Forming and joining of semitubular self-piercing rivets made of high strength steel with adapted mechanical properties and numerical analysis of the process chain

• Applicants: Professorin Dr.-Ing. Marion Merklein; Professor Dr.-Ing. Gerson Meschut
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 328853593

The goal of the research project is the identification of cause‑effect relationships between the joinability of different material combinations and the geometric and mechanical properties of semi‑tubular self‑pericing rivets after forming using high strain hardening materials. For this purpose, influencing factors are identified and effects on the dimensional accuracy, the local strengthening due to forming and the resulting impacts on the joining process are investigated. The correlation of the occurring effects allows for developing methods for increasing the dimensional accuracy and influencing the mechanical properties of the rivets according to the requirements of the joints. In this way, the joint strength and the reliability of the joining process are improved. Dimensional deviations due to process variations or due to the limited loadability of the forming tools generally have a negative impact on the required joint parameters like the interlock. Therefore, the influence of process parameters like the forming temperature on the rivet geometry is investigated. Knowing the impact of volume variations is necessary to separate the effects. Regarding the cross process analysis, the correlation between variations related to the forming process, the joint quality and the joint behaviour under different loading conditions is of importance. The influence of corrosive environments and the combination of riveting with adhesive bonding must also be considered. Besides the geometry of rivet and die, the mechanical properties of the rivet are a decisive factor for the joint quality, especially for the defined flaring of the rivet, which is indispensable for improving the joint strength. The use of high strain hardening material leads to a non‑homogeneous strength distribution, as the usual hardening and tempering of the rivets is not necessary anymore. Hence, there is the potential for directly influencing the mechanical properties during forming. The prerequisite for this is a deeper understanding of the local strengthening. Thus, the mutual influence of process parameters and material specific characteristics regarding the mechanical properties is investigated. Correlating the strength of the rivets, the rivet geometry and the strength and thickness of the joined sheets is the basis for identifying a strength distribution of the rivet, which results in an improved joint strength.

• DFG Programme: Research Grants