Influencing Intermetallic Layers of Hot Stamped Steel for Adhesively Bonded Plastic Metal Hybrids

Abstract

Hot stamping of high strength steel parts is an established way to realize today ́s structural lightweight components in car bodies through sheet thickness reduction. The steel typically used for hot stamping is a boron-manganese 22MnB5 steel reaching up to 1,900 MPa in strength. New boron-manganese alloys achieving higher strength are expected to be developed, generating challenges for the manufacturing process by means of formability. Thus, a further reduction of steel thickness resulting in lighter components is not expected. Hybrid lightweight design approaches aim at weight reduction by reducing steel thickness and applying fiber-reinforced plastics (FRP) to regain structural stiffness and strength. The use of residual heat remaining from the hot stamping process allows to activate adhesives to bond FRP to hot stamped steel. The performances of adhesive bonds depend strongly on surface characteristics. To avoid scaling and decarburization during the heating process several coatings for hot stamped steels are used forming intermetallic layers through heat treatment. The most common coating in today’s automotive application is an Al-Si coating. Thus, the overall performance of the adhesive bonded hot stamped metal polymer hybrid is not only depending on adhesives performance but also strongly on the hot stamped steel’s coating performance. In this paper, the characteristics of hot stamped steel 22MnB5 Al-Si coating are investigated with regard to adhesion performance. Therefore, hot stamped specimens are manufactured under realistic industrial conditions investigating the influence of furnace temperature and dwell time on the overall coating and intermetallic layers of Al-Si coating. The specimens are investigated with respect to Al-Si coating thickness, lap shear strength of hybrid specimen and tensile strength of hot stamped steel demonstrating the dependency of the overall hybrid specimen performance from the coating performance.