## 1 Introduction  
Aluminum alloys are modern engineering materials used in many industries. The application potential of aluminum alloys results primarily from their low density and good strength properties. The density of aluminum is 2.7 g/cm\({}^{3}\), which is only 34% of the density of iron. The strength-to-density ratio makes aluminum alloys an effective replacement for steel in industries such as transportation, aviation, and automobile production. High corrosion resistance, and good thermal and electrical conductivity, in addition to full recyclability, complement the characteristics of aluminum alloys and emphasize their versatility. Aluminum alloys are most commonly used in the aviation and automotive industries, but these materials are present in virtually every industry. What limits the application potential of these alloys is primarily their low hardness and resistance to abrasive wear. Therefore, various actions are taken to improve these properties. One such solution is to modify the surface layer. Inducing specific changes in the microstructure of the surface layer allows enhancement of its mechanical and tribological properties while maintaining a ductile core, which is particularly important in many applications. That is why surface engineering solutions enjoy unflagging interest. In addition to coating technology, concentrated heat sources are increasingly employed to melt the surface layer and induce rapid crystallization in the material [1; 2; 3]. A relatively new solution is modification of the surface layer using the modern FSP method. FSP technology was developed by [PERSON] et al. [4; 5], although the essence of this method comes from the FSW technology developed in the early 1990s by [PERSON] from the Welding Institute (TWI) in Cambridge, UK [6], and is based on the same principles as FSP. The difference between FSW and FSP technology lies in the purpose of these methods. FSW technology is used to join materials [7; 8; 9; 10; 11], while FSP technology is used to modify the surface layer of materials [12; 13; 14; 15; 16]. In both technologies, the source of heat is the friction of a special tool against the surface of the modified material and the strong plastic deformation of the material [17]. The dimensions of the pin and shoulder, as well as their shape and surface profile, determine the amount of heat generated, the mixing efficiency, and material flow intensity, and, consequently, also determine the microstructural effects of the processing and the resulting properties of the surface layer, including the material hardness and wear resistance. When designing and constructing a tool, it should be remembered that as much as 80-90% of the heat generated when the tool rubs against the material surface is generated by the shoulder, and only 10-20% of this heat is generated as a result of the friction of the remaining surfaces of the tool against the material [18]. During processing, the melting point of the modified material is not exceeded, and all changes in the microstructure and properties of the material occur during one short technological operation, which makes the FSP method a unique technology in many respects. One of the significant advantages of FSP is the ability to produce composite surface layers. The first information about the use of the FSP method for the production of composites appeared in the works of the creators of this method [19]. The results presented by the authors of the paper initiated a new important trend and direction in the development of the FSP method and paved the way for further methodological solutions enabling the production of a wide range of composite surface layers with various chemical compositions and purposes [20; 21; 22; 23; 24]. To date, a number of solutions have been developed to effectively introduce the reinforcing phase into the metal matrix.