Polymer-based functionally graded multi-material additive manufacturing via material extrusion: Interfacial mechanisms and gradient design strategies

Due to their ability to achieve spatially varying material and enhanced mechanical properties, reduce stress concentration and enable multifunctional behaviour within a single component, the functionally graded multi-materials (FGMMs) have attracted significant attention in additive manufacturing (AM). Among the various AM techniques, material extrusion (MEX) has emerged as a promising approach for the fabrication of graded polymer systems because of its accessibility, material versatility and capability for multi-material processing. This study provides a unified process-structure-property framework of the recent developments in the fabrication of FGMM structures using MEX-AM. The study examines fundamental concepts of functionally graded materials (FGMs), operating principles of material fabrication and different strategies for achieving graded structures, including variations in material composition, structural geometry and lattice architectures. Recent research on multi-material extrusion systems, co-extrusion techniques and gradient control methods was also summarised. Furthermore, key application areas demonstrating the significant potential, including lightweight structural components, energy absorption structures, biomedical implants and soft robotic systems, were discussed. Additionally, the advantages of graded architectures in enhancing mechanical, structural and functional performance and the major challenges associated with such as interfacial bonding, material compatibility, process limitations and design complexity were analysed, respectively. Finally, future research opportunities with emphasis on advanced multi-material extrusion technologies, computational design and optimisation methods, machine learning-assisted material design and the development of sustainable graded polymer systems were identified and discussed. Overall, this study highlights the growing importance of MEX-based AM in enabling the design and fabrication of next-generation FGMM structures for advanced engineering applications.