distribution
1 Introduction
Al-Si alloys are some of the most widely used materials for the casting of domestic, military, automotive, and aerospace components [1-2]. The hypereutectic Al-Si alloy has been developed as a kind of in-situ composite. The primary Si particles in this alloy act as the in-situ reinforcement phase, resulting in excellent physical and mechanical properties. The primary Si in hypereutectic Al-Si alloys has a high microhardness (HV 1 000-1 300), while the phase α(Al) has a much lower microhardness (HV 60-100).
To produce a wear resistant alloy that has considerable strength, a component with a heterogeneous microstructure can be designed. Such component should have a high volume fraction of hard particles at the surface (or outer side) where better wear properties are needed, and a gradually decreasing volume fraction of hard particles below the surface (or on the inner surface) to provide good strength and ductility. The centrifugal casting technique is, therefore, a good choice for the preparation of materials with good tribological properties such as engine cylinders. Efforts have been made to produce microstructurally-graded materials by centrifugal casting, e.g. Al-SiC[3-4], Al-Al3Ni[5], and Al-Al3Ti[6]. In many aluminum alloys containing particles, the density of the reinforcement particles is larger than that of the matrix. The hypereutectic Al-Si alloy system is one possible system for the production of tubes with a concentration of hard particles across the tube. Because the density of reinforcement Si is lower than that of Al, the production of such microstructurally graded tubes is impossible by means of traditional centrifugal casting.
It is well known that the electromagnetic filed can exert a strong influence on the segregation of various elements in alloys during the electromagnetic stirring (EMS) solidification process[7-9]. In previous researches[10-11] it is considered that EMS enhances the liquid flow in front of solid/liquid interface, and changes the migration of the solutes in the melt. Therefore the distribution of particles in an alloy matrix can be controlled by the combined process of centrifugal casting and EMS. By this approach it is expected that tubes with good wear resistance at the outer periphery can be produced.
2 Experimental
Pure Al（99.7%）and pure Si ( 99.9%) were used as the starting materials to prepare a Al-Si hypereutectic binary alloy. Two groups of tubes were produced in the present research. In the first group, the melt was centrifugally cast in a pre-heated stainless steel mold at about 500 ℃ with different rotating speeds of 1 500, 1 800, 2 100 and 2 400 r/min. In the second group, the castings were prepared by using the same pre-heated stainless steel mold at the same preheat temperature but using a electromagnetic field intensity of different values at a fixed rotation speed of 2 400 r/min. In each case, tubes with 60 mm in outer diameter, approximate 10 mm in thickness and about 100 mm in length were obtained. A schematic illustration of the casting set-up is shown in Fig.1.
Fig.1 Schematic illustration of centrifugal casting set-up with DC EMF: (a) Centrifugal casting under EMF; (b) Design of solidification mold
All tubes were cast at 1 023 K. For each casting, the mold was rotated at the set speed before casting and for at least 2 min after the liquid metal was poured into the mold to allow adequate solidification.
The microstructure of each casting was examined by optical microscopy. The volume fraction of Si particles was measured by estimating their area. Measurements were made on cross-section of the centrifugal tubes along the radial direction. The particle segregation profile along the radial direction was obtained from the average of at least five measurements in each selected regions.
3 Results and discussion
3.1 Morphologic differences of matrix without and with EMF
The matrix of the sample without EMF can be divided into three layers from the outer to the inner. The first layer contains some Si particles at the surface. In the second layer dendrites are clearly observed. In the third layer a higher Si particle density is seen. This arrangement is illustrated in Fig.2 (a).
Fig.2 Difference in morphology along radial direction of tubes: (a) Without EMF; (b) With EMF
For casting with an applied EMF (corresponding to an induced current of 10A), the morphology is changed greatly. The particles of Si are distributed more homogenously and α(Al) changes from a dendrite to rose-like morphology, as seen in Fig.2 (b).
The particles on the outer periphery in the sample without EMF are probably caused by the abrupt cooling when the molten alloy is poured into the mold. Because the density of Si(about 2.33 g/cm3) is lower than that of the molt alloy (about 2.4 g/cm3), the centrifugal force on Si is lower than that on Al, then the Si particles are likely to move toward inner. The fact that the volume fraction of Si in the inner is larger than that in the outer is caused by the moving of Si during the solidification process without EMF.
The conductivity of Al is larger than that of the semiconductor Si, consequently the Lorentz force acting on Al is larger than that on Si, which results in a relative retardancy of Al. The particles and molten alloy are therefore likely to have a more similar rotation speed and thus solidify together, resulting in a homogeneous particle distribution in the matrix, when the Lorentz force on Al is increased to a certain value.
3.2 Particle distribution at different rotating speeds with and without EMF
A gradient in the distribution of plate-like Si particles along the radial direction is observed in all centrifugal casting tubes. Fig.3 shows the particle distribution profile for centrifugal tubes produced at different rotating speeds without EMF. The volume fractions at the inner and outer side are regarded as 0 and 1, respectively. The volume fraction at the inner surface is higher than that at the outer surface and is the lowest in the middle region for a rotation speed higher than 1 500 r/min. The volume fraction is the maximum in the middle region at rotation speed of 1 500 r/min. The peak volume fraction near the inner surface is most likely caused by the difference in rotation speed between Si and molten alloy not being large enough to move the particles towards the inner surface.
The particle distribution profile produced with different EMF intensities is shown in Fig.4.
Fig.3 Volume fraction distribution of Si without EMF
Fig.4 Volume fraction distribution of Si with EMF
The distribution of silicon can be understood by the combined effect of the centrifugal casting and the application of an EMF. Both Al and Si are subjected to Lorentz force, and the force exerted on Al by EMF is larger than that on Si, which leads to a relative slowing down of the rotation velocity of Al. This is counter-balanced by the effect of density on the rotation speed. At a certain value of the EMF the silicon particles can be driven from the inner to the outer of the casting.
4 Conclusions
1) The morphology of sample can be divided into three layers without EMF. The dendrites in the second layer is very clear, while the particles distribute homogenously and α(Al) becomes rose-like under EMF with the intensity of 10A.
2) The particles are driven from inner to outer with increasing intensity of EMF and the volume fraction achieves its peak when the intensity is 20A at the surface and changes little with increasing intensity.
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(Edited by YANG Bing)
Foundation item: Project (50474087) supported by the National Natural Science Foundation of China
Corresponding author: LIU Wei; Tel: +86-10-62772853; E-mail: liuw@mail.tsinghua.edu.cn
