Process-adapted temperature application within a two-stage rivet forming process for high nitrogen steel

Abstract

Mechanical joining technologies like self-piercing riveting are gaining importance with regard to environmental protection, as they enable multi-material design and lightweight construction. A new approach is the use of high nitrogen steel as rivet material, which allows to omit the usually necessary heat treatment and coating and thus leads to a shortening of the process chain. Due to the high strain hardening, however, high tool loads must be expected. Thus, appropriate forming strategies are needed. Within this contribution, the influence of applying different temperatures for each forming stage in a two-stage rivet forming process using the high nitrogen steel 1.3815 is investigated. The findings provide a basic understanding of the influence of the temperature management when forming high nitrogen steel. For this purpose, the rivets are not formed at the same temperature in each stage, but an elevated temperature is applied selectively. Different process routes are investigated. First, cups are manufactured in stage 1 at room temperature, followed by stage 2 at 200°C. Second, cups are formed in stage 1 at 200°C and used for stage 2 at room temperature. By comparing the findings with results when applying the same temperature in both stages, it is shown that the temperature during the first forming operation has an effect on the forming behaviour during the second forming stage. The required forming forces and the resulting rivet hardness can be influenced by process-adapted temperature application. Furthermore, the causes for the temperature impact on the residual cup thickness in stage 1 are evaluated by a cause and effect analysis, which provides a deeper process understanding. The thermal expansion of the tool and the billet as well as the improved forming behaviour at 200°C are identified as the main influencing causes on the achieved residual cup thickness.

Mechanical joining of high-strength multi-material systems − trends and innovations

“…On the basis of the material characterization in [91] it can be concluded that the yield strength and the true stress level during forming of 1.3815 can be reduced by forming the material at elevated temperature and that this is already the case with a temperature increase up to 200 °C compared to room temperature. Consequently, the application of different forming temperatures in both forming stages was investigated in [90] in order to identify a suitable process-adapted temperature application for the rivet manufacture. For this purpose, the influence of various temperature combinations including room temperature and 200 °C in the two-stage rivet forming process on the necessary process forces but also on the resulting mechanical properties of the rivets after forming were analyzed.…”

Study on the forming process and shear mechanical behavior of CFRP/Al self-piercing riveting employed 3D modeling

“…The SPR forming principle is that the rivet pierces the upper plate and forms an interlocking structure with the lower plate, so it can be used to join plates of the same or dissimilar materials. 7,8 Hoang et al 9 realized the use of aluminum self-piercing rivets to join two Al plates of the same type together by conducting an experimental design and investigated the effect of the die geometry on the performance of aluminum SPR joints. Franco et al 10 investigated the possibility of joining CFRP and Al plates by SPR.…”