Electrochemical and thermal-induced degradation of additively manufactured titanium alloys: a review

Abstract

Titanium (Ti)-based alloys wield unique combination of mechanical, chemical, and high temperature properties, which place them at the forefront of engineering applications ranging from biomedical to aerospace. Among these properties, electrochemical and thermal-induced degradation involving corrosion and high-temperature oxidation, respectively, are critical as they impact service life of the component. Advanced manufacturing techniques under additive manufacturing (AM) offer capabilities of fabricating complex structural and functional, near-net shaped engineering components. Owing to the excellent weldability, and ease of precursor (powder/wire) formability, Ti alloys are especially suitable for production using AM techniques. In contrast to narrow range of near-equilibrium thermokinetic conditions in conventionally processing techniques of these alloys, AM fabricated materials encompass vast range of regimes of near to fully non-equilibrium thermokinetic and thermomechanic factors including multiple, extremely rapid heating/cooling cycles, steep thermal gradient, and severe thermal stress cycles controlled via distinct precursor morphology, processing atmosphere, and process parameters. Consequently, AM components exhibit characteristic microstructures including but not limited to heterogenous grain structure, non-equilibrium phase evolution, and presence of 3D macro/micro defects like crack networks, porosity, and crystallographic and atomic defects. These characteristics have been suggested to impact electrochemical and thermal-induced degradation of Ti alloys. Hence, there exists AM process induced variation in results and differed views about the mechanisms underlying these variations. The considerable prospect of AM for optimized fabrication of corrosion-resistant Ti alloys remains partly unrealized and provides plenty of room to explore. In this review, we discuss the present scenario of corrosion and high-temperature oxidation in AM Ti alloys. The process-induced peculiarities associated with AM and influence of these peculiarities and ambient media have been highlighted. Further, efforts to mitigate the corrosion/oxidation of AM components via post processing are reviewed. The review concludes comprehensively on the AM process-induced variation in corrosion and high temperature oxidation of Ti alloys.