The Production-Related Influence of Iron Oxides on Steel Surfaces on the Adhesion of Fusion-Bonded Hybrid Structures

In view of e-mobility and the still challenging weight of lithium-ion battery packages as well as the demand for a reduction of exhaust emissions, in particular carbon dioxide, the development of innovative lightweight solutions is required. Therefore the manufacture of tailored hybrid structures fabricated within a continuous production line represents a promising approach for the automotive industry. Roll forming of tailored metal sheets is state of the art and already implemented in the automotive production of roof headers, rocker panels, lower radiator support beams or rear seat back beams. The continuous roll forming process of hybrid structures, consisting of steel sheets locally reinforced by carbon fiber reinforced thermoplastic tapes, offers a high degree of automation as well as decreased weight compared to a roll-formed part of pure metal. With the focus on minimizing the process steps in the present work the steel sheets and the thermoplastic tape are joined by fusion bonding whereby the thermoplastic matrix is melted on the steel surface. With the objective of improving the joint strength of the hybrid structure both materials are heated separately by different heating technologies and joined in a subsequent process step. As a consequence the formation of iron oxides on the steel surface, depending on the heating temperature, can be observed. Depending on various heating temperatures, the steel surface is analyzed physico-chemically by Drop-Shape Analysis as well as chemically by using X-ray Spectroscopy. In addition, the evaluation is carried out by mechanical testing of lap shear specimens. The chemical examinations show the formation of iron oxides on the steel surface, based on prior tempering, resulting in improved adhesion to the carbon fiber reinforced thermoplastic tape.