Numerical simulation of hybrid joining processes: self-piercing riveting combined with adhesive bonding

Reliable simulation of hybrid joining processes using conventional finite element (FE) tools is challenging, because the liquid adhesive must be somehow included in the model. Thus, in this work the viscoelastic properties of the adhesive are substituted with “equivalent” mechanical properties. The complex viscosity of an epoxy-based single-component adhesive was determined at five temperatures between 20‒55 °C and at seven shear rates between 1‒150 s-1 using a rheometer. Flow stresses and strain rates were calculated from the complex viscosities and from the shear rates. For each temperature investigated the relationship between flow stress and strain rate was fitted with a power-law, which enables modeling the actually liquid adhesive as solid with strain rate-dependent flow stress. In order to validate the material model, a defined volume of adhesive was uniaxially compressed. This testing setup was also modelled using the FE software Simufact Forming 15. In the model the Young’s modulus of the adhesive was iteratively adapted until good agreement between the numerical and the experimental force-displacement curves was achieved. The obtained mechanical properties were finally used for modeling the adhesive layer between two 2.0 mm-thick 6xxx aluminum alloy blanks in the hybrid riveting-bonding process. An axisymmetric model including deformable (rivet, upper blank, lower blank, adhesive layer) and rigid (punch, die, blankholder) components was built in Simufact Forming. The cross-section of the hybrid joint obtained from simulation showed very good geometrical agreement with cross-sections obtained from the joining experiments, and just small differences between the calculated and the measured force-displacement curves was observed.