Parametric optimization-based design and testing of 3D printed stainless steel circular X-joints

The integration of topology optimization (TO) and metal 3D printing offers transformative opportunities for the design and fabrication of steel joints in spatial structures. This study develops a parametric joint TO-based design workflow, incorporating subdivision surface technology, the Bi-directional Evolutionary Structural Optimization (BESO) algorithm and advanced re-engineering techniques. Four X-joints for connecting circular tubes were optimized and then fabricated using Selective Laser Melting (SLM) with 316 L austenitic stainless steel powder. To characterize the mechanical properties of the printed material, uniaxial tensile coupon tests were conducted in five loading orientations. The 3D printed steel optimized X-joints were tested under axial compression, with the deformations and the strains captured using 3D Digital Image Correlation. The structural response was assessed in terms of strain distribution, load-deformation behavior, joint strength and ductility, as well as failure mode. The results demonstrate that the 3D printed TO designed X-joints exhibit a more uniform distribution of stress, superior ductility and more efficient load transfer compared to conventional tubular joints. This excellent structural performance is due to the inherent high ductility of SLM-fabricated 316 L stainless steel, the smooth geometric transitions achieved by means of subdivision surface technology, and the optimized material layout from BESO-based TO. The findings validate the feasibility of 3D printing TO designed joints for next-generation structural systems, with potential benefits in structural performance, fabrication efficiency and design flexibility.