FABRICATION OF CAST Al-BASE LEAD BEARING ALLOYS AND THEIR WEAR CHARACTERISTICS

Abstract

Among the aluminium base bearing alloys aluminium-tin alloys are widely used in industrial bearings. The replace-ment of tin by lead is of interest because, besides being cheaper lead ensures a better interfacial film of lubricant than tin. However, the production of Al-Pb alloys by conven-tional casting techniques poses problems of segregation due to mutual immiscibility and large difference in density. In view of the'above, the present investigation attempts to disperse lead in aluminium rich matrix by the following techniques; (a) Liquid-liquid dispersion at elevated tempera-tures by stircasting, (b) Liquid-solid dispersion by rheo- , casting, (c) Inserting the lead alloy wires into the liquid aluminium and quenching it. The friction and wear characteristics o.f the alloys thus produced have been studied in the context of the particle distribution and morphology. To understand the mechanism the friction and wear tracks are examined under SEM and roughness of the tracks are determined by Philips Roughness tester. The investigation finally culminated in comparison of wear properties of aluminium base lead bearing alloys with those of Al-Sn, Al-Pb-Sn and Al-(Pb-Sn). The problem under investigation has been introduced in Chapter 1. Chapter 2 deals with the critical review of -iv the literature in respect of liquid-liquid and solid- liquid dispersion along with various parameters influencing particle size distribution of lead rich phase, Morphology of stircast and rheocast alloys, gravity segregation, friction and wear mechanism, friction and wear behaviour of materials with a special emphasis on the effect of soft metallic thin films. In Chapter-3, the experimental methods followed in stircasting, rheocasting and reinforced wire casting for fabricating Aluminium-base lead bearing alloys are discussed. The alloys are cast at different temperatures in a permanent mould filled by bottom pouring melt from a crucible in Kanthal resistance furnace controlled to an accuracy of ±2K and quenched in iced brine solution. The specimens of suitable sizes are cut for metallographic studies and for determining wear and friction behaviour by techniques explained in Chapter 3. The qualitative analysis of the elements present in these alloys are carried out by Inductively Coupled Plasma (ICP) technique and wet Chemical analysis. The microstructures of the alloys are observed under optical and Scanning Electron Microscope and the particle size distribution has been carried out by Saltykova method as outlined in Chapter 3. The methods for the study of wear and friction charac-teristics of stircast Al-Pb alloys, rheocast Al-Pb alloys, -v- stircast Al-Pb-Sn alloys and reinforced Al-(Pb-Sn) alloy performed on Timken wear and lubricant testing machine at different loads & sliding velocities are also given in this chapter. Chapter 4 deals with the morphology of the stircast and rheocast Al-Pb alloys. The effect of the lead content and stirring speed during casting on microstructure. In stircast alloys, due to liquid-liquid dispersion, lead rich phase is distributed in aluminium rich phase as spherical particles. It has been observed that with increase in lead content average particle size of lead rich phase is increased whereas interdendritic arm spacing of the aluminium rich phase is decreased. With increase in lead content the number of particles in the size range of 60-80 pm is increased. It is also observed that size range of particles of lead rich phase increases but average particle size is decreased with the increase in stirring speed. It has been observed that average particle size of lead rich phase increases with increase in lead content but interdendritic arm spacing of aluminium rich phase is reduced. In rheocast Al-Pb alloys due to solid-liquid dispersion, lead rich phase is observed at the boundaries of aluninium rich particles. However, some spherical particles of lead rich phase are embedded in Al-rich phase. It has been observed that for a particular agitator speed the thickness as well -vi- as length of the lead rich phase increases with increase in lead content whereas particle size of aluminium rich phase is decreased. To examine the stability of liquid-liquid dispersion, stirring is ceased and specimens have been taken out at regular intervals and the .growth rate of the lead rich phase has been studied. The friction and wear characteristics of. the alloys thus produced have been studied in the context of the particle distribution and morphology to understand the mechanism of friction and wear in subsequent sections. Chapter 5 deals with the friction- and wear charac-teristics of stircast Al-Pb alloys containing about 4 to 56 wt% lead and wear characteristics of rheocast Al-Pb alloys containing about 4 to 20 wt% lead at different loads of 14 to 25 kgf and sliding velocities ranging from 12.83x10-2 to 51.33x10-2 m/s. It has been observed that in stircast alloys friction coefficient remains almost steady with increase in load or sliding velocity for all the Al-Pb alloys and commercially pure aluminium but friction coeffi-cient of pure lead increases with increase in load as well as with sliding velocity. It is also observed that friction coefficient reduces with increase in lead content and attains a minima around 20 wt% lead but beyond this composition —vii— friction coefficient starts increasing sharply with lead content. The bulk wear in all the Al-Pb alloys is observed to increase with increase in load but even with small amount of lead in aluminium bulk wear is drastically reduced. Bulk wear continuously increases with increase in sliding velocity for pure aluminium and Al-Pb alloys containing below 8 wt% lead but beyond this corrposit ion a minima in bulk wear with sliding velocity appears and this minima in bulk wear shifts towards higher sliding velocities with increase in lead content in stircast as well as rheocast alloys. However, with increase in lead content bulk wear is observed to decrease and between 10 to 20 wt% of lead the minimum bulk wear is observed but beyond 20 WO lead bulk wear increases with further increase in lead content. To understand the mechanism of friction and wear the friction and wear tracks have been studied under SEM. Roughness of the wear tracks is determined and wear debris has been studied under M. X-ray analysis of debris shows the presence of Pb2AL205.