Electrochemical polishing of additively manufactured titanium alloys: a mechanism-to-parameter framework for process design

Abstract

Additive manufacturing (AM) has expanded the use of titanium and its alloys in aerospace, biomedical, marine, and automotive components, but the rapid melting–solidification cycles inherent to AM leave characteristic surface defects—balling, partially melted particles, scan-track ridges, and near-surface porosity—that make post-process finishing essential. Electrochemical polishing (ECP) is a leading candidate because material is removed by anodic dissolution, allowing treatment of geometrically complex surfaces inaccessible to mechanical methods. The literature available to newcomers remains divided: classical electropolishing theory addresses idealized surfaces prior to AM, while recent AM reviews focus empirically on electrolytes and roughness outcomes rather than current-distribution and mass-transport physics. Consequently, researchers often reproduce published conditions as empirical recipes, relying on trial-and-error optimization. This review is therefore organized not as a chronological survey but as a mechanism-to-parameter design framework for engineers adopting ECP for Ti and Ti-6Al-4V. It (i) relates classical current-distribution theory (primary, secondary, and tertiary distributions; the Wagner number (Wa)) and salt-film and acceptor-based mass-transport mechanisms to macro- and micro-smoothing of AM Ti surfaces and to the diffusion-limited plateau of the J–V curve that defines the practical process window; (ii) decomposes electrolytes into functional units—solvent, supporting electrolyte, acid, and additive—so that composition becomes a set of design rules rather than a list of recipes; (iii) consolidates acid-based, organic, ionic-liquid, and deep-eutectic systems, and AM and wrought substrates, into a unified condition database; and (iv) positions finite-element (COMSOL and others) prediction of fields, current-density distributions, and material removal as a means of narrowing the experimental search space. Viewed through this framework, the diversity of reported ECP conditions converges on one objective: forming and sustaining a stable, mass-transport-controlled interfacial layer. Viscosity, additive chemistry, agitation, temperature, and waveform are not independent variables but alternative routes to controlling the diffusion layer, and specimen geometry sets how much geometry-driven current concentration can contribute to leveling. Framed this way, condition selection becomes a reasoned choice of where to sit within the process window rather than searching across a catalogue of recipes—the basis on which ECP can be transferred from coupon-level demonstrations to component-scale Ti parts.