Electrochemical behaviour of laser solid formed Ti–6Al–4V alloy in a highly concentrated NaCl solution

Accepted Manuscript Title: Electrochemical behaviour of laser solid formed Ti–6Al–4V alloy in a highly concentrated NaCl solution Authors: Jiaqiang Li, Xin Lin, Pengfei Guo, Menghua Song, Weidong Huang PII: DOI: Reference:

S0010-938X(18)30283-X https://doi.org/10.1016/j.corsci.2018.07.023 CS 7622

To appear in: Received date: Revised date: Accepted date:

11-2-2018 15-7-2018 16-7-2018

Please cite this article as: Jiaqiang L, Xin L, Pengfei G, Menghua S, Weidong H, Electrochemical behaviour of laser solid formed Ti–6Al–4V alloy in a highly concentrated NaCl solution, Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.07.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electrochemical behaviour of laser solid formed Ti–6Al–4V alloy in a highly concentrated NaCl solution Jiaqiang Li a, b, Xin Lin a, b, *, Pengfei Guo a, b, Menghua Song a, b, Weidong Huang a, b a

State Key Laboratory of Solidification Processing; Northwestern Polytechnical University,

127 Youyixilu, Xi’an, Shaanxi 710072, P. R. China b

Key Laboratory of Metal High Performance Additive Manufacturing and Innovative Design,

MIIT China, Northwestern Polytechnical University, 127 Youyixilu, Xi’an, Shannxi 710072,

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P. R. China

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Highlights:

The grain characteristics of laser solid forming Ti-6Al-4V is characterized.

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The corrosion resistance relies on the grain characteristics of the deposit.

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The electrochemical dissolution is affected by the grain characteristics.

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The transpassive dissolution potential is determined by phase constituent.

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Abstract: Grain characteristics exhibit significant effects on electrochemical machining. The effects of the surface of grains on the electrochemical behaviour of laser solid formed Ti-6Al-

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4V deposits were investigated using a 15 wt.% NaCl solution. The epitaxial growth behaviour caused the deposits to generally exhibit columnar grains near the lateral surface and equiaxedshape grains near the top surface. The coarser columnar grains presented inferior corrosion

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resistance, while the equiaxed grains exhibited slightly better corrosion resistance but distinctly poorer electrochemical machinability. The increase in the β/α volume ratio of the grain decreased its corrosion resistance but increased the transpassive dissolution potential.

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Keywords: Titanium; Polarization; EIS; Anodic dissolution; Additive manufacturing (AM). 1 Introduction

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Ti and Ti-based alloys, particularly Ti-6Al-4V, have wide applications in the aerospace,

marine, automotive, medical and petrochemical industries because of their high specific strength, excellent corrosion performance, and biocompatibility [1-4]. In the light of the recent development of additive manufacturing (AM), laser AM has become an important net and near-net shape forming method during the process of Ti-6Al-4V [1, 5]. Laser solid forming (LSF) is a near-net shape AM technology based on laser cladding that can be used to fabricate highly performant three-dimensional (3D) near-net shape metal components using high deposition rates, and without size limitations, directly from computer aided design

(CAD) files [6]. Moreover, LSF offers distinct advantages over conventional manufacturing, such as design flexibility, short lead times, efficient use of materials, substantial cost savings, and it eliminates the need for tooling [7, 8]. However, the contradiction between the deposition rate and fabrication accuracy is intrinsic for AM. Thus, post-processing is still necessary to achieve desired surface quality and final part geometry to accomplish the AM of net shape structures with high efficiency and high accuracy. Electrochemical machining (ECM) is a type of non-contact special machining technique for removing materials that can be applied to all types of metallic materials regardless of their mechanical properties [9-11].

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Furthermore, ECM is a fast and efficient technology that removes material by controlled

anodic dissolution, shaping high quality products from metals and alloys. In addition, ECM

has several advantages: good surface integrity, high material removal rate, no internal stress,

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and no tool wear. It is believed that the combination of LSF and ECM could represent the

method with the highest potential for manufacturing metal components with high efficiency and accuracy. Fig. 1 shows the schematic diagram of a combination of LSF and ECM. Recently, the combination the LSF and ECM has achieved good results when used on the Inconel 718 superalloy [12, 13]. Fraunhofer IPT, together with WZL and EMAG ECM

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GmbH found that the manufacturing chains combining LSF and ECM could significantly

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reduce production costs. The costs of machining 800 blisks (bladed disks) per year using LSF combined with ECM were 60% lower than those associated with complete cutting machining

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processes [14, 15].

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In general, anodic dissolution behaviour plays a central role in selecting the parameters of ECM and evaluating the electrochemical machinability of materials. Meanwhile, corrosion is inevitable during ECM process and impacts the quality of the surface of the machined part.

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The processing behaviour of the Ti-6Al-4V alloy in different solutions has been widely researched because solution properties affect the dissolution behaviour of machined materials and greatly influence processing quality. Weinmann et al. [16] used electrochemical

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measurements to study the anodic dissolution behaviour of titanium, and pointed out that increasing the concentration of chloride ions in NaCl solutions can accelerate the dissolution

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of Ti-6Al-4V during ECM. Researchers have reported the effect of grain size on the electrochemical behaviour of Ti alloys and other metals. Klocke [17] found that fine grains in Ni-based alloys resulted in a better electrochemical machinability. Both Al and Zn castings with coarse microstructures possess superior corrosion resistance [18]. However, most studies demonstrated that grain refinement resulted in increasing the corrosion resistance of Ti alloys

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[19-21]. It is generally reckoned that a thick and homogeneous oxide layer formed on the surface of coarse grains improved the corrosion performance of Ti alloys [22]. In addition, the grain morphologies (equiaxed or columnar structures) has also affected the electrochemical behaviour of alloys. Equiaxed structures present slightly higher corrosion resistance than columnar structures for both Zn and Al castings [18]. By contrast, Hoseini [23] studied changing the texture and grain size of pure Ti and suggested that texture exhibited a dominant effect on corrosion resistance.

It should be noted that grain characteristics, such as size, morphology, and texture, are closely related to the processing method. The parts fabricated by LSF exhibited coarser grain, finer sub-microstructure and no macro-segregation compared to conventional cast and wrought parts [24, 25]. In fact, Ti-6Al-4V is one of the few alloys that can be reliably manufactured by LSF [26]. Laser solid formed (LSFed) Ti-6Al-4V generally tends to form columnar grains, and its grain characteristics are very sensitive to changes in processing parameters such as laser power and scan velocity, and the location of the deposit [27, 28]. Few studies have been conducted so far on the anodic dissolution behaviour of Ti-6Al-4V

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fabricated by AM. Pimenova and Starr [29] demonstrated that the corrosion rates of LSFed Ti alloys were lower than those of Ti alloys prepared by traditional melting. Dai et al. [30, 31] found that the selective laser melted (SLM) Ti-6Al-4V alloy possessed poorer corrosion

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resistance than the forged Ti-6Al-4V (Grade 5) in NaCl solution and exhibited anisotropic corrosion properties in harsher solution system (1 M HCl). According to these results, the corrosion behaviour of AM Ti alloys in corrosive media has been preliminarily explored. However, the anodic dissolution behaviour of LSFed Ti-6Al-4V alloy in solutions used

during ECM processes has been rarely reported. More importantly, the relationship between

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the grain characteristics and electrochemical behaviour plays a major role in improving the

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quality of the parts obtained by ECM.

In general, during ECM, the non-machined surface of the workpiece will always be kept

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in the electrolyte environment where it is corroded, for a long time, while the machined

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surface undergoes high-speed dissolution. Therefore, the corrosion behaviour of machined materials in electrolytes used for ECM significantly affects overall quality of the workpiece. Although material dissolution during static electrochemical tests is very different than that

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occurring during actual ECM, which is controlled by the electrolyte flow and small gap electric field, the intrinsic anodic dissolution behaviour of the material obtained using static electrochemical tests still holds important guiding significance on the dissolution mechanism

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of high-speed dissolution ECM [32]. Polarisation measurements typically used to select the proper ECM processing voltage for a material [10]. In this study, open circuit potential

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(OCP), Tafel polarisation, potentiodynamic polarisation, and electrochemical impedance spectroscopy (EIS) experiments were performed to investigate the electrochemical behaviour of LSFed Ti-6Al-4V in highly concentrated NaCl solutions. The purpose of this study was to obtain information about the dissolution properties and corrosion resistance of the LSFed Ti6Al-4V alloy. As a prerequisite for the present study, surface grain evolution of the LSFed Ti-

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6Al-4V alloy was also investigated. 2 Materials and methods 2.1 Sample preparation The deposits were fabricated on an LSF-VII system equipped with a Laserline LDF6000 diode laser, five-axes computer numerical control (CNC) table, powder delivering system equipped with a coaxial nozzle, and glove box filled with pure Ar that controlled the oxygen

content and maintained it under 50 ppm. The Ti-6Al-4V powders used as feed powder were composed of 150–180 μm spherical particles. The powders were produced using plasma rotating electrode processing (PREP). A 110 mm × 50 mm × 10 mm wrought Ti-6Al-4V alloy plate was used as substrate for the LSF process. The substrate was first ground using SiC paper to remove the oxide film on its surface and then cleaned with acetone before deposition. The dimensions of the deposits were 90 mm × 30 mm × 20 mm, as shown in Fig. 2. To investigate the evolution of the grain characteristics of LSFed Ti-6Al-4V, experiments were conducted using laser powers of 2, 3, 4, and 5 kW, while the other

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processing parameters were maintained constant: beam scanning velocity of 15 mm/s, layer thickness of 0.7 mm, powder feed rate of 8.2 g/min, beam diameter of 5 mm, and hatch spacing of 2.5 mm.

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2.2 Characterization Grain morphology was investigated using optical microscopy (OM, GX71, Olympus,

Japan). Samples for OM were cut from the original samples along the vertical plane, as shown in Fig. 2(b). The microstructure of the samples was revealed using an etchant consisting of 2

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mL HF, 3 mL HNO3, and 15 mL H2O after grinding them using SiC paper and polishing using a SiO2-H2O2 solution. The statistical analysis software Image-Pro Plus (IPP) was used

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to analyse the size of the β grains in the deposits. For the equiaxed grain size, the average

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value of the maximum and minimum diameters was used, and the grain width was measured for the columnar grain size (20 measurements to yield a mean value). After electrochemical (VHX-2000, Keyence, Japan).

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testing, the surface morphology of the samples was observed using a digital microscope

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The structural characteristics of the deposits were detected using X-ray diffraction (XRD, D8 Discover, Bruker, Germany) at room temperature. The XRD instrument used Co Kα radiation and 2θ ranging from 35 to 95°. To calculate the volume fraction of the

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constituent phases, the integrated areas of both the α and β diffraction peaks in the XRD

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spectra were fitted using the Fityk computer software with Pearson VII function [33-35].The volume fraction of the α phase, Vf, α , was estimated using the following equation: Aα Aα +Aβ where Aα and Aβ are the total integrated areas of the α and β phases, respectively. Vf, α =

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2.3 Electrochemical measurements The deposits prepared using laser powers of 2 and 5 kW were chosen for electrochemical measurements because of the significant differences in the characteristics of their grains. The sampling locations and shapes are shown in Fig. 2(c). All samples used for electrochemical measurements were ground up to 1500 grit using SiC paper, cleaned with ethanol, dried, and loaded into a special electrochemical test fixture to be used as working electrode. Electrochemical tests were performed utilizing an electrochemical workstation (PARSTAT 4000, Princeton Applied Research, USA) using a conventional three-electrode

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cell. A standard calomel electrode (SCE) was used as reference electrode while a Pt sheet was employed as counter electrode. The working area of the samples serving as working electrode was 1.0 cm2. The distance between the working electrode and the Luggin capillary was 2 mm. The anodic dissolution process was conducted by anodic potentiodynamic polarisation measurements. Electrochemical measurements were conducted in a NaCl solution at the concentration of 15 wt.%, which is a typical solution used for the ECM of Ti alloys. The 15 wt.% NaCl solution was prepared using analytical grade reagents and ultra-pure water. First, the OCP was measured for sufficient time to attain a stable OCP value.

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Subsequently, EIS was conducted using an AC amplitude of 10 mV versus the OCP in the

10−2–105 Hz frequency range. Afterwards, Tafel measurements were performed in the −0.5 to

+0.5 V potential range versus the OCP at a sweep rate of 0.1 mV s−1. Lastly, potentiodynamic

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polarisation tests were conducted within the −0.5 to +8 V potential range versus the OCP at a

sweep rate of 1 mV s−1. The Zview and VersaStudio software were used to determine the EIS curve fitting and corrosion rate, respectively. All the potentials reported in this paper were measured against the SCE. Triplicate experiments were carried out for all samples to ensure

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reproducibility.

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3 Results and discussion

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3.1 Surface grain evolution of the LSFed Ti-6Al-4V alloy Fig. 3 shows the microstructures of the top and lateral surfaces LSFed Ti-6Al-4V. The grains of the LSFed Ti-6Al-4V alloy were generally dominated by columnar grains throughout the deposit. At the lateral surface of the deposit (Fig. 3(a)), the grain morphology

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was generally columnar, growing at an angle of approximately 30° relative to the deposition direction. In addition, at the top surface of the deposit (Fig. 3(b)), the prior β grains exhibited columnar morphology and grew along the deposition direction from the bottom to the top

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surface when the laser power was 2 kW. However, the morphology of the grains at the top of the deposit became equiaxed when the laser power was greater than 2 kW. Grain boundaries were straight when the laser power was lower than 4 kW, while they were tortuous when the

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laser power increased to 5 kW. According to the morphological features of the grains of LSFed Ti-6Al-4V for the different laser powers presented in Fig. 3, the schematic diagrams of the morphology of the grains were established and presented in Fig. 4. The schematic diagrams clearly displayed the morphology of the LSFed Ti-6Al-4V grains fabricated using

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different laser powers in different regions of the deposits, including their centre, top, and lateral surface. The lateral surface, which was fabricated using all the laser power values utilized in this study, was composed of deflected columnar grains. The top surface of the deposits obtained using a laser power of 2 kW did not contain equiaxed grains and displayed as a pseudo-equiaxed structure, while equiaxed grain zones occurred when the laser power exceeded 2 kW.

Fig. 5 shows the profile of the lateral wall and shape of the deposition layer during LSF process. As can be observed, the deflected columnar grains were formed near the lateral surface. Based on the investigation conducted by Song et al. [36], both lateral walls of the bulk sample would tilt inward during the LSF process, which led to the deformation of the molten pool on the lateral walls. As shown in Fig. 5, the deformation of the molten pool on the lateral wall changed its temperature field. The distribution of the temperature field of the molten pool on the lateral wall varied from that of the centre of the deposit. This changed the direction of the temperature gradient from vertically downwards in the centre of the deposit to

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tilted inwards close to the lateral surface. Consequently, the direction of heat dissipation

(marked using orange arrow in Fig. 5(b)) from the outer edge of the deposit formed an angle with the centre. As a result, the columnar grains near the lateral surface were deflected

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towards the outer wall, while, in the centre of the deposit, they grew along the deposition

direction, as indicated by the arrow in Fig. 5(a) and the schematic diagram of the columnar grains in Fig. 5(b).

Notably, the as-deposited prior β grains presented two types of distinct morphologies

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from the centre to the top surface of the deposit as the laser power increased. Nucleation and growth conditions play decisive roles in the grain morphology of the deposits. The grain

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morphology is also strongly influenced by the metallurgical and thermodynamic states of the molten pool. During LSF, the bottom of the molten pool represents the beginning point of

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solidification, where the temperature gradient occurs mainly along the deposition direction.

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Thus, columnar grains, favoured the dendritic growth characteristic, might be formed at the bottom of the molten pool. In most cases, the dendrite stalks lie between the direction of the temperature gradient and the preferred growth direction [37]. As a result, the grain structure

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consists of columnar dendrites along the deposition direction. However, the morphology of the grains during solidification is affected by some complicated factors. Lin et al. [38] described the columnar-to-equiaxed transition (CET) model and concluded that the

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solidification rate and temperature gradient determined the morphology of solidification. The CET curve [38] of Ti-6Al-4V and the range of solidification conditions during LSF are shown

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in Fig. 6. During the rapid solidification stage of the LSF process, the solidification rate increased and the temperature gradient decreased from the bottom to the top of the molten pool, depending on heat dissipation and solidification characteristics. Taking into account the scope of the solidification condition, as shown in Fig.6, large columnar grains were formed at the bottom of the molten pool, and exhibit epitaxial grain growth from the parent grains at the

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bottom of the pool. By contrast, at the upper part of the molten pool, equiaxed grains predominated. However, during the LSF process using a low laser power (e.g. 2 kW), unidirectionally aligned full-columnar grains were obtained throughout the entire deposit. This was attributed to the scope of solidification conditions located at the single columnar zone of the CET curve for the entire molten pool. As the laser power increased (exceeding 2 kW), the accumulated heat increased, the temperature gradient decreased, and the solidification rate increased. Thus, equiaxed grains occurred at the upper part of the molten pool. While the equiaxed grains on the underlying deposited top layer could be re-melted by

the subsequent deposited layer, and equiaxed grains were only retained on the top in the last deposited layer of the deposit. As a result, the microstructure of the deposit fabricated using laser powers exceeding 2 kW was composed of an equiaxed grain zone at the top surface and unidirectionally aligned columnar grains below the top equiaxed grain zone. The statistical analysis of the grain size in the LSFed Ti-6Al-4V alloy is illustrated in Fig. 7. As shown in Fig. 7(a), the grain size increased as the laser power increased. The width of the columnar grains near the lateral surface gradually coarsened (from 247 to 985 μm) as the laser power increased. The size of the equiaxed grains increased (from 503 to 990 μm)

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and the equiaxed grains zone thickened (from 0 to 2.8 mm) as the laser power increased. Both the columnar and equiaxed grains of the deposit became coarser as the laser power increased

due to the greater heat accumulation and melt superheating, which prolonged the existence of

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the molten pool and the growth time of the grains. Fig. 7(b) shows that the thickness of the equiaxed grain zones at the top obviously increased as the laser power increased, while the

grain sizes slowly increased. When the laser power and deposited height increased, the heat accumulation also increased, which increased the depth of the molten pool. As a result, CET and the thickness of the equiaxed grain zone increased.

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occurred more easily during solidification, the volume of the equiaxed grains became larger,

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3.2 Corrosion behaviour of the LSFed Ti-6Al-4V alloy As shown in section 3.1, the evolution of the surface grains of the LSFed Ti-6Al-4V alloy suggested that the deposits prepared using laser powers of 2 and 5 kW exhibited

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significant differences in grain morphology and size. Thus, these two deposits were selected for electrochemical tests. In this paper, ‘2 kW-lateral’, ‘2 kW-top’, ‘5 kW-lateral’ and ‘5 kW-

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top’ were used to refer to the lateral and top surfaces of the deposits prepared using LSF employing laser powers of 2 and 5 kW, respectively. The microstructural characteristics of the surface during electrochemical tests are shown in Fig. 8. The grain morphology of the 2

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kW-top sample exhibit fine pseudo-equiaxed structure (386 ± 32 μm) which represent a transversal section of columnar structure, as shown in Fig. 4. The 5 kW-top sample exhibited coarse equiaxed structure (990 ± 62 μm). The grain morphologies of the 2 kW-lateral and 5

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kW-lateral samples were fine columnar (247 ± 39 μm) and coarse columnar structures (985 ± 59 μm), respectively. We performed XRD measurements to examine the phase constituents of the LSFed Ti-6Al-4V alloy of all samples before electrochemical tests. Fig. 9 illustrates the XRD patterns for the LSFed Ti-6Al-4V alloys with different grain characteristics. It can be seen from Fig. 9, that the LSFed Ti-6Al-4V alloy consisted mainly of α-Ti phase and only a

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small percentage of β phase from the prior β grain. Although all the samples exhibited the same constituent phases, the intensities of several peaks of the main phases of the 2 kWlateral, 2 kW-top, 5 kW-lateral and 5 kW-top samples were slightly different. This revealed that these samples presented different volume fractions of the constituent phases. The volume fractions of the constituent phases calculated from the XRD fitting results based on Eq. (1) are summarized in Table 1.

Fig. 10 presents the variations in the OCP values with time for the samples of deposits immersed in the 15 wt.% NaCl solution. The solution is one of the typical solutions used for Ti alloys machined utilizing ECM. As shown in Fig. 10, the OCP values of the deposits shifted positively during immersion in the NaCl solution. This indicated that the electrochemical anodic process was significantly inhibited by the passive film formed on the surface of the alloy. The formation of the film led to the electrode process being controlled by anodic process. The passive film formed on the surface of the Ti-based alloys has been demonstrated to be a layer of oxide [39]. The OCP values of the 5 kW-top and 5 kW-lateral

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samples had the tendency to become stable after approximately 1 h, while approximately 2 h

was required for the 2 kW-top and 2 kW-lateral samples. For the Ti-6Al-4V produced by AM, the immersion time to obtain a stable OCP in 15 wt.% NaCl, was apparently shortened

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compared to that required to obtain a stable OCP in a 3.5 wt.% NaCl solution (25 h) [30].

This suggested that a protective passive film formed more rapidly on the surface of Ti when it was immersed in more concentrated NaCl solution. In addition, the formation rates of the passive film for 5 kW-lateral and 5 kW-top LSFed Ti-6Al-4V alloy samples were higher than those for the 2 kW-lateral and 2 kW-top samples immersed in the 15 wt.% NaCl solution. The

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final stabilized potentials for the 5 kW-lateral (−219.5 ± 12.3 mV) and 5 kW-top (−231.2 ±

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16.5 mV) LSFed Ti-6Al-4V alloy samples were very similar. These values were obviously more negative than those of the 2 kW-lateral (−69.0 ± 8.9 mV) and 2 kW-top (−45.6 ± 10.2

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mV). For Ti alloys, the more positive OCP values implied superior corrosion resistance when compared to all the other samples.

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immersed in NaCl solutions [40].The 2 kW-top sample exhibited the best corrosion resistance

Fig. 11 shows the Tafel plot polarisation curve for the samples immersed in the 15 wt.%

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NaCl solution. Both the cathodic and anodic branches on the Tafel curves were used to determine the corrosion current density (jcorr) and corrosion potential (Ecorr) values of the materials [41]. In addition, polarisation resistance (Rp) was calculated from the Tafel plots

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near the corrosion potential using a single small potential step of ±10 mV [42]. The corrosion parameters obtained from the Tafel plots of the deposits are listed in Table 2. Generally, it

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was documented that the more positive value of the Ecorr implied superior corrosion resistance for the corresponding material [43]. From Fig. 11(a), the Ecorr values suggested that the corrosion resistance of the 5 kW-top sample (−219 ± 12.3 mV) was the lowest, while that of the 2 kW-lateral sample (−96 ± 8.6 mV) was the best. The Ecorr value of the 5 kW-lateral sample (−208 ± 6.7 mV) was very similar to that of the 5 kW-top sample, which indicated

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that the two samples possessed similar corrosion resistance. The values of jcorr reflected the corrosion rate of the material for each solution system according to Faraday's law [44]: jcorr K EW d where K is a constant (3272 mm/(A cm year)), EW represents the equivalent weight (11.758 Corrosion rate =

g/equiv.), the d is the density of the workpiece (4.420 g/cm3). Based on Eq. (2), the corrosion rate increased as the jcorr increased. However, Ti alloys could very easily form passive films on their surfaces, and their actual corrosion rates might not correspond to jcorr [45]. Thus, the

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jcorr values listed in Table 2 were only used to rank the corrosion resistance of the sample. The jcorr values of the 5 kW-lateral (12.89 ± 0.03 μA cm−2) and 5 kW-top (10.77 ± 0.02 μA cm−2) samples exhibited an improvement of two orders of magnitude compared to those of the 2 kW-lateral (0.75 ± 0.02 μA cm−2) and 2kW-top (0.87 ± 0.02 μA cm−2) samples. This suggested that the corrosion resistance of the LSFed T-6Al-4V alloys in 15 wt.% NaCl was similar for the 5 kW-lateral and 5 kW-top samples, however these were inferior to those of the 2 kW-lateral and 2 kW-top samples. In addition, Rp calculated from the Tafel plots represented the resistance of the metal to oxidation while applying an external potential.

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Based on the Stern-Geary equation [46], as shown in Eq. (3), the corrosion rate of the material is inversely related to Rp.

βa βc 2.303 Rp ( βa +βc ) where βa and βc represent the anodic and cathodic Tafel slope, respectively. The corrosion rate

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jcorr =

estimated from the Rp values was found to increase as follows: 2 kW-lateral (88 ± 0.63 kΩ cm−2) < 2 kW-top (75 ± 0.56 kΩ cm−2) < 5 kW-top (13 ± 0.42 kΩ cm−2) < 5kW-lateral (8 ± 0.25 kΩ cm−2). As such, the electrochemical corrosion parameters obtained from the Tafel

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plots of the LSFed Ti-6Al-4V alloys immersed in 15 wt.% NaCl suggested that the 2 kW-

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lateral sample exhibited almost the same corrosion resistance as the 2 kW-top sample, and the 5 kW-lateral sample presented a slightly lower corrosion performance than the 5 kW-top

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sample. The top surface and lateral surface samples produced using a laser power of 2 kW

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possessed remarkably superior corrosion resistance compared to the 5 kW samples. We used EIS to measure the general corrosion resistance of the deposits immersed in the 15 wt.% NaCl solution. The EIS experimental data and fitted results are presented in Fig. 12

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using Nyquist and Bode plots. To obtain information about the corrosion of the samples, electrical equivalent circuit (EEC) was used to model the EIS data. Most passivated metals, such as Ti-based alloys, usually form a duplex structure oxide film on the surface. The film

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consists of a compact inner layer and a relatively porous outer layer [30, 44, 45]. Additionally, as seen in Fig. 12(b), the Bode plots exhibit plateaus over wide frequency

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ranges. Thus, a two time-constants EEC model consisting of resistance of solution (Rs), charge transfer (Rct), and film (Rf), along with a capacitance resistance (Cf), constant phase element (Qdl), and Warburg impedance (Ws), was used to fit the EIS data, as shown in Fig. 13. The Qdl was used because the surface of the electrode was not perfectly flat. The fitted results of the EIS data are summarized in Table 3. The sum of Rf and Rct can describe the overall

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corrosion resistance of the material, and represents Rp. As seen in Table 3, the values of Rct were significantly higher than those of Rf for all of the LSFed samples. Thus, Rct could be used to approximately represent Rp. Moreover, the higher values of Rct implied that the sample possessed better corrosion resistance. As such, there were no obvious distinction in corrosion resistance performance of the 2 kW-lateral and 2 kW-top samples. The corrosion resistance of the 5 kW-top sample was slightly higher than that of 5 kW-lateral sample. The LSFed samples produced using a laser power of 2 kW exhibited great improvement in

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corrosion resistance compared to the samples produced using a laser power of 5 kW. The EIS results on corrosion behaviour were very consistent with those obtained from the Tafel tests. Corrosion, as a surface-dependent property, is affected by the grain size, morphology, phase constituents and texture of the material [20, 30, 47]. The corrosion resistance of the 5 kW-lateral sample was the lowest of all samples, and the 5 kW-top sample exhibited slightly better corrosion resistance than the 5 kW-lateral sample. As shown in Figs. 7(a) and 8, the two samples presented almost the same grain size, while the corroded surface grain morphology exhibited equiaxed grain for the 5 kW-top sample and columnar grain for the 5

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kW-lateral sample. For the LSFed Ti alloy, the crystallographic orientation of the equiaxed prior β grains was random, while the columnar prior β grain presented obvious texture

features. Zhang [48] found that the LSFed Ti alloy was characterized by coarse columnar

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prior β grains. Simultaneously, the prior β grains showed strong <100> fibre texture. Since

LSF is a near-rapid solidification process, where Ti and its alloys are cooled down from the β phase field, the β → α phase transformation follows the Burgers orientation relationship. As shown in Fig. 5, the columnar β grains near the lateral of the deposit exhibited a small

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deflection angle from the deposition direction, which was parallel to the test section of the lateral surface. Therefore, the crystallographic orientation of the β grains at the lateral surface

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formed a small angle with the deposition direction. Thus, the texture characteristics of the 5 kW-lateral lateral and 5 kW-top samples were different. Grains oriented in the direction of

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close-packed atoms presented superior corrosion resistance because their removal was more

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difficult compared to the removal of grains oriented in a more loosely packed direction [47, 49]. The different corrosion resistances between the 5 kW-lateral and 5 kW-top samples might be attributed to the grain texture. From the corrosion resistance results, it can be

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predicted that the lateral surface of the sample exhibited a lower packing density compared to the top surface. Osorio [18, 50] has reported that the equiaxed structure exhibited a slightly better corrosion resistance than columnar structures for Al and Zn castings. In addition, Levy

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[51] has investigated the electrochemical behaviour of α+β Ti alloys and found that galvanic coupling was created between the α-Ti and β phases because of the potential difference

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between them. Furthermore, the composition of the constituent phases and their effective interface area were the primary factors affecting the intensity of galvanic coupling [39]. As shown in Table 1, the volume fraction of the β phase of the 5 kW-lateral sample (11.2%) was higher than that of 5 kW-top sample (6.5%). This resulted in an increase in the effective α/β interface area for the 5 kW-lateral sample. As a result, the corrosion behaviour of the 5 kW-

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lateral sample was inferior compared to that of the 5 kW-top sample. While the 5 kW-lateral and 2 kW-lateral samples presented almost the same grain

textures, their corrosion performances were notably different. The higher content of β phase of the 5 kW-lateral sample led to the decrease in its corrosion resistance due to the increase in galvanic coupling between the α and β phases [39]. On the other hand, the columnar grain width of the 5 kW-lateral sample was much larger than that of the 2 kW-lateral sample. Balyanov, Balakrishnan, and Lu et al. [19, 21, 52] considered that reducing the grain size

presented an important effect on enhancing the electron activity [53] near grain boundaries, resulting in decreasing the electron work function. Therefore, the surface characterized by higher grain boundaries became more reactive and prone to form a more stable passive film. Thus, we concluded that the corrosion resistance of the coarser columnar grain of Ti-6Al-4V produced by LSF was inferior to that of the finer one due to the mechanism mentioned above. In addition, the corrosion resistance of the 5 kW-top sample was obviously lower than that of the 2 kW-top. According to the information above, the equiaxed grain morphology and coarser grain size of the 5 kW-top sample contributed to its superior corrosion resistance.

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However, because the 5-kW lateral and 5-kW top samples exhibited little difference in

corrosion performance, we determined that the influence of grain morphology on corrosion resistance was not significant. Thus, we concluded that grain size was a more influential

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factor than grain morphology on the corrosion properties of the Ti-6Al-4V alloy produced by LSF.

3.3 Electrochemical dissolution behaviour of LSFed Ti-6Al-4V alloy As previously shown, the phase constituents and grain morphology and size significantly

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impact corrosion resistance. However, ECM was carried out in the transpassive region, therefore, when the anode potential became higher than a certain value, a steep increase in

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current density would occur because of breaking the passive film. To investigate the anodic dissolution behaviour of the LSFed Ti-6Al-4V alloy, potentiodynamic measurements were

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conducted. The polarization potential represents the additional potential required to dissolve a

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metal [54]. In addition, measuring the polarization curve was also used to estimate the range of processing potential required for ECM [10]. This set of measurements was aimed to characterize the dissolution potential (Ediss), and transpassive current density (jdiss), which

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existed at the inflexion point of the curve where the material started to transpassively dissolve. In general, a material with a lower Ediss in a solution system would exhibit a better electrochemical machinability. Here, jdiss was defined as the initial current density of

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transpassive dissolution for the film formed on surface of the alloy, and, typically, a lower jdiss would indicate easier passivation.

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Fig. 14 shows the potentiodynamic polarisation curves of deposits with various grain

morphologies immersed in 15 wt.% NaCl solutions. Table 4 lists the results of the potentiodynamic polarization curves. As shown in Fig. 14, the anodic current density exhibited very low increases in the 1–5, 1–5.3, 1–4.9, and 1–6 V potential ranges for the 2

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kW-lateral, 2 kW-top, 5 kW-top, and 5 kW-lateral samples, respectively. The current density increasing slowly over a wide potential range was primarily attributed to the oxide film formed on the surface, which prevented the anodic material from dissolving [55]. The region where the anodic current changed slowly was called a passive region in the polarization curve. As the potential increased to Ediss for each sample, the current exhibited a sharp inflection and continued to increase until a maximum was reached, indicating that the oxide layer was broken and the anodic material began to dissolve [56, 57]. The corresponding region of the polarization curve was considered to be a transpassive region, and was followed

by a stable dissolution region. The dissolution potentials of the samples inferred from the polarization curve were approximately 5 V for the 2 kW-lateral, 2kW-top, and 5kW-top samples, and 6 V for the 5 kW-lateral sample. These values of Ediss represented the minimum potentials required for machining the LSFed Ti-6Al-4V alloy using ECM. The values of Ediss were in good agreement with the results of the volume fractions of the phases. This implied that the content of β phase was positively related to the dissolution potential. As shown in Fig. 9, the XRD results revealed that all test samples were composed of α and β phases. The volume fraction of the β phase for the 5 kW-lateral sample (11.2 %) was much higher than

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those of other samples (approximately 6%) calculated from the XRD patterns. The difference in the α/β volume ratios in the microstructure of the Ti-based alloy resulted in different

electrochemical activities of the alloy in NaCl solutions [30]. The V content the β phase was

higher than that of α phase, and played a crucial role in increasing the resistance to dissolution

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of the Ti-alloy, since the oxide film that formed on the β phase could be more stable than that formed on the surface of the α phase [58]. Therefore, it was reasonable to deduce that the 5

kW-lateral sample exhibited the poorer anodic dissolution performance than the other samples because of the higher amount of β phase in its microstructure. Therefore, the higher the α/β

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volume ratio of the deposits was, the more inferior their electrochemical machinability. The

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jdiss values for the 5 kW-lateral (4.10 ± 0.03 mA cm−2) and 5 kW-top (3.93 ± 0.02 mA cm−2) samples exhibited an improvement of an order of magnitude compared to those of the 2 kW-

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lateral (0.90 ± 0.02 mA cm −2) and 2kW-top (0.75 ± 0.01 mA cm−2) samples. In general, lower jdiss values suggested a slow dissolution of the material or an easy passivation of the film. This

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revealed that the passive films of the 2 kW-lateral and 2 kW-top samples possessed better protective properties than those of the 5 kW-lateral and 5 kW-top samples. These results were

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similar to the corrosion behaviour data obtained from Tafel measurements and EIS. Fig. 15 illustrated the morphologies and 3D optical topographic maps of the dissolved surfaces after potentiodynamic polarization tests for the 2 kW-top and 5 kW-top LSFed

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samples. As shown in Fig. 15(a) and (b), the grain boundaries of the 2 kW-top sample were preferentially dissolved during the potentiodynamic polarization test. This could be confirmed

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by the clearly observed pseudo-equiaxed grain characteristic. However, only a few noncontinuous grain boundaries were observed for the 5 kW-top sample. According to the literature, the grain boundaries of the LSFed Ti-6Al-4V alloy consisted of continuous α-Ti phase [25]. The grain boundary α phase was coarser than the intragranular α phase [59]. The morphologies of the dissolved surfaces suggested that grain boundaries of fine pseudo-

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equiaxed grains preferentially dissolved, while the dissolution rates of the grain boundaries and intragranular phase of the coarse equiaxed grains were not significantly. The grains of the 2 kW-top samples exhibited a strong fibre texture, offering nearly the same atomic packing density between adjacent grains on the surface. Therefore, the higher energy grain boundary became the region where dissolution resistance was the weakest. However, the 5 kW-top sample consisted of more randomly orientated equiaxed grains than the samples consisted of columnar grains. Thus, the competitive dissolution among grains with different atomic packing densities on the surface obscured the effect of grain boundary, implying that grain

texture played a more effective role in electrochemical dissolution than grain size. Furthermore, the 3D optical macro-topography map of the 2 kW-top was smoother than that of the 5 kW-top sample, as shown in Fig. 15 (a)–(d). The local 3D optical micro-topography within a grain of the 2 kW-top exhibited a smaller scattered discrete bulge compared to that of a grain of the 5 kW-top sample, as shown in Fig. 15(e) and (f). This indicated that the fine pseudo-equiaxed grain dissolved more uniformly than the coarse equiaxed grain when subjected to an applied potential. Generally, the formation of a compact oxide passive layer, which restrains the

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dissolution process, presented the main factor affecting the ECM of the deposits. As described in section 3.2, the passive film also played an important role in corrosion behaviour. The

mechanism of electrochemical dissolution using an applied potential was similar to that of

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corrosion. The grain boundary, crystallographic orientation, and phase constitute at the

machined surface directly affected the formation of passive film. The transpassive dissolution potential was determined by the phase constitute. The pseudo-equiaxed grain was actual the observation of the cross section of columnar grain, which exhibited the orientated growth and

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strong texture. The pseudo-equiaxed grains on the dissolved surface exhibited the same lattice plane. By contrast, the equiaxed grains were randomly oriented, and the atomic packing

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density of exposed grains on the dissolved surface was different. Thus, the dissolved surface morphology of fine pseudo-equiaxed grain was more uniform than that of coarse equiaxed

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grain.

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3.4 Electrochemical dissolution rate The influence of grain morphology on the corrosion behaviour of the deposits might

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have been caused by grain orientation. In theory, the effect of grain orientation on corrosion performance could be estimated by comparing the surface energy levels in closely and loosely packed crystallographic planes [60]. More-closely packed planes are associated with higher

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atomic coordination and higher binding energy but lower surface energy and higher dissolution activation energy [61]. In fact, the relationship between corrosion rate and surface energy has been well investigated for many materials [60, 62]. In this study, the LSFed Ti-

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6Al-4V alloy presented similar lattice structure and lattice constant to pure Ti. Moreover, the Ti lattice was not significantly distorted by the limited amounts of Al and V as substitutional solute atoms in the matrix phase. Although the phase composition of Ti-6Al-4V (α+β) was different than that of pure Ti (α), the β phase content of the LSFed Ti-6Al-4V alloys

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estimated from XRD patterns was very small. Thus, using the unit crystal cell of α-Ti to understand the electrochemical dissolution properties of LSFed Ti-6Al-4V in different crystallographic planes would introduce an error, but the error would not be too significant. For α-Ti, the (0001) basal plane possessed the highest atomic density (1.33 × 1015 atoms cm−2), followed by the (112̅0) and (011̅0) prismatic planes (8.36 × 1014 and 7.24 × 1014 atoms cm−2, respectively) [63]. Closely packed crystallographic plane possessed higher binding energy but lower surface energy. For the α-Ti (0001), (011̅0) and (112̅0) planes, the surface energy were 0.933 × 10−3, 1.359 × 10−3 and 1.187 × 10−3 J m−2 [64], respectively. The

electrochemical anodic dissolution rate for a metal can be determined using the following equation [60]: Q+αnFE Ia =nFk exp ( ) RT where n represents the number of electrons related in the electrochemical process; k, F, and R

are the reaction, Faraday and gas constants, respectively; T represents the absolute temperature for the electrochemical process; E is the electrode potential; Q is the dissolution activation energy; and α is the transit coefficient calculated using the surface energy instead

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of the activation energy. The effect of the surface of the crystallographic plane exposed to the electrolyte solution on Q was the same as that on E. In this study, it was reasonably assumed that n and k were the same for different crystallographic planes. At a given value of E at 35 °C, the ratio of I5α kW-lateral of the 5 kW-lateral surface consisting of columnar grains to I5α kW-lateral 5 kW-top Iα

=exp [

α (Q5 kW-lateral − Q5 kW-top ) RT

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I5α kW-top of the 5 kW-top surface consisting of equiaxed grains should be

]. In general, Ti alloy produced by AM exhibit strong

<001> fibre texture for the prior β grain, and their β → α phase transformation follows the

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Burgers orientation relationship [48, 65], as shown in Fig. 16. As mentioned in section 3.1, the prior β columnar grains also showed obvious epitaxial growth along the deposition

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direction, which was consistent with the preferred growth direction. The lateral surfaces of the

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deposits were dominated by the {001} plane for prior β grain, and the prismatic plane for the α-Ti phase according to the Burgers orientation relationship. However, the orientation of the

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equiaxed β grains was random. It could be reasonably assumed that each crystallographic plane (including the basal and prismatic planes) of α-Ti equiprobably appeared on the surface 1

̅

̅

3

̅

̅

2

1

[Q(0110) +Q(1120) +Q(0001) ]. If α= , after the t surface energy values of pure Ti were used, we 2

obtained:

I5α kW-lateral

1

= exp {2

5 kW-top Iα

1 2

1 3 19

×10−3 ×[ (1.359+1.187)− (1.359+1.187+0.933)]×6.02×1023

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1

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of the equiaxed prior β grains. Thus, Q5 kW-lateral = [Q(0110) +Q(1120) ]; Q5 kW-top =

1.6×10 ×8.31×308

} =1.001,

suggesting that the theoretical dissolution rate of the 5 kW-lateral sample was slightly higher

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than that of the 5 kW-top sample. Namely, the corrosion resistance of the equiaxed grains was slightly higher than that of the columnar grains. These findings matched the experimental results very well. Therefore, we concluded that the influence of grain morphologies on the

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electrochemical corrosion properties mainly contributed to grain orientation. 4 Conclusions This study focused on the relationship between the grain characteristics and electrochemical behaviour of an LSFed Ti-6Al-4V alloy in a 15 wt.% NaCl solution. Metallographic analysis was used to analyse the grain characteristics of the deposits. In addition, EIS, Tafel, and potentiodynamic polarisation tests were performed on the surfaces of deposits with various grain characteristics. The conclusions can be summarized as follows.

(4)

(1) The Ti-6Al-4V alloy deposits contained three types of grain: (a) coarse β columnar grains that grew along the deposition direction in the centre of the deposit, (b) columnar grains deflected towards the outer wall near the lateral surface of the deposit, and (c) equiaxed grains or epitaxial columnar grains near the top surface of the deposit. The width of the columnar grains, diameter of equiaxed grains, and thickness of equiaxed grain layers increased as the laser power increased. (2) According to the EIS results and Tafel polarisation curves, the corrosion behaviour of the LSFed Ti-6Al-4V alloy was related to the phase constituent, grain size, and texture. The

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coarse grain containing higher percentages of β phase exhibited inferior corrosion performance.

The corrosion resistance of the equiaxed grains was slightly higher than that of the columnar grains due to grain texture. Lastly, corrosion resistance decreased as the grain size increased.

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(3) The potentiodynamic polarisation curve indicated that the anodic dissolution behaviour

of the LSFed Ti-6Al-4V alloy was also affected by its grain characteristics. The columnar morphology grain boundary was preferentially dissolved compared to that of equiaxed morphology. The quality of the protective film on the surface of the fine grain was better, and thus the film hindered dissolution. The fine pseudo-equiaxed grains dissolved more uniformly

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than the coarse equiaxed grains. The higher content of β phase on the machined surface resulted

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in a nobler transpassive dissolution potential.

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Acknowledgements

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Funding: This work was supported by the National Key Research and Development Programme of China (Grant Number 2016YFB1100104); and the National Natural Science

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Foundation of China (Grant Numbers 51323008, 51501154 and 51565041). References:

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Figure and Table Caption

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Fig. 1. Schematic diagram of the LSF and ECM combination.

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Fig. 2. Schematic diagram of (a) LSF; (b) metallographic sample; and (c) electrochemical

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measurements sample.

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Fig. 3. OM of (a1)–(a4) lateral and (b1)–(b4) top surface of LSFed Ti-6Al-4V (The 1, 2, 3 and 4 subscripts represent samples deposited using the laser powers of 2, 3, 4, and 5 kW, respectively).

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Fig. 4. Schematic diagram of grain morphologies produced by LSF using laser powers of (a) 2

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and (b) 3–5 kW.

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Fig. 5. Lateral wall profile and shape of LSF deposition layer: (a) experimental results and (b)

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schematic diagram of columnar crystallographic orientation.

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Fig. 6. CET curve [38] of Ti-6Al-4V and scope of solidification conditions using LSF.

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Fig. 7. Statistical data of LSFed Ti-6Al-4V: (a) histogram of columnar grain width and (b)

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thickness of equiaxed grains zone and diameter of equiaxed grain.

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Fig. 8. Surface grain morphologies of electrochemical measurements for (a) 2 kW-top surface;

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(b) 5 kW-top surface; (c) 2 kW-lateral surface; and (d) 5 kW-lateral surface.

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Fig. 9 XRD patterns of LSFed Ti-6Al-4V samples with different grain characteristics.

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Fig. 10. OCP dependence of time for various grain characteristics of the LSFed Ti-6Al-4V alloy

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immersed in a 15 wt.% NaCl solution.

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Fig. 11. Tafel plot polarisation curve of LSF Ti-6Al-4V: (a) top or lateral surface at different

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laser powers and (b) schematic diagram of Tafel fit.

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Fig. 12. Measured (discrete) and fitted (solid lines) impedance spectra of LSFed Ti-6Al-4V

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alloys at their corresponding OCP during exposure to a 15 wt.% NaCl solution.

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Fig. 13. EEC used to fit the impedance data.

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Fig. 14. Potentiodynamic polarization measurements of the LSFed Ti-6Al-4Valloy with

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different grain characteristics in a 15 wt.% NaCl solution.

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Fig. 15. Dissolved surface morphologies and 3D optical topographic maps after

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potentiodynamic polarization tests for the LSFed Ti-6Al-4V alloy: (a) and (b) Dissolved surface morphologies of the 2 kW-top and 5 kW-top samples, respectively; (c) and (d) 3D optical

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macro-topographic maps of the 2 kW-top and 5 kW-top samples, respectively; and (e) and (f) Local 3D optical micro-topographic maps of the 2 kW-top and 5 kW-top samples within a

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grain, respectively.

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Fig. 16. Schematic representation of the Burgers orientation relationship between the α and β

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phases during the phase transformation of the LSFed Ti-6Al-4V alloy.

Table 1. Phase constituents and their volume fraction (Vf) of LSFed Ti-6Al-4V alloy estimated from the XRD patterns. Sample

Vf, α /% 93.7 94.2 88.8 93.5

Phase constituents α+β α+β α+β α+β

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2kW-lateral (Fine columnar structure) 2kW-top (Fine pseudo-equiaxed structure) 5kW-lateral (Coarse columnar structure) 5kW-top (Coarse equiaxed structure)

Vf. β /% 6.3 5.8 11.2 6.5

Table 2. The corrosion parameters evaluated from Tafel plots of LSFed Ti-6Al-4V alloy in a 15 wt.% Ecorr/mV -96 ± 8.6 -148 ± 9.3 -208 ± 6.7 -219 ± 12.3

jcorr/μA cm-2 0.75 ± 0.02 0.87 ± 0.02 12.89 ± 0.03 10.77 ± 0.02

βc/mV dec-1 265 ± 11 216 ± 9 488 ± 13 625 ± 12

βa/mV dec-1 370 ± 15 300 ± 11 461 ± 12 649 ± 10

Rp/kΩ cm-2 88 ± 0.63 75 ± 0.56 8 ± 0.25 13 ± 0.42

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NaCl solution. Sample 2kW-lateral 2kW-top 5kW-lateral 5kW-top

Table 3.

0.64 0.02 0.47 0.01 0.37 0.01 0.33 0.01

Cf Qdl (Fcm-2) (S Secn cm-2) 1.02 0.01 1.15 0.01 1.09 0.01 1.05 0.01

4.36 0.03 9.28 0.05 3.29 0.04 5.21 0.12

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χ2 ×104

0.72 0.01 0.76 0.01 0.65 0.01 0.75 0.01

3.54

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The fitting parameters of EIS for the LSFed Ti-6Al-4V alloy. Rct Rs Rf W-R ×10-3 -3 Sample ×10 W-T -2 -2 (Ω cm ) (Ω cm ) (Ω cm-2) -2 (Ω cm ) 2kW-lateral 2.32 26.04 11.91 41.96 74.13 Error 0.01 1.47 0.21 0.47 1.29 2kW-top 2.64 21.11 13.24 52.17 2179 Error 0.01 1.01 0.22 0.80 71.74 5kW-lateral 0.91 23.13 3.77 14.14 2328 Error 0.01 1.77 0.18 0.26 116.48 5kW-top 0.81 55.10 5.72 30.90 5363 Error 0.01 5.27 0.20 0.56 287.47

2.40 9.20 2.10

Table 4 j diss/mA cm-2 0.90 ± 0.02 0.75 ± 0.01 4.10 ± 0.03 3.93 ± 0.02

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Results of potentiodynamic curve for LSFed Ti-6Al-4V alloy in 15 wt% NaCl solution. Sample Grain morphology Grain size/μm E diss/V columnar 386 ± 32 2kW-lateral 5.0 ±0.6 pseudo-equiaxed 247 ± 39 2kW-top 5.3 ± 0.4 columnar 990 ± 62 5kW-lateral 6.0 ± 0.5 equiaxed 985 ± 59 5kW-top 4.9 ± 0.6