## 2 Materials and Experimental Work
### Tool Design and Lubrication  
For this research, the test conditions for sheet deep drawing are summarised in Table 2. The diameter of the blank and punch, the punch radius, and the test speed were kept constant, while the die radius, holding force, and forming temperature were varied. Two dies with radii of ten and fifteen millimetres and separate inner diameters were prepared (see Figure 3). In order to obtain the predesigned temperature, the test machine was modified; the blank holder was equipped with heating cartridges to provide the required forming temperature. In addition, the samples, punch, and die were preheated in an external furnace to 20 \({}^{\circ}\)C above the predesigned temperature to compensate for the temperature loss occurring during their transfer. It should be mentioned that these three tool components were separated from the machine by means of insulating ceramics to avoid excessive heating of the machine and to maintain the temperature in the sample. The forming temperatures were adjusted at 235 \(\pm\) 5 \({}^{\circ}\)C and 200 \(\pm\) 5 \({}^{\circ}\)C; herewith, (quasi) isothermal warm forming could be assumed and considered. For further results, the real-time temperature progress is depicted in [31]. In addition, early defects such as tears and wrinkles in the deep-drawing process can be minimised or even avoided if suitable lubrication conditions are present. Therefore, different lubrication conditions were tested for cold and warm deep drawing:  
1. In Figure 4a, using a pure thermoplastic film (TP film: 0.05 mm), the drawing behaviour of a monolithic TS290 was significantly limited, with a maximum drawing depth of 15 mm at RT. 2. With the addition of grease in the setting shown in Figure 4b, the drawing depth was slightly improved after reaching 25 mm. Moreover, waviness on the steel sheet was observed following the configuration in Figure 4c. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Blank** & **Punch** & **Punch Radius** & **Die Radius** & **Test Speed** & **Holding Force** & **Temperature** \\
**Diameter** & **Diameter** & **r\({}_{\text{p}}\) (mm)** & **r\({}_{\text{d}}\) (mm)** & **v (mm/s)** & **F\({}_{\text{h}}\) (kN)** & **T (\({}^{\circ}\)C)** \\ \hline
180 & 100 & 15 & 10/15 & 0.5 & 6–100 & RT, 200/235 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Test conditions for the deep drawing experiments. Figure 2: Surface treatments of cover and core layers and the production scheme of the sandwich panels by means of hot pressing. A further lubricant configuration was tested, as shown in Figure 4d, and led to a significant improvement, as the drawing depth reached 50 mm. This was the lubrication setting used for further cold deep-drawing experiments. 4. Furthermore, the lubrication settings for the warm deep drawing of the monolithic steel sheet and FML were found to be similar to those at RT, where only the TP film and grease were replaced by a Teflon film (0.05 mm) and Molycote spray (graphite based), as shown in Figure 4e,f. Figure 5 illustrates the force-displacement curve of the deep-drawn TS290 and FML for lubrication conditions shown in Figure 4a-f. In the absence of grease, the slope of the force-displacement curve was highest for the lubrication setting shown in Figure 4a. Despite the addition of grease in Figure 4b, the drawing depth was limited to 23 mm and the slope of the force-displacement curve was only slightly reduced compared to the setting shown in Figure 4a. Despite the high drawing depth and lower forming force of the setting  
Figure 4: Different lubricant settings for deep drawing at RT (**a–d**): F\({}_{\rm{h}}\) = 6 kN and r\({}_{\rm{d}}\) = 10 mm and at 235 \({}^{\circ}\)C (**e**,f): F\({}_{\rm{h}}\) = 100 kN and r\({}_{\rm{d}}\) = 10 mm. Figure 3: Tool design for warm deep drawing. in Figure 4c, waviness was observed in the sidewalls.