## 2 Materials and Methods  
The matrix of the designed composite was the AA7075 aluminum alloy (Al-5.5 Zn-2.4 Mg-1.6 Cu-0.20 Cr) in the T6 condition (an alloy treated with a supersaturated solution and artificially aged). The samples had the shape of cuboids with the dimensions 100 \(\times\) 70 \(\times\) 15 mm (length \(\times\) width \(\times\) sample thickness). The chemical composition of the alloy is presented in Table 1, and an example microstructure is shown in Figure 1.  
Microstructural investigations were carried out on cross sections perpendicular to the tool traverse direction. The metallographic specimens were etched with [PERSON]'s reagent (2 mL HF + 3 mL HCl + 5 mL HNO\({}_{3}\) + 190 mL distilled water). Microscopic examinations were performed using a Keyence VHX-7000 light microscope (Keyence Ltd., Osaka, Japan). The aluminum alloy had a typical rolling microstructure with highly elongated grains typical of rolled material, which reflected the rolling direction and gave it a banded feature. The average length of the grains of the \(\alpha\) primary solution in the analyzed alloy was approximately 290 \(\upmu\)m. Moreover, numerous precipitates of intermetallic phases were found in the alloy. These precipitates were located both inside the grains and at the grain boundaries of the solid solution. Identification of the chemical composition of the precipitates was carried out using an Oxford Instruments EDS microanalyzer (Oxford Instruments Plc., Abingdon, UK). These studies showed that the AA7075 alloy contains precipitates rich in Al, Zn, and Cu, but also precipitates containing Al, Fe, and Cu and less numerous phases rich in Al, Si, and Zn and Al, Zn, and Mg. Studies of the phases occurring in the AA7075 aluminum alloy were presented in a previous work [16]. Two ashes obtained in the process of coal and biomass combustion were used as the reinforcing phase. In the case of the ashes marked as A-1, the share of coal and biomass in the fuel was 70-90% in the case of coal and 10-30% in the case of biomass, respectively. These shares in the case of the second ash (marked as A-2) were as follows: 90-95% in the case of coal and 5-10% in the case of biomass. The chemical composition of the ashes is presented in Table 2. The chemical composition was determined by X-ray fluorescence spectroscopy. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Alloy** & & & & & & **Element Content, wt\%** & & & \\ \hline
7075 & Zn & Fe & Cu & Mn & Mg & Cr & Si & Ti & Al \\
5.5 & 0.3 & 1.6 & 0.15 & 2.4 & 0.2 & 0.2 & 0.1 & 89.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of AA7075 aluminum alloy. Figure 1: Microstructure of AA7075 alloy. Etched, light microscopy. SiC powder was also used to produce the composite. Technical SiC powder was utilized with an average particle size of approximately 20 \(\upmu\)m and a polyhedral shape. The composite reinforced with SiC particles was the reference material in this work. Important parameters characterizing each material used as a reinforcing phase in a composite are its physicochemical properties, especially the size, shape, and geometry of the surface. These properties result from the employed production technology, but also from the origin of the material. Too complex a shape and a highly developed geometric structure of the surface make it difficult to distribute the reinforcing phase in the composite matrix and to achieve an even distribution of such a phase in the matrix. In such a case, the risk of unfavorable clusters and agglomerates occurring in the composite also increases as a result of the mutual adhesion of neighboring particles with surface irregularities. The occurrence of agglomerates is often accompanied by material discontinuities. These discontinuities are usually located inside the agglomerates due to the inability or limited ability of the plasticized matrix to penetrate such a place. Therefore, in the process of designing composites and selecting the reinforcing phase, its physicochemical properties are also assessed.