## 3 Results and Discussion
### Mechanical Properties of Drawpieces  
Table 9 presents the mechanical properties of the drawpieces measured in the directions parallel and transverse to the drawpiece axis, depending on the temper state of the workpiece material and additional heat treatment (HT) after forming. The values of yield stress, ultimate tensile strength and elongation were higher for the parallel direction compared to the transverse direction. Figure 14 shows the change in the value of mechanical properties (\(\lambda_{1}\)) of the drawpieces in relation to the values determined for the sheet metal. Figure 15 shows the effect of additional heat treatment on the properties of the drawpieces (\(\lambda_{2}\)). The coefficients \(\lambda_{1}\) and \(\lambda_{2}\) were determined according to the following relationships:  
\[\lambda_{1}=\frac{(YS;UTS;A)_{drawpiece}}{(YS;UITS;A)_{sheet}} \tag{2}\]  
\[\lambda_{2}=\frac{(YS;UTS;A)_{drawpiece+HT}}{(YS;UTS;A)_{drawpiece}} \tag{3}\]  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Temper Designation** & **Sample Orientation** & **YS, MPa** & **UTS, MPa** & **A, \%** \\ \hline \multirow{3}{*}{O} & transverse & 109 & 158 & 8.2 \\  & parallel & 146 & 169 & 10.4 \\  & transverse & 269 & 324 & 4.4 \\  & parallel & 275 & 338 & 5.4 \\  & transverse & 282 & 339 & 7.2 \\  & parallel & 289 & 351 & 6.6 \\  & transverse & 321 & 368 & 6.6 \\  & parallel & 333 & 390 & 6.8 \\  & transverse & 131 & 161 & 7.7 \\  & parallel & 133 & 162 & 7.8 It was found that as a result of incremental sheet forming, the strength of the material increased, especially in the case of annealed sheet. The lowest increase in the \(\lambda_{1}\)-value for the _YS_ and _UTS_ was noted for artificially aged sheet metal (Figure 14). The opposite situation was found for the case of elongation. The smallest decrease in the \(\lambda_{1}\)-value for elongation A was determined for the material in the T6 tempering state. This effect was mainly related to the work-hardening phenomenon as a result of incremental forming. In the case of the drawpiece made of annealed sheet, a larger limit-forming angle was obtained, and consequently, a larger degree of deformation, which in turn contributed to a greater work-hardening of the material in relation to the sheet in the as-received state. The work-hardening phenomenon is associated with the increase in flow stresses caused by permanent (plastic) deformation. There are two mechanisms of plastic deformation in metallic materials in cold working: twinning and slip. Twinning consists in twinning one part of the crystal relative to the neighbouring ones by a certain angle. Slip consists in shifting parts of the crystals relative to each other along specific lattice planes under the influence of external forces. Slip occurs in slip planes with the densest arrangement of atoms. Slip is the process of dislocation movement. During deformation, the number of active slip planes and free dislocations, which are generated during deformation, increases. The stresses causing twinning are greater than the stresses necessary for dislocation movement. Figure 14: Change in the mechanical properties of the EN AW-6082 aluminium alloy as a result of incremental forming. Figure 15: Change in the mechanical properties of the drawpieces as a result of additional heat treatment. Additional ageing after incremental forming had various effects on the properties of the drawpieces (Figure 15). In the case of the drawpieces made from annealed sheet, additional heat treatment contributed to slight changes in strength properties (increase in YS and decrease in UTS), with a simultaneous clear decrease in elongation A. On the other hand, additional natural ageing performed on the drawpiece made of solutionised sheet (W + T4) resulted in an increase in the material properties. Artificial ageing of the solutionised material (W + T6) and in the T6 condition only resulted in an increase in elongation A of the material, with a simultaneous decrease in the YS and UTS. This can probably be associated with the ageing of the material.