Effect of Electron Beam Surface Remelting on Surface Microstructure and Properties of Wire-feed Additive Manufactured Titanium Alloy

Electron beam surface remelting is an advanced high-energy beam surface-modification technology that allows to locally manipulate the microstructure and performance without affecting the matrix material. It has attracted much attention for the fabrication of active and refractory materials, since a vacuum working environment can effectively avoid the introduction of extrinsic contaminants. The work aims to address the problems of insufficient wear resistance and friction instability resulted from the inherent microstructure of wire-feed additive manufactured titanium alloy through electron beam surface remelting to, broaden its application fields. The square specimens (15 mm×15 mm×4 mm) were extracted from the electron beam free form fabricated Ti6Al4V alloy along the build direction. The polished specimens were treated by surface melting technique with an electron beam gun under a vacuum of 10–3 Pa, with fixed beam power of 180 W and moving speed of 5 mm/s. The overlap ratio between the two adjacent passes was 50%. The variation of macroscopic morphology, microstructure and phase composition of the titanium alloy before and after remelting were analyzed with stereomicroscope, optical microscope and X-ray diffractometer, respectively. The surface nanomechanical properties and wear resistance of the remelted layer were investigated with nanoindenter and wear testing machine, respectively. The worn features were characterized with three-dimensional optical profilometer and scanning electron microscope. A favorable remelted layer was successfully prepared on the surface of deposited titanium alloy via electron beam surface remelting. Under the irradiation of electron beam, the as-received surface was melted completely into a molten pool, and then was rapidly solidified afterwards by self-quenching. Therefore, the initial heterogeneous microstructure (basket-wave structure and Widmanstatten structure composed of α+β) was transformed into homogeneous and fine acicular martensite (α′) after surface remelting treatment. The nanoindentation results showed that the remelted alloy provided a uniform and enhanced nanohardness (3.8 GPa), which was over 15% higher than that of the un-remelted sample, exhibiting a superior hard elasticity. The enhanced hardness was contributed to the formation of fine acicular martensite. Due to the heterogeneous microstructure and insufficient hardness, a worn track with different degree of abrasion occurred easily on the surface of as-deposited alloy during the wear process. It resulted in the increase of roughness on the contact area between the alloy and counterpart, leading to the instability of friction and further deterioration of wear. As expected, a stable friction coefficient and a better wear resistance were available after surface remelting. The average friction coefficient was 0.45 and the wear rate was 3.59×10–13 mm3/(N·m), which was 19.6% and 22.1% lower than that of the un-remelted one, respectively. The remelted layer possessed a superior tribological performance, which was attributed to its uniform microstructure and hard phase. Shallow grooves, minor adhesive traces and oxidation generated on its worn surface. The wear mechanism was not changed after the surface remelting treatment, and its dominant failure mechanism was abrasive wear, oxidation wear and adhesive wear. This study shows that the microstructure homogeneity of wire-feed additive manufactured titanium alloy can be improved by electron beam surface remelting, and a favorable surface mechanical and tribological performance is achievable. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.