Wear and corrosion behaviour of Inconel 718 laser surface alloyed with rhenium

Materials and Design 132 (2017) 349–359

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Wear and corrosion behaviour of Inconel 718 laser surface alloyed with rhenium Tomasz Kurzynowski ⁎, Irina Smolina, Karol Kobiela, Bogumiła Kuźnicka, Edward Chlebus Centre for Advanced Manufacturing Technologies/Fraunhofer Project Center, Wrocław University of Technology, ul. Łukasiewicza, 550-371 Wrocław, Poland

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Complete dissolution of Re powder in the substrate enhances corrosion resistance. • Alloying with 14 wt% Re increases corrosion potential by 100 mV and pitting potential by 270 mV. • Partial dissolution of Re powder in the substrate improves wear resistance. • Alloying with 28 wt% Re decreases sliding wear rate by 82%.

a r t i c l e

i n f o

Article history: Received 13 April 2017 Received in revised form 10 July 2017 Accepted 11 July 2017 Available online 12 July 2017 Keywords: Laser surface alloying Inconel 718 Wear Corrosion Rhenium Microstructure

a b s t r a c t Laser surface alloying (LSA) with rhenium was applied to produce wear and corrosion resistant layers on Inconel 718 alloy. Using a high-power diode laser and 50% overlapping ratio of laser tracks, two kinds of alloyed layers were produced: IN718–14 wt% Re layer with completely dissolved Re powder in substrate, and IN718–28 wt% Re layer with partially dissolved Re powder in substrate. Examinations of microstructure and tribological properties were carried-out of both alloyed layers and substrate under sliding and abrasive wear conditions, as well as electrochemical measurements were made in 3 wt% NaCl solution. It was found that, alloying with 14 wt% Re favourably affects corrosion resistance of IN718, as indicated by significant increase of critical pitting potential (+270 mV) at 24% drop of corrosive current, and anodic displacement of corrosion potential (+100 mV). In comparison to the IN718 substrate, the layer alloyed with 28 wt% Re has 160% higher hardness, 82% lower sliding wear rate, and 25% higher abrasive wear resistance index. © 2017 Published by Elsevier Ltd.

1. Introduction Inconel 718 (IN718) is an age-hardenable Ni-Cr-Fe alloy that combines high-temperature strength up to 700 °C with corrosion resistance and excellent fabricability [1,2]. Because of these attributes, IN718 is one of the most widely used nickel-based superalloys in the aircraft engine industry [3–5]. Moreover, it is also commonly used for critical rotating ⁎ Corresponding author. E-mail address: [email protected] (T. Kurzynowski).

http://dx.doi.org/10.1016/j.matdes.2017.07.024 0264-1275/© 2017 Published by Elsevier Ltd.

parts, supporting structures, high-speed airframe parts, hightemperature bolts and fasteners, as well as for nuclear engineering as cast, wrought and powder metallurgy products. In spite of its otherwise excellent properties, IN718 (as well as other superalloys) has unsatisfactory wear characteristics. Thus, surface treatment that could improve tribological behaviour without adversely affecting corrosion resistance has high potential to improve performance and expand the application field of both this one and other grades of superalloys. Laser surface alloying (LSA) is a material-processing method changing chemical composition of metal by melting a thin superficial layer of

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a substrate with a laser beam, simultaneously adding a desired alloying element [6]. Apart from a change of chemical composition, the inherently rapid heating and cooling rates (l05 to 1011 K/s) [6,7] associated with this process result in creation of metastable and nonequilibrium phases, offering a possibility to create new materials with novel properties. An additional advantage is that some expensive, scarce and strategically important metals can be saved. However, obtaining a laser-alloyed zone (LAZ) with desired depth, solute content and microstructure requires determination of a suitable processing window determined by laser power, beam diameter, traverse speed, amount of alloying elements and degree of overlapping [8]. Rhenium is a refractory metal that possesses an exclusive combination of physical, mechanical and chemical properties. It is distinguished, among others, by low coefficient of friction, one of the highest strain hardening exponents, resistance to a wide range of harsh environments, and absence of ductile-to-brittle transition at cryogenic temperatures [9]. Consequently, Re and its alloys are considered high-performance engineering and coating materials [10,11] in a diverse range of defense and civilian industries that include aerospace, energy, nuclear, electrical, chemical and biomedical production. It is also used as a very effective modifier of properties of the alloys. The materials with rhenium have good high-temperature stability of microstructure and mechanical properties [12]. Rhenium is the most effective solid-solution hardening element and this is why it is used in the second- and third-generation superalloys at 3 wt% and 5–6 wt%, respectively [13]. Even though solubility of rhenium in nickel in solid state reaches 30 wt%, its limited percentages in superalloys result mainly from its negative effect on hightemperature oxidation [13] and large price fluctuations of this scarce strategic element [14]. According to Fleischmann et al. [15], rhenium has strong solidsolution hardening efficiency in matrices of superalloys by reducing the stacking fault energy (SFE). In turn, El-Bagoury et al. [16] attribute hardening action of rhenium to its unique ability to create clusters in solid solution of Inconel 718. Results obtained by Chaudhuri et al. [17] showed that lowered SFE value of nickel-based alloys plays a significant role in increasing their sliding wear resistance. However, no information can be found in literature concerning attempts of utilizing rhenium to improve tribological properties of nickel-based superalloys. On this ground, the authors of this paper assumed that LSA with controlled Re concentration would make it possible to give excellent tribological properties and corrosion resistance to superficial IN718 layers, as well as stability of tribological properties at wide range of temperatures. Disadvantages of the other applied techniques of modifying IN718 surface layers, like plasma nitriding [18,19], paste-boriding [20] or ion implantation of hard coatings [21], are their low thickness and inclination to brittle cracking and to delamination. In comparison to these techniques, an undoubted advantage of applying LSA is precision of operations and significant reduction of the time when a layer with a determined thickness and fine-grained microstructure is created with no change of the substrate microstructure. An additional advantage is favourable distribution of residual compression stresses in the lasertreated surface layer of IN718 substrate [22]. In comparison to laser cladding (LC), an attribute of the LSA process is higher flexibility of laser power selection, not restricted by depth of the substrate melting. Moreover, layers with higher surface smoothness can be created with unchanged substrate dimensions. However, rapid self-quenching and rapid resolidification of the substrate melted during LSA can obstruct dissolving in it the desired amount of the powdered alloying element, especially a high-melting element like rhenium. This makes a technological difficulty that must be considered during selection of LSA parameters but, at the same time, it gives a possibility to create both surface alloyed and composite layers. In this study, it is shown that laser melting is a successful technique of superficial alloying parts made of IN718 with rhenium and that rhenium is an attractive modifier of wear and corrosion properties of this

alloy. Influence of rhenium (over 6 wt%) on properties of IN718 has not been so-far explored. Therefore, a goal of this research was to obtain better understanding of relationship between rhenium content, microstructure and properties in order to guarantee suitability of surfacemodified components in aerospace, nuclear or chemical industries. When designing abrasive wear resistant and corrosion resistant layers, modified with rhenium, differently influencing factors must be considered (i.e. Re content and microstructure). Therefore, creation of two kinds of surface alloyed layers was assumed. The layer with improved corrosion resistance was created by reaching possibly highest solution degree of Re powder in the substrate, with no Re particles. In turn, the layer with improved abrasion resistance was designed as a composite layer in that solid solution strengthening was intensified by presence of dispersed rhenium particles. First of all, correct selection of LSA parameters was verified on the grounds of microstructural observations, EDS analysis of Re content and microhardness measurements. Next, influence of rhenium on wear and corrosion behaviour of the created alloyed layers was analysed on the grounds of tests carried-out in dry sliding and abrasive conditions, electrochemical measurements, and microstructure examinations.

2. Experimental procedure 2.1. Materials and LSA parameters LSA was performed on substrates of Inconel 718 (UNS N07718). Specimens 150 × 150 × 5 mm were cut-out of a plate in as-delivered hot-worked condition. Commercially pure Re powder (99.9 wt% Re; 0.25 at% O and 0.020 at% H) was subjected to plasma-atomization process to obtain spheroidal particles. The fraction of Re particles ≤40 μm was separated by sieving. LSA was carried-out using a system of a high-power diode laser LDF 4000-30 with the head coaxial with a powder-feeding nozzle, with no preheating of the substrate. Shielding was provided by argon supplied coaxially with the laser beam. The gas served also for transporting Re powder via the feeder. This oxidation prevention system appeared highly effective. SEM observations confirmed absence of oxides on surfaces and in microstructures of the surface alloyed layers. Process parameters were preliminarily selected on the grounds of trial manufacture of single melt tracks at various variants of laser power and powder feeding rate. Selected variants of parameters were used for manufacturing a series of 20-tracks layers whose quality (thickness, microstructure, hardness and lack of defects) made a base for a subsequent selection of parameters. Accepting effectiveness of dissolving rhenium particles in the superficially melted substrate as a criterion, two variants of parameters were finally selected for manufacture of alloyed layers: V1 (1100 W / 1.92 g/min) and V2 (1300 W / 4,79 g/min). The variant V1 was to ensure complete dissolution of rhenium particles in the melted substrate. The variant V2 was to ensure formation of composite microstructure, consisting of matrix (with possibly high dissolved rhenium content) and residual particles of rhenium. The remaining constant LSA parameters are given in Table 1.

Table 1 Parameters of laser surface alloying. Power

1100 and 1300 W

Diameter/surface area of laser spot Beam focusing diameter Shielding/carrying gas Number/length/overlap of melt tracks Nozzle travel speed Powder feeding

2.5 mm/4.91 mm2 250 mm argon 20/27 mm/50% 10 mm/s 1.92 and 4.79 g/min

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Fig. 1. Geometry of alloyed layers produced on IN718 at various LSA parameters: a) 1100 W; 1.92 g/min, 1300 W; 4.79 g/min. Cross-sectional light micrographs.

2.2. Microstructure examination Examinations were carried-out on cross-sections perpendicular to melt tracks, etched with 87 Glyceregia (standardized acc. to ASTM E 407) consisting of 15 ml HCl, 10 ml glycerol and 5 ml HNO3. The etchant was used fresh, 5 to 15 min after preparation. A laser scanning confocal microscope Olympus LEXT OLS4000 was used in microstructure examinations and surface roughness measurements. A scanning electron microscope Zeiss SEM EVO MA25, equipped with an EDS analysis system, was used for observations of higher magnification images and of alloyed layer surfaces after wear and corrosion tests. For microstructure analysis, an X-Ray diffraction system (D8 ADVANCE powder diffractometer with Cu Kα radiation and a Vantec detector) was used. Concentration of rhenium dissolved in the IN718 substrate was determined by quantitative spot-EDS analysis. The analysis was carried-out in dendrites of γ phase, repeating it several times. Accuracy of the analysis was considered sufficient, taking into account high solubility of rhenium in γ phase and susceptibility to dendritic microsegregation.

100Cr6) with hardness 62 HRC was used. The ball was pressed-down against the disc rotating at 80 rpm (v = 0.15 m/s) with the force F = 24.5 N. The total sliding distance L = 785 m was accepted. According to ASTM G 99, specific wear rate was accepted as the measure of wear of the alloyed layer: Z¼

  V m mm3 ¼ F∙L F∙L∙ρ N∙m

ð1Þ

The measured quantity was loss in mass m. Density ρ of the layers alloyed with rhenium was evaluated using the linear rule of mixtures. Three-body abrasion tests were conducted in accordance with GOST 23.208-79 (close to ASTM G 65-04), with use of the Tester T-07 corresponding to the standard. The main parameters of the abrasion tests were: rubber-clad steel wheel with a diameter of 50 and 15 mm thick; particle size of abrasive medium (aluminium oxide) 241 to 89 μm; load 44 N; rotational speed 60 rpm. The measured quantity was loss in mass of the specimens compared with that of the reference specimen. Acc. to GOST 23.208-79, the reference specimen was made of normalized steel C45 (EN 10083-2) with hardness 200 HV10. The measure of abrasive wear resistance in these conditions is abrasion rating index:

2.3. Wear tests and microhardness measurements To select a proper method of wear resistance testing, it is necessary to accept the criteria considering service conditions and contact conditions, in that the new coatings can be particularly effective. It is assumed in this work that the layers alloyed with rhenium will show effective wear resistance in harsh conditions thanks to their high susceptibility to strain hardening, low coefficient of friction and resistance to surface degradation as a result of local temperature increase. Thus, two types of sever wear tests were performed: dry sliding friction (ball on disc) and abrasive friction (three-body abrasion), so that evaluation of rhenium as a modifier of wear resistance of the alloy IN718 covered also its influence on various mechanisms of wear. At selecting the wear tests, their popularity was taken into account, so that the obtained results could be compared with those published for other materials [18,20]. All the tests were carried-out on the surfaces of laser alloyed layers cleaned by shot blasting, at ambient temperature. Dry sliding tests were carried-out on specimens in form of discs with a diameter of 40 mm using a ball-on-disc Tribometer T-01. Test parameters were selected considering recommendations of the standard ASTM G 99. To simulate harsh service conditions, higher load and smaller ball radius were chosen in order to increase the contact pressure. As a counter body, a ball with a diameter of 2 mm of bearing steel (ISO

Kw ¼

Mref ∙ρ Nref M∙ρref N

ð2Þ

where: Mref and M – loss in weight of the reference steel specimen and the examined specimen, ρref and ρ – density values of the reference steel specimen and the examined specimen, Nref and N – total number of wheel revolutions during the tests of the reference steel specimen and the examined specimen (Nref = 600; N = 600 or 1800 depending on hardness of the examined specimen). Vickers microhardness profiles on cross-sections of the alloyed layers were determined at 2.94 N, surface hardness was measured at 98.1 N and microhardness of undissolved Re particles was measured at 0.49 N. 2.4. Electrochemical measurements Tafel curves and cyclic polarization curves were used to characterize corrosion behaviour of IN718 substrate and two variants of the layers alloyed with rhenium. Electrochemical measurements were performed at ambient temperature using 400 ml of 3 wt% NaCl solution. Potentiodynamic polarization curves were determined using a three-electrode

Table 2 Characteristics of IN718 laser surface alloyed with rhenium. Variant No.

Power [W]

Powder feed rate [g/min]

V1 1100 1.92 V2 1300 4.79 Average roughness of reference IN718: Ra = 0.85 μm

Concentration of Re

Surface geometrical parameters [μm]

[wt%]

[at%]

Ra

Rz

Wz

14 ± 5 28 ± 5

4.9 ± 1 9.8 ± 1

11.8 12.1

164.3 155.2

13.0 87.6

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Fig. 2. BSE micrograph of IN718 substrate.

cell assembly consisting of the examined specimen as the working electrode, stainless steel SS316 as the counter electrode and an Ag/AgCl electrode in Haber-Luggin capillary as the reference electrode. Surface area of the specimen subjected to polarization was 0.762 cm2. A potentiostat PGSTAT 320 N was used for electrochemical measurements. Before the measurements, the surfaces to be examined were ground on abrasive paper No. 1000 and ultrasonic cleaned with acetone. Open circuit potential (OCP) was monitored for 30 min after immersion of the specimens in test solution. Polarization curves within the Tafel range for three variants of specimens were obtained by polarizing each of them with voltage scan rate of 1 mV/s between − 150 mV from OCP and +150 mV from OCP. Cyclic polarization measurements were extended just after Tafel measurements. The potential scan was started at 100 mV below the OCP and reversed below 2 V. 3. Results and discussion 3.1. Microstructure characteristics Cross-sectional light micrographs of alloyed layers obtained in two selected variants of LSA parameters are shown in Fig. 1. Concentrations of alloyed rhenium determined by spot EDS analysis and geometrical parameters of the layer surfaces (roughness Ra, total roughness Rt and waviness Wz) measured along the lines perpendicular to the direction of laser scanning are given in Table 2. As can be seen, the alloyed layers had similar thickness and surface geometry, although they were significantly different in rhenium content. It should be emphasised that EDS analysis covered the microareas free from undissolved, residual particles of Re powder. The results of chemical analysis are estimated

because of their large scatter caused by non-uniform intermixing of rhenium in melted IN718 substrate, as visible in Fig. 1 in form of bright and dark streaks. The alloy IN718 belongs to the group of multicomponent alloys of the system Ni-Cr-Fe-Nb-Mo, precipitation-hardened by the coherent phases γ′ and γ″, i.e. secondary phases Ni3(Al,Ti) and Ni3Nb. Niobium and molybdenum are susceptible to microsegregation, so this alloy in as-cast, hot-worked or laser-melted condition can include NbC carbides, δ-Ni3Nb phase and brittle Laves-type phase (Ni, Cr, Fe)2(Nb, Mo,Ti) [2, 23]. Microstructure of wrought IN718 substrate (Fig. 2) consisted of recrystallized grains of solid solution γ with almost exclusively needlelike δ phase at grain boundaries. XRD analysis did not show any δphase peak in the spectrum (Fig. 3) because its amount was too small to be detected. Occurrence of Laves phase and carbides on grain boundaries makes IN718 an alloy sensitive to constitutional liquation due to high heat input [24]. No Laves phase or NbC carbides significant with respect to quantity, sizes and distribution of particles were observed. In a back-scatter electron (BSE) image, the carbides should be visible as black-coloured, globular particles. The alloyed layers formed at lower laser power (1100 W) and powder feed rate (1.92 g/min) were characterised by chemically nonhomogeneous, non-equilibrium dendritic microstructure of solid solution γ (Fig. 4a). This fact and absence of new peaks in the XRD spectrum (Fig. 3) prove that 14 wt% Re is contained within solubility of rhenium in γ phase. Only sporadically occurring residual rhenium particles were observed, because they had not enough time to dissolve in liquid metal due to their relatively large initial size. Unlike Nb and Mo (the elements susceptible to segregation, with partition coefficient k b 1), partition coefficient of rhenium k is above 1. Thus, concentration of rhenium (as well as of Cr and Fe) is higher in dendrite axes and concentration of Nb Mo and Ti is higher in interdendritic spaces (Fig. 5). As was found in [25], diversified concentration of these elements in γ phase is more intensive at higher cooling rate and higher concentration of rhenium. This is confirmed by relatively large fraction of dispersive particles of Laves phase (products of the eutectic reaction) distributed in interdendritic spaces of very fine-grained γ phase (Fig. 4). When the laser power was increased to 1300 W and powder feed rate to 4.97 g/min, concentration of rhenium in solid solution γ reached ca. 28 wt% and composite-type microstructure was formed. This certainly results from higher viscosity of molten metal accompanied by larger amount of dosed rhenium and thus from lower effectiveness of mixing. As can be seen in Fig. 4b and in Fig. 6, the composite-type microstructure was composed of dendritic matrix of γ solid solution with high fraction of a rhenium particles or rhenium-based solid solution (judging from distribution of Cr in Fig. 6) that solidified in the areas of molten metal with high concentration of rhenium. Significantly smaller number of Laves phase particles in interdendritic spaces was observed. It could be seen at higher magnifications (inset in Fig. 4b) that the Re-based solid solution was in form of dendrites with strongly diversified sizes solidified mostly in situ or on partially dissolved Re particles. The interfaces between alloyed zones and substrate were defect-free and continuous in nature, as shown in Fig. 7. The unmixed zones with similar width ca. 60–100 μm occurred in both variants of the fabricated layers. No thermally induced changes of substrate microstructure in the zones adjacent to outer edges of unmixed zones (so-called “true” heat affected zone) were observed. No grain growth occurred, either, undoubtedly due to the precipitates of δ phase on grain boundaries. Partially melted regions were locally found, indicated by marks of grainboundary liquation (described in [26]), see the arrows in Fig. 7a and b. 3.2. Microhardness

Fig. 3. XRD spectra of IN718 substrate and rhenium-alloyed layers.

Hardness measured on surfaces of the layers and substrate showed higher values for the layer with composite microstructure

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Fig. 4. BSE micrographs of rhenium-alloyed layers on IN718 substrate, processed at various parameters: a) 1100 W, 1.92 g/min; the arrow indicates a Laves particle, b) 1300 W, 4.79 g/min; the arrow indicates a Re particle. Cross-sections.

Fig. 5. Distribution maps of main alloying elements in the IN718-14 wt% Re layer. Distribution of Mo and Ti is identical as that of Nb. EDS analysis.

Fig. 6. Distribution maps of main alloying elements in the IN718–28 wt% Re layer. Distribution of Mo and Ti is identical as that of Nb. EDS analysis.

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Fig. 7. BSE micrographs of interfaces between alloyed zones and IN718 substrate, processed at various parameters: a) 1100 W, 1.92 g/min; b) 1300 W, 4.79 g/min. The arrows indicate marks of grain-boundary liquation.

(674 ± 84 HV10) in comparison to the layer with rhenium almost completely dissolved in the substrate (534 ± 34 HV10) and for the IN718 substrate itself (260 HV10). According to [15], this higher hardness resulted from lowering of the SFE value proportionally to concentration of rhenium and was considered an indicator of considerable ability of rhenium to support strain-hardening during plastic deformation. Microhardness distribution on cross-sections of the layers is shown in Fig. 8. Each point on the microhardness profile represents the average of five measurements. Interfaces between the fusion and the unimixed zones for individual profiles were located at 1.1 to 1.5 mm from the layer surface, depending on the measurement place. Microhardness of the layer containing 14 wt% Re at the depth over 15 μm is relatively lower than microhardness of IN718 substrate and also than microhardness of SLM-processed IN718 in as-fabricated condition (ca. 320 HV as reported in [27,28]). This results from the fact that microstructure is less refined due to dendritic solidification of the melted alloy during LSA in comparison to the SLM-processed microstructure formed in the conditions of cellular-dendritic grain growth. In the case of the layer with composite-type microstructure, the undoubtedly strong effect of rhenium on reaching microhardness values over 400 HV0.3 at the depth to 1 mm results from solution hardening and strengthening with resolidified Re particles with microhardness of 464 ± 12 HV0.05. At the depth to 0.4 μm, this microhardness is higher than microhardness (420 HV0.3) of the very thin, 20 μm thick layer formed by surface laser melting of IN718 under protective nitrogen atmosphere [23]. The so distinct improvement of surface hardness obtained by Yilbas et al. [23] resulted from grain refinement and dispersion hardening with nitrides of Ti, Cr and Mo. It should be added that hardness of the wrought IN718 or SLMprocessed can reach values between 430 and 470 HV [1,27–29], depending on parameters of supersaturation and ageing. Thus, attempts

Fig. 8. Microhardness profiles of laser surface alloyed IN718.

to improve properties of IN718 surface-alloyed with rhenium by postprocess thermal treatment are planned in a subsequent stage of the research. 3.3. Wear behaviour 3.3.1. Ball-on-disc test Results of ball-on-disc tests are shown in Fig. 9. As can be seen, specific wear rates of both variants of the alloyed layers are much lower than specific wear rate of IN718 substrate, namely over two times lower for the layer IN718–14 wt% Re and eight times lower for the layer IN718–28 wt% Re. This is reflected in different widths of wear tracks and topography of worn surfaces compared in Fig. 10. The obtained effect of wear rate lowered by LSA seems to be only attributed to increased hardness of alloyed layers as a result of solution hardening with rhenium and refinement of structure. Topography of worn surfaces of three examined specimens shows similar features of longitudinal grooving, compacted layers of debris (highlighted on magnified fragments of images in Fig. 10), transfer material and mixedmode fractures of these layers. As can be seen in Fig. 10, the higher initial surface hardness, the more intensive its smoothing by sliding friction, smaller sizes of flake-like debris and lower specific wear rate. This mechanism of sliding wear is different from wear mechanisms (abrasion, adhesion, oxidation) of two Inconel HX and Inconel 625 alloys revealed by Fox et al. [30], subjected to ball-on-disc tests with reciprocating motion of the ball. When comparing subsurface microstructures on cross-sections to wear tracks (Fig. 11), one can notice that alloying with rhenium changes

Fig. 9. Wear rates of reference IN718 and the layers alloyed with rhenium.

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Fig. 10. BSE micrographs of wear tracks in: a) Ref. IN718 substrate; b) IN718–14 wt% Re layer; c) IN718–28 wt% Re layer.

frictional behaviour of IN718 in spite of phase stability of its microstructure. Beneath a wear track of IN718 substrate, a layer strongly plastically deformed (named “white layer”) was observed, with dispersed particles of δ and Laves phases. Boundary of this layer is marked with broken line in Fig. 11a. The arrow indicates an example of a propagating shear crack, along which the particle will be detached. Referring to the delamination theory suggested in [31] it can be assumed that relatively low SFE, changing propagation modes of dislocations, suppresses creation of cellular dislocation structure under the surface in the places of severe plastic deformation caused by contact pressure. Since SFE influences cross slip of screw dislocation segments and, consequently, intensifies the strain hardening effect of subsurface area, moving the point of failure closer to the surface reduces thickness of flake-like debris. Therefore, alloying with rhenium, reducing SFE value, increases sliding wear resistance of IN718 by rapid hardening of subsurface, which confirms the thesis of Chaudhuri et al. [17]. The adherent, featureless white layer was observed under a worn track of IN718 alloyed with 14 wt% Re (Fig. 11b). As Blau [32] explains, featureless nature of this layer results from its severely deformed structure caused by the mechanism of micro-plowing. Making SFE stable at elevated temperatures [33], rhenium can also impede softening at the surface due to heating caused by friction. As can be seen in Fig. 11c, no white layer occurs on the surface of the alloyed layer IN718–28 wt% Re. At the surface visible are deformed particles of undissolved rhenium and resolidified dendrites of rhenium in the matrix clearly more intensively deformed just under Re particles. This evidences strong strain hardening of Re particles during sliding friction and taking by them the carrying function. It is highly probable that Re particles have a significant part in reducing sliding wear of IN718, also by lowering its kinetic friction coefficient and thus reducing micro-plowing and increasing its abrasive wear resistance. While 2.4-fold reduction of wear rate of the alloyed layer with 14 wt% Re in relation to the IN718 substrate is a joint result of structure refining and influence of rhenium, the over 3-fold difference in wear rates of both variants of alloyed layers results from the difference in Re concentrations in solid solution and the presence of undissolved Re particles and resolidified dendrites. It is interesting that the obtained effect of 96-% reduction of specific wear rate of IN718 by laser surface alloying with Re is larger than such a reduction by 93% obtained by Deng et al. [20] by paste-boriding technology.

3.3.2. Three-body test Results of abrasion tests are shown in Fig. 12 in form of wear resistance indexes. The Re-alloyed layers show significantly higher wear resistance in comparison to that of both reference materials, steel C45 and hot-worked IN718. Their wear indexes Kw in relation to those of wearresistant iron alloys (Table 3) are surprisingly high, in spite of larger roughness. In turn, surprisingly low is wear resistance of cobalt alloy F-75 that has higher SFE and lower strengthening coefficient in comparison to IN718 Three-body abrasion is a combination of the micro-cutting wear mechanism and the plastic wear mechanism (plowing and wedge forming) [1]. The abraded surfaces shown in Fig. 13 exhibit both groove marks and a large number of pits dented by abrasive particles. The grooves are of two kinds: relatively wide ones delineated by ridges generated by plastic deformation and much narrower scratches. The layer IN718–28 wt% Re shows much smaller and fewer dents and higher smoothing degree in comparison to the layer with lower Re concentration. The dents are indicative of the obstacles to motion of abrasive particles, resulting in termination of some grooves at the dents. This is especially visible on the rhenium particle indicated with an arrow in Fig. 13c. High wear resistance of the examined Re-alloyed layers should be attributed, like in the case of sliding friction, to the effect of surface strain hardening of nickel austenite γ, and in the case of a composite microstructure – to the additional effect of undissolved and resolidified Re particles. This effect does not exist in the Co-Cr-Mo alloy (see Table 3), in spite of its high strengthening coefficient (low SFE). Cobalt austenite is not stable and under pressures can undergo transformation to martensite whose nature often leads to lower performance [1]. This confirms the principle that a high abrasive wear resistance of the material guarantees high strength together with considerable ductility/toughness.

3.4. Corrosion resistance Results of electrochemical measurements in form of cyclic potentiodynamic polarization (CPP) curves for the substrate and two variants of Re-alloyed layers are shown in Fig. 14. Corresponding corrosion parameters (potential Ecorr and corrosion current density icorr) determined by Tafel extrapolation are given in Table 4. Pitting potential Epit is defined

Fig. 11. Subsurface microstructures on surfaces perpendicular to sliding direction: a) Ref. IN718: “featureless” layer with crack; b) IN718–14 wt% Re: “white layer” and deformed dendrites beneath this layer; c) IN718-28 wt% Re: superficial deformations of rhenium particles and their surrounding matrix.

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Fig. 12. Variation of wear resistance index.

as the potential at that current density increases significantly and quickly above the passive current density. The Ecorr and icorr values determined for IN718 substrate are very close to the values obtained by Chen et al. [36] for IN718 as solutionannealed, in similar measuring conditions. As can be seen in Table 4, alloying the IN718 surface with rhenium shifted corrosion potential to more positive values, more significantly (+101 mV) in the layer with lower Re concentration. For the alloyed layer with 14 wt% Re, corrosion potential increased 1.9 times with 1.3-fold decrease of corrosive current in comparison to parameters of the IN718 substrate. However, for two times higher Re concentration in the alloyed layer, corrosion potential increased only 1.6 times with 3.6-fold increase of corrosive current. The above results differ from those of Amin's et al. [37] examinations of influence of rhenium on electrochemical behaviour of IN718 in the solution 1 M H2SO4 + xM NaCl, indicating that an addition of 3.5 wt% Re results in a very strong anodic shift of polarization curves, while an addition of 6.0 wt% Re acts in opposite direction. Alloying with rhenium clearly changed course of CPP curves, as can be seen in Fig. 11. An increase of passivation current density along with increasing Re concentration is visible. This suggests that the passive film created on surfaces of Re-alloyed layers is less protective than that created on the IN718 alloy. However, as can be seen in Table 4, an addition of rhenium caused larger difference of potentials (Epit − Ecorr), which indicates increased resistance against pit nucleation [38]. Existence of a large “negative” hysteresis loop in the CPP curve of IN718 (Fig. 14a) can prove its low resistance against growth of existing pits due to their low ability for repassivation when the potential is scanned towards the negative direction. On the other hand, “positive” hystereses (Fig. 14b, c) in the CPP curves of Re-alloyed layers indicate resistance to growth of pre-existing pits.

Table 3 Comparison of wear resistance indexes of the examined materials and wear-resistant iron and cobalt alloys. Material

Vickers Hardness

Wear resistance index, Kw

Referencea

Ref. IN718 IN718–14 wt% Re IN718–28 wt% Re Hardox 400 X37CrMoV5 − 1 D2 (X153CrMoV12) High chromium cast iron F-75 (Co-Cr-Mo) SLM-ed

260 534 674 430 641 705 685 408

1.12 1.20 1.41 1.13 1.12 1.44 1.68 0.59

This work This work This work [34,35] [34] [34] [35] Unpublished data

a The same test conditions and the same reference material were used in the listed items.

Since, in the case of each CPP curve in Fig. 14, anodic-to-cathodic transition potential (reverse portion of the scan) is more noble than corrosion potential, it can be believed acc. to the Silverman's interpretation [39] that any film created on the reference IN718 surface and the Realloyed layers at the corrosion potential might not be very passivating, especially in the Re-alloyed layers, and would allow uniform corrosion to occur at the corrosion potential. The above analysis can be summarized (in a similar way as in Davydov et al. [40]) with the statement that alloying with rhenium significantly increases resistance to pitting corrosion of IN718, but does not increase its ability for passivation in NaCl solution. In order to explain the role of Re addition in increasing pitting corrosion resistance of IN718, it should be considered that three examined specimens differed from each other not only in rhenium content, but also in features of the microstructures described in Subsection 3.1, see Figs. 2 and 4. Fig. 15 shows BSE/SEM images of microstructures under corroded surfaces after the CPP experiments. It is visible that corrosion processes run in different ways determined mostly by segregation of rhenium and niobium [37]. In the case of IN718, irrespective of development of pits, corrosion proceeds preferentially in the zones along the grains depleted in Nb and Mo as a result of precipitated on them cathodic δ phases and Laves phases. It is interesting that, in the alloyed layer IN718–14 wt% Re, concentration of rhenium in centres of γ-phase dendrites results in their anodic dissolution, whereas interdendritic spaces with increased concentrations of Nb and Mo, with high susceptibility to passivation, are cathodic. The single pit visible in Fig. 15b became after anodic dissolution of a residual particle of rhenium, which explains presence of irregularities on the CPP curve. Increased saturation degree of γ-phase in the layer IN718–28 wt% Re with rhenium, as well as large fraction of residual and resolidified Re particles make anodic dissolution of these particles the dominating corrosive process. Such corrosion behaviour of IN718 with addition of rhenium, in comparison with that described by Amin et al. [37], is worth to be explained in a future complementary research. 4. Conclusions Laser melting is a successful technique of superficial alloying with rhenium parts made of IN718 to enhance their wear and corrosion resistance. Using a high power diode laser and 50% overlapping ratio of laser tracks, one can obtain alloyed layers with uniform thickness, very narrow unmixed zone, and with no grain coarsening zone. Thanks to high solubility of rhenium in nickel, it is possible to control quantity of rhenium dissolved in solid solution γ by changing laser power and powder feed rate at other process parameters kept unchanged. A restriction is increasing number of residual particles of rhenium along with increasing concentration of rhenium in γ phase. Laser surface alloying of IN718 with Re leads to evolution of strongly refined microstructure, which can be designed as a mainly corrosion resistant (without residual Re particles) or a mainly wear resistant (with residual or undissolved Re particles) microstructure. In the first case, dissolution of 14 wt% of rhenium powder in solid solution enhances corrosion resistance of the IN718 substrate (in 3-wt% NaCl solution) demonstrated by anodic displacement of corrosion potential (+100 mV), as well as causes 24% reduction of corrosive current and significant shift of critical pitting potential. In the second case, alloying with 28 wt% of rhenium enhances wear resistance of the IN718 substrate reducing its sliding wear rate by 82% and increasing its abrasive wear resistance index by 25%. The obtained results encourage continuing examinations of IN718 surface-alloyed with rhenium after post-processing heat treatment to prove microstructural stability of rhenium-alloyed layers in high temperatures [41]. An additional encouragement is the fact that high price of Re powder can be compensated not only by a significant increase of the part lifetime but also by a reduction of laser alloying to small,

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357

Fig. 13. Secondary electron images of the abraded surface of: IN718 substrate (a) layer alloyed with 14 wt% Re (b) and layer alloyed with 28 wt% Re (c).

Fig. 14. Cyclic potentiodynamic polarization curves in 3 wt% NaCl solution for: a) IN718 substrate; b) alloyed layer IN718–14 wt% Re, c) alloyed layer IN718–28 wt% Re.

discrete regions. For example, consumption of Re powder for creation of a 3-track layer 6 mm wide on sealing surface of a heavy duty combustion engine valve (diameter of the valve head 42 mm, mass of the IN718 valve 171 g) is 0.3 g with use of the variant 1300 W /

4.79 g/min. This consumption is at least 2.5 and 4 to 5 times lower than that required for making such a valve of IN718 with additions of 3 wt% and 5 to 6 wt% Re, respectively, like it is in the second- and third-generation superalloys.

Table 4 Electrochemical parameters of alloyed layers on IN718 substrate after 30 min exposure in 3% NaCl solution. Laser alloying parameters

Tested material

icorr [A / cm2]

Ecorr [mV]

Epit [mV]

Epit − Ecorr [mV]

1100 W/1.92 g/min 1300 W/4.79 g/min

Ref. IN718 IN718–14 wt% Re IN718–28 wt% Re

1.21 × 10−7 0.92 × 10−7 4.32 × 10−7

−215 −114 −135

328 602 594

543 716 729

358

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Fig. 15. BSE image of microstructure on cross-sections perpendicular to corroded surfaces of: a) IN718 substrate; b) alloyed layer IN718–14 wt% Re; c) alloyed layer IN718–28 wt% Re. Obtained after the experiments of cyclic potentiodynamic polarization, see Fig. 14.

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