Laser-cladded coatings on titanium alloys: processing-microstructure relationships, phase evolution, and interfacial mechanisms

Titanium (Ti) alloys are attractive for aerospace, biomedical, and advanced engineering applications because they combine high specific strength with excellent corrosion resistance. However, their intrinsically low hardness and poor tribological performance severely limit long-term reliability under frictional loading. Laser cladding provides a promising route for improving Ti alloy surfaces by forming compact, metallurgically bonded coatings whose microstructures can be tailored through rapid laser-induced melting and solidification. This review re-examines recent progress in laser-cladded coatings on Ti alloys from a microstructure-oriented perspective, with particular emphasis on non-equilibrium solidification, phase constitution, interfacial reactions, defect evolution, and structure-dependent degradation during service. Five representative coating systems, namely self-fluxing alloys, ceramic-particle-reinforced composites, rare-earth-oxide-modified coatings, self-lubricating composite coatings, and high-entropy alloy (HEA) coatings, are comparatively discussed not simply in terms of wear performance, but according to their characteristic phase architecture, strengthening pathways, and damage-evolution behavior. Special attention is given to how load transfer, crack initiation, tribofilm stability, oxidation-assisted protection, and delamination resistance are controlled by phase distribution, interface stability, and structural compatibility across multiple length scales. Accordingly, the evolution of laser-cladded coatings is viewed as a shift from isolated hardening or single-purpose functional design to coordinated control of phase configuration, interface reliability, and damage accommodation. Current challenges associated with brittle reaction products, compositional complexity, thermo-mechanical mismatch, and microstructural instability are also discussed. Finally, future directions are proposed toward gradient and multiphase architectures, interface-controlled failure mitigation, advanced microstructural characterization, and data-assisted coating design. This review provides a unified microstructure-based framework for understanding and designing advanced laser-cladded wear-resistant coatings on Ti alloys for demanding tribological environments.