Influence of the shank geometry on the joint formation of the versatile self-piercing riveting of ultra-high-strength steel-aluminium and aluminium-aluminium assemblies To reduce CO₂ emissions, the automotive industry is adopting multi-material structures. Fusion-based joining reaches its limits for aluminium–steel due to brittle intermetallic phases and mismatched thermophysical properties; therefore, mechanical joining (e.g., SPR) is used. Though conventional SPR requires tool changes for different stack-ups. Versatile self-piercing riveting (V-SPR) addresses this with an extended punch actuator and a multi-range-capable rivet (Kappe in PERD16:363–378, 2022), enabling joints up to 600 MPa across varying thicknesses without retooling. With the use of ultra-high-strength steels up to 1000 MPa, optimisation is required. This study quantifies how rivet shank geometry affects joint formation using a design of experiments and validated 2D axisymmetric FE simulations. The optimum depends strongly on the material system. For CP1000–EN AW-6014, maximum interlock f is predicted for a medium shank thickness of about 0.73 mm, a small internal foot radius of 0.620 mm, and a deeper drill depth of 3.136 mm, yielding f fc =0.4503 mm with a desirability of 0.954. For EN AW-6014–EN AW-6014, the optimum shifts to a thinner shank of 0.670 mm, a larger internal foot radius of 0.820 mm and a shallow drill depth of 2.30 mm, giving ffc = 0.3023 mm with a desirability of 1.0. A compromise geometry of 0.713 mm shank thickness, 0.776 mm internal foot radius and 2.755 mm drill depth achieves ffc = 0.3641 mm for CP1000–aluminium and ffc = 0.1851 mm for aluminium–aluminium with an overall desirability D = 0.6378, expanding V-SPR to ultra-high-strength steel–aluminium joints while maintaining aluminium joinability. 5 1 Springer Science and Business Media LLC