Simulation of the joining process of graded hardened multi-range capable rivets

Abstract. The versatile self-pierce riveting (V-SPR) is a further development of semi-tubular self-pierce riveting. V-SPR enables adaptation to changing boundary conditions, such as a change in the material thickness combination, without varying the rivet die combination due to increased punch actuation and the use of multi-range capable rivets [1]. The inner punch first sets the rivet. The outer punch then forms the rivet head to the respective sheet thickness. For this, the rivet requires a hard shank and a ductile rivet head, which is achieved by an inductive local hardening process [2]. Until now, the joint formation of rivets with graded hardness profile has been challenging to estimate in the FEM simulation due to the inhomogeneous material conditions in the rivet. In this study, a method capable of reproducing the experimentally determined hardness levels of rivets in detail is shown. This FE model enables the realistic modelling of the mechanical properties of the rivet on the basis of the hardness profile in order to predict the correct deformation processes and stresses during the riveting process. First, the detailed experimental hardness mapping of the locally heat-treated rivets is transferred into the FE model. The FEM material model can predict the local strength of the rivet based on hardness by scaling the flow curves. To estimate the predictive capability of the FEM model, the joint formation of rivets with different graded hardness profiles is compared experimentally and simulative. Based on the validated model, the influence of different rivet hardness profiles on the joint formation is analysed numerically. By adapting the material model, a high level of correlation between the experiment's joint formation and the simulation can be achieved.