Forming behaviour of multilayer metal/polymer/metal systems

Abstract

Due to the growing demands to reduce the fuel consumption and improve the environmental impact of the vehicles, developing alternative lightweight materials plays an essential role. Here, Metal/pol-ymer/metal (MPM) multilayered composite materials are an innovative substitute to the used com-mercial sheets. MPM provide the desired lightweight potential with comparable structural stiffness in addition to an improved thermal and acoustic isolation properties. The current study is concerned with providing design guidelines and fundamental correlations regarding the production and shaping of low-density, flat, multi-layered and formable MPM systems. With the aid of the proposed investi-gation approaches, it was possible to produce tailored MPM following an ascending scaling approach starting from the monolithic materials up to multilayered structures. A thermoplastic polyolefin foil with different thicknesses was common in the produced MPM, as it offers an outstanding forming potential at room temperature. In order to describe the correlation between the characteristic properties of the skin/core layers and their influence on the forming be-haviour, diverse skin sheet materials, thicknesses and mechanical properties were considered. They include stainless, dual-phase and deep-drawable steel grades in addition to aluminium and titanium ones for different application fields. Furthermore, the applied production technology by roll bonding allowed the flexibility tailoring arbitrary MPM combinations via utilizing a compatible metal/polymer adhesive agent. Symmetric MPM systems (same skin sheet grade and thickness) and asymmetric ones were produced and characterized aiming at better understanding the forming behaviour for diverse MPM structures. In addition to determining the characteristic properties of the monomaterials – like the mechanical properties, bending and deep drawability, and forming limit curves – additional specific properties of the MPM, like the adhesion quality and service life durability, were evaluated as well. Accordingly, the principal correlations could be derived for predicting and interpreting the forming behaviour. The metal/polymer interface and failure surface showed an outstanding durable adhesion quality that assured delamination-free forming operations that was confirmed by the performed experiments. Furthermore, the mechanical tensile properties were correlated primarily to the skin/core thickness ratio and the MPM symmetry and additionally could be verified with simple approaches like the rule of mixtures. The formability in terms of the deep drawability and the stretch forming showed a signif-icant correlation with the MPM structure. The limiting drawing ratio (LDR) was reduced for higher core/skin thickness ratios and became more limited for thinner skin sheets. The formability is signif-icantly reduced if the polymer core volume fraction exceeds 50%. In this case, the interlaminar shear-ing of the MPM layers and the thickness irregularities are unavoidable. The formability was further restricted, if the thinner skin sheets are positioned as the outer skins in asymmetric MPM sheets either in three or five layered systems. Furthermore, the drawing forces and the LDR could be pre-dicted with a deviation up to ±15% utilizing empirical approaches. Additionally, the core of the MPM sheets was locally reinforced with metallic reinforcements (RE) to enable applying mechanical and thermal joining methods and to reduce the local thinning especially of the thin skin sheets used in the MPM. It was found that inhomogeneity regions were arisen at the RE boundary under deep drawing and stretching conditions leading to reduced forming limits of the skin sheets and accordingly earlier failure. Different parameters were considered like the RE size, geometry, location and type. It was found that the RE location and the MPM thickness or rather the skin sheet thickness are the critical parameters: when the RE is positioned close or in the forming or rather the bending region, the forming limits are significantly reduced. There is still a need to improve the forming behaviour of the reinforced MPM by means of further solution approaches to avoid the inhomogeneity regions at the interface between the RE and neigh-bour non-reinforced core and the shifting of the RE during forming. Further motivation is to develop approaches to predict the material flow especially in the surrounding of the reinforced regions con-sidering the RE size and type. The outlook is based in particular on the research needs and questions regarding the applicability of the of sandwich materials in a form (size and geometry) closer to the application scale utilizing the gained correlation from the current study. In this concern, investigating the crash behaviour of stand-ard profiles like a double hat or Z-profiles is essential.