Multi-Performance Optimization in Friction Stir Welding of Aluminum Alloy Using Response Surface Methodology

1. Introduction

Industrial advancements and development of new materials describes the progress of human beings. There are numerous newly developed advanced and composite materials which are used in aerospace, space-crafts, automobiles etc and require various operations like machining, welding, riveting etc. The joining of these advanced materials is highly challenging when the conventional fusion welding and riveting is not permitted. Joining of materials through welding is preferred more over the riveting and bolting. The aerospace alloys (aluminium alloys) are not permitted to be joined by fusion welding as the microstructure of weld represents poor quality. There are different welding processes to join the materials depending on the applications and characteristics. Friction stir welding (FSW) is a solid state welding process used to join such types of alloys for special applications. The FSW process was invented by the Welding Institute UK, in 1991. In FSW, a non-consumable rotating tool is plunged into the profile line of two materials, which are placed in butt configuration with the help of fixture as shown in experimental setup mentioned in Figure 1. The frictional heat generated by the advancement of rotating tool in work material causes the material softening and adiabatic plastic deformation around the rotating tool. During the tool movement in forward direction, the material of two plates flows towards each other and forms the solid state joint.

The FSW is most suitable for joining of light weight and low melting materials which cannot be joined by fusion welding process. The components joined through FSW possess good dimensional stability, excellent joint properties, low structural distortion, no requirement of shielding gas, minimum power requirement, etc. (Mishra, & Ma, 2005; Fukuda, 2001; Gemme, Verreman, & Dubourg, 2009). The several researchers has investigated and described the benefits of FSW on light weight and low melting materials like aluminium alloys, magnesium alloys etc. Karthikeyan and Balasubramanian (Karthikeyan, & Balasubramanian, 2010) used FSW for the joining of AA 2024. RSM was applied to optimize the FSW parameters for the tensile shear fracture load. The optimum load of 9.39kN was obtained at 13.56mm/min of plunge rate, 1000r.p.m. of tool rotational speed and 5.178mm of plunge depth. Arora et al. (Arora, Pandey, Schaper, & Kumar, 2010) joined AA2219 by FSW using vertical milling machine. It has been founded that forging force depends upon the rotational speed and shoulder diameter. Welding speed and shoulder diameter significantly affect the tensile strength while percentage elongation affected by welding speed only. The forced was evaluated using an in-house developed load cell. L₉ orthogonal array was selected for planning and performing the experiments. Lorrain et al. (2010) studied the FSW of AA 7020-T6 using two different types of tools. The first tool is unthreaded straight cylindrical while other tool consist tapered cylindrical pin with no threads on it. The material flow in case of unthreaded pin was consisting the same features as consist by threaded pins. Some of the material was deposited on the upper part of the weld during advancing side while lower part of the weld during retreating side. Also the analysis revealed that the size of shoulder dominated zone is largely affected by the product of rotational speed and plunge force.