Hypereutectic aluminium alloy tubes with graded distribution of Mg Si particles prepared by centrifugal casting

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1 Ž. Materials and Design Hypereutectic aluminium alloy tubes with graded distribution of Mg Si particles prepared by centrifugal casting Jian Zhang a,b,, Zhongyun Fan a, Yuqing Wang b, Benlian Zhou b a Department of Materials Engineering, Brunel Uni ersity, Uxbridge UB8 3PH, UK b Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China Received 9 September 1999; accepted 17 November 1999 Abstract Hypereutectic Al-Mg Si alloy tubes were produced by a centrifugal casting process. Primary Mg Si particles were formed in the tube periphery with graded distribution. Influence of the rotation speed and cooling rate on formation and graded distribution of Mg Si particles was discussed. It is found that in the outer periphery the particle distribution profile at different rotation speeds is quite different from each other, while it is similar in the inner periphery. Extremely higher cooling rates during centrifugal casting resulted in a better particle distribution gradient and a much finer microstructure in the outer periphery of the tube. 000 Elsevier Science Ltd. All rights reserved. Keywords: Al-Mg Si; Centrifugal casting; Graded distribution 1. Introduction Recently, some hypereutectic alloys have been developed as a kind of in situ composite 1 3. One of the most significant advantages of such materials is that the readily available casting and thermo-mechanical processing techniques developed for alloys can be directly applied to such composites. Among these alloy systems, the hypereutectic Al-Mg Si alloy has attracted more and more attention as a light, low cost material 4 8. The primary Mg Si particles in the alloy was used as the in situ reinforcement because of its excellent physical and mechanical properties compared with silicon Ž see Table 1.. The microstructure and mechanical properties of such as-cast material have been investigated previously 6,7, and advanced processing techniques such as rapid solidification 9, hot extrusion 4 Corresponding author. Tel.: ; fax: address: jian.zhang@brunel.ac.uk Ž J. Zhang.. and mechanical alloying 10 were applied for better properties. To produce a wear resistance alloy with considerable strength, an alloy with heterogeneous microstructure can be designed: a high volume fraction of hard particles at the surface where better wear properties are needed, and a gradually lowered volume fraction of hard particles below the surface where better strength and ductility can be achieved. The well-developed centrifugal casting technique is, therefore, the ideal processing for such a purpose where tribological surfaces are required in the periphery of the cylinders. Efforts have been made to produce the graded materials, e.g. Al-SiC 11,1, Al-Al Ni 13, and Al-Al Ti by centrifugal casting. However, among the aluminium alloys containing particles, the density of most particulate reinforcements is larger than that of the matrix. Thus, few systems are available for production of the tubes with a concentration of hard particles in the inner periphery of the tube through a centrifugal casting process. Meanwhile, good wear resistance is required in the inner periphery of a tube casting in most $ - see front matter 000 Elsevier Science Ltd. All rights reserved. Ž. PII: S

2 150 J. Zhang et al. Materials and Design Table 1 Physical and mechanical properties of Mg Si and Si a Material Density Strength T.E.C. Elastic modulus Melting point Ž 10 kg m. Ž MPa. Ž 10 K. Ž GPa. Ž K. MgSi Si a Thermal expansion coefficient cases from a practical point of view. Fortunately, the low density of Mg Si compared with aluminium alloy made this possible 15. Tubes with different Mg Si content in aluminium alloy matrix have been prepared by centrifugal casting in our previous work 15. Generally, the centrifugal tubes were found containing three parts: Ž. 1 fine particles in an aluminium alloy matrix near the outer periphery of the tube; Ž. a particle free region in the middle area of the tube wall; and Ž. 3 gradually distributed high volume fraction of Mg Si particles in the inside wall of the tube. However, the influence of processing parameters such as rotation speed and cooling rate have not been reported. The objective of the present work is, therefore, aimed at the analysis of the influence of processing parameters on the formation and graded distribution of the Mg Si primary particles.. Experiment Fig. 1. Schematic illustration of the centrifugal casting process: Ž. a graphite mould; and Ž b. copper mould with water cooling system. Industrially pure Al, Mg and Si were used as starting materials to prepare Al-15Mg Si hypereutectic alloys. ŽAll data given are in weight percent Ž wt. %., unless otherwise stated.. Detailed preparation procedure was described elsewhere 7,8,15. The melt was centrifugal cast in a cold graphite mould at different rotation speeds of 800, 100 and 1600 rev. min. Centrifugal tubes with 80 mm in outer diameter, 15 mm in thickness and 100 mm in length were obtained. Centrifugal casting of the same alloy was also conducted under a rapid cooling rate. A copper mould with a water cooling system during casting was used. The rotation speed was 1600 rev. min. Tubes with a 60-mm outer diameter, 15 mm in thickness and 60 mm in length were obtained. Schematically illustrated diagrams of the centrifugal sets are shown in Fig. 1. All alloys were cast at 1073 K. For all casting processes, the rotating mould was adjusted to 5 rev. min of the set rotation speed before casting and continued to be rotated for at least 3 min after the liquid metal was poured into it to allow adequate solidification. Microstructure was examined by optical microscopy. Fig.. Particle distribution in Al-15Mg Si hypereutectic alloy tubes obtained from different rotation speeds of the graphite mould.

3 Volume fraction of Mg Si primary particle in the cen- trifugal tube was measured by an image analysis system. The cross-section of a centrifugal tube along the radial direction was polished and divided into different regions every mm from the inner-to-outer surface. The particle segregation profile along the radial direction was obtained from the average of at least five measurements in each of these regions. J. Zhang et al. Materials and Design Results and discussion 3.1. Particle distribution at different rotation speeds A graded distribution of Mg Si in situ particles along the radial direction was observed in all centrifugal cylinders. Fig. illustrates the particle distribution profile in the centrifugal tubes obtained at different rotation speeds. Similar to our previous work 15, the centrifugal tubes can be divided into three parts according to their microstructural features. It seems that in the outer periphery, the particle distribution profile at different rotation speeds is quite different from each other. While in the inner periphery, not very much difference can be observed. The change from the outer surface that contained particles to the middle particle free area was rather abrupt in all samples. However, the change from the middle region to the inner region varies. In the composite with a rotation speed of 800 rev. min, the outer region that contained Mg Si parti- cles was approximately 1.5 mm in thickness. There was a gradual transformation from the particle free area to inner area in this sample when compared with other ones. However, more casting defects were observed in Fig. 3. Dendritic morphology of Mg Si primary phase in the outer region of a centrifugal tube obtained at 100 rev. min. Fig. 4. Calculated position of Mg Si particles during centrifugal casting conducted at different rotation speeds as a function of time and temperature. this sample due to an insufficient rotation speed. When the rotation speed increased, the thickness of the outer periphery containing particles increased. A similar trend in this area was observed in samples obtained at 100 and 1600 rev. min. The volume fraction of Mg Si in situ particles increased to a higher level and then dropped rapidly to zero. It is well known that the solidification rate increases with an increase in the rotation speed during centrifugal casting. Therefore, different cooling rates were achieved in the present experiments. Since the graphite mould was not pre-heated, the initial stage of solidification was rapid: Mg Si as a primary phase tends to crystallise as a dendritic structure as shown in Fig. 3. The movement of most of the Mg Si particles was very limited after they were produced in the liquid alloy at the outer surface of the centrifugal tube. A small amount of them might be broken from the dendritic crystals by impact of the liquid alloy and moved into the melt again. The particles formed by subsequent solidification, as well as the small amount of broken dendrites, were soon captured by the rapidly moving solid liquid interface when the cooling rate was sufficiently high. This resulted in the volume fraction peaks in the outer region of the tubes obtained at 100 and 1600 rev. min as shown in Fig.. With a higher cooling rate at 1600 rev. min, a smaller movement of the particles was allowed in the outer region. Therefore, the volume fraction peak in this alloy was in front of the peak in the tube obtained at 100 rev. min. When the cooling rate is low, i.e. at 800 rev. min, the movement of the solid liquid interface is not rapid enough to capture the moving particles, and this resulted in a continuous movement of these particles to the inner region of the centrifugal tubes. After the rapid cooling period at the outer surface, the liquid alloy was solidified at a lower speed. The

4 15 J. Zhang et al. Materials and Design movement of the solid liquid interface was nearly fixed at a constant speed 16. Then, the particle position, r as a function of time t in this solidification stage can be obtained by simple calculation. Using Stokes law, assuming the angular velocity is a constant and omitting the particle growth during solidification, r can be expressed as 17,18 : r r e A t where r is the position of particle at time t 0, A is the constant related to the viscosity of the melt for a given system. By a similar method illustrated by Zhang et al. 15, the particle position in the melt, at time t, in the centrifugal field can be calculated as seen in Fig. 4. It is found that the particle movement along the radial direction during centrifugal casting is completed within a short time. The particles move at a nearly constant speed at a given temperature in all cases, although the velocity is expressed as a function of time t: dr dt r A e A t. The in situ character of the system shows an apparent influence on the movement behaviour of the particles as various volume fractions of particles are produced during solidification. The change in volume fraction of the particles effects the viscosity of the melt significantly, which results in a significant difference of the moving speed of the particles at different temperatures. 3.. Particle distribution at different cooling rates Fig. 5 shows the particle distribution profile in the composite tubes obtained at different cooling rates. The volume fraction of Mg Si particles in the outer region increased significantly with an increase in the cooling rate. Furthermore, a much finer particle size was obtained at an extremely high cooling rate. Fig. 6 Fig. 6. Microstructures in outer surfaces of Al-15Mg Si alloy tubes obtained by using: Ž. a graphite mould; and Ž. b copper mould with water cooling. Fig. 5. Particle distribution profile in Al-15Mg Si centrifugal tubes Ž. obtained at different cooling rates 1600 rev. min. compares the morphology of the outer region in the Al-15Mg Si centrifugal tube obtained by using a graphite mould and a copper mould with a water cooling system. The particle size in this area reduced from 70 m to5 10 m according to Fig. 6. It is suggested that a better gradient in the inner region was achieved in the centrifugal tube with a higher cooling rate. This is because at the same rotation speed of the centrifugal mould, the influence of the centrifugal field on the particle-moving pattern is similar. However, a much larger temperature gradient in front of the liquid solid interface resulted in a different viscosity of the melt in the inner regions of the centrifugal tubes. The high viscosity reduced the movement of the Mg Si particles and induced a better

5 J. Zhang et al. Materials and Design graded distribution along the radial direction rather than an abrupt change in the tubes produced at a lower cooling rate. 4. Summary Hypereutectic Al-Mg Si alloy tubes with graded dis- tribution of the primary Mg Si particles were produced by centrifugal casting. Influence of rotation speed of the centrifugal mould and cooling rate during casting has been discussed. Lower rotation speed resulted in better gradient distribution of Mg Si primary particles, but also accompanied with higher volume fraction of the casting defect. With an increase in rotation speed an apparent change of the particle distribution profile in the outer periphery can be observed. Meanwhile, the influence of rotation speed on the particle distribution in the inner periphery is rather small. The extremely high cooling rate achieved by using a copper mould with water cooling leads to a very fine microstructure in the outer periphery when compared with the same area in the other tubes obtained by using graphite mould. It is suggested that both the microstructure and the macro segregation in the centrifugal tubes are tailorable. However, further investigations are needed and optimised technical routes are only available when based on a particular application background. Acknowledgements The authors would like to thank Professor Jia Guanglin and Mr Qiu Yiqing from the North-east University, China and Dr Wu Xinqiang from the Institute of Metal Research, Chinese Academy of Sciences for their help during the preparation of the centrifugal samples. References 1 Li GH, Gill HS, Varin RA. Metall Trans 1993;4A: Abramov VO, Abramov OV, Straumal BB, Gust W. Mater Des 1997;18: Kiser MT, Zok FW, Wilkinson DS. Acta Mater 1996;44: Mabuchi M, Kubota K, Higashi K. Mater Lett 1994;19: Mabuchi M, Higashi K. Acta Mater 1996;44: Mabuchi M, Kubota K, Higashi K. J Mater Sci 1996;31: Zhang J, Wang YQ, Yang Bin, Zhou BL. J Mater Res 1999; 1: Zhang J, Fan Z, Wang YQ, Zhou BL. J Mater Sci Lett 1999; 18: Mabuchi M, Kubota K, Higashi K. Scripta Metall Mater 1995; 33: Frommeyer G, Beer S, Oldenburg K. Z Metallkd 1994;85: Fukui Y, Watanabe Y. Metall Mater Trans 1996;7A: Sui X, Luo C, Luo Z, Ouyang L. J Mater Proc Technol 1997;63: Fukui Y, Takashima K, Ponton CB. J Mater Sci 1994;9: Watanabe Y, Yamanaka N, Fukui Y. Z Metallkd 1997;88: Zhang J, Wang YQ, Zhou BL. Mater Sci Tech 1999;11: Jiao YN. Dissertation. Institute of Metal Research. Chinese Academy of Sciences, Kang CG, Rogatgi PK. Metall Mater Trans 1996;7B: Liu QM, Jiao YN, Yang YS, Hu ZQ. Metall Mater Trans 1996;7B: