Single-step self-punching lockbolt process and equipment development for pre-hole-free joining of aluminum sheets using a dual-die system

This study presents a self-punching lockbolt (SPLB) process for pre-hole-free joining of aluminum sheets in lightweight aerospace structures. A dual-die concept reduces the required joining force, enabling compact, automated equipment. Experiments and simulations confirm feasibility and delineate the operating window. Joint formation is highly sensitive to punch–die angular misalignment; for EN AW-2024 the maximum allowable deviation is 0.3°, beyond which bolt or collar failure prevents interlock. Under lap-shear loading, the dominant failure mode is bolt-head fracture followed by shear-out. Optimizing the bolt to a conical shank introduces beneficial compressive stresses during forming and increases shear load capacity by 22 % relative to the reference. An automated prototype—combining servo-electric actuation, die kinematics, and automated fastener feeding within a numerically optimized C-frame—achieves stable multi-point joining, with force–time histories consistent with simulation. These results indicate that SPLB enables high-strength, low-force spot joining while meeting aerospace constraints on equipment size and alignment.