Mechanistic investigation of the reforming clinch process for joining steel-aluminum dissimilar materials

To clarify the mechanism of the reforming clinch process and improve the quality of reformed joint, a comprehensive mechanistic investigation was conducted on the reforming process of the clinched joint for joining steel and aluminum materials. A novel multi-step finite element model was developed to simulate the entire clinching and reforming process; Material flow behavior and stress distribution were systematically analyzed for two distinct sheet stacking configurations: (i) steel sheet positioned above the aluminum sheet, and (ii) aluminum sheet positioned above the steel sheet. During the reforming phase, the interlock parameters of the joint and reforming force were analyzed, and the influence of tool parameters on the joint reformation was studied. The results show that reforming process can significantly improve the mechanical interlock while simultaneously reducing joint protrusion height. Notably, interlock development accelerates significantly during the latter half of the reforming stroke. This improvement is primarily attributed to inward material flow in the upper region of the S-shaped mechanical interlock. Tool parameters exert an indirect yet critical influence on the final reformed joint geometry by modulating the initial clinched joint’s shape. This study provides a theoretical basis for optimizing the mechanical interlocking structure of the reformed joint and applying the two-stroke clinching process.