Fatigue life enhancement by residual stress engineering in refill friction stir spot welds: Experimental investigation and industrial application

Autor*in: Niklaas Becker

ISBN: 978-3-69030-139-8

Dissertation, Leuphana Universität Lüneburg, 2025

Herausgeber*in der Reihe: Prof. Dr.-Ing. Benjamin Klusemann

Band-Nr.: IPTS 06/2025

Umfang: 133 Seiten, 61 Abbildungen

Schlagworte: Refill friction stir spot welding, Failure mechanism, Fatigue performance, Residual stress Engineering, Lightweight design

Kurzfassung: In times of increasing resource scarcity and the global push to reduce greenhouse gas emissions, lightweight design plays a crucial role in achieving sustainable mobility. However, higher material utilization in lightweight structures also raises challenges for structural integrity, particularly under dynamic loading conditions in the transport sector. This dissertation investigates the failure of spot welds performed as Refill Friction Stir Spot Welding (Refill FSSW), an advanced solid-state joining process suitable for high-strength aluminum alloys which are known as not weldable using conventional fusion-based methods. By avoiding melting, Refill FSSW prevents hot cracking and porosity and promotes fine, recrystallized grain structures, making it highly relevant for lightweight applications. The research focuses on understanding the failure mechanisms and fatigue performance of Refill FSSW joints. Detailed experimental and analytical studies reveal that residual stresses, rather than microstructural features, are the dominant factor influencing fatigue behavior. Based on this finding, a novel geometric approach to control and exploit residual stresses was developed, resulting in fatigue life improvements of up to 4000% under consistent load conditions.

The industrial applicability of Refill FSSW is demonstrated through its implementation within the LESSMAT research project, including process qualification and the production of a full-scale lightweight demonstrator in railway vehicle engineering. Overall, this work provides new insights into the role of residual stresses in fatigue performance and establishes strategies for enhancing the structural integrity and durability of lightweight welded joints.