Enabling interlocking joint design for multi-material additive manufacturing via topology optimization with discontinuous interfacial property

Multi-material additive manufacturing (MMAM) offers the potential to create functionally graded structures with tailored properties; however, its structural integrity is often compromised by weak bonding at material interfaces, which leads to premature delamination or structural failure. To address this challenge, geometric interlocking has been identified as the optimal reinforcement strategy for connections characterized by weak interfaces; however, existing approaches rely on human intuition to design interlocking geometry. This work proposes a multi-material topology optimization method to autonomously generate such interlocking features by explicitly modeling the interfacial weakness. Specifically, the method utilizes a Heaviside-based interpolation to model the material interface as a discrete region with reduced elastic modulus or strength limit. This formulation compels the optimizer to evolve from a standard flat boundary between two materials into an interlocking joint in order to maximize overall stiffness of the structure. Comparative analysis demonstrates that while standard continuous interpolation results in disjointed interfaces, the proposed Heaviside-based interpolation method successfully drives the formation of interlocking geometry with extensive interconnected interfaces throughout the design domain autonomously. The performance of the optimized structure is shown to outperform robust human-designed interlocking joints produced by MMAM through tensile testing. The results demonstrate that the proposed approach offers a simple yet effective solution for enhancing the interfacial strength and reliability of multi-material additively manufactured components.