The effect of aging on wear characteristics of rheocast-leaded aluminum alloys
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INTRODUCTION
IN the last few decades, the use of aluminum-base alloys as bearing materials has increased considerably because of their light weight. In these alloys, metals like lead, tin, indium, or cadmium with low melting points are dispersed to work as solid lubricants. During sliding motion, these metals form a thin film at the interface of mating surfaces to restrict metal-metal contact, and hence, resistance to wear and friction is improved. Among these alloys, A1-Sn alloys are already in use. Besides being cheaper, lead ensures a better interfacial film of lubricant than tin at the interface, ILl which has drawn widespread attention of research workers; hence, attempts have been made to replace tin by lead. But almost negligible mutual solubility, even at elevated temperatures, and the large difference in specific gravities of the constituent present restrict a uniform distribution of lead in aluminum by conventional casting techniques; 12] these have given way to several other techniques, like rheocasting of A1-Pb alloys by Ichikawa and Ishizuka t3] and, more recently, by Mohan. [41 Dobatkin e t a l . tSj could produce a uniform distribution of lead granules in aluminum by sparging the melt superheated to above the stratification temperature. Inoue e t al. [6] produced A1-Pb alloys by arc welding and spinning the liquid in the form of ribbon with the help of a copper wheel. Borgunov e t al. tTj dispersed lead by ultrasonic mixing. A1-Pb composites have been produced by powder metallurgy techniques also. tS1 In these alloys, shearing takes place in soft-phase lead, whereas aluminum acts as a load-bearing component; however, aluminum does not have adequate strength to run at higher loads and sliding velocities as revealed by extensive studies on wear characteristics of the A1-Pb alloys in as-cast condition. 19-~5j Further, replacement of pure aluminum by LM 1 1 in these alloys has shown marked improvement in wear resistance, u~ Earlier studies I~637't81 have revealed that strength of partially homogenized rheocast AI-Cu alloys is improved with aging, and a peak in strength is observed during aging. In the S.K. SRIVASTAVA, Research Scholar, Mechanical Engineering Department, and S. MOHAN, Lecturer, Metallurgical Engineering Department, are with IT-BHU, Varanasi, UP, India. V. AGARWALA and R.C. AGARWALA, Readers, are with the Department of Metallurgical Engineering, University of Roorkee, Roorkee-247667, UP, India. Manuscript submitted June 24, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
present investigation, A1-4.5 wt pct Cu alloy was taken as a base material with minor additions of silicon and magnesium, and the lead was dispersed in this alloy by the rheocasting technique. Further, this alloy was aged at 473 K for different periods of time after partial homogenization at 763 K for 2 hours. Specimens aged for different extents of time were tested against a carburized steel ring for wear behavior. In order to understand the wear mechanism, wear tracks have been studied under a scanning electron mi
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