Influence of die morphology on microstructure and mechanical performance of friction forge riveted aluminum alloy sheets

• Friction forge riveting was applied to solve the excessive force and high noise issues in conventional interference fit riveting. • Compared with the concave die, the flat die effectively reduces the interaction between the forged rivet head and the die, avoiding defect formation. • The flat die enhances local heat dissipation of the joint, mitigates thermal softening, and achieves a joint strength coefficient of 1.0. To address the issues of excessive riveting force, high noise and insufficient strength of aluminum alloy interference fit riveting, this study applies the friction forge riveting (FFR) process to join AA2A12-T4 sheets using AA7075-T651 rivets, investigating die morphology effects. Results indicate that concave die FFR joints are prone to cracks and forged rivet head damage under specific process parameters, showing a broader grain refinement zone and significant softening of the forged rivet head material with 64% hardness of base material. In contrast, flat die FFR joints achieve defect-free forming across all parameters, with a smaller fine-grained area and higher overall microhardness. Concave die FFR joints exhibit inferior tensile-shear performance yet superior cross-tension performance. Both die types share the same tensile-shear failure mode, characterized by rivet shank fracture at the interface between the upper and lower workpieces. In terms of cross-tension failure modes, the concave die FFR joint exhibits dual modes: overall fracture of the forged rivet head under extreme process parameters, and detachment of the outer ring from the forged rivet head under other conditions. In contrast, the flat die FFR joint consistently undergoes cross-tension failure via outer ring detachment from the forged rivet head across all process parameters.