Metallic multi-material structures are heterogeneous structures characterized by changing composition, microstructures, and site-specific characteristics, advantageous for numerous applications where multifunctionality is desired. Metallic multi-material structures are known as bimetallic structures (BSs), which are functionally graded materials (FGMs). In recent years, wire-arc additive manufacturing (WAAM) advanced as a promising additive manufacturing process to realize the fabrication of these structures due to its high deposition rate, cost-effectiveness, and material utilization efficiency. This review presents a comprehensive overview of the recent progress, processing strategies, and scientific challenges in WAAM of multi-material structures. The paper begins with an introduction to multi-material structures, followed by a bibliometric analysis of the current research landscape. Conventional and additive manufacturing fabrication approaches are presented. The review highlights key developments in processing strategies and critically evaluates research studies on WAAM of BS and FGMs. Major scientific challenges, including porosity, lack of fusion, residual stresses, cracking, material compatibility, and brittle intermetallic phase formation, are critically analyzed. Additionally, modeling, simulation, and process automation issues are discussed as barriers to industrial-scale implementation. The paper concludes with an outlook on future research directions to address existing challenges and accelerate the adoption of WAAM for complex multi-material components.

Metallic multi-material structures are heterogeneous structures characterized by changing composition, micro-structure, and site-specific characteristics, advantageous for numerous applications where multifunctionality is desired. Metallic multi-material structures are known as bimetallic structures (BS), functionally graded materials (FGMs). In recent years, Wire Arc Additive Manufacturing (WAAM) advanced as a promising additive manufacturing process to realize the fabrication of these structures due to its high deposition rate, cost-effectiveness, and material utilization efficiency. This review presents a comprehensive overview of the recent progress, processing strategies, and scientific challenges in WAAM of multi-material structures. The paper begins with an introduction to multi-material structures, followed by a bibliometric analysis of the current research landscape. Conventional and additive manufacturing fabrication approaches are presented. The review highlights key developments in processing strategies and critically evaluates research studies on WAAM of BS and FGMs. Major scientific challenges, including porosity, lack of fusion, residual stresses, cracking, mate-rial compatibility, and brittle intermetallic phase formation, are critically analyzed. Additionally, modeling, simulation, and process automation issues are discussed as barriers to industrial-scale implementation. The paper concludes with an out-look on future research directions to address existing challenges and accelerate the adoption of WAAM for complex multi-material components.

Shape memory alloys (SMAs), such as Ni–Ti, are valuable in structural and aerospace applications for their shape memory and superelastic properties. However, their performance can be degraded by corrosive and high-temperature environments. Although surface coatings such as thermal barrier coatings (TBCs) and anti-corrosion coatings (ACCs) are used for protection, their effects on the phase transformations of SMAs are not well understood. This study evaluates the impact of commercial polymer-based TBC and ACC coatings on the behavior of Ni–Ti SMAs through thermal, mechanical, and microstructural analysis. Results indicate that both coatings maintain the SMA’s phase transformation temperatures. However, the transformation enthalpy decreased for ACC-coated samples and increased for TBC-coated samples compared to uncoated ones. In thermal recovery tests, uncoated samples showed the highest shape recovery, followed by ACC-coated and TBC-coated samples, suggesting a compromise between protection and transformation efficiency. After 100 cycles, ACC coatings deteriorated significantly, while TBC coatings showed only minor cracking. Electrochemical tests confirmed the anti-corrosion effectiveness of the ACC, and thermal analysis verified the insulating property of the TBC. These findings demonstrate the trade-offs between coating functionality and SMA performance, offering practical insights for selecting coatings in SMA systems under thermal cycling conditions.