Homogeneous dispersion of graphite in a 6061 aluminum alloy by ball milling H.T. Son a , T.S. Kim b , C. Suryanarayana c, *, B.S. Chun a a Rapidly Solidified Materials Research Center, Chungnam National University, Taedok Science Town, Taejon 305-764, South Korea b ITRI & School of Applied Materials Engineering, Kongju National University, Kongju 314-701, South Korea c Department of Mechanical, Materials, and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450, USA Received 1 January 2002; received in revised form 17 September 2002 Abstract A composite of rapidly solidified Al-6061 alloy powder with graphite particle reinforcements was prepared by ball milling and subsequent hot extrusion. Proper choice of the processing parameters ensured a homogeneous distribution of the graphite particles in the aluminum alloy matrix. The microstructure and mechanical properties of these composites were investigated as a function of milling time. With increasing milling time, the spherical powder became elongated and acicular, which subsequently became spherical with a lamellar structure inside it. The shape change and intermixing process of the two constituent powders, and consequently the microstructure of the milled powder and extruded bars, were dependent on the milling time. The best compression and wear properties were obtained in the powder milled for 70 h, associated with the increased fine and homogeneous distribution of graphite particles in the aluminum alloy matrix. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Al /graphite composite; Mechanical alloying; Extrusion; Microstructure; Mechanical properties; Wear 1. Introduction A number of research investigations have been carried out in recent years to develop engineering materials that are stronger, stiffer, and more wear-resistant than the presently available commercial materials. Metal matrix composites (MMCs) are regarded as excellent materials to obtain properties that are superior to those of the constituent phases and also to satisfy the above require- ments [1,2]. However, the higher cost of processing the composites limits their use. Powder metallurgy is an important processing tech- nique for MMCs, which can eliminate the segregation of the reinforcement that is typical of the ingot metallurgy process [3,4]. Segregation can adversely affect the properties of the MMCs, and thus a homogeneous distribution of the reinforcement is essential for improv- ing the strength and other properties of the composites [5]. The main reasons for the segregation are differences in particle size, densities, geometries, flow characteris- tics, and development of an electrical charge [6].Even though the subsequent processing techniques such as powder extrusion can improve the reinforcement dis- tribution, it is desirable to achieve a uniform distribu- tion of the reinforcement particles in the powder itself [7,8]. Even though a number of processing techniques are available for producing MMCs, mechanical alloying (MA) has been frequently used to synthesize them due to the relative ease of the process in addition to its low processing costs. MA is a solid-state powder processing technique that has been used to synthesize a variety of stable and metastable materials [9]. In fact, this techni- que was originally developed in the late 1960s to produce a fine and uniform dispersion of oxide particles in a nickel-base superalloy to produce oxide-dispersion * Corresponding author. Tel.:  /1-407-823-6662; fax:  /1-407-823- 0208 E-mail address: csuryana@mail.ucf.edu (C. Suryanarayana). Materials Science and Engineering A348 (2003) 163  /169 www.elsevier.com/locate/msea 0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0921-5093(02)00749-9