Finite Element Implementation of a Bonding Model and Application to Roll Bonding of Aluminum Sheets of Largely Different Yield Strength

Abstract

Roll bonding is a joining-by-forming operation, in which two or more metallic strips or plates are bonded permanently through the pressure and plastic deformation in the roll gap. Although roll bonding has been successfully used in industrial production over many years, difficulties occur especially when materials of largely different yield strength are roll-bonded, e.g. when hard aluminum alloys are clad with soft commercially pure aluminum. Examples are AA2024 sheets used in wing and fuselage structures of aircrafts, which are clad with AA1050 to improve the corrosion resistance. Likewise, aluminum sheets for heat exchangers consist of a hard base material that is clad with a soft solderable aluminum alloy. In these cases, the strength difference may influence the bonding behavior since the softer face sheet has to transmit the deformation to the harder core material. To analyze and optimize such cases, a bonding model integrated into a numerical framework for the simulation of the roll bonding process is required. In this paper, a finite element model is presented, in which the development of bond strength is simulated using a cohesive contact formulation. The model is used to study the bonding behavior of laboratory-scale roll bonding trials of two aluminum alloys with a large difference in yield strength. It is found that shear stresses are generated towards the end of the roll gap that may exceed the shear bond strength created earlier in the roll gap such that no firm bond is obtained. The conditions under which bonding is successful are analyzed using a finite element simulation study with varying yield stress differences and pass reductions and summarized in a map.