Incremental Sheet Forming on Tailored Hybrid Steel/Polymer/Steel Sandwich Materials; Formability and Residual Stress Investigation

Metal/polymer/metal (MPM) sandwich composites have recently attracted interest for lightweight applications due to their mechanical performance. This study investigates the formability of MPMs with different configurations using the single point incremental forming (SPIF) process in a conical geometry. Various core and skin layer combinations were tested to examine the effects of process parameters such as incremental step size and tool diameter. The sheets were evaluated for forming force, thickness reduction, cracking depth, and residual stresses. The results showed that MPMs, owing to their higher bending stiffness from the increased thickness, require greater forming forces than monolithic steel, with the force nearly doubling for a sandwich with a 1.2-mm core. Increased thickness, step size, and tool diameter elevated forming forces and reduced formability by decreasing cracking depth. Statistical analysis identified core thickness as the most influential factor on cracking depth. Residual stress measurements revealed a more uniform stress distribution in MPMs compared to steel, attributed to the polymer core reducing stress concentration. This trend was consistent with spring-back results, where steel showed greater spring-back than MPM sheets. Thicker cores enhanced stress uniformity, particularly in the mid-wall region, while the upper-wall region showed more variation due to complex deformation and higher bending and stretching.