L Journal of Alloys and Compounds 319 (2001) 233–241 www.elsevier.com / locate / jallcom Growth rate of intermetallic compounds in Al / Cu bimetal produced by cold roll welding process a b, a * M. Abbasi , A. Karimi Taheri , M.T. Salehi a Metallurgical Engineering Department, Iran University of Science and Technology, Narmak, Tehran, Iran b Material Science and Engineering Department, Sharif University of Technology, Azadi Ave., P .O. Box 11365-9466, Tehran, Iran Received 25 September 2000; accepted 4 December 2000 Abstract The aim of this article is to study the growth rate of intermetallic compounds at the interface of cold roll bonded Al / Cu bimetal at 2508C and compare the results with a similar study performed on friction welding of Al to Cu. Samples of tri-layered Cu–Al–Cu composite were produced by the cold roll welding process and heat treated at constant temperature of 2508C for 1 to 1000 h. The thickness, morphology and composition of the intermetallic compound at the interface of Cu and Al were studied by optical and scanning electron microscope and EDX analyzer. The presence of various intermetallic compounds (Cu Al, Cu Al , CuAl and CuAl ) was 3 4 3 2 detected and the priority of formation of each compound was studied. The bond strength and electrical resistivity of different samples were measured by peeling test and a high precision micro-ohmmeter, respectively. The variations of resistivity and bond strength versus thickness of intermetallic compound were plotted. It was observed that the strength and electrical conductivity are greatly reduced by increasing the thickness of intermetallic compounds. Moreover, the growth rate of intermetallic compounds in roll welded bimetal composite is lower compared with that reported for similar friction welded specimens.  2001 Elsevier Science B.V. All rights reserved. Keywords: Aluminum; Copper; Roll welding; Intermetallic growth; Bond strength; Bimetal resistivity 1. Introduction Cu are incompatible metals because they have a high affinity to each other at temperatures greater than 1208C Metallurgically welded Al to Cu is widely used as a and produce brittle, low strength and high electrical transition piece in high direct-current bus systems to resistance intermetallics on their interface [2,3]. Thus, transmit the electricity [1]. Such a bimetal transition piece fusion-welding processes are not applicable for welding of is shown in Fig. 1 and acts as a connecting bridge between Al to Cu and solid state welding processes such as the copper terminal and the aluminum bus bar conductor. explosion, friction and cold roll welding have been consid- 2 Each bimetal with 20 cm connecting surface can conduct ered as the qualified welding processes of these metals. 2 a high electrical current up to 2000 amp (100 amp/cm ) Although, according to the reports of previous authors, [1]. If a copper terminal and aluminum bus are connected during both the friction and roll welding processes, the to each other directly, without an Al / Cu bimetal transition formation of intermetallic compounds are inhibited [1–6], piece, the service life of the electrical connection is about 1 however, the interface of solid state welded Al / Cu is year, but by using Al/Cu bimetal the service life is susceptible to the nucleation and growth of intermetallic increased to more than 10 years [1,2]. In the manufacturing compounds at temperatures greater than 1208C [2–4,6]. process of a high quality transition piece, the electrical This is a thermally activated process and by increasing the resistance of the Al / Cu bimetal should be kept as low as temperature the nucleation and growth of compounds are possible to minimize the wasting of electrical energy and accelerated. These compounds have a nonmetallic coval- prevent the increase of temperature. ence bond and therefore are brittle and have high electrical From the point of view of the welding process, Al and resistance. In both processes, there is a nonequilibrium state or metastable condition at the weld interface. This metastable condition provides a driving force for nucleation and *Corresponding author. E-mail address: ktaheri@sina.sharif.ac.ir (A. Karimi Taheri). growth of intermetallic compounds and the amount of 0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388(01)00872-6