## 3 Results and Discussion  
### Residual Stress  
This process of welding aluminum alloys enables the production of aircraft structures while reducing labor consumption, costs, and weight, while maintaining comparable or higher strength parameters compared to conventional joining methods. However, it introduces tensile stresses in the joint. Tensile stresses improve dislocation mobility and, as a result, reduce hardness [34; 35]. Plastic deformation occurring in the surface layer of the weld under the influence of the burnishing tool with a diamond tip can be considered a process of generating new dislocations and their displacement. Dislocationgeneration occurs when a certain level of stress is reached. Since the deeper, non-deformed layers do not change, compressive stresses arise in the surface layer of the joint, increasing the joint's fatigue strength. Figure 3 and Table 4 present the results of measurements of residual stresses of butt joints subjected to slide burnishing with a diamond tool. The analysis of the test results shows that, regardless of the adopted setting parameters of the process, slide burnishing introduces compressive stresses both in the direction transverse \(\sigma_{x}\) to the weld axis and along the weld axis (\(\sigma_{y}\) direction). In the x-axis direction, the largest increase in compressive stresses from \(\sigma_{x}=2\) MPa to \(\sigma_{x}=-130\) MPa was recorded for experimental run no. 4 (\(P=50\) N and \(f=500\) mm/min). In turn, in the direction of the y-axis, as a result of applying a force of \(P=60\) N and a feed rate of \(f=500\) mm/min (experimental run no. 7), compressive stresses of \(\sigma_{y}=-242\) MPa were introduced. It should be noted that before the burnishing treatment in the considered direction, only small compressive stresses of \(\sigma_{y}=-6\) MPa. For stresses in the transverse direction, at feed rates of \(f=500\) mm/min and \(f=1000\) mm/min, an increase in contact force causes an initial increase in compressive stress by \(51.16\%\) at a feed rate of \(f=500\) mm/min and \(13.75\%\) at a feed rate of \(f=1000\) mm/min, and next decrease in compressive stress (Figure 4a). In the case of a \(f=1500\) mm/min, the highest value of compressive stresses was recorded at a burnishing force of \(P=40\) N. In this case, a further increase in the burnishing force causes only a decrease in the stress value. In the case of stresses in the longitudinal direction, the opposite trend can be observed. At the feed rate of \(f=500\) mm/min and \(f=1000\) mm/min, the increase in the tool clamping force causes an increase in the value of compressive stresses. In the case of the feed rate of \(f=500\) mm/min, as a result of changing the force from \(P=40\) N to \(P=60\) N, the compressive stress increased by \(53.16\%\) (to \(\sigma_{y}=-242\) MPa), while at the feed rate of \(f=1000\) mm/min by \(26.59\%\) to the value of \(\sigma_{y}=-238\) MPa). Atthe feed rate \(f=1500\) mm/min, the increase in force from \(P=40\) N to \(P=50\) N initially results in an increase in the compressive stress value by 20.10% to \(\sigma_{y}=-221\) MPa, and then with the increase in force, a decrease in the stress value by 13.38% (Figure 4b).