*4.1. Analysis of the Unloaded Geometry*

The influence of the partial heating on the unloaded bending geometry is shown focusing on springback and warping (Figure 9a) From 25 to 300 ◦C, the warping angle and springback ratio stay approximately constant. With further increasing temperature springback and warping angle are decreasing. For higher feed velocity the profile springback increases by a mean value of 12% (see Figure 9b). The increase of springback at higher feed velocity can be attributed to the strain rate hardening (Figure 4). The higher feed velocity of 8 mm/s leads to a doubled strain rate compared to the lower velocity of 4 mm/s, resulting in a change of the highest strain rate from 0.055 1/s to 0.11 1/s. This results in an increase of the flow stress by 9% at 0.2 strain for the 600 ◦C case.

**Figure 9.** (**a**) Profile specimens and corresponding cross-sections for a 600 mm loaded bending radius (change of moment arm through the heating area is neglected) and profile feed of 8 mm/s for different temperatures in the sprung back state. (**b**) springback ratio for varying partial heating temperatures and profile feed velocities.

Until a temperature of 300 ◦C in the heated area, the maximum change between the room temperature case and the partial heated case is 2%. For temperatures higher than 300 ◦C, the springback reduces approximately linearly with a maximum reduction of 56% at 600 ◦C.

To validate the numerical model, the experimental data are compared to the numerical data (Figure 10a). The numerical simulation can approximate the experimental springback

**Figure 10.** Comparison of springback (**a**) and related warping angle (**b**) between experiments and Simulation for 8 mm/s feed and loaded radius of 600 mm.

The related warping angle increases linearly over the arc length for each parameter set until a saturation value (Figure 10b) on the arc length of 600–800 mm is reached, after saturation, the related warping angle is constant with a deviation from a constant value of maximum 13%. Related warping angle is decreasing with increasing temperature. After saturation (*x* = 1100 mm), the decrease for 300 ◦C is 9% and for 600 ◦C the decrease is 76% compared to the room temperature curve. The mean deviation between experimental and numerical results for all parameter sets is 10%. The springback is only reduced at temperatures higher than 300 ◦C. This can be attributed to the flow stress reduction in the cross section. To further discuss this effect, the position of the principal forming zone must be determined.
