Preparation and properties of Al-alloy coconut shell char particulate composites
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I.
INTRODUCTION
C A R B O N is an excellent bearing material due to its heat resistance, wear resistance, and corrosion resistance, as well as its high thermal conductivity and low thermal expansion.l However, carbon cannot readily be used owing to its inherent brittleness and low mechanical properties. Carbon is generally used in the form of composites wherein carbon is dispersed or impregnated in the matrices of metals like copper, zinc, aluminum, and aluminum-alloy. 1,2,6 Coconut shell char is a specially light and porous form of carbon (84 to 85 pct C). Resources of coconut shells in India and in the world are reported to be 0.85 million tons per year, and 4.25 million tons per year, respectively.3 This could yield about 0.3 million tons per year of shell char in India, and about 1.4 million tons per year in the world. Therefore, it will be advantageous if a large renewable resource like shell char can be used as a solid lubricant in the form of a dispersoid in suitable metal matrices. Kieslev4 reports that birchwood charcoal wastes have a coefficient of friction value of 0.12 and have antifriction properties approaching that of graphite. An earlier work l reports that wear loss of apex seals made of AI-11.8 pet Si-38 vol pet coconut shell char composite made by infiltration techniques in an autoclave is low when used in a Wankel engine. Further addition of 30 vol pet shell char particles to Al-ll.8 pct Si alloy is reported to have decreased the density of the alloy from 2.7 to 2.1 gm/cm3, l Al-graphite,6 Al-mica,7 Al-alumina,8 and Al-silica 9 composites have been produced by other workers by simple foundry techniques and have been successfully tested in
various tribological applications. However, most of the earlier investigations were confined to incorporation of low vol pct (3 to 5 pet) of dispersoids in A1 and AI alloy matrices. In addition, in none of the previous work, have attempts been made to add a renewable tree product like coconut shell char particle to molten Al-alloy to make cast aluminum alloy char composites. In this paper we describe the process of dispersing a relatively large volume percentage (up to 40 pet) of shell char particles in Al-ll.8 pct Si alloy melts. In addition, the mechanical, tribological, and electrical properties of cast aluminum alloy shell char composites have been measured and reported. II.
MATERIALS PREPARATION
Materials used: (a) A l - l l . 8 pct Si alloy of composition, Cu-0.1 pct, Mg-0.1 pct, S i - l l . 8 pct, Fe-0.6 pct, Mn-0.5 pct A1remainder (b) Cu-0.03 pct, Mg-0.03 pct, Si-0.3 pct, Fe-0.4 pct, Mn-0.03 pet, AI-99.5 pet rain. (c) Coconut shell char of size 125/zm.
Preparation of Coconut Shell Char About 15 kg of shell powder were charged into the ceramic coated mild steel container which is placed inside a pit type electrical resistance furnace. The container is closed airtight with the help of suitable gaskets to prevent the leakage of volatile gases through the lids of the container evolved during the process of charring. The maximum temperature of charring
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