**1. Introduction**

Aluminium alloys are known to be good candidates for different applications in various fields like aerospace, food packaging, automotive industries, etc. Their good candidacy came from the fact that these metals are light in weight, have good mechanical properties, good corrosion resistance, etc. Various aluminium alloys possess different mechanical and thermal properties and these differences are influenced by the alloying elements used in producing each alloy [1,2]. Most industries are opting towards using dissimilar alloys in producing various components. This option is meant to reduce the costs that are involved in using similar alloys [3]. In as much as this approach is a cost-saving measure, however, there are also challenges associated with it. This includes the welding technique suitable at welding dissimilar alloys. The most used welding techniques involve tungsten inert gas (TIG) welding and friction stir welding (FSW). The TIG welding technique has been dominant in joining similar and dissimilar aluminium alloys until the birth of FSW.

There have been some challenges that were involved in joining dissimilar alloys through TIG technique. Those challenges include porosity, solidification cracking, thermal residual stresses, etc. These challenges have led to the discovering of the post-processing technique called friction stir processing. Friction stir processing (FSP) is a technique used to modify the microstructure of a metal through the use of a non-consumable rotating tool. FSP originated from friction stir welding which was initially established by The Welding Institute. FSP uses the same principle as FSW but does not join metals rather modifies the local microstructure in the near-surface layer of metals [4]. Figure 1 shows the schematic diagram of FSP and FSW techniques.

**Figure 1.** (**a**) FSW (friction stir welding) and (**b**) FSP (Friction stir processing) technique.

FSP works by plunging a specific cylindrical non-consumable tool into the plate and kept stationary for a few seconds. This is done so as to allow the stabilization in an input temperature required for processing. The rotating tool gets released so that it travels along the surface of the metal being processed. The tool travels from the start to the end of the plates resulting in the attainment of the processing. When the processing is finished, the tool is then unplunged, leaving a small hole, or rather travels to an offset distance to avoid leaving a hole. The side in which the tangential velocity of the tool surface is parallel to the traverse direction is called the advancing side, and the non-parallel side is called the retreating side [5,6].

This paper is aiming at reviewing the works that deals with the processing of similar and dissimilar joints produced by the TIG and FSW welding techniques.
