## 4 Results and Discussion
### Hardness Measurement  
The key parameter verifying the correctness of the selection of the processing parameters and indicating the extent to which the friction modification and the reinforcing phase introduced into the matrix affect the material properties is hardness. This is a basic parameter in assessing the utility value of surface layers and the effectiveness of methods  
Figure 7: Microstructure of composite reinforced with A-1 particles (**a**,**b**), A-2 particles (**c**,**d**), and SiC particles (**e**,**f**). Stirring zone (**a**,**c**,**e**), boundary of stirring zone, and thermomechanically affected zone (**b**,**d**,**f**). Etched, light microscopy. employed to modify these surfaces. Therefore, great importance was attached to the hardness measurement results. Hardness measurements were made on cross-sectional surfaces in the central part of the stirring zone. The tests were conducted using a Shimadzu hardness tester (Shimatzu Corp., Kyoto, Japan) with a load of 980.7 mN. All the produced samples were tested. The results of hardness measurement as a function of distance from the surface are shown in Figure 8.  
The hardness increased in all the samples, regardless of the type of reinforcement used. Despite the dissolution of some of the intermetallic phases found during the microscopic examination, no sample had a hardness lower than that of the initial aluminum alloy. In the case of the composite reinforced with the A-1 powder, the growth in hardness was approximately 41%, while in the case of the A-2 powder, the composite achieved an average value of HV0.1 = 132, which was approximately 36% higher than the hardness of the starting material. In the case of the material reinforced with SiC particles, the rise in hardness was the highest and exceeded 43%. The change in the hardness of the material after FSP was primarily a consequence of introducing the hard particles of the reinforcing phase into the matrix and the strong refinement of the grains caused by the dynamic recrystallization of the material. According to the Hall-Petch relationship, as the degree of grain refinement grows, the hardness of metals and their alloys increases [53; 54]. This is because, as a result of reducing the grain size, the number of grain boundaries rises, which constitute a barrier to dislocations and act as pinning points, inhibiting their further propagation. This mechanism is the basis of so-called edge amplification or Hall-Petch amplification. However, it is worth noting here that an equally important factor influencing the course of dynamic recrystallization and its kinetics is the presence of second-phase particles. The role these particles play depends on their size. Large particles can constitute recrystallization nuclei and facilitate recrystallization of the material, while small particles lead to slower recrystallization and the inhibition of grain growth, thus contributing to a greater degree of grain refinement in the material. The powders used in the study were characterized by a very large variation in particle size, but it should be noted that small particles predominated, their dimensions usually not exceeding a few micrometers. Such particles, located on the boundaries of newly formed grains, block the movement of grain boundaries and recrystallization fronts, and thus limit grain growth. Moreover, as a result of the mechanical interaction of the tool with the reinforcing phase, it cannot be ruled out that additional refinement of the particles of this phase may occur, resulting in an increment in the number of smaller particles. The effect of additional refinement of the reinforcing phase during FSP was, among others, noted by the authors of this work during the FSP of a cast composite based on an aluminum alloy reinforced with SiC particles [55]. Large differences in the particle size and shape among the powders used in the research  
Figure 8: Hardness measurement results.