Effects of annealing temperature on the forming limits of T2 copper/304 stainless steel composite thin strips under double-sided stamping

T2 copper /304 stainless steel (Cu/SS) composite thin strips are considered promising materials for applications in flexible electronics and aerospace industries. The forming limit diagram (FLD), which serves as a comprehensive indicator of material formability, allows for rapid assessment of the deformation state—whether safe, critical, or failed—based on the major and minor strains in the formed sheet. This study examines the effect of annealing temperature on the double-sided stamping behavior of Cu/SS composite thin strips by systematically evaluating the interfacial bonding strength, strain hardening exponent (n-value), and bending stress distribution. Results show that annealing at 400 °C provides adequate interfacial bonding strength to prevent interface cracking during stamping. Moreover, higher annealing temperatures significantly modify the n-value through microstructural evolution. As the annealing temperature increases from 400 °C to 900 °C, the n-value rises from 0.042 to 0.253, resulting in a marked upward shift of the forming limit curve and optimal formability at 900 °C. A notable asymmetry in double-sided forming limits is also revealed: at 800–900 °C, the higher n-value of the steel side, combined with non-uniform bending stress distribution and a shift of the neutral layer toward the steel side, leads to superior stamping performance on the steel side. In contrast, at 400–700 °C, the lower n-value of the steel side reduces its resistance to plastic deformation, rendering the copper side more formable. By adopting the FLD as a principal formability metric, this work offers a theoretical foundation for optimizing the forming processes of Cu/SS composite thin strips.