Comparative multiscale evaluation of the temperature-dependent interfacial strength in aluminum alloy/carbon fiber-reinforced thermoplastic joints

Lightweight structural systems increasingly rely on dissimilar joints between aluminum alloys and carbon fiber-reinforced thermoplastics (CFRTPs); however, their mechanical reliability remains highly sensitive to the bonding temperature and the length scale of fracture. In this study, a comparative multiscale framework was established to elucidate the temperature-dependent evolution of strength and fracture mechanisms in dissimilar 6061 aluminum alloy (A6061)/polyamide 6 (PA6)-CFRTP joints. A6061 surfaces were nanostructured via hydrochloric acid etching to enhance mechanical interlocking, and thermocompression bonding was performed at temperatures ranging from 230 to 350 °C. Mechanical characterization was conducted across three scales: macroscopic tensile-shear, sub-millimeter-scale miniature tensile, and microscale tensile testing. These were complemented by thermal analysis, microscopy, X-ray nano-computed tomography, and nanoindentation. The tensile-shear strength increased moderately from near the melting point of PA6 to approximately 330 °C (reaching ≈ 8 kN), but declined to ∼ 6 kN at 350 °C. Cross-sectional observations revealed numerous voids within ∼ 1 mm of the interface above 300 °C, which was consistent with the onset of thermal decomposition of PA6. Miniature-scale specimens exhibited an earlier onset of strength loss than joint-scale specimens, which was attributed to void-induced stress concentration under a loading axis nearly perpendicular to the in-plane fiber orientation. Microtensile tests were used to isolate the intrinsic degradation region of PA6 adjacent to the interface. Nanoindentation confirmed a near-interfacial hardness reduction of ≈ 200 MPa at 350 °C. These findings elucidate the scale-dependent and temperature-driven fracture transition from interfacial to CFRTP-side failure and define an optimized process window.