## 5 Conclusions  
The basic criteria for assessing the suitability of reinforcing phases were the changes observed in the material microstructure and composite properties. Despite the differences in the shape, size, and morphology of the reinforcing phases used, a composite surface layer was obtained in all researched material variants, and the FSP treatment was accompanied by favorable changes in the matrix, expressed in strong grain refinement, a more uniform distribution of intermetallic phases, and the disappearance of the rolling microstructure.  
Figure 10: Worm surface of composite with A-1 reinforcing phase (**a**), and A-2 reinforcing phase A-2 (**b**). Light microscopy.  
Figure 9: Results of tribological tests.  
The average grain size in the starting material was 290 \(\upmu\)m, while in the stirring zone of the friction stir processed material, it ranged from 1 to 4 \(\upmu\)m, regardless of the reinforcing phase used.  
A side effect of FSP treatment was a reduction in the share of intermetallic phases owing to their partial dissolution in the solid solution. However, reducing the content of intermetallic phases did not lower the hardness or resistance of the material to abrasive wear. As the studies showed, the effects of dissolving the phases in the solid solution were effectively compensated for by the introduction of the reinforcing phase into the matrix, strong grain refinement, and more even redistribution of the remaining, not yet dissolved, precipitates.  
Changes in the microstructure caused by FSP treatment and the introduction of a reinforcing phase resulted in a significant rise in hardness and abrasive wear resistance compared to the properties of the AA7075 aluminum alloy. The average hardness of the material reinforced with the A-1 phase was HV0.1 = 137, and the material reinforced with the A-2 phase was HV0.1 = 132, which corresponded to growth in the hardness of the material after FSP treatment and the introduction of the reinforcing phase by 41% and 36%, respectively.  
The linear loss of the sample reinforced with the A-1 phase in the tribological test was 118.5 \(\upmu\)m, and the sample containing the A-2 phase was 134 \(\upmu\)m. This loss in the case of the initial aluminum alloy was 176.6 \(\upmu\)m; therefore, it was higher by 49% and 32%, respectively, compared to the linear loss recorded for the composite samples reinforced with phases A-1 and A-2. The abrasive wear resistance, expressed by the size of the registered linear losses of the samples, was therefore 32.9% higher in the case of the composite reinforced with the A-1 phase than that of the abrasion resistance of the AA7075 aluminum alloy and by 24.1% in the case of the A-2 reinforcing phase.  
The conducted research has shown that the reinforcing phases investigated in this work in the form of ash from the combustion of fossil fuels and biomass can constitute a valuable component in the production of metal matrix surface composites. Nonetheless, when deciding to use them, one should take into account the risk of the high variability of the chemical composition and morphology of these materials depending on the type of raw material used or the parameters of the technological process.  
Conceptualization, J.I.; methodology, J.I. and K.K.; material research, J.I., K.K. and M.S.; writing, J.I., K.K. and M.S.; review and editing, J.I., K.K. and M.S.; supervision, J.I. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors declare no conflicts of interest.