Influence of contour strategy and chemical immersion on the surface characteristics and mechanical behaviour of electron beam powder bed fusion Ti-6Al-4 V

The International Journal of Advanced Manufacturing Technology https://doi.org/10.1007/s00170-026-18437-3 ORIGINAL ARTICLE Influence of contour strategy and chemical immersion on the surface characteristics and mechanical behaviour of electron beam powder bed fusion Ti-6Al-4 V Andre Giordimaina1 · Eucharist Bajada1 · Qiang Wang3 · Guoxin Lu4 · Bonnie Attard1,5 · Ann Zammit1 · Arif Rochman2 · Glenn Cassar1 Received: 2 October 2025 / Accepted: 1 June 2026 © The Author(s) 2026 Abstract Parts produced via electron beam powder bed fusion often exhibit higher lateral surface roughness relative to other metal-based powder bed processes due to partially sintered powder and the inherent layered building process. Increased roughness acts as stress concentrators that degrade mechanical performance, particularly fatigue life, and detached sintered particles can exacerbate tribological wear. While post-processing methods such as machining and surface grinding are commonly employed to mitigate these effects, alternative strategies are of interest, especially when conventional methods cannot be used to treat complex part geometries fabricated via additive manufacturing. This study investigates continuous contouring as an in-process strategy to modify surface morphology, alongside chemical immersion in hydrofluoric (HF) and nitric (HNO₃) acid as a post-processing treatment. Surface roughness was characterised using optical profilometry and SEM, and tensile and fatigue properties were evaluated under axial loading. Continuous contours showed broadly comparable amplitude-type roughness values (Ra, Rq, Rz, Rp) to multispot, though with a higher mean spacing Rsm (425 μm vs. 304 μm). Chemical immersion reduced most roughness parameters by about 33% for multispot and 24% for continuous contours. The combined use of continuous contouring and chemical immersion produced modest improvements in tensile strength, ultimate tensile strength, and ductility, but significantly enhanced fatigue resistance as chemically immersed continuous samples exhibited a mean fatigue life 64% longer than as-built samples (sinusoidal loading, σmax = 275 MPa, σmin = 27.5 MPa and R = 0.1). Keywords Electron beam powder bed fusion · Surface roughness · Chemical immersion · Continuous contours · Titanium · Additive manufacturing Andre Giordimaina Glenn Cassar 1 Department of Metallurgy and Materials Engineering, Faculty of Engineering, University of Malta, Msida, Malta 2 Department of Industrial and Manufacturing Engineering, Faculty of Engineering, University of Malta, Msida, Malta 3 Aviation Key Laboratory of Science and Technology on Advanced Corrosion and Protection for Aviation Material, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China 4 Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, School of Materials Science and Engineering, Ministry of Education, Shandong University, Jinan, Shandong 250061, China 5 Institute of Engineering & Transport, Malta College of Arts, Science & Technology (MCAST), Paola, PLA 9032, Malta The International Journal of Advanced Manufacturing Technology 1 Introduction Additive manufacturing (AM) of metals has seen increasing adoption across industries including aerospace, medical, and high-performance engineering applications [1]. AM enables the creation of complex geometries that are difficult or impossible to produce using conventional subtractive methods, while also producing very little material waste. Among structural metals, titanium alloys such as Ti6Al-4 V are especially attractive. Ti-6Al-4 V offers a high strength-to-weight ratio, retains its mechanical properties across a broad temperature range, and exhibits good corrosion resistance, making it suitable for demanding applications [2, 3]. Unfortunately, it is also well known to be difficult to machine due to its high strength and low thermal conductivity [4]. Additive manufacturing using powder bed fusion (PBF) has been widely adopted in many sectors because it enables the fabrication of highly complex geometries that are extremely difficult or impossible to produce using conventional subtractive methods, using materials which may be difficult to process using traditional means. Electron beam powder bed fusion (EB-PBF) is a widely used AM technology for processing Ti-6Al-4 V. Due to the nature of the process and the heat source used, significant pre-heating is needed to sinter the surrounding powder to avoid electrostatic charging, which can cause an explosive repulsion of charged particles in the powder bed, causing build failure [5, 6]. To further suppress electrostatic charging and repulsion under the electron beam, EB-PBF requires coarser powder feedstock, typically in the range of 40–150 μm. In addition, the electron beam has a more diffuse spot size compared with the beam size used for L-PBF. Factors such as the larger powder size coupled with the sintering process and the larger spot size limit the achievable resolution and surface quality in EB-PBF [7]. This negatively influences mechanical properties such as fatigue life, and as a result, samples produced by EB-PBF have inferior fatigue properties compared to L-PBF [8]. Post-processing methods are commonly adopted to improve the surface quality of AM parts produced by powder bed fusion techniques such as L-PBF and EB-PBF. Mechanical finishing techniques such as machining and shot blasting can improve surface finish and subsequent mechanical properties, but are limited to accessible external surfaces and relatively simple part geometries. In contrast, chemical immersion, also referred to as chemical polishing, offers a significant advantage in that it can effectively treat internal surfaces and complex geometries without requiring electrical contact or counter-electrodes, which are necessary for electropolishing [9]. Tyagi et al. demonstrated 13 that chemical immersion significantly reduced the surface roughness of both external and internal surfaces of stainless steel 316 L AM components, even outperforming electrochemical polishing for internal cavities [10]. Due to the excellent chemical resistance of Ti-6Al-4 V, aggressive etchants are required in order to significantly dissolve the surface features of Ti-based samples. Hydrofluoric acid (HF) effectively dissolves the oxide layer to then dissolve the titanium surface without the oxide reforming. Its exclusive use is problematic due to hydrogen evolution, which can lead to hydride formation and embrittlement [11]. During the acid-metal reaction, hydrogen gas evolves and may be absorbed into the titanium matrix, potentially forming brittle hydride phases such as TiH₂. Adding a strong oxidiser such as nitric acid (HNO₃) mitigates this by re-passivating the surface and suppressing hydrogen absorption [12]. In addition to post processing, the alteration of surface properties and surface morphology through the alteration of processing parameters h (...truncated)