Copper and aluminum materials are extensively used in different industries because of its great conductivities and corrosion resistant nature. It is important to join dissimilar materials such as copper and aluminum to permit maximum use of the special properties of both the materials. The joining of dissimilar materials is one of the most advance topic, researchers have found from last few years.
Friction stir welding (FSW) technology is feasible to join dissimilar materials because of its solid state nature. Present article provides a comprehensive insight on dissimilar copper to aluminum materials joined by FSW technology. FSW parameters such as tool design, tool pin offset, rotational speed, welding speed, tool tilt angle and position of workpiece material in fixture for dissimilar Cu-Al system are summarised in the present review paper. Additionally, welding defects, microstructure and intermetallic compound generation for Cu-Al FSW system have been also discussed in this article.
Furthermore, the new developments and future scope of dissimilar Cu-Al FSW system have been addressed. KEYWORDS: Aluminum, copper, dissimilar, friction, materials, welding Downloaded by University of Colorado – Health Science Library at 04:18 31 March 2015 2 INTRODUCTION Joining of dissimilar materials by any welding process is always difficult because of the enormous differences in mechanical and metallurgical properties. The joints of dissimilar materials are increasingly employed in different sectors of industries due to its technical and economic advantages. Copper (Cu) and aluminum (Al) materials having good electrical and thermal conductivities which tend it to use for electrical and thermal applications. Electrical connectors, bus-bars, foil conductor in transformers, capacitor and condenser foil windings, refrigeration tubes, heat-exchangers tubes and tube-sheets etc.
are some common applications of Cu and Al joints. Fusion welding processes are not recommended to join Cu and Al together because of a tendency to form large intermetallic compounds (IMCs). These IMCs are very hard and brittle, which causes many defects.
The solidification and liquefaction cracking are some common problems associated with fusion welding 1. Solid state welding processes such as friction welding, ultrasonic welding, cold rolling, explosive welding, diffusion welding and friction stir welding are feasible methods by which Cu-Al dissimilar materials can be joined together 2, 3. Since, the last few years, researchers have focused on friction stir welding (FSW) technology to join Cu-Al materials together. FSW is a solid state welding process, invented and patented by The Welding Institute (TWI), London, UK in 1991 4–10. Friction and stirring of material produced by the non-consumable rotating tool consists of specially designed pin and shoulder as shown in Fig. 1. Tool pin is totally inserted between abutting surfaces of workpiece with suitable rotational speed until the shoulder makes a contact with the workpiece surfaces. Rubbing action between tool and Downloaded by University of Colorado – Health Science Library at 04:18 31 March 2015 3 workpiece surfaces generate a large amount of frictional heat which softens the workpiece materials.
This softened material travelled from front to back and top to bottom of the pin when the tool is travelled along with the certain rotational speed 4–10. The basic process principle of dissimilar FSW technology is shown in Fig. 1. This paper comprehensively reviews recent work along several significant aspects of dissimilar Cu-Al FSW system such as process parameters and its effect, material flow and microstructure changes, defects, mechanical properties and variant of FSW. FSW TOOL FSW tool is a heart of the process. The functions of the FSW tool are heating and softening of base materials, extruding the base materials from front to back and from top to bottom of the tool, and finally make the bonding of the softened material to form a solid state joint 11.
The tool material and tool geometry are the important components of FSW tool. Tool Material Tool material should be such that, the geometry and features remain unchanged during the process. Requirement of tool material is critical for higher melting workpiece materials. Important characteristics of tool material such as ambient and elevated temperature strength, elevated temperature stability, wear resistance, tool reactivity, fracture toughness, machinability, uniformity in microstructure and density, availability of materials are required for getting success in the FSW process 6. Downloaded by University of Colorado – Health Science Library at 04:18 31 March 2015 4 Simplest tool design and cost effective tool material are the greatest advantages of dissimilar Cu-Al FSW system.
Available literatures indicate that the tool steel (with tempered and quenched conditions and usually hardened from 45 HRC to 62 HRC) is commonly used as a tool material for dissimilar Cu-Al FSW system. The required hardness of the tool material depends on alloys and the thickness of the workpieces. Literature summary of different recommended tool materials for dissimilar Cu-Al FSW systems are shown in Table 1. Esmaeilia et al. 15, 16 described that the tool steel of H13 grade eroded off at a higher rotational speed for dissimilar brass-AA1050H16 FSW system. The consistent results are addressed by Agrawal et al.
37 for the Cu-AA6063 FSW system. The reason for this may be attributed to lower wear resistance and elevated temperature stability at higher rotational speed. The rubbing action with high strength alloys such as brass and AA6063 materials at a higher rotational speed may result in wear of the tool. Additionally, the sticking of Cu-Al mixed material on the surface of the tool after every welding run is a big issue which causes defects. However, this problem can be avoided by inserting the FSW tool into the fresh Al material after every experiment.
Insertion of tool in fresh material helps to react Cu-Al mixed material with fresh Al material which in turn clean the tool pin and prevent the defects 15, 62. Downloaded by University of Colorado – Health Science Library at 04:18 31 March 2015 5 Tool Design And Geometry The FSW tool has two basic parts: (I) Pin and (II) shoulder. Important elements of these parts are shoulder diameter, shoulder surface angle, pin geometry, including its shape and size, and the nature of tool surfaces 11. Tool design and geometry affects the heat input, force & torque variations and plasticized material flow in FSW technology 7. Different tool designs and geometries for dissimilar Cu-Al FSW system are discussed as below. Tool Shoulder In FSW, the shoulder diameter is maximum responsible for heat generation.
It has been found that the shoulder generates around 87% heat by rubbing action between the shoulder surface and the workpiece 9. Tool shoulder diameter and geometry/surface features affect the quality of weld in FSW as it contributes to maximum heat generation. For achieving good quality FSW joint, the optimum shoulder diameter is one of the important parameters needed into consideration before the welding 5. Tool shoulder diameter affects the peak temperature variation, material deformation, plunge load variation, mechanical properties, microstructural variation and formation of intermetallic compounds (IMCs) in dissimilar Cu-Al FSW system.
Akinlambi et al. 29 claimed that uniform mixing between Cu and Al with a proper material flow pattern can be obtained with 15 mm and 18 mm diameters while improper material mixing was observed with 25 mm