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Aluminium alloy-rice husk ash particle composites

S. DAS, T. K. DAN, S. V. PRASAD, P. K. ROHATGI Regional Research Laboratory, Council of Scientific and Industrial Research, Bhopal - 462 026, India

Liquid metallurgy techniques, wherein the second- phase particles are added to the melt above the liquidus temperature prior to solidification, have been devel- oped to produce metal matrix particle composites. A large number of aluminium alloy composites contain- ing dispersions of graphite [1], alumina [2], zircon [3], and carbon microballoons [4] have been synthesized for a variety of applications. Mechanical [5] and wear [6, 7] properties of some of these composites have been extensively studied. However, little attention has been paid to the solidification behaviour of these alloys in the presence of second-phase particles. Krishnan and Rohatgi [8] have reported the refinement of eutectic silicon in aluminium-sil icon alloys due to the presence of graphite particles. The pr imary object of the present study is to disperse rice-husk-ash particles in molten aluminium-sil icon alloys and study the effects of second-phase particles on the solidification behaviour of aluminium silicon alloys.

Rice husk in an agricultural waste and is available in very large quantitites throughout the world. Rice husk, which essentially protects the rice grain during growth, contains approximately 50 to 60% fixed car- bon, the rest being silica and minor oxides [9]. In recent years considerable efforts have been made to utilize the silicon-rich rice-husk ash (a product of the controlled burning of rice husk) for making cement and a variety of other useful products [10-13].

In the present study, rice husk was procured from local sources in Bhopal (India) and was thoroughly washed with water to remove the dust and dried at room temperature for 3 days. Washed rice husk was then heated to 200 ° C for 1 h in order to remove the moisture and organic matter. During this operation, the colour of the husk changed from yellowish to black because of charring of organic matter. It was then heated to 600 ° C for 12 h to remove the carbonaceous material. After this operation, the colour changed from black to greyish white. The silica-rich ash, thus

TAB L E I Analysis of rice husk ash

obtained, was used as a filler material in the prep- aration of composites. Chemical composition of the rice husk ash after the above treatments is shown in Table I. Chemical composition of rice husk ash from other sources [12] is also reported in Table I for com- parison.

The chemical composition of the aluminium-silicon alloy (LM 13) used in the present investigation as the matrix alloy is shown in Table II. This alloy is of interest because it exhibits a low coefficient of thermal expansion and good bearing properties. About 15 kg LM 13 alloy was melted in a pit furnace, degassed with nitrogen and stirred by a steel impeller. During mech- anical stirring of the melt, 450g of rice husk ash particles were added to the surface of the melt near the Centre of the vortex. Simultaneously, 1 wt % mag- nesium (in 10 g pieces) was added to the melt in order to enhance the wettability between rice husk particles and the alloy melt. It was noticed that without the addition of magnesium, the particles of rice husk ash were rejected. The composite melt containing 3 wt % rice husk ash particles was cast in a cylindrical cast iron mould of 18mm i.d. Specimens of LM 13-rice husk ash composite were metallographically polished, etched with Keller's reagent and examined using both optical and scanning electron microscopes (SEM). Rice husk ash particles were also observed in the SEM. Before SEM examination, rice husk ash particles and the composite surfaces were coated with a thin layer of silver in order to prevent charging effects.

Scanning electron micrographs of a typical rice husk ash particle which has retained the original shape of the rice husk, are shown in Fig. la. A micrograph of another particle viewed from the opposite direction is shown in Fig. lb. Fig. la and b clearly shows the hull-like shape of the ash particle. However, these particles are quite fragile and some particles tend to break down during handling, thereby loosing the hull- like shape. A higher magnification micrograph of the

Component From USA From North From Southern From Bhopal (wt %) Eastern India tip of India (wt %)

(wt %) (12) (wt %) (12)

SiO2 94.50 94.50 CaO 0.25 0.48 MgO O.23 O.23 K z O 1.10 Trace Na 2 O 0.78 Trace P205 0.53 Trace SO4 1.13 Trace A1203 AI (trace) A1 = 1.21 FezO 3 Fe (trace) Fe = 0.54 Carbonaceous - - and moisture

91.40 90.61 0.17 1.24 1.30 0.21

Not examined Not examined Not examined Not examined Not examined Not examined Not examined Not examined A1 = 1.57 0.31 Fe = 0.62 2.40

562 0261-8028/86 $03.00 + .12 © 1986 Chapman and Hall Ltd.

ash particle (Fig. lc) shows the internal skeleton structure of silica, which has been left behind after the volatile matter and most of the carbon have been burnt down during the ashing of rice husk. Fig. 2a and b shows optical micrographs of LM 13 and a typical LM 13-rice husk ash content cast under similar con- ditions. The necklace-shaped cross-section of the rice husk ash particles can be clearly seen in two places in Fig. 2b in the centre of the picture. In addition, alu- minium dendrites, eutectic silicon and some small fragmented rice husk ash particles can also be seen in the Fig. 2b. A scanning electron micrograph of the metallographic surface of LM 13 alloy is shown in Fig. 3. Aluminium dendrites and the eutectic silicon in the inter-dendritic region can be clearly seen in the figure.

The change in microstructure, solidified in the presence of rice husk ash particles, can be estimated by measuring the secondary dendrite arm spacing (centre-

Figure 1 Scanning micrographs of typical rice husk ash particles.

to-centre distance between two neighbouring den- drites). The average secondary dendrite arm spacing of primary ~-aluminium is found to be of the order of 12 #m (Fig. 3) in plain LM 13 alloy (without rice husk ash). On the other hand, the average dendrite arm spacing in LM 13-rice husk ash composite was found to be 20 #m (Fig. 4a). A magnified view of the region solidified inside the necklace-shaped rice husk is shown by an arrow in Fig. 4b. Fig. 4c shows the magnified view of the microstructure just outside the necklace shown in Fig. 4a.

It can be seen from the optical micrograph in Fig. 2b that the aluminium dendrites are oriented differently within the necklace-shaped rice husk ash particles as compared to their orientation outside the necklace, there is no continuity in the dendritic struc- ture across the rice husk particles. This observation indicates that the liquid alloy entered the cavity of the hull-shaped rice husk ash and solidified as a result of nucleation events separately from the bulk liquid, out- side the ash particles. This kind of structure, where molten alloy enters the cavities of rice husk ash and freezes as a separate entity as a result of a separate nucleation event, points out the possibilities of producing new kinds of cast microstructures where the large bulk of liquid is subdivided into smaller parts which freeze as independent entities while maintaining the structural continuity in the bulk. This filling of hull-shaped rice husk ash particles with molten alloy prior to solidification might have prevented these ash particles from floating. The apparent density of ash

Figure 2 Optical micrographs of (a) LM 13 alloy, (b) LM 13-rice husk ash composite.

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T A B L E II Chemical composit ion of LM 13 alloy

Element wt %

Silicon 10-12 Copper 1 Magnesium 1 Nickel 1.5 Iron 0.8 Magnesium 0.5 Aluminium Balance

Figure 3 Scanning electron micrograph of the metallographic surface of LM 13 alloy.

particle was found to be 2.0 g cm 3 whereas the density of the molten aluminium-sil icon alloy is 2.65 g cm 3.

Rice husk ash particles are conconducting and, therefore, do not allow the heat to be dissipated immediately, resulting in molten metal remaining at higher temperatures for a longer time in the vicinity of the ash particles. Consequently, the dendrites are coarser within the rice husk particles in the com- posites than in the base alloy.

Acknowledgements The authors thank Dr M. Saxena, Mr A. C. Karera and Dr M. Patel for their help in chemically analysing the rice husk ash particles. They also thank Mr K. K. S. Gau tam for technical assistance in SEM.

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(1969) 402. 2. K. J. BHANSALI and R. M E H R A B I A N , J. Metal. 34

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(1984) 41. 9. P. C. K A P U R , "Metallica" (Department of Metallurgical

Engineering, Indian Institute of Technology, Kanpur , 1982 85)p. 1.

10. M. H. VENNETI and G. RAO, J. Sci. lnd. Res. 39 (1980) 495.

11. P. C. K A P U R , " A s h m o h " (Department of Metallurgy, Indian Institute of Technology, 1976).

12. G. R A M A R A O and P. K. ROHATGI , to be published. 13. Machine Design (1984) 4.

Received 1 November and accepted 25 November 1985

Figure 4 (a) Scanning electron micrograph of metallographic surface of LM 13-rice husk as composite. The necklace-shaped rice husk ash particle can be seen. (b) Magnified view of the region marked by arrow in (a). (c) Magnified view of region just outside the necklace shown in (a).

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