Surface modification of WAAM Ti6Al4V via organic chemical machining for improved gold PVD coating adhesion

Abstract

Wire Arc Additive Manufacturing (WAAM) is a versatile and sustainable manufacturing technique for producing titanium components, with promising applications in sectors such as jewelry and interior design. WAAM technology offers advantages such as high deposition rates and efficient material usage. However, it often leads to the formation of thermal oxide layers and an alpha case on the surface, which can adversely affect mechanical properties and hinder coating adhesion. This study explores the effectiveness of environmentally sustainable chemical machining treatments based on oxalic acid and oxalic-ascorbic acid solutions for the surface preparation of WAAM-fabricated Ti6Al4V components before gold Physical Vapor Deposition (PVD). Samples were fabricated without inert atmosphere protection to minimize environmental impact. Treatments were performed at 60 °C, 75 °C, and 90 °C, and their effects on surface characteristics were evaluated through thickness loss measurements, roughness analysis, and elemental composition assessment. Conventional hydrofluoric-nitric acid treatments achieved complete oxide and alpha case removal within one hour. In contrast, organic treatments required 24 h at 90 °C to achieve similar results, with the addition of ascorbic acid accelerating the process. Organic treatments resulted in distinct surface morphologies, characterized by increased roughness, beneficial for coating adhesion. The deposited gold layer had a thickness of 198 ± 5 nm, as determined through X-ray fluorescence analysis. The results of the tape test indicated that coatings applied to samples treated with organic acids exhibited excellent adhesion, while those deposited on samples subjected to conventional treatments experienced delamination exceeding 65%.