Ultrasonic cavitation erosion of gas nitrided Ti–6Al–4V alloys

Ultrasonic cavitation erosion of gas nitrided Ti–6Al–4V alloys

Ultrasonics Sonochemistry 21 (2014) 1544–1548 Contents lists available at ScienceDirect Ultrasonics Sonochemistry journal homepage: www.elsevier.com...

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Ultrasonics Sonochemistry 21 (2014) 1544–1548

Contents lists available at ScienceDirect

Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Ultrasonic cavitation erosion of gas nitrided Ti–6Al–4V alloys I. Mitelea a, E. Dimian a, I. Bordeasßu b, C. Cra˘ciunescu a,⇑ a b

‘‘Politehnica’’ University of Timisoara, Department of Materials and Manufacturing Engineering, Bd. Mihai Viteazul 1, 300026 Timisoara, Romania ‘‘Politehnica’’ University of Timisoara, Department of Mechanical Machines, Equipments and Transportation, Bd. Mihai Viteazul 1, 300026 Timisoara, Romania

a r t i c l e

i n f o

Article history: Received 19 June 2013 Received in revised form 16 December 2013 Accepted 6 January 2014 Available online 13 January 2014 Keywords: Cavitation Ti-alloys Nitriding Oxidation Surface Erosion

a b s t r a c t Ultrasonic cavitation erosion experiments were performed on Ti–6Al–4V alloys samples in annealed, nitrided and nitrided and subsequently heat treated state. The protective oxide layer formed as a result of annealing and heat treatment after nitriding is eliminated after less than 30 min cavitation time, while the nitride layer lasts up to 90 min cavitation time. Once the protective layer is removed, the cavitation process develops by grain boundary erosion, leading to the expulsion of grains from the surface. The gas nitrided Ti–6Al–4V alloy, forming a TixN surface layer, proved to be a better solution to improve the cavitation erosion resistance, compared to the annealed and nitrided and heat treated state, respectively. The analysis of the mean depth of erosion rate at 165 min cavitation time showed an improvement of the cavitation erosion resistance of the nitrided samples of up to 77% higher compared to the one of the annealed samples. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The resistance to cavitation erosion is an actual subject, influencing the wear behavior of materials used in naval, nuclear, aerospace, and industries that use high pressure fluids [1]. Among the materials considered for this purpose, Ti-alloys show good corrosion resistance, low specific weight, and high strength. However, their wear resistance needs to be significantly improved by special thermal [2], thermomechanical [3], and thermochemical treatments [4], aiming to increase the surface resistance to erosion phenomena. The thermochemical treatments, in particular, increase the fatigue resistance and the hardness in the superficial layers, improve the corrosion behavior and reduce the friction coefficient. The most effective such treatments applied to Ti-alloys for this purpose are those aiming to generate oxide, carbide or nitride Tibased superficial layers [5]. The cavitation erosion is a real issue of Ti-alloys and considerable efforts have been made to improve their resistance by surface alloying [6] or by applying thermochemical treatments with the goal to change the composition and the properties of the surface. Nitriding is known to provide a good protection against wear; it was mentioned that the resistance to erosion wear can be increased in the range of 18–300%, depending on the angle of erodent stream compared to the resistance of titanium alloy Ti–6Al-4V [7]. ⇑ Corresponding author. Tel.: +40 256 403655; fax: +40 256 403523. E-mail addresses: [email protected] (I. Mitelea), [email protected] (E. Dimian), [email protected] (I. Bordeasßu), [email protected] (C. Cra˘ciunescu). 1350-4177/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2014.01.005

The effects of nitriding as a way to increase the cavitation erosion resistance of Ti-alloys are not fully studied so far [8]. TiN superficial layers have been generated by laser coatings [9], gas [10,11] or plasma [12] nitriding. Alloying elements appear to limit nitrogen diffusion; in Ti-based alloys, Al and V reduce the nitrogen diffusions [13] even below the one corresponding to pure Ti, due to an aluminum segregated layer that hinders the diffusion of nitrogen toward the core [14]. A nitride layer provides an effective protection against oxygen intake at the surface of the Ti–6Al-4V alloy and is limited by the formation of a rutile layer at the surface of the nitride layer [15,16]. Most Ti-alloys show an improved resistance to oxidation, as result of nitriding. The analysis of the hydrogen content for the samples nitrided in ammonia atmosphere showed that it intensively penetrated the alloy, without adverse effects [11]. The analysis of the surface cavitation erosion is performed, in laboratory conditions, via hydrodynamic or ultrasonic tests. Compared to the hydrodynamic experimental set up, the ultrasonic equipment is simpler, the experimental parameters (frequency, amplitude, fluid temperature) can be easily controlled and the reproducibility of the process provides grounds for reliable comparisons between samples with different properties tested under the same conditions. The ultrasonic cavitation equipments allow both direct (with the specimen attached to the ultrasonic vibrating horn) and indirect (with stationary specimen) standardized measurements of the cavitation erosion. In assessing the erosion of the specimens surface and the weight changes are measured by interrupting the cavitation process at preset times so that a history of the mass loss / surface

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changes versus time can be plotted. The erosion process can usually be characterized by three different periods: an incubation one where the weight loss is not relevant and the effects are mostly related to microcracks formation around grain boundaries and inclusions, an accumulation period with the cracks growth leading to an expulsion of material and a steady-state period with the surface eroding at a slower rate. An incubation time may be observed for some specimens, depending on the testing conditions and the quality of the material tested. Ti–6Al-4V alloys show an incubation period that depends on the surface treatments, with a negligible period for the untreated state [17]. The oxidation of the nitrided layer in Ti alloys is not elucidated so far and is considered to occur either via oxygen diffusion or via nitrogen desorbtion from the interface [15]. The work in this paper aims to identify the contribution of gas nitriding on ultrasonic cavitation erosion of Ti–6Al-4V alloys and the effects resulting from subsequent annealing. 2. Experimental details The Ti–6Al-4V alloy was selected due to its very good mechanical and corrosion resistance properties. The aptitude for improving the properties by heat treatments makes this Ti-alloy a primary choice for the fabrication of components for aircraft, marine, power generation or biomedical fields. Three sets of samples made out of ASTM B348 Grade 5 Ti–6Al-4V alloy manufactured by Zirom S.A., Romania, were machined for the experiments, according to ASTM G32-2010 standardized test. The nominal composition, determined by energy dispersive X-ray spectrometry (EDS) using a Bruker Quantax with XFlash 4010 detector was: 6.27% Al, 3.98% V, 0.009% C, 0.008% N2, 0.004% H2, 0.138% O2, and 0.098% Fe, Ti- rest. The samples were polished on the frontal surface and cleaned in ethanol, prior to the thermal and thermochemical treatments. Two of the three sets of samples were gas nitrided in vacuum. The nitriding was performed in a flow of 5 l/h nitrogen (grade 4.8 and 99.9998% purity) and 2 l/h NH3 (grade 3.8, with NH3 > 99.98%, H2O < 200 ppm, oil < 10 ppm), supplied by Linde Gas. The nitriding process is detailed in Fig. 1. One set of samples was further annealed at 700 °C for 240 min, in order to increase the nitrogen diffusion in the surface layers. Nanoindentation test were performed using a CSM Indentation Tester type NOX S/N, equipped with a Berkovich indenter, to assess the effect of the nitriding and subsequent heat treatment on the hardness of the samples used in the experiments. The cavitation tests were made according to ASTM G32-2010 standards, using an ultrasonic vibratory equipment with 20 kHz vibration frequency and 50 lm peak-to-peak amplitude, by immersion in tap water at constant temperature (23 ± 2 °C) [7]. o

600

570 C Furnace cooling

400

Nitrogen flush

300 200 100

Nitriding during heating

o

Temperature [ C]

500

Air cooling

Nitriding 480 min

0 0

200

400

600

800

1000

Time [ min] Fig. 1. Cyclogram of the nitriding thermochemical treatment showing the parameters used for the N and NA experiments.

The peak-to-peak amplitude at the surface of the sample was measured using a micrometric system (APSONIC, Switzerland) that allowed the control of the mechanical resonator amplitude in the ±2 lm range around the set amplitude of 50 lm. The cavitation time was up to 165 min, with 5, 10 and 15 min increments and the samples were periodically weighted to assess the weight loss by erosion and to plot the mean depth of erosion (MDE) and the mean depth of erosion rate (MDER) as a function of the cavitation time. X-ray diffraction (XRD) was performed using CuKa radiation in a Bruker D8 HRXRD, system. The surface of the cavitated samples was observed macroscopically and the cross section as well as the surface was examined in a Zeiss model DSM 962, scanning electron microscope (SEM). 3. Results and discussion As a result of the gas nitriding process, the formation of TiN on the surface of the samples occurs, via a complex process, involving reactions at the interface between the gas and the Ti-alloy. The adsorbed nitrogen diffuses in Ti and forms an a(N) interstitial solution in a-Ti. For higher nitrogen concentration on the surface, a totally nitrided Ti2N superficial layer forms. For even higher nitrogen concentration the Ti2N transforms into TiN [16]. Thus, the nitride layer results from the reduction of solubility and from nitrogen diffusion in the solid solution. The macroscopic images of the samples investigated, prior and after the cavitation test and in different states are shown in Table 2. Before cavitation (at zero cavitation time), the surface of the gas nitrided samples (N) has a blue color and, respectively, a grey color after the subsequent heat treatment (NA). The effect of nitriding and of the subsequent annealing is analyzed based on the nanohardness and elastic modulus evolution as a function of the applied force presented in Fig. 2a and b. For lower forces, the hardness of the nitrided samples appears higher than the one of additionally heat treated samples, but for higher forces this difference diminishes. The effect of the cavitation erosion time on the macroscopic aspect is analyzed based on the corresponding samples shown in Table 1. For the sample tested for 30 min, the surface observations indicate only a limited change in the original color that resulted from the nitriding and the nitriding and heat treatment. The cavitation phenomenon starts to show its effect from the outer discolored ring toward the center. For the NA sample a full deterioration of the surface can be observed for the 90 min sample, while for the N sample a surface not fully eroded can still be seen even after 165 min exposure. The N sample exposed for 120 min shows an external discolored ring compared to the inner surface where the cavitation process fully removed the nitride protective layer. The cross sectional microscopic images of the cavitated samples are shown in Fig. 3, revealing a typical mechanism that develops at grain boundary and leads to grain expulsions from the surface. For both the N (Fig. 3a) and NA (Fig. 3b) samples the grain are dislocated and the resulting surface appears unevenly eroded (Fig. 4c and d) due to the propagation of fatigue cracks.

Table 1 Parameters of the treatments applied to Ti–6Al–4V sets of samples. Sample

Treatment

Parameters

Surface layer

A N

Annealing Nitriding

Oxidized Nitrided

NA

Annealing

730 °C, 240 min Ammonia nitriding according to Fig. 1 700 °C, 240 min

Nitrided and oxidized

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Table 2 Cavitation effects on nitrided (N) and nitrided and annealed (NA) samples. Sample

Cavitation time [min] 0

30

90

120

165

Nitrided(N)

Nitrided and annealed(NA)

Sample diameter: 16 mm, according to ASTM G32-2010.

(a) 25000

Hardness [MPa]

20000 nitrided (N) nitrided and annealed (NA)

15000

10000

5000 0

10

20

30

40

50

60

Force [ mN]

Elastic modulus [GPa]

(b)

180 160 nitrided (N) nitrided and annealed (NA)

140 120 100 80 60 10

20

30

40

50

Force [ mN] Fig. 2. The effects of the applied treatments on the hardness (a) and elastic modulus (b) for the nitride (n) and nitride and annealed (NA) samples.

(a)

The X-ray diffraction data for the samples before and after the cavitation tests is shown in Fig. 5a and b. The a-Ti and TiN0.26 phases are present in the nitrided sample prior to the cavitation, respectively a-Ti and TiO2 were detected in the annealed sample. For both cases, the X-ray diffraction patterns indicate that the nitrided and the oxidized layers are eliminated during the cavitation process, leaving only the Ti–6Al-4V substrate. This additionally confirms the macroscopic observations for the sets of samples in Fig. 1, where the color of the surface changes after 180 min exposure. A study of the mean depth of erosion (MDE) detailed in Fig. 6a indicates a significantly improved behavior of the nitrided samples, compared to the nitrided and heat treated sample, both having a substantially better cavitation resistance than the one of the annealed Ti–6Al-4V sample tested under same conditions. A similar observation results from the analysis of the mean depth erosion rate (MDER) in Fig. 6b. Table 3 details the estimation of the eroded mass loss for three sets of samples studied for each material. All measurements for the cavitated samples were in the error band with the highest mass loss – both experimental and estimated – indicating a slightly better behavior for the nitride sample compared to the annealed one. Based on the experimental results, no incubation time was observed for any of the of the Ti–6Al-4V samples tested for cavitation erosion according to the direct ASTM G-32 method. The absence of an incubation time is not unusual and was recently reported for Cr60Ni40 coatings generated by friction surfacing [18]. Additionally it is known for Ti–6Al-4V that in the annealed state, there is a negligible incubation period [17]. Our results also indicate that such an incubation time is not detected for the nitrided and the nitrided and annealed samples. The absence of an incubation time appears to be influenced by the severity the cavitation erosion test, the

(b)

Fig. 3. Cross sectional details of the Ti–6Al–4V cavitation eroded samples reflecting the crack growth and the expulsion of the grains during the accumulation period of the cavitation erosion process. (a) Gas nitrided (N), (b) gas nitrided and annealed (NA).

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(a)

(b)

2µm

30µm

Fig. 4. Surface morphology of the NA sample, revealing the cavitation erosion effects. (a) Eroded surface (SEM – 1K magnification), (b) details of the crack development along the grain boundary (SEM – 20K magnification).

Fig. 5. X-ray diffraction patterns of the Ti–6Al–4V alloy before (bottom) and after (top) the applied treatments. (a) Gas nitrided (N), (b) gas nitrided and annealed (NA).

thickness of the nitrided layer and its hardness as well as the microstructure of the sample tested. Regardless of the surface microstructure (i.e. A, N or NA) there is no significant difference in the MDE for shorter cavitation erosion times. For longer times, both the N and the NA samples provide a better cavitation

Fig. 6. The effects of the applied treatments on the cavitation erosion resistance of Ti–6Al–4V alloy in annealed (A), nitrided (NA) and nitrided and annealed (NA) state. (a) Mean depth erosion (MDE) as a function of the cavitation time, (b) mean depth erosion rate (MDER) as a function of the cavitation time.

Table 3 Comparison between experimental and maximal estimated loss. Sample code

N

NA

Experimental cumulative mass loss [mg] Maximal estimated mass loss [mg]

7.77 8.61

7.85 8.80

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protection when compared to the annealed Ti–6Al-4V alloy, ranging from 37%, for the oxide layer up to 77% for the nitrided layer, respectively. 4. Conclusions The gas nitriding of Ti–6Al-4V alloy forming TiN coating proved to be a better solution to improve the cavitation erosion resistance, compared to annealed and nitrided and heat treated state. The protective oxide layer formed as a result of annealing and heat treatment after nitriding is eliminated after less than 30 min cavitation time, while the nitride layer lasts up to 90 min cavitation time. Once the protective layer is removed, the cavitation process develops by grain boundary erosion, leading to the expulsion of grains from the surface. A comparison of the mean depth of erosion and mean depth of erosion rate as a function of the cavitation time for the Ti–6Al-4V, subjected to different heat treatments, lead to the conclusion that gas nitriding provides the best cavitation erosion resistance. The analysis of the mean depth of erosion rate at 165 min cavitation time revealed that nitrided samples showed an improvement of up to 77% higher than the one of annealed samples. Further oxidation of the surface by annealing the nitrided samples reduced the benefits of nitriding by half, leading to a better cavitation erosion resistance of up to only 37% than the one of the annealed alloys. Acknowledgment The support of the Romanian National Authority for Scientific Research, Grant CNCS – UEFISCDI, project number PN-II-ID-PCE2011-3-0837 is also acknowledged. References [1] S. Hattori, N. Mikami, Cavitation erosion resistance of satellite alloy weld overlays, Wear 267 (11) (2009) 154–1960.

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