Friction Stir Welding/Processing of Aluminum Alloys with and without Adding Nanoparticles: A Review on the Microstructure, Texture, and Hardness

Friction stir processing (FSP) and friction stir welding (FSW) methods are two types of severe plastic deformation (SPD) processes. SPD methods are useful in producing nanoparticle or ultrafine-grained materials (UFG) microstructure. In FSP and FSW, a rotating cylinder tool (pin), which could take the form of various geometries, pierces into the workpiece with a particular angle and depth. Furthermore, it refines grains by moving in the direction of interest along with the tool's movements. The uniform distribution of nanoparticles in the stir zone is one of the main challenges of using nanoparticles. Controlling variables such as tool rotational speed, tool travel speed, number of passes, etc., the distribution of nanoparticles and the grain size can be changed in the stir zone. Microstructure, texture, and grain size directly affect the hardness of the stir zone. Recent studies have shown that using nanoparticles enhances the mechanical properties of the stir zone. The main aim of this review article is to collect the results of previous articles focused on analyzing the operation of FSW and FSP, the microstructure of the stir zone in FSW and FSP, the impact of effective parameters on the microstructure after adding nanoparticles to the stir zone, and the applications of FSW and FSP in various industries. Moreover, the fundamental mechanisms of grain refinement throughout FSW and FSP, including morphology and grain boundaries forming, were discussed.