Water–material interactions in underwater laser additive manufacturing: process physics, microstructure evolution, and performance

Abstract A harsh marine environment causes severe surface damage to underwater infrastructure, jeopardizing its long-term reliability and necessitating in-situ underwater repair. Underwater laser additive manufacturing (ULAM) is a promising technique that enables direct deposition of metallic materials on water-exposed components. While there is a consensus that ULAM fundamentally deviates from land-based processing due to extreme boundary conditions such as strong optical attenuation, intensified heat dissipation, and environment-assisted chemical reactions, significant scientific controversies remain. Current studies present contradictory conclusions regarding the exact mechanisms of defect nucleation, the stability of the gas-liquid interface in local dry cavity, and the competing effects of thermal gradients on microstructural phase evolution versus residual stress. This review evaluates these disputed mechanisms. By dissecting the multi-physics coupling among the laser, melt pool, and underwater environment, we identify the fundamental causes of discrepancies in existing literature. This review establishes a comprehensive process-structure-property framework that systematically links transient thermophysical phenomena to microstructural characteristics and surface degradation behavior. By delineating established knowledge from unresolved contradictions, this work highlights the critical scientific gaps and provides a clear roadmap for achieving controllable quality and reliable engineering deployment of the ULAM technique.