American Journal of Materials Science 2012, 2(5): 147-152 DOI: 10.5923/j.materials.20120205.03 Effect of Heat Input on the Properties of Dissimilar Friction Stir Welds of Aluminium and Copper Esther T. Akinlabi * , Stephen A. Akinlabi Department of Mechanical Engineering Science, University Of Johannesburg, Auckland Park Kingsway Campus, P. O. Box 524, Auckland Park, Johannesburg, 2006, South Africa Abstract This paper reports the effect of heat input on the resulting properties of joints between aluminium and copper produced with the friction stir welding process. The welds were produced using three different shoulder diameter tools, viz: 15, 18 and 25 mm by varying the rotational speed between 600 and 1200 rpm and the traverse speed between 50 and 300 mm/min in order to vary the heat input to the welds. The microstructures, the grain sizes and the microhardness of the joint interfaces were characterised, and the electrical resistivities of the joints were also measured. The resulting microstructural characterization revealed that good metallurgical bonding was achieved at the joint interfaces of the welds produced; this is evident with the presence of interlayers of aluminium and copper observed in the stir zones of the welds. The grains at the interfacial regions were recrystallized leading to a sharp decrease in the grain sizes compared to the parent materials. Higher Vickers microhardness values were measured at the joint interfaces resulting from strain hardening and the presence of intermetallics. It was also observed that the electrical resistivities of the joints increased as the heat input to the welds increases. Keywords Electrical Resistivity, Friction Stir Welding, Microhardness, Microstructure 1. Introduction Friction Stir Welding (FSW) is a solid–state joining technique invented and patented by The Welding Institute (TWI) in 1991 for butt and lap welding of ferrous and non–ferrous metals and plastics[1]. Since its invention, the process has been continually improved upon as its scope of application becomes expanded. FSW is a continuous process that involves plunging a portion of a specially shaped rotating tool between the abutting faces of a joint. The relative motion between the tool and the substrate generates frictional heat that creates a plasticised region around the immersed portion of the tool. In addition, the shoulder prevents the plasticised material from being expelled from the weld, therefore, the tool is moved relatively along the joint line, forcing the plasticised material to coalesce behind the tool to form a solid–phase joint[1]. Figure 1 illustrates the process definitions for the tool and work piece. The inserted picture also depicts the tool shoulder and the tool pin. The tool pin is sometimes referred to as the probe. The advancing side is on the right, where the tool rotation direction is the same as the tool travel direction (opposite the direction of metal flow), while * Corresponding author: etakinlabi@uj.ac.za (Esther T. Akinlabi) Published online at http://journal.sapub.org/materials Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved the retreating side is on the left, where the tool rotation is opposite to the tool travel direction (parallel to the direction of the metal flow). Figure 1. Schematic diagram of the friction Stir Welding process[2] The tool serves three primary functions; the heating of the workpiece, the movement of material to produce the joint, and the containment of the hot metal beneath the tool shoulder[1]. The heat generated during the FSW process is often assumed to occur predominantly under the shoulder; due to its greater surface and to be equal to the power required to overcome the contact forces between the tool and the workpiece[3]. To an extent, the heat input into the welds increases as the shoulder diameter increases[4]. The three different shoulder diameters used in this research study were