CRYOGENIC ADDITIVE MANUFACTURING OF HIGH-ENTROPY ALLOYS FOR NEXT-GENERATION SPACE PROPULSION SYSTEMS: A REVIEW

ABSTRACT: The new method, known as cryogenic additive manufacturing (Cryo-AM), has recently become a potential avenue for manufacturing metals with complex, high-performance properties for use in extreme cold and high stress conditions. Meanwhile, the high-entropy alloys (HEAs) have been gaining more and more interest because of their high strength, phase stability, oxidation resistance, and improved mechanical properties at cryogenic temperatures. This renders HEAs very suitable for next-generation space propulsion systems such as fuel tanks, combustion chambers, nozzles, thermal protection structures, and reusables for propulsion. This review aims to discuss the recent progress of cryogenic AAM of HEAs in the field of space propulsion applications critically. The basics of AM technologies and the principles of HEA design, the state of the art in cryogenic processing conditions, microstructural evolution, mechanical and thermal properties, characterization techniques and propulsion-related case studies are discussed. Particular focus will be given to the interrelationship of process parameters, microstructure, defect formation and the performance in cryogenic service. The review also highlights a number of issues, such as residual stress, cracking, porosity, short fatigue testing data, lack of extended cryogenic testing, and a lack of standardized qualification procedures for space-rated AM-HEA parts. Finally, the future research directions are proposed, such as alloy design, in-situ monitoring, digital twins, machine learning to assist in process optimization and space-environment simulation testing. This review outlines a roadmap for the development of Cryo-AM HEAs from the lab to reliable implementation in future space propulsion applications in a structured way