The anisotropic corrosion behaviour of wire arc additive manufactured Ti-6Al-4V alloy in 3.5% NaCl solution
Introduction
Titanium and its alloys have been increasingly utilized for biomedical, marine, industrial, and aerospace application due to their good mechanical properties, great biocompatibility and excellent corrosion resistance [1]. In particular, Ti-6Al-4V alloy exhibits outstanding tensile strength, which makes it the most commonly used titanium alloy for many applications. However, conventional fabrication methods, such as CNC machining and casting, generally suffer from high manufacturing costs due to either low utilization rate or long construction periods, precluding the use of Ti-6Al-4V alloy in a wide range of commercial applications [2]. Hence, considerable effort has been invested into developing innovative and more economical manufacturing methods [[3], [4], [5]].
Recently, wire arc additive manufacturing (WAAM) has attracted interest from the industrial manufacturing sector for the production of large Ti-6Al-4V components, as it offers a number of potential benefits such as relatively high deposition rate, low equipment cost, high material utilization, and consequent environmental friendliness. For example, recent breakthrough in WAAM technology has made it possible for Bombardier Inc. to fabricate landing gear rib, with approximately 78% raw material saving compared with traditional subtractive machining process [6].
For many applications, corrosion resistance is a primary consideration because it often determines the service life of the component. A number of studies have assessed the corrosion behaviour of Ti-6Al-4V under various conditions. Dai et al. [7] evaluated the corrosion resistance of selective laser melted (SLM) Ti-6Al-4V in NaCl solution, and found that the SLM-produced samples had poorer resistance to corrosion than commercial grade 5 wrought alloy due to the acicular α’ martensite formed within the microstructures. The authors further reported that anisotropic corrosion behaviour exists in SLM-fabricated Ti-6Al-4V alloy [8]. In 1 M HCI solution, corrosion resistance in the planes perpendicular to the build direction is superior to that in planes parallel to the build direction, while in 3.5 wt.% NaCl solution, the corrosion resistances show only a very slight difference. Recently, a group of electrochemical corrosion tests in NaCl solution were carried out by Yang et al. [9] to compare the corrosion resistance of Ti-6Al-4V specimens that were processed by SLM, SLM followed by heat treatment (SLM-HT), WAAM and also traditional rolling conditions. The SLM-HT sample exhibited the highest corrosion resistance, followed by rolled, WAAM and finally SLM. The differences in performance were attributed to the formation of distinctive microstructures by the different manufacturing processes. In the cause of WAAM-produced Ti-6Al-4V, the corrosion mechanism is still not well understood due to its complex microstructural distribution and limited information reported in earlier literature. It is believed that the electrochemical corrosion resistance of WAAM-fabricated Ti-6Al-4V has anisotropic characteristics due to its directional microstructures. A better understanding of the underlying mechanism may be beneficial from the perspective of optimizing microstructural control in the WAAM process.
In this study, a comprehensive investigation on the electrochemical corrosion behaviour of WAAM-fabricated Ti-6Al-4V part has been conducted by means of optical microscopy (OM), X-ray diffraction (XRD), hardness testing and electrochemical impedance spectroscopy (EIS) analysis. Specimens were produced from the fabricated part in orientations parallel and perpendicular to the build direction, to assess the influence of directional microstructure on corrosion behaviour. Samples were also taken from the build substrate, to assess the comparative corrosion behaviour of conventional roll-processed or wrought Ti-6Al-4V. The findings provide an insight into the corrosion mechanism of WAAM fabricated Ti-6Al-4V, which gives direction to future improvements of process control and optimization.
Section snippets
Sample and solution preparation
The apparatus used in this study mainly consists of a 200 A-rated GTAW power source, “cold” wire feeder, water cooling unit and travel mechanism, as shown in Fig. 1. The feedstock was commercial ASTM B863 grade 5 Ti-6Al-4V alloy wire with a diameter of 1.2 mm deposited onto a Ti-6Al-4V substrate with dimensions of 200 mm × 150 mm × 6 mm to ASTM B265 specification. The chemical composition is listed in Table 1, which is according to the manufacturer’s specifications. A straight wall structure of
Metallographic microstructure
Fig. 3 shows the optical micrographs of BM, VP and HP regions of the fabricated specimen. The main microstructure of wrought Ti-6Al-4V alloy presents a mixed α/β equiaxed phase structure distributed uniformly. For the WAAM-fabricated Ti-6Al-4V samples, a secondary α morphology (acicular martensite α’) is preferentially formed in HP regions, while a fully lamellar α morphology forms in VP regions, interwoven with a Widmanstätten structure inside the prior-grain boundary α. The microstructure of
Microstructural evolution
For the VP, HP and BM regions, the main variation in microstructure is the generation of martensite lamellae α with Widmanstätten structure and acicular α’ structure due to the differences in thermal behaviour at each location. As layers are successively deposited during the WAAM process, the cooling rate progressively decreases for higher layers, and this contributes to the variation in microstructural evolution.
The conductive thermal resistance from the uppermost deposited layer to the
Conclusion
The effects of directional microstructures on the corrosion properties of Ti-6Al-4V parts fabricated with wire are additive manufacturing (WAAM) technology have been investigated, and these corrosion properties have been compared against the performance of ASTM standard wrought-Ti-6Al-4 alloy. As the WAAM component is built up from the substrate, the conductive thermal resistance through the deposited material varies along different orientations. A relatively slower cooling rate occurs in the
Acknowledgements
The authors gratefully acknowledge the China Scholarship Council for financial support (NO. 201506680056) and the UOW Welding and Industrial Automation Research Centre for the use of their facilities. The authors are also grateful to Sina Jamali and Yue Zhao for their help in discussion and experiments.
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These authors contributed equally to this work.