"WEAR CHARACTERISTICS OF RHEOCASTLEADED ALUMINIUM ALLOY"

Abstract

Among the aluminium base bearing alloys, aluminium-tin alloys are widely used in industrial bearings.. The replacement of tin by lead is of interest because, besides being cheaper, lead ensures a better interfacial film of lubricant than tin. Special techniques such as rheocasting, stircasting etc. are generally employed for the production of leaded aluminium alloys since the production of these alloys by conventional casting techniques poses problems of lead segregation due to mutual immiscibility and large difference in specific gravities which causes nonuniform dispersion of lead in the alloy. The present investigation attempts to study the wear characteristics of a rheocast leaded aluminium alloy with 2 wt.% lead in the context of the morphology of the alloy. To understand the wear mechanism in the alloy, wear tracks are examined under SEM and EPMA. The roughness of wear tracks is determined by Philips roughness tester and it has been correlated with the bulk wear by using a mathematical. relationship. The problem under study has been introduced in chapter-l. Chapter-2 deals with the critical review of the morphology, general properties, wear mechanism and wear characteristics of the leaded aluminium alloys as determined and studied by other workers earlier. (iv) In chapter-3, the details of experimental methods followed in the study of the morphology and wear characteristics of the alloy are discussed. The specimens of required size are prepared for metallographic studies and for determining wear characteristics by techniques explained in chapter-3. The chemical composition of the alloys is determined by Atomic Absorption Spectroscopy and wet chemical analysis. The distribution of constituent elements of the alloy is determined by line and point analysis under EPMA. The microstructure of the alloy is observed under SEM. The method for the study of wear characteristics of rheocast leaded aluminium alloy, performed on Timken Wear and Lubricant Testing Machine at different loads, different sliding velocities and different sliding distances alongwith wear tracks study under SEM and EPMA is also given in this chapter. This chapter also includes the method adopted for roughness measurements. Chapter-4 deals with the morphology and wear characteristics of the rheocast leaded aluminium alloy. In rheocast alloy, due to liquid-solid dispersion, almost spherical particles of a-aluminium are observed in the microstructure. The lead-rich phase is observed in, the form of a partly continuous network in the inter a-particle regions. Copper-rich phase is observed in the form of divorced eutectic CuAl2 in the inter a-particle regions. Line and point anlaysis also confirm the microconstituents of the rheocast leaded aluminium alloy under study. (v) This chapter also deals with the wear characteristics of rheocast leaded aluminium alloy with about 2 wt.% lead at different sliding distances ranging from 38.5 m to 92.4 m, different applied loads ranging from 18.1 Kgf to 24.4 Kgf and different sliding velocities ranging from 12.83x10-2 m/s to 51.33x102 m/s. Wear characteristics of base aluminium alloy is also determined to study the effect of lead addition in aluminium base alloy. The bulk wear in base aluminium alloy is observed to increase with increase in sliding velocity but addition of lead in base aluminium alloy reduces the bulk wear. In rheocast leaded aluminium alloy, the bulk wear increases with sliding distance, applied load and sliding velocity. To understand the mechanism of wear, wear tracks are studied under SEM and EPMA. Roughness, R, of wear tracks has been determined and correlated with bulk wear, W, by using a mathematical relationship of the type; W=aR2-b. The values of a and b are found to be different for mild and severe wear regions. Chapter-5 outlines the conclusions drawn from the present study.