Single-step self-punching lockbolt process for aluminum sheets without pre-hole

Abstract

For the mechanical joining of aviation-grade aluminum alloys, the study introduces an innovative approach using self-punching lockbolts. This method involves the utilization of a profiled collar and a specially designed bolt to seamlessly join EN AW-2024 T42 aluminum sheets, up to a thickness of 8.0 mm, without the need for pre-holes. In this process, when the aluminum sheets are punched, the collar undergoes slight deformation, effectively functioning as a die. Subsequently, the collar is systematically formed by applying increased force. By fine-tuning the design of the bolt ring groove, it becomes feasible to create a joint with a bolt length that accommodates sheet thickness variations of up to 1.6 mm. To facilitate this development, the joining process was meticulously designed and the auxiliary part was optimized with the aid of numeric simulation tools. The process simulation enabled the identification of deformation mechanisms within the collar, accounting for both geometry and material properties. The important factors influencing collar deformation include collar thickness, shoulder height, and shoulder diameter. Furthermore, experimental investigations were conducted to assess the mechanical properties of the joint and evaluate how material-specific factors influence joint failure behavior. It can be seen that the formation of the mechanical fit is influenced by the ring groove depth and the fitting position. The joints failed under shear tensile load exclusively at the bolt head.