3.1. Microstructure characterization
Observations of the microstructure of the cross-section of the zone after FSP modification depending on the rotational speed of the tool (Fig. 2) revealed a change in the structure and shape of the stir zone with an increase in the rotational speed of the tool. The FSP zone width and depth shrunk with increasing rotational speed. It is also worth drawing attention to the limitation of the depth of the impact of the tool shoulder with the increase in the rotational speed, which is related to the change in the amount of heat introduced to the stir area and its temperature. Increasing the rotational speed of the tool results in the fact that the material just below the surface of the tool reaches a higher temperature, and thus is able to bear smaller loads; while becoming more plastic it is "sheared" faster, and the depth of the shoulder impact is smaller. Previous studies show that the microstructure in the process zone depends on the process parameters (rotational speed of the tool, travel speed, pressure force), the type of material being processed and the shape of the tool [16–18]. The state of knowledge regarding the phenomena occurring during FSP is not yet fully known, which makes it difficult to select the optimal parameters for various alloys. In addition, observations of the microstructure after FSP modification revealed the presence of a nugget in the centre of the FSP zone, characterized by a lower degree of particle refinement, which is clearly noticeable when modifying with the lower tool rotational speed of 280 RPM (Fig. 2a) and disappears with increasing the rotational speed (Fig. 2b).
Arrangement of the material in the form of "onion rings" is visible, which is observed as a repeating pattern in the cross-section of the weld. They arise as a result of a rhythmic change in the size and distribution of the strengthening phase. The patterns repeat at intervals (seen in the cross-section) equal to the linear distance travelled by the tool during each rotation. The information presented in the literature indicates that it is related to oscillation of the tool rotation axis around its linear axis of motion . Higher rotational speeds generate more heat, in addition, in the stir zone, layers with different material temperatures should be expected [18, 20], which leads to the formation of bands richer in precipitates (layers with a higher temperature) and bands poorer in secondary phases (layers with a lower temperature ) .
Figure 3a, b shows the microstructure of the studied alloy, while Fig. 3c and d presents the micrographs of the microstructure showing the stir zone depending on the rotational speed of the tool used. The microstructure of the alloy before modification consists of a few rhomboidal/cubic precipitates with stoichiometry corresponding to the SnSb phase and numerous needle-shaped and globular-shaped precipitates with stoichiometry corresponding to the Cu6Sn5 phase distributed on the background of a tin-rich matrix (Figs. 3a, b), which is confirmed by the results of the EDS analysis (Fig. 4) and the X-ray phase analysis (Fig. 5). As a result of FSP modification, fine and equiaxed recrystallized grains of the tin-rich matrix (Fig. 3c, d) with a size of approx. 5–40 µm were formed in the microstructure. FSP generates a significant rise in temperature due to frictional forces, intense plastic deformation, and material flow caused by the movement of the tool, thus favouring dynamic recrystallization in the stir zone.
The EDS analysis of the chemical composition (Fig. 4) and XRD analysis of the phase composition (Fig. 5) showed no change in the phase composition of the alloy as a result of modification, while both the SnSb and Cu6Sn5 precipitates were significantly refined. The presented micrographs of the microstructure (Figs. 3, 4) show a change in the morphology of the SnSb and Cu6Sn5 phases present in the alloy. The shape of the Cu6Sn5 phase particles changed to more regular, close to globular, while the SnSb phase occurs in the form of very numerous, small globular particles, evenly distributed in the matrix - inside the grains and at the grain boundary (Fig. 3c, d, Fig. 4 - point 1). Lead is evenly distributed in the tin matrix, and also occurs in the form of small precipitates with a stoichiometry corresponding to the eutectic composition of Sn-Pb (Fig. 4 - point 4).
One of the advantages of the FSP method is the possibility of obtaining a microstructure that changes in a gradient manner from the surface into the depth of the product to prevent defects, e.g. spalling. The transition zone between the stir zone and the thermo-mechanically affected zone and the area directly under the pin are determined mainly by adhesion, which is very important in the case of surface engineering . Linear analysis of the chemical composition was performed to accurately characterize the transition areas from the stir zone towards the base material. The results of the linear analysis are shown in Figs. 6, 7. Additionally, in Fig. 6 the point corresponding to the clear transition boundary (point A) is marked with a dashed yellow line on the graph. Similarly, in Fig. 7 the area of significant refinement is between points A and B marked in the figure. The tests confirmed a significant increase in the refinement of the Cu6Sn5 and SnSb phase particles in the material stir zone after friction treatment on the advancing side (Fig. 6) and in the lower transition zone under the pin (Fig. 7). This is confirmed by the numerous peaks in the graphs corresponding to the composition of these phases.
In order to determine the influence of FSP on the microstructure of the studied alloy, including the process parameters, statistical analysis of the SnSb and Cu6Sn5 phases present in the microstructure was also performed. The results of the quantitative analysis including determination of the cross-sectional area of the particles and the nearest neighbour distance for the Cu6Sn5 phase are presented in Fig. 8, and for the SnSb phase particles in Fig. 9. The obtained results confirm the refinement of the microstructure as a result of FSP modification.
The particle size histogram of the Cu6Sn5 precipitates is characterized by an extremely asymmetric shape (Fig. 8a). A clear maximum in the histogram occurs for particle sizes under 25 µm2, which in the case of the initial material account for about 76% of the studied population. On the other hand, as a result of FSP modification, this range includes approx. 81–90% of the analysed precipitates. The increase in rotational speed slightly influenced the refinement of the Cu6Sn5 phase. In addition, the results of NND measurements showed a shift in the population towards smaller distances between the precipitates for the material after FSP modification compared to the cast material (Fig. 8b). Moreover, the influence of rotational speed on the nearest neighbour distance was noticed. For the speed of 450 RPM, over 95% of the analysed population is below 10 µm, of which the nearest neighbour distance, less than 5 µm, is shown by approx. 50% of the studied precipitates. For comparison, for the initial material, only 32% of the population was in this range.
The results of measuring the size of the SnSb phase precipitates presented in Fig. 9a confirm its considerable refinement after FSP modification. There was an almost tenfold increase in the percentage of the smallest particles with a size below 5 µm² in the material after FSP treatment compared to the initial material. The use of FSP modification practically eliminates precipitates larger than 20 µm², while in the initial alloy these particles account for nearly 78% of the studied population of precipitates. The nearest neighbour distance measurement results also confirm a significant increase in particle refinement as a result of FSP modification. The NND histogram (Fig. 9b) indicated a significant shift towards shorter distances. Virtually the entire population of SnSb precipitates for the material after FSP modification is in the range below 15 µm, including 40–50% - below 5 µm, while in the case of the initial material no particle was recorded in the smallest distance interval; in contrast, 91% of the studied particles are at a distance greater than 15 µm.
3.2 Mechanical properties
The results of the Brinell hardness measurements for the material after FSP modification depending on the rotational speed of the tool are shown in Fig. 10. Modification of the SnSbCu alloy by FSP caused a slight fall in hardness in relation to the initial alloy (20 HB), which is related to recrystallization of the alloy (Fig. 3). No significant influence of the tool rotational speed on the hardness result was noticed; the obtained hardness values remain at the level of 17–18 HB, of which the alloy after modification with the speed of 450 RPM has the highest hardness value (Fig. 10). At the same time, the 450 RPM modified alloy is characterized by the highest degree of refinement of hard particles of Cu6Sn5 precipitates and slightly larger SnSb precipitates compared to the material modified at the lower rotational speed (Fig. 9), which may justify the highest hardness for this material.
The results obtained in the static compression test depending on the state of the SnSbCu alloy (as cast, after FSP treatment with different speeds) are shown in Fig. 11. Table 3 summarizes the values of real stresses determined for all the considered variants of the alloy, which correspond to the actual strain amounting to 0.1 and 0.3. The initial alloy is characterized by higher stress values in relation to the alloy after FSP modification, which can be explained by recrystallization of the tin-rich matrix. As a result of dynamic recrystallization of the material during FSP, the density of structural defects in the material is reduced. Therefore, there is a reduction in the strength properties of the material and improvement in its plastic properties.
The results of the flexural strength test (Fig. 12) showed its increase after FSP modification compared to the initial alloy, for which the flexural strength value was 214 MPa, and after the modification process, 307 MPa (280 RPM) and 290 MPa (450 RPM), respectively. The increase in flexural strength in the case of the alloy after FSP modification is caused by refinement of and a change in the morphology of the Cu6Sn5 phase particles to a more globular shape. It is worth noting that in the case of the samples after FSP treatment, cracks appeared on the advancing side, as shown in the photos of the samples after bending (Fig. 12). Due to the turbulent and highly dynamic flow, the stir zone is characterized by variable strains and strain rates around the rotating and simultaneously moving tool, generating the presence of various stress states (and cyclic forces) . The tool rotation and travel directions are consistent on the advancing side, while on the retreating side they are opposite directions, with the result that the modified zone area on the advancing side is usually the hotter side of the process zone and has a higher stress level than the retreating side in the modified material [20, 21], at the same time it is more prone to cracking.
The fractures of the samples after bending have a transcrystalline malleable character with visible elevations and depressions resulting from plastic deformation within the plastic matrix, both for the initial alloy and the samples after FSP modification, and are brittle within the numerous intermetallic phases located in the alloy.
In the case of the cast material, numerous acicular precipitates of hard particles of Cu6Sn5 phases are visible, which undergo brittle fracture  much easier than the finer ones without sharp edges, formed after FSP modification (Fig. 13a). The sharp edges of the Cu6Sn5 acicular precipitates may favour the formation of cracks during the flexural test. After FSP modification, an increase in the share of the ductile nature of cracking was observed.