## 2 Materials and Methodology  
### Materials for Investigation  
As a material for investigation, 0.99 purity mint silver was used. Two metal powders were applied for alloying: chlorine (Cr) and nickel (Ni). The properties of the powders are listed in Table 1.  
To investigate the treated Ag surface layer, a specially chosen laser system was applied. Surface treatment was performed using the fibre Ytterbium laser system YLS-4000 (IPG Photonics, Oxford, MA, USA), equipped with a head to obtain a wavelength of \(\lambda\) = 1070 nm (Figure 2). The highest power value of the laser beam was 4 kW. The head was placed on the special six-axis REIS RV30-26 (Reis Robotics, Obernburg, Germany) mounted on the dedicated tilt-and-swivel positioning system. The laser alloying itself was conducted in a protective argon gas shielding atmosphere. The gas was supplied with two independent nozzles. The first of the nozzles was tilted at a 30\({}^{\circ}\) angle to the sample plane, and the second was tilted at 90 degrees. The first nozzle was placed to provide the alloying of chromium or nickel powder in the molten metal area. The second nozzle was set to provide the protective gas for air oxidation protection. The maximum power value of the laser beam was 4 kW. The laser head was placed on a special six-axis REIS RV30-26 mounted on a special tilt-and-rotate sample mounting system. The laser alloying process was performed in a protective argon atmosphere to protect against the effects of the external environment. Gas was supplied using two independently located feed nozzles. The first one was inclined at an angle of 30\({}^{\circ}\) to the sample plane, while the second one was inclined at an angle of 90\({}^{\circ}\). The first nozzle was placed to ensure proper dosing of the alloying powder of chromium or nickel (Figure 3) in the molten metal area, and the second nozzle was designed to supply a shielding gas to protect against oxidation of the treated surface. \begin{table}
\begin{tabular}{c c c} \hline
**Property** & **Cr** & **Ni** \\ \hline Density, kg/m\({}^{3}\) & 15.69 & 4.25 \\ \hline Vickers hardness, HV\({}_{0,05}\) & 3400 & 1550 \\ \hline Melting point, \({}^{\circ}\)C & 2870 & 3140 \\ \hline Thermal expansion coefficient \(\alpha\), 10\({}^{-6}\)/\({}^{\circ}\)C & 23.5 & 8.3 \\ \hline Grain size, \(\mu\)m & 80 & 5 \\ \hline Mixture ratio & 2 & 1 \\ \hline \end{tabular}
\end{table} 
Table 1: Selected physical and mechanical properties of the materials. Figure 2: YLS-4000 fibre ytterbium laser used for remelting and alloying. The grain sizes of the chromium and nickel powders used ranged from approximately 60 \(\upmu\)m to 130 \(\upmu\)m. These were determined to be appropriate because too small a granulation resulted in complete alloying in the matrix, while too large a powder particle size caused difficulties in mixing the powder with the matrix during alloying as a result of the relatively low wettability of large powder grains. Particular attention should be paid to removing moisture and/or dirt from the treated surface and powder prior to the process. The presence of moisture may cause significant increases in surface porosity and roughness, which adversely affect the obtained strength properties and quality of the microstructure of the surface layer. Before processing, the metal powders were dehydrated in a laboratory dryer, at a temperature of 110 \({}^{\circ}\)C for 12 h. Before surface treatment, the aluminium samples were mechanically ground on a Tegramin 30 Struers laboratory grinder using sandpaper with a gradation of 80 \(\upmu\)m, 40 \(\upmu\)m, and finally 15 \(\upmu\)m. In the next stage, the polished surfaces were cleaned in an ultrasonic scrubber and washed with ethyl alcohol. The prepared surface was covered with a flux (lithium chloride with methyl alcohol). To remove alcohol and moisture immediately before the process, the sample was dried on a heating plate at a temperature of 423 K. The flux used (paste) had a double effect.