A novel mechanical joining method for sheet-tube structures by annular ring compression

Joining by forming has lately received great attention for connection between sheets and tubes due to its high joining efficiency, simple operation, and excellent joining strength. However, protrusion or indentation often occurs adjacent to the joints and, in some cases, the sheet warps, seriously affecting the performance of the connector. To overcome this problem, we propose a novel joining by forming process by compressing an accessory ring to produce the annular squeezing of the sheet, thus forming high-quality joints with smooth surfaces. A numerical-experimental analysis was adopted to identify the influence of the ring width and height on the joint cross-section, effective strain, tube thickness, and flow velocity. To assess the performance of the enhanced joint, its mechanical properties, microstructure, and failure modes were also analyzed. The results showed that the ring width and height have a significant effect on joint strength. Compared with the original joining process without the ring, the joint utilizing a wider ring can obtain higher pull-out loads (58.9% higher) to detach the sheet from the tube, while no sheet warpage happens for excessive local deformation. The utilization of larger and higher rings also motivates the increase of the pull-out strength. In some cases, tube plastic instability can be triggered during the pull-out tests, thus avoiding the full detachment of the components and contributing to the safety of the mechanical joint. Compared with the non-deformation zone, the grains in the deformation zone of the sheet are compressed and become elongated, and there are inclined separation bands between the two regions. This paper provides a new and efficient method for manufacturing sheet-tube structures with high joining strength.