## 4 Conclusions and Outlook  
In this paper, the influence of material properties (cover sheets: TS275-0.4 mm and TS290-0.3 mm, and core layers: PA6, RG and RGUD), thicknesses of core materials (PA6: 0.5, 1.0 and 1.5 mm, and organosheets: 0.5, 1.0 and 2.0 mm) and forming temperatures  
Figure 22: FLC levels for: (**a**) FML02 with different core thicknesses and (**b**) at different temperatures, influence of cover sheets on FLC levels: (**c**) FML01 and (**d**) FML02. (RT, 200 and 235 \({}^{\circ}\)C) on stretch-forming behaviour and FLC-levels were experimentally determined and characterised. Based on the obtained results, the following conclusions can be made:  
* Stretch forming of the organosheets at RT was highly restricted. A further increase of the stretching force was due to its resistance of unfractured fibres in warp direction of RGUD. * The formability of the MPMs was found to be comparable to that of their monolithic steel cover sheets, yet it decreased when core thickness increased. The same effect was found regarding the FLC-levels. * The stiffness of FML increased when core thickness increased, but its formability decreased accordingly. * At 235 \({}^{\circ}\)C, i.e., above the melting point of PA6, no remarkable difference of drawing depth and stiffness of FML with varied core thicknesses and different organosheets RG and RGUD can be stated. The formability of FML was comparable to that of the monolithic steel covers. * At 235 \({}^{\circ}\)C, irregular thickness distribution of thicker core layer (>1.0 mm) in FML led to decreasing the dynamic friction coefficient and earlier failure of the inner steel sheet. * At 235 \({}^{\circ}\)C, no fibre cracking in the FML was detected, but severe distortion in the flange region due to the restriction of the fibres' movement due to the drawing beads was seen. * Regarding the FLC of MPM, no remarkable improvement of the FLC-level was observed by increasing the forming temperature. * The FLC-levels of FML significantly improved (almost > 300%) at the elevated forming temperatures. The influence of the core thickness (0.5-1.0 mm) on the FLC-level was ignorable at RT, 200 and 235 \({}^{\circ}\)C. However, organosheets dominated the FLC-level at RT and the steel cover sheets dominated at elevated temperatures. For the warm stretch forming of FML, further improvement approaches can be undertaken. For example, by using a multi-step variothermal stretching process, sandwiches are stretched below the melting point (e.g., 200 \({}^{\circ}\)C) of the matrix PA6 to obtain the maximum forming limit, and then the shaped specimen can be bonded sufficiently by increasing the temperature (\(\geq\)230 \({}^{\circ}\)C) and applying counter-pressure via a support tool part. This eliminates the non-uniformity of thickness distribution that occurs after the FML has been formed in the molten state of matrix and therewith to avoid the change of dynamic friction state and the earlier cracking of inner cover sheet. In addition, reliable simulation models are needed to be developed to predict the fluidity of the core layer in the molten state and the failure modes of FML during the variothermal forming process, such as thickness change, fibre fracture and cover sheet cracking. Conceptualization, W.H., M.H., G.Z. and H.P. ; methodology, W.H. and M.H. ; software, W.H. ; validation, W.H. ; investigation, W.H. and M.H. ; resources, G.Z. and H.P. ; data curation, W.H. and M.H. ; writing--original draft preparation, W.H. and M.H. ; writing--review and editing, M.H., G.Z., A.C. and H.P. ; visualization, W.H. ; supervision, G.Z., A.C. and H.P. ; project administration, G.Z., A.C. and H.P. ; funding acquisition, G.Z. and H.P. All authors have read and agreed to the published version of the manuscript. This research was funded by the German Research Foundation, grant number 330043166. Not applicable. The authors are ready to share the research data upon request. We acknowledge financial support by Open Access Publishing Fund of Clausthal University of Technology.