Abstract
Friction self-piercing riveting (FSPR) is a unique hybrid joining technique that combines the advantages of mechanical interlocking, frictional heat, and solid-state joining (if metallurgically compatible) to produce crack free joints in high strength and/or low-ductility alloys at room temperature. In the current study, Al-7055 sheets were joined using FSPR for lightweight automotive applications and significant microhardness variations were observed across the joint cross-section. A detailed microstructural characterization at multiple length scales was carried out using advanced electron microscopy and X-ray scattering techniques to provide a fundamental understanding of the process-structure-property relationships. The relative contributions of microstructural characteristics at various length scales (i.e., grain size, dislocation density, solute concentration, precipitate nature) to strengthening were estimated using existent formulations (i.e., Hall-Petch, Taylor, precipitate bypass/shear equations) and correlated to the observed microhardness values across different regions. Small-angle X-ray scattering and scanning transmission electron microscopy revealed significant changes in the size and volume fraction of precipitate species, i.e., GP-I Zones, η′, and Mg/Zn solute co-clusters, depending on the process region. It was observed that the dissolution of the small η′/GP-I zones (T ∼ 150–200 °C) in the heat-affected zone were the key reason for the hardness drop. Further, it was shown that solid-solution, dislocation, grain size and solute co-cluster strengthening played a key role in the thermo-mechanically affected zone and grain-refined zone (GRZ). Finally, these observations were leveraged along with the Zener-Holloman relationship and grain size in the GRZ to estimate the peak joining temperature of the GRZ (∼ 350 °C) near the steel rivet.
| Original language | English |
|---|---|
| Article number | 116166 |
| Journal | Materials Characterization |
| Volume | 234 |
| DOIs | |
| State | Published - Apr 2026 |
Funding
A novel hybrid technique called “friction self-piercing riveting (FSPR)” combines the benefits of friction stir welding (FSW) — frictional heat and solid-state joining — and self-piercing riveting (SPR) — mechanical interlocking. This method allows for the joining of low-ductility materials, such as Mg alloys and high-strength 7xxx Al alloys [8,9] . In FSPR, a rotating rivet is pierced into the sheets to be joined, generating frictional heat that locally enhances their ductility, resulting in a robust and crack-free joint [8–10] . Moreover, extensive studies on modified rivet and die designs have helped in improving mechanical joint properties for joining of different grades of Al alloys (i.e., AA7075, 2060-T8E30, AA5083-O, and AA6063-T6) [11–13] . The FSPR technique has been succesfully demonstrated to join various lightweight material combinations, such as CFRC–Mg Alloy, CFRC–7xxx Al, 7xxxAl–dual phase 980 steel, and 7xxx Al–7xxx Al with good mechanical joint properties [10,14–20] under the Joining Core Program funded by the U.S. Department of Energy Vehicle Technology Office. This study used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. This work was funded by the U.S. DOE's Office of Energy Efficiency and Renewable Energy's (EERE) Vehicle Technologies Office (VTO) as a part of the Joining Core Program. This work was performed at Argonne National Laboratory, operated under Contract No. DE-AC02-06CH11357 by the UChicago Argonne, LLC. The part of the research performed at the Oak Ridge National Laboratory was managed by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 for the U.S. DOE. R.R.K is grateful to Sagar Bhatt (Argonne National Laboratory) for the interesting discussions on strengthening mechanisms and useful comments on the graphical summary of this work. The submitted manuscript has been created by Argonne National Laboratory, a U.S. Department of Energy laboratory managed by UChicago Argonne, LLC, under Contract No. DE-AC02-06CH11357 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.
Keywords
- Aluminum 7xxx alloy
- Friction self-piercing riveting
- Precipitation strengthening
- Small-angle scattering
- Solute co-clusters
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