Post-print annealing: a simple strategy for enhancing adhesion in FDM-printed PLA-SMA composites

Embedding shape memory alloy (SMA) wires within 3D-printed polymers enables 4D printing; however, achieving strong interfacial adhesion is challenging without complex surface treatments. This study investigates a simple post-print annealing strategy to enhance the adhesion between SMA wires and 3D-printed PLA matrices. SMA wires were embedded in fused deposition modelling-printed PLA cylinders. A full factorial design of experiments was conducted, varying annealing temperatures (90 degrees C, 110 degrees C, 130 degrees C) and time (30, 60, 90 min). Adhesion was quantified via pull-out tests measuring interfacial shear strength (ISS). Volumetric changes and degree of crystallinity by differential scanning calorimetry were assessed. Tensile tests evaluated the effects of annealing on PLA properties. Results demonstrated that annealing temperature is the dominant control parameter, while annealing time had a negligible main effect. Optimal conditions (130 degrees C, 60 min) resulted in a threefold increase in ISS (0.8 MPa vs 0.3 MPa control). This enhancement is driven by the synergy of volumetric shrinkage (1.2%-1.3%), which creates circumferential compression on the wire, and increased Young's Modulus (from 2.13 to 2.42 GPa), which amplifies the mechanical grip. Although crystallinity increased substantially (from 14% to 39%-52%), this alone does not explain the ISS improvement, indicating crystallinity is a necessary enabler but not the direct driver. Notably, the maximum pull-out force exceeded the wire's martensitic detwinning threshold; the resulting radial contraction likely triggered detachment, suggesting the true interfacial strength exceeds the measured values. Prolonged annealing (130 degrees C, 90 min) reduced performance, indicating an optimal window where beneficial crystallisation balances against polymer relaxation. SEM analysis confirmed purely adhesive failure with no polymer residue. This non-invasive, easily implementable method preserves the integrity of SMA wire while enabling the fabrication of more robust 4D-printed SMA-polymer actuators and composites.