Biomimetic 4D printing of heterogeneous metals: Stress-mismatch-driven self-morphing for stimuli-free dynamic architectures

To address the challenges in the metal intrinsic stimuli-free responsiveness and the thermal behavior mismatch between heterogeneous metals, this study proposes a biomimetic 4D printing strategy that activates the shape-morphing capabilities of inert metal through stress-mismatch-driven mechanisms. Inspired by pod-like bilayer structures, the heterogeneous bilayer precursor composed of Fe and Cu filament uses direct ink writing (DIW) alternate deposition. The precursor deformation utilizes the sintering-induced shrinkage differences between axial and radial filaments. By synergistically tuning the powder-to-binder ratio and printing parameters, such as the layer number ( n ), aspect ratio ( L/W ), porosity and printing angle ( α ), the bending curvature and twisting angle can be precisely programmed, creating complex shape self-morphing structures (SMSs). The mean bending curvature and twisting angle of the SMS can be controlled within the ranges of 1.0–17.6 × 10 −4 /mm and 16.9°–50.2°, respectively. The minimum error of curvature between the simulated and experimental values of the bending SMS is 6.7%, and the error in twisting angle is 13.7%. Microstructure analysis reveals a crack-free Fe Cu interface, achieved through pore-mediated stress accommodation and element diffusion. This work further demonstrates potential applications in integrated turbine blades and mold-assisted forming, providing a new paradigm for high-strength and shape-programmable metal structures in fields such as aerospace and microsystems. This work extends the 4D printing material system to metals without response to stimulation, opening a new path for heterogeneous material design and multi-functional structure manufacturing.