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Multi-layer laser solid forming of Zr65Al7.5Ni10Cu17.5 amorphous coating: Microstructure and corrosion resistance
Optics & Laser Technology 69 (2015) 17–22
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Multi-layer laser solid forming of Zr65Al7.5Ni10Cu17.5 amorphous coating: Microstructure and corrosion resistance Yu Gan a,b, Wenxian Wang a,b,n, Zhuosen Guan a,b, Zeqin Cui a,b a b
College of Material Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials of Ministry of Education, Taiyuan, Shanxi 030024, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 September 2014 Received in revised form 3 December 2014 Accepted 8 December 2014
Multi-layer Zr65Al7.5Ni10Cu17.5 amorphous coatings were produced by laser solid forming on A283 substrate. The coatings with few pores and free of cracks had good metallurgical bonding with the substrate. The microstructural characterization, phase composition, chemical component distribution and corrosion behavior of the coatings were investigated. The results revealed that the amorphization degree increased from the substrate to the coating surface mainly due to the dilution and stir influence from the melted substrate. In the five layers coating, the volume fraction of amorphous phase in the 5th layer, 3rd layer and 1st layer was about 77%, 64% and 49% respectively. With regard to corrosion property, potentiodynamic polarization plots, Nyquist plots and the equivalent circuits were employed in 3.5 wt% sodium chloride solution. Attributing to the presence of amorphous phase and passivation, the LSF coatings exhibit excellent corrosion resistance. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Amorphous Laser processing Corrosion resistance
1. Introduction Amorphous alloys have attracted great attention owing to their exceptional physical, chemical and mechanical properties [1,2]. Such properties encourage researchers to focus on preparation of amorphous coatings in order to exhibit better performance in harsh environment [3–5]. However, the critical dimension of the amorphous coating is limited by the conventional coating preparation methods, which restricts its widespread application [6–9]. So an urgent desire calls the preparation of thick amorphous coating [10]. As a novel additive manufacturing technology, laser solid forming (LSF) can be used to fabricate complex components with full density and high performance in its near-net-shape manufacturing process [11–13]. The critical cooling rates to form amorphous are generally range from 1 K/s to 100 K/s according to different amorphous composition systems [14], LSF is a potential candidate to prepare amorphous coatings owing to the large cooling rate 3000 K/s generated by continuous wave laser [6,8,15,16]. To date, many researchers have attempted to prepare amorphous coating using laser processing technology. Sahasrabudhe and Bandyopadhyay [17] successfully employed a Laser Engineered Net Shaping (LENS) technique to process Fe-based amorphous alloy coating on zirconium substrate. But the thickness of the amorphous
n Corresponding author at: College of Material Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China. Tel./fax: þ 86 351 6010076. E-mail address: [email protected] (W. Wang).
http://dx.doi.org/10.1016/j.optlastec.2014.12.008 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
coating was only about 612 μm. Thermal effects and corrosion resistance of the laser assisted Fe-based bulk amorphous coating were investigated by Katakam et al. [18]. Similar to our previous work [19], visible pores and cracks were observed in their coating. Wang et al. [20] prepared Zr–Al–Ni–Cu amorphous composite coatings with some graphite additions on Ti substrate using a laser cladding method and found the amorphous fraction was about 58%. In order to evaluate the spherulitic crystallization mechanism of Zr– Cu–Ni–Al–Nb metallic glass during laser processing, a numerical and experimental study was carried out by Sun and Flores [21]. Among the various amorphous alloy systems, Zr-based multicomponent amorphous alloy has been a promising coating material because of its high glass forming ability (GFA) and excellent corrosion resistance [22–24]. Unfortunately, little information of thick Zr-based amorphous coating especially of its corrosion behavior can be found. On this point, multi-layer Zr-based amorphous coatings were prepared using LSF in this study. Further, microstructural characterization, phase composition, chemical component distribution and corrosion behavior of the coatings were investigated.
2. Material and methods The Zr65Al7.5Ni10Cu17.5 powder (at%) which was chosen for present work possesses a extremely low critical cooling rate at about 1.5 K/s to form amorphous phase [16]. The powder was
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Y. Gan et al. / Optics & Laser Technology 69 (2015) 17–22
Table 1 Parameters of laser beam. Laser Power (kW)
Scanning speed (mm/min)
Spot diameter (mm)
3.2
180
3
atomized with a nominal size range 10–60 μm in a purified argon atmosphere. The A283 (melting temperature 1673 K) carbon steel plate was cut, polished and cleaned into 50 mm 50 mm 5 mm rectangle plates, which were used as substrates. The HUST-JKR 5170 5 kW continuous transverse flow CO2 laser processing system manufactured by National Engineering Research Center for Laser Processing, Huazhong University of Science and Technology, Wuhan City, China was used for the laser solid forming experiments. Argon was used as the shielding gas. The laser beam was focused by means of an aspheric paraboloid focusing lens (f ¼400 mm) onto a spot with a diameter of 3 mm at the surface of the specimen. In present work, one, three and five layers of amorphous coatings of various thicknesses were prepared. Based on the previous work [19], the optimum operation parameters shown in Table 1 were applied in the LSF process. The five layers coating sample was polished and etched with etchant. Then the microstructure of the sample was studied using a JEOL scanning electron microscope (SEM, JSM-7001F). The distribution of elemental composition was analyzed through energy dispersive X-ray spectroscopy (EDS) attached to the SEM. The phase composition was examined by X-ray diffraction (XRD, TD-3500) with the 2θ range of 20–801. The analysis was conducted in different planes prepared at the 5th layer, 3rd layer and 1st layer of the five-layers-coating sample. To evaluate the corrosion resistance, electrochemical potentiodynamic polarization experiments on various layers LSF amorphous coatings and the substrate were carried out in 3.5 wt% sodium chloride solution (pH ¼7) at room temperature using a CS350 electrochemical measurement system. In addition, electrochemical impedance spectroscopy (EIS) was examined with sinusoidal amplitude of 10 mV in the frequency ranging from 10 kHz to 0.01 Hz.
3. Results and discussion 3.1. Microstructure The microstructure of the LSF Zr65Al7.5Ni10Cu17.5 coating with five layers is shown in Figs. 1 and 2. The 1.7 mm coating with few pores and cracks forms a good metallurgical bonding with the substrate. And the cross-section of the coating presents a complex structure with three different regions with distinct microstructures. It is shown that dendritic crystals epitaxially grew on the substrate. Isometric crystals were formed in the 2nd and 3rd layer. In the upper region of the coating, a featureless matrix distributed by a few crystals can be observed. Moreover, several crystals existed around the interface between two contiguous layers because of the reheating. In order to take a detailed observation of the morphology, magnified images of the rectangle area A and B are shown in Fig. 2(a) and (d). As Fig. 2(a) shows, an obvious boundary between the 1st layer and 2nd layer can be observed. When the 1st layer was deposited, a melt pool was created by the temperature of the laser beam. Due to the low solidification rate and high temperature gradient at the bottom of the melt pool, dendritic crystals grew on the substrate [25,26]. The melted substrate fluid forced the dendritic crystals pointing to different directions. EDS was conducted to analyze the
Fig. 1. SEM image of the five layers LSF Zr65Al7.5Ni10Cu17.5 coating at low magnification.
dilution and stir influence of the melted substrate, and the result is shown in Section 3.3 When deposited the 2nd layer, on one hand, the influence caused by the substrate was reduced. On the other hand, the laser beam reheated on the previous layer, isometric crystals nucleated due to the decrease of temperature gradient [27]. Magnification of the crystals is shown in Fig. 2(b) and (c), concave morphology was presented on account of the metallographic etchant. As shown in Fig. 2(d), in the upper region of the coating, a featureless constituent is predominantly formed, with a few refined isometric crystals embedded [8,25]. Fig. 2(e) and (f) shows a further exhibition of the typical featureless morphology. Compared with the XRD results, the featureless matrix is believed to be amorphous phase. Therefore, the amorphization ratio increased gradually from the substrate to the coating surface. Moreover, as shown in Figs. 1 and 2(a), (d), and (e), few visible pores are distributed randomly in the coating. From this distribution characteristic, it is believed that the convection caused by the surface tension gradient and gravity gradient of the melt pool formed the porosity dominantly, and the influence of recoil effect from the evaporation is negligible. So the temperature developing in the deposited layer is estimated to be about 2400 K, which is between the melting point and the boiling point of the alloy component element. 3.2. Phase composition Fig. 3 demonstrates the X-ray diffraction patterns of different planes created at 1st, 3rd and 5th layers, which aims to show the phase composition of the bottom, middle and upper region in the five layers coating. Each pattern shows a halo with a maximum at 2θ¼ 381which confirms the existence of the amorphous phase in the coatings [28], although several crystalline diffraction peaks which indexed as Zr, Zr oxide, Ni, ZrCu, Ni10Zr7, Al4Cu9, NiZr2, AlZr2, Al4Zr5, and Fe oxide can be observed. In the patterns, the
Y. Gan et al. / Optics & Laser Technology 69 (2015) 17–22
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Fig. 2. SEM images of microstructure in the coating. (a) Magnified images of the rectangle area A, (b) isometric crystals in the 2nd layer, (c) dendritic crystals in the 1st layer, (d) magnified images of the rectangle area B, (e), (f) magnification of the amorphous featureless morphology.
amorphous halo becomes gradually accentuated from the bottom to the surface. These crystalline phase forms due to: firstly, the different thermal cycles during the LSF process of multi-layer coating, crystallization was caused by the accumulation of structural relaxation in the HAZ [6]. Secondly, the dilution effect between the coating and the substrate cannot be neglected. The element of the substrate such as Fe will be introduced into the coating and disrupt the nominal composition of Zr65Al7.5Ni10Cu17.5.
It can be interpreted by the formation of Fe2O3 and Fe3O4 phase in the middle and bottom regions of the coating. And this influence will be explained further in the Section 3.3. The volume fraction of amorphous phase in the 5th layer, 3rd layer and 1st layer is approximately 77%, 64% and 49% according to Eqs. (1) and (2) [29], where XC and XA are volume fraction of crystalline phase and amorphous phase respectively, IC and IA are XRD intensity of crystalline phase and amorphous phase
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respectively, and K is experimental constant (set as K ¼1 for qualitative calculation) XC ¼
IC IC þ K IA
XA ¼ 1 XC
ð1Þ ð2Þ
Thus, it can be concluded that the volume of amorphous phase increases along with the increase of coating thickness. 3.3. Chemical component distribution Fig. 4 shows the distribution of elements measured by line-scan EDS of the five layers coating. When the 1st layer is deposited, the laser beam energy creates a melt pool and forms a dilution zone with a maximum depth of about 520 μm, which is identified in Fig. 4. It is noted that the Zr–Al–Ni–Cu composition varies with the dilution effect at the interface region. Therefore, crystallization occurs due to the dilution of Fe. The distribution of Zr, Al, Ni and
Fig. 3. X-ray diffraction patterns of the 5th layer, 3rd layer and 1st layer of the five layers coating.
Cu is reasonably close to the nominal composition of Zr65Al7.5Ni10Cu17.5 in the middle and the upper parts of the coating, which guarantee the formation of amorphous phase [30]. As shown in Fig. 4, base on the thermal cycle during the multi-layer laser solid forming process, the chemical component of the coating has a little fluctuation especially at the interface between two contiguous layers. 3.4. Corrosion resistance Fig. 5 shows the potentiodynamic polarization plots of various layers LSF amorphous coatings in comparison with the substrate. The corrosion potential (Ecorr) and current density (icorr) values are listed in Table 2. Ecorr and icorr were determined by the intersection of the vertical line and the Tafel extrapolation method respectively [31]. The various layers LSF amorphous coatings exhibit both higher Ecorr and lower icorr than the substrate, which means higher chemical stability, lower corrosion tendency and lower corrosion rate. It indicates that the formation of the amorphous phase could significantly improve the corrosion resistance. For the various layers LSF amorphous coatings, icorr should be taken as the analytical factor based on the corresponding Ecorr value. According to the decrease of the order of magnitude of icorr, the LSF coatings exhibit better corrosion resistance along with the layers number increase. The five layers coating exhibits the best corrosion resistance owing to the high volume of amorphous phase. Due to the formation of protective passive film with low electronic conductivity, which is a typical behavior of valve-metals e.g. Zr, Ti, Al, Hf, Nb, and Ta [32–34], all the corrosion curves of various layers LSF amorphous coatings display obvious passivation platform in region ranging from 0.4 V to 0.1 V. It should be noted that there are two different passivation processes existing in the anode region of the five layers coating. When the first passivation occurs, icorr sharply reduces from 1.74 10 6 A/cm2 to 3.86 10 7 A/cm2 owing to the corrosion inhibition of the passive film. Then icorr increases indicating that the passive film has been broken up. As the coating being consumed and dissolved, the second passivation appears at icorr ¼5.65 10 5 A/cm2. In the later stage of the
Fig. 4. Line-scan EDS result across the substrate and the five layers coating.
Y. Gan et al. / Optics & Laser Technology 69 (2015) 17–22
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Fig. 7. Equivalent circuit for the EIS experiment.
Fig. 5. Comparison of the potentiodynamic polarization plots of LSF coatings and substrate.
Table 2 Corrosion potentials (Ecorr) and current density (icorr) of LSF amorphous coatings and substrate.
Substrate One layer Three layers Five layers
Ecorr (mV)
icorr (A/cm2)
659 418 461 445
7.974 10 5 5.944 10 5 2.252 10 6 8.565 10 7
displays excellent impedance performance owing to the high amorphization ratio and the passivation. The equivalent circuit shown in Fig. 7 was employed to interpret the EIS results. Rs represents solution resistance, Q is constant phase angle element in double layer of solution and coating, R1 is resistance of substrate, R2 is resistance of coating, and C is capacitance associated with the coating and substrate. Based on the description of the equivalent circuit in this case, with the past of immersion time, the aggressive Cl will corrode the coating and distribute in the coating surface with micropores. Finally the solution will contact the substrate, the resistance of the microporous coating surface should be taken into consideration in a long time immersion case.
4. Conclusions Multi-layer Zr65Al7.5Ni10Cu17.5 amorphous coatings were successfully prepared on A283 substrate using the LSF method. The microstructural characterization, phase composition, chemical component distribution and corrosion behavior of the coatings were investigated. The main results are summarized as follows: 1) The LSF coatings with few of pores and free of cracks achieved good metallurgical bonding with the substrate. 2) In the coating, the ratio of amorphous phase increased from the bottom to the surface. The volume fraction of amorphous phase in the 5th layer, 3rd layer and 1st layer was about 77%, 64% and 49% respectively. 3) The chemical component distribution in the coating demonstrates that crystallization occurred mainly due to the dilution and stir influence from the melted substrate. 4) Attributing to the presence of amorphous phase and passivation, the LSF coatings exhibit excellent corrosion resistance.
Fig. 6. Comparison of the Nyquist plots of LSF coatings and substrate.
Acknowledgments corrosion experiment, the increase tendency of icorr maintains basically at 0.1 A/cm2 which is similar to the substrate. That means cracks occur at the surface of the coating, passes have been opened up between the solution and the substrate on account of the consumption of the coating. As a consequence, formation of the Zr-based amorphous phase and the passivation contribute to the corrosion resistance of the LSF coating significantly. EIS was carried out at open circuit condition. Fig. 6 shows the Nyquist plots of various layers LSF coatings and the substrate. For the substrate, the smallest radius indicates the poorest impedance performance compared with the other three coating specimens. For the one layer and three layers coatings, one capacitive loop is evident on an Nyquist plot. It means that the system shows one time constant. This high-frequency loop could be physically related to the coating characteristics. It is noted that the five layers coating shows a particularly large radius of the Nyquist plot. Considering the limitation of experimental time, it could be interpreted as the coating
This project is supported by National Natural Science Foundation of China (Nos. 51075293 and 51405324) and Research Project Supported by Shanxi Scholarship Council of China (No. 2013-029). References [1] Byrne CJ, Eldrup M. Bulk metallic glasses. Mater Sci 2008;321:502–3. [2] Inoue A, Takeuchi A. Recent development and application products of bulk glassy alloys. Acta Mater 2011;59:2243–67. [3] Yue TM, Su YP. Laser cladding of SiC reinforced Zr65Al7.5Ni10Cu17.5 amorphous coating on magnesium substrate. Appl Surf Sci 2008;255:1692–8. [4] Chokethawai K, McCartney DG, Shipway PH. Microstructure evolution and thermal stability of an Fe-based amorphous alloy powder and thermally sprayed coatings. J Alloy Compd 2009;480:351–9. [5] Li Ruifeng, Li Zhuguo, Zhu Yanyan, Qi Kai. Structure and corrosion resistance properties of Ni–Fe–B–Si–Nb amorphous composite coatings fabricated by laser processing. J Alloy Compd 2013;580:327–31. [6] Yang Gaolin, Lin Xin, Liu Fencheng. Laser solid forming Zr-based bulk metallic glass. Intermetallics 2012;22:110–5.
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[7] Inoue A, Zhang T, Nishiyama N, Ohba K, Masumoto T. Preparation of 16 mm diameter rod of amorphous Zr65Al7.5Ni10Cu17.5 alloy. Mater Trans 1993;35:1234–7. [8] Sun H, Flores KM. Microstructural analysis of a laser-processed Zr-based bulk metallic glass. Metall Mater Trans A 2010;41:1752–7. [9] Lu ZP, Liu CT, Thomspon JR, Porter WD. Structural amorphous steels. Phys Rev Lett 2004;92(245503):1–4. [10] Wu Xiaolei, Hong Youshi. Fe-based thick amorphous-alloy coating by laser cladding. Surf Coat Technol 2001;141:141–4. [11] Liu Fencheng, Lin Xin, Leng Han, Cao Jun, Liu Qiang, Huang Chunping, et al. Microstructural changes in a laser solid forming Inconel 718 superalloy thin wall in the deposition direction. Opt Laser Technol 2013;45:330–5. [12] Lin X, Yue TM, Yang HO, Huang WD. Microstructure and phase evolution in laser rapid forming of a functionally graded Ti-Rene88DT alloy. Acta Mater 2006;54:1901–15. [13] Boisselier Didier, Sankaré Simon. Influence of powder characteristics in laser direct metal deposition of SS316L for metallic parts manufacturing. Phys Proc 2012;39:455–63. [14] Telford M. The case for bulk metallic glass. Mater Today 2004;7:36–43. [15] Tan H, Chen J, Lin X, Zhao XM, Huang WD. Research on molten pool temperature in the process of laser rapid forming. Mater Sci Forum 2007;2301:546–55. [16] Inoue A. Stabilization of metallic supercooled liquid and bulk amorphous alloys. Acta Mater 2000;48:279–306. [17] Sahasrabudhe Himanshu, Bandyopadhyay Amit. Laser processing of Fe based bulk amorphous alloy coating on zirconium. Surf Coat Technol 2014;240: 286–92. [18] Katakam Shravana, Kumar Vivek, Santhanakrishnan S, Rajamure Ravishankar, Samimi P, Narendra B Dahotre. Laser assisted Fe-based bulk amorphous coating: thermal effects and corrosion. J Alloy Compd 2014;604:266–72. [19] Zhuosen Guan, Wenxian Wang, Zeqin Cui. Laser cladding of Zr63.8Ni17.2Al11.4Cu7.6 amorphous alloy on carbon steel. Rare Met Mater Eng 2013;42:64–7. [20] Wang Yanfang, Li Gang, Shi Zhiqiang, Liu Mingxing, Zhang Xiuyun, Liu Yi. Effects of graphite addition on the microstructure and properties of laser cladding Zr–Al–Ni–Cu amorphous coatings. J Alloy Compd 2014;610:713–7.
[21] Sun Hongqing, Flores Katharine M. Spherulitic crystallization mechanism of a Zr-based bulk metallic glass during laser processing. Intermetallics 2013;43:53–9. [22] Ningshen S, Kamachi Mudali U, Krishnan R, Raj Baldev. Corrosion behavior of Zr-based metallic glass coating on type 304 L stainless steel by pulsed laser deposition method. Surf Coat Technol 2011;205:3961–6. [23] Kamachi Mudali U, Baunack S, Eckert J, Schultz L, Gebert A. Pitting corrosion of bulk glass-forming zirconium-based alloys. J Alloy Compd 2004;377:290–7. [24] Padhy N, Ningshen S, Mudali U Kamachi. Electrochemical and surface investigation of zirconium based metallic glass Zr59Ti3Cu20Al10Ni8 alloy in nitric acid and sodium chloride media. J Alloy Compd 2010;503:50–6. [25] Gargarella P, Almeida A, Vilar R, Afonso CRM, Peripolli S, Rios CT, et al. Formation of Fe-based glassy matrix composite coatings by laser processing. Surf Coat Technol 2014;240:336–43. [26] Vilar R. Laser cladding. J Laser Appl 1999;11:64–80. [27] Kurz W. Solidification microstructure-processing maps: theory and application. Adv Eng Mater 2001;3:443–52. [28] Audebert F, Colaco R, Vilar R, Sirkin H. Production of glassy metallic layers by laser surface treatment. Scr Mater 2003;48:281–6. [29] Hang Shengtao. X-ray physics of solid. Beijing: Higher Education Press; 1990. [30] Carvalho D, Cardoso S, Vilar R. Amorphisation of Zr60Al15Ni25 surface layers by laser processing for corrosion resistance. Scr Mater 1997;37:523–7. [31] Amin Mohammed A, Khaled KF, Fadl-Allah Sahar A. Testing validity of the Tafel extrapolation method for monitoring corrosion of cold rolled steel in HCl solutions-experimental and theoretical studies. Corros Sci 2010;52:140–51. [32] Padhy N, Ningshen S, Mudali U Kamachi, Baldev Raj. Electrochemical and surface investigation of zirconium based metallic glass Zr59Ti3Cu20Al10Ni8 alloy in nitric acid and sodium chloride media. J Alloy Compd 2010;503:50–6. [33] Baunack S, Mudali U Kamachi, Gebert A. Characterization of oxide layers on amorphous Zr-based alloys by Auger electron spectroscopy with sputter depth profiling. Appl Surf Sci 2005;252:162–6. [34] Ningshen S, Mudali U Kamachi, Krishnan R, Raj Baldev. Corrosion behavior of Zr-based metallic glass coating on type 304 L stainless steel by pulsed laser deposition method. Surf Coat Technol 2011;205:3961–6.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Multi-layer laser solid forming of Zr65Al7.5Ni10Cu17.5 amorphous coating: Microstructure and corrosion resistance Yu Gan a,b, Wenxian Wang a,b,n, Zhuosen Guan a,b, Zeqin Cui a,b a b
College of Material Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials of Ministry of Education, Taiyuan, Shanxi 030024, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 September 2014 Received in revised form 3 December 2014 Accepted 8 December 2014
Multi-layer Zr65Al7.5Ni10Cu17.5 amorphous coatings were produced by laser solid forming on A283 substrate. The coatings with few pores and free of cracks had good metallurgical bonding with the substrate. The microstructural characterization, phase composition, chemical component distribution and corrosion behavior of the coatings were investigated. The results revealed that the amorphization degree increased from the substrate to the coating surface mainly due to the dilution and stir influence from the melted substrate. In the five layers coating, the volume fraction of amorphous phase in the 5th layer, 3rd layer and 1st layer was about 77%, 64% and 49% respectively. With regard to corrosion property, potentiodynamic polarization plots, Nyquist plots and the equivalent circuits were employed in 3.5 wt% sodium chloride solution. Attributing to the presence of amorphous phase and passivation, the LSF coatings exhibit excellent corrosion resistance. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Amorphous Laser processing Corrosion resistance
1. Introduction Amorphous alloys have attracted great attention owing to their exceptional physical, chemical and mechanical properties [1,2]. Such properties encourage researchers to focus on preparation of amorphous coatings in order to exhibit better performance in harsh environment [3–5]. However, the critical dimension of the amorphous coating is limited by the conventional coating preparation methods, which restricts its widespread application [6–9]. So an urgent desire calls the preparation of thick amorphous coating [10]. As a novel additive manufacturing technology, laser solid forming (LSF) can be used to fabricate complex components with full density and high performance in its near-net-shape manufacturing process [11–13]. The critical cooling rates to form amorphous are generally range from 1 K/s to 100 K/s according to different amorphous composition systems [14], LSF is a potential candidate to prepare amorphous coatings owing to the large cooling rate 3000 K/s generated by continuous wave laser [6,8,15,16]. To date, many researchers have attempted to prepare amorphous coating using laser processing technology. Sahasrabudhe and Bandyopadhyay [17] successfully employed a Laser Engineered Net Shaping (LENS) technique to process Fe-based amorphous alloy coating on zirconium substrate. But the thickness of the amorphous
n Corresponding author at: College of Material Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China. Tel./fax: þ 86 351 6010076. E-mail address: [email protected] (W. Wang).
http://dx.doi.org/10.1016/j.optlastec.2014.12.008 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
coating was only about 612 μm. Thermal effects and corrosion resistance of the laser assisted Fe-based bulk amorphous coating were investigated by Katakam et al. [18]. Similar to our previous work [19], visible pores and cracks were observed in their coating. Wang et al. [20] prepared Zr–Al–Ni–Cu amorphous composite coatings with some graphite additions on Ti substrate using a laser cladding method and found the amorphous fraction was about 58%. In order to evaluate the spherulitic crystallization mechanism of Zr– Cu–Ni–Al–Nb metallic glass during laser processing, a numerical and experimental study was carried out by Sun and Flores [21]. Among the various amorphous alloy systems, Zr-based multicomponent amorphous alloy has been a promising coating material because of its high glass forming ability (GFA) and excellent corrosion resistance [22–24]. Unfortunately, little information of thick Zr-based amorphous coating especially of its corrosion behavior can be found. On this point, multi-layer Zr-based amorphous coatings were prepared using LSF in this study. Further, microstructural characterization, phase composition, chemical component distribution and corrosion behavior of the coatings were investigated.
2. Material and methods The Zr65Al7.5Ni10Cu17.5 powder (at%) which was chosen for present work possesses a extremely low critical cooling rate at about 1.5 K/s to form amorphous phase [16]. The powder was
18
Y. Gan et al. / Optics & Laser Technology 69 (2015) 17–22
Table 1 Parameters of laser beam. Laser Power (kW)
Scanning speed (mm/min)
Spot diameter (mm)
3.2
180
3
atomized with a nominal size range 10–60 μm in a purified argon atmosphere. The A283 (melting temperature 1673 K) carbon steel plate was cut, polished and cleaned into 50 mm 50 mm 5 mm rectangle plates, which were used as substrates. The HUST-JKR 5170 5 kW continuous transverse flow CO2 laser processing system manufactured by National Engineering Research Center for Laser Processing, Huazhong University of Science and Technology, Wuhan City, China was used for the laser solid forming experiments. Argon was used as the shielding gas. The laser beam was focused by means of an aspheric paraboloid focusing lens (f ¼400 mm) onto a spot with a diameter of 3 mm at the surface of the specimen. In present work, one, three and five layers of amorphous coatings of various thicknesses were prepared. Based on the previous work [19], the optimum operation parameters shown in Table 1 were applied in the LSF process. The five layers coating sample was polished and etched with etchant. Then the microstructure of the sample was studied using a JEOL scanning electron microscope (SEM, JSM-7001F). The distribution of elemental composition was analyzed through energy dispersive X-ray spectroscopy (EDS) attached to the SEM. The phase composition was examined by X-ray diffraction (XRD, TD-3500) with the 2θ range of 20–801. The analysis was conducted in different planes prepared at the 5th layer, 3rd layer and 1st layer of the five-layers-coating sample. To evaluate the corrosion resistance, electrochemical potentiodynamic polarization experiments on various layers LSF amorphous coatings and the substrate were carried out in 3.5 wt% sodium chloride solution (pH ¼7) at room temperature using a CS350 electrochemical measurement system. In addition, electrochemical impedance spectroscopy (EIS) was examined with sinusoidal amplitude of 10 mV in the frequency ranging from 10 kHz to 0.01 Hz.
3. Results and discussion 3.1. Microstructure The microstructure of the LSF Zr65Al7.5Ni10Cu17.5 coating with five layers is shown in Figs. 1 and 2. The 1.7 mm coating with few pores and cracks forms a good metallurgical bonding with the substrate. And the cross-section of the coating presents a complex structure with three different regions with distinct microstructures. It is shown that dendritic crystals epitaxially grew on the substrate. Isometric crystals were formed in the 2nd and 3rd layer. In the upper region of the coating, a featureless matrix distributed by a few crystals can be observed. Moreover, several crystals existed around the interface between two contiguous layers because of the reheating. In order to take a detailed observation of the morphology, magnified images of the rectangle area A and B are shown in Fig. 2(a) and (d). As Fig. 2(a) shows, an obvious boundary between the 1st layer and 2nd layer can be observed. When the 1st layer was deposited, a melt pool was created by the temperature of the laser beam. Due to the low solidification rate and high temperature gradient at the bottom of the melt pool, dendritic crystals grew on the substrate [25,26]. The melted substrate fluid forced the dendritic crystals pointing to different directions. EDS was conducted to analyze the
Fig. 1. SEM image of the five layers LSF Zr65Al7.5Ni10Cu17.5 coating at low magnification.
dilution and stir influence of the melted substrate, and the result is shown in Section 3.3 When deposited the 2nd layer, on one hand, the influence caused by the substrate was reduced. On the other hand, the laser beam reheated on the previous layer, isometric crystals nucleated due to the decrease of temperature gradient [27]. Magnification of the crystals is shown in Fig. 2(b) and (c), concave morphology was presented on account of the metallographic etchant. As shown in Fig. 2(d), in the upper region of the coating, a featureless constituent is predominantly formed, with a few refined isometric crystals embedded [8,25]. Fig. 2(e) and (f) shows a further exhibition of the typical featureless morphology. Compared with the XRD results, the featureless matrix is believed to be amorphous phase. Therefore, the amorphization ratio increased gradually from the substrate to the coating surface. Moreover, as shown in Figs. 1 and 2(a), (d), and (e), few visible pores are distributed randomly in the coating. From this distribution characteristic, it is believed that the convection caused by the surface tension gradient and gravity gradient of the melt pool formed the porosity dominantly, and the influence of recoil effect from the evaporation is negligible. So the temperature developing in the deposited layer is estimated to be about 2400 K, which is between the melting point and the boiling point of the alloy component element. 3.2. Phase composition Fig. 3 demonstrates the X-ray diffraction patterns of different planes created at 1st, 3rd and 5th layers, which aims to show the phase composition of the bottom, middle and upper region in the five layers coating. Each pattern shows a halo with a maximum at 2θ¼ 381which confirms the existence of the amorphous phase in the coatings [28], although several crystalline diffraction peaks which indexed as Zr, Zr oxide, Ni, ZrCu, Ni10Zr7, Al4Cu9, NiZr2, AlZr2, Al4Zr5, and Fe oxide can be observed. In the patterns, the
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Fig. 2. SEM images of microstructure in the coating. (a) Magnified images of the rectangle area A, (b) isometric crystals in the 2nd layer, (c) dendritic crystals in the 1st layer, (d) magnified images of the rectangle area B, (e), (f) magnification of the amorphous featureless morphology.
amorphous halo becomes gradually accentuated from the bottom to the surface. These crystalline phase forms due to: firstly, the different thermal cycles during the LSF process of multi-layer coating, crystallization was caused by the accumulation of structural relaxation in the HAZ [6]. Secondly, the dilution effect between the coating and the substrate cannot be neglected. The element of the substrate such as Fe will be introduced into the coating and disrupt the nominal composition of Zr65Al7.5Ni10Cu17.5.
It can be interpreted by the formation of Fe2O3 and Fe3O4 phase in the middle and bottom regions of the coating. And this influence will be explained further in the Section 3.3. The volume fraction of amorphous phase in the 5th layer, 3rd layer and 1st layer is approximately 77%, 64% and 49% according to Eqs. (1) and (2) [29], where XC and XA are volume fraction of crystalline phase and amorphous phase respectively, IC and IA are XRD intensity of crystalline phase and amorphous phase
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respectively, and K is experimental constant (set as K ¼1 for qualitative calculation) XC ¼
IC IC þ K IA
XA ¼ 1 XC
ð1Þ ð2Þ
Thus, it can be concluded that the volume of amorphous phase increases along with the increase of coating thickness. 3.3. Chemical component distribution Fig. 4 shows the distribution of elements measured by line-scan EDS of the five layers coating. When the 1st layer is deposited, the laser beam energy creates a melt pool and forms a dilution zone with a maximum depth of about 520 μm, which is identified in Fig. 4. It is noted that the Zr–Al–Ni–Cu composition varies with the dilution effect at the interface region. Therefore, crystallization occurs due to the dilution of Fe. The distribution of Zr, Al, Ni and
Fig. 3. X-ray diffraction patterns of the 5th layer, 3rd layer and 1st layer of the five layers coating.
Cu is reasonably close to the nominal composition of Zr65Al7.5Ni10Cu17.5 in the middle and the upper parts of the coating, which guarantee the formation of amorphous phase [30]. As shown in Fig. 4, base on the thermal cycle during the multi-layer laser solid forming process, the chemical component of the coating has a little fluctuation especially at the interface between two contiguous layers. 3.4. Corrosion resistance Fig. 5 shows the potentiodynamic polarization plots of various layers LSF amorphous coatings in comparison with the substrate. The corrosion potential (Ecorr) and current density (icorr) values are listed in Table 2. Ecorr and icorr were determined by the intersection of the vertical line and the Tafel extrapolation method respectively [31]. The various layers LSF amorphous coatings exhibit both higher Ecorr and lower icorr than the substrate, which means higher chemical stability, lower corrosion tendency and lower corrosion rate. It indicates that the formation of the amorphous phase could significantly improve the corrosion resistance. For the various layers LSF amorphous coatings, icorr should be taken as the analytical factor based on the corresponding Ecorr value. According to the decrease of the order of magnitude of icorr, the LSF coatings exhibit better corrosion resistance along with the layers number increase. The five layers coating exhibits the best corrosion resistance owing to the high volume of amorphous phase. Due to the formation of protective passive film with low electronic conductivity, which is a typical behavior of valve-metals e.g. Zr, Ti, Al, Hf, Nb, and Ta [32–34], all the corrosion curves of various layers LSF amorphous coatings display obvious passivation platform in region ranging from 0.4 V to 0.1 V. It should be noted that there are two different passivation processes existing in the anode region of the five layers coating. When the first passivation occurs, icorr sharply reduces from 1.74 10 6 A/cm2 to 3.86 10 7 A/cm2 owing to the corrosion inhibition of the passive film. Then icorr increases indicating that the passive film has been broken up. As the coating being consumed and dissolved, the second passivation appears at icorr ¼5.65 10 5 A/cm2. In the later stage of the
Fig. 4. Line-scan EDS result across the substrate and the five layers coating.
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Fig. 7. Equivalent circuit for the EIS experiment.
Fig. 5. Comparison of the potentiodynamic polarization plots of LSF coatings and substrate.
Table 2 Corrosion potentials (Ecorr) and current density (icorr) of LSF amorphous coatings and substrate.
Substrate One layer Three layers Five layers
Ecorr (mV)
icorr (A/cm2)
659 418 461 445
7.974 10 5 5.944 10 5 2.252 10 6 8.565 10 7
displays excellent impedance performance owing to the high amorphization ratio and the passivation. The equivalent circuit shown in Fig. 7 was employed to interpret the EIS results. Rs represents solution resistance, Q is constant phase angle element in double layer of solution and coating, R1 is resistance of substrate, R2 is resistance of coating, and C is capacitance associated with the coating and substrate. Based on the description of the equivalent circuit in this case, with the past of immersion time, the aggressive Cl will corrode the coating and distribute in the coating surface with micropores. Finally the solution will contact the substrate, the resistance of the microporous coating surface should be taken into consideration in a long time immersion case.
4. Conclusions Multi-layer Zr65Al7.5Ni10Cu17.5 amorphous coatings were successfully prepared on A283 substrate using the LSF method. The microstructural characterization, phase composition, chemical component distribution and corrosion behavior of the coatings were investigated. The main results are summarized as follows: 1) The LSF coatings with few of pores and free of cracks achieved good metallurgical bonding with the substrate. 2) In the coating, the ratio of amorphous phase increased from the bottom to the surface. The volume fraction of amorphous phase in the 5th layer, 3rd layer and 1st layer was about 77%, 64% and 49% respectively. 3) The chemical component distribution in the coating demonstrates that crystallization occurred mainly due to the dilution and stir influence from the melted substrate. 4) Attributing to the presence of amorphous phase and passivation, the LSF coatings exhibit excellent corrosion resistance.
Fig. 6. Comparison of the Nyquist plots of LSF coatings and substrate.
Acknowledgments corrosion experiment, the increase tendency of icorr maintains basically at 0.1 A/cm2 which is similar to the substrate. That means cracks occur at the surface of the coating, passes have been opened up between the solution and the substrate on account of the consumption of the coating. As a consequence, formation of the Zr-based amorphous phase and the passivation contribute to the corrosion resistance of the LSF coating significantly. EIS was carried out at open circuit condition. Fig. 6 shows the Nyquist plots of various layers LSF coatings and the substrate. For the substrate, the smallest radius indicates the poorest impedance performance compared with the other three coating specimens. For the one layer and three layers coatings, one capacitive loop is evident on an Nyquist plot. It means that the system shows one time constant. This high-frequency loop could be physically related to the coating characteristics. It is noted that the five layers coating shows a particularly large radius of the Nyquist plot. Considering the limitation of experimental time, it could be interpreted as the coating
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