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Characterization of Al0.5FeCu0.7NiCoCr high-entropy alloy coating on aluminum alloy by laser cladding
Optics and Laser Technology 105 (2018) 257–263
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Characterization of Al0.5FeCu0.7NiCoCr high-entropy alloy coating on aluminum alloy by laser cladding Cong Ni, Yan Shi ⇑, Jia Liu, Genzhe Huang School of Electromechanical Engineering, Changchun University of Science and Technology, Changchun 130022, Jilin, China Engineering Research Center of Laser Processing for Universities of Jilin Province, Changchun 130022, Jilin, China National Base of International Science and Technology Cooperation in Optics, Changchun 130022, Jilin, China
a r t i c l e
i n f o
Article history: Received 3 December 2017 Received in revised form 24 January 2018 Accepted 26 January 2018
Keywords: High-entropy alloy coatings Laser induced breakdown spectroscopy Substrate dilution Microhardness
a b s t r a c t Al0.5FeCu0.7NiCoCr high-entropy alloy (HEA) coatings were synthesized on aluminum by laser cladding, aiming at enhancing surface properties. Samples were characterized by using scanning electron microscopy with spectroscopy (SEM/EDS), X-ray diffraction, laser induced breakdown spectroscopy (LIBS), microhardness. The results showed that the HEA coatings exhibited good metallurgical bonding to the matrix by using optimized laser processing parameters. The HEA coatings were composed of fcc + bcc phases. All the composed elements can be noted in mapping through the calibration of the plasma, and the plasma of the collected Al confirmed that come from substrate dilution. The intensity change of Al-II reflected the depth variety of the cladding layer. The microstructure of clad layer was consisted of dendrite. The microhardness of HEA layer reached 750HV0.2 that was about 8 times larger than that of the substrate. Ó 2018 Published by Elsevier Ltd.
1. Introduction Aluminum and its alloys have been widely used in the fields of aircraft, household, transportation and military, because of its high intensity, excellent lightweight, high forming quality, and good thermal conductivity [1,2]. However, the low hardness, easy oxidation, poor corrosion and wear resistance limit its wide application. Moreover, it is not feasible to enhance their mechanical properties via conventional solid-state heat treatments and phase transformations, because they have not allotropic transformations [3]. Laser cladding is one of the important surface modification techniques in today’s industry due to its several advantages such as high velocity of heating and cooling, low dilution, small heat affected zone and allows the precise adaptation of surface properties [4]. Laser cladding on aluminum and its alloys have been widely investigated. The materials systems that have been studied, including: molybdenum [3], tungsten [2], Ni-Ti-C [5], MMC [6], TiAl-Fe-B [7], WC-Co-NiCr [8], TiB2-TiC-Al [9]. However, these researches were only aimed at improving wear or corrosion resistance through coating materials. The concept of high-entropy alloy (HEA) was presented by Taiwan scholar Yeh and his colleague that break through the tradi-
⇑ Corresponding author. E-mail address: [email protected] (Y. Shi). https://doi.org/10.1016/j.optlastec.2018.01.058 0030-3992/Ó 2018 Published by Elsevier Ltd.
tional alloy design framework, which was on the base of one or two major alloy element [10–12]. With the effects of the lattice distortion, sluggish diffusion, cocktail effect, and high mixing entropy, the HEA easily form simple solid-solution phases and nanostructures during solidification, rather than intermetallic or other complicated phases [13–15]. Owing to these characterizations, the HEAs usually possess several properties, such as high hardness, super corrosion resistance and better wearing resistance [16–19]. Previous HEAs studies focus on arc melting and casting, while the size and costs of HEAs restricted its usage. So high entropy alloys also considered attractive coating materials for enhancing surface behavior. With high temperature and rapid cooling rate during laser processing, the HEA phase was easy to be synthesized, leading to the enthusiasm of study HEA coatings by laser processing. Zhang et al. [20] deposited FeCoCrAlNi highentropy alloy coating on 304 stainless steel by laser surface alloying. The parameters of mixing entropy (DSmix), mixing enthalpy (DHmix), atom-size difference (d) and valence electron concentration (VEC) play a major role in the formation of simple solid solutions. Shon et al. [21] synthesized AlFeCrCoNi HEA alloy on aluminum by laser cladding through using a pre-coating process. They controlled over dilution by combinations double layered coating and higher energy input, while the coating still existed cracks. Meng et al. [22] fabricated AlCoCrCuFeNi high-entropy alloy on the AZ91D surface by using the LMI techniques and the wear resistance was significantly improved. The quality of a
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cladding layer mainly depends on several parameters including laser power, laser beam size, scanning speed, and powder feed rate, which has an influence on the temperature of the clad interaction zone. Moreover, laser cladding is an open-loop technique, which means the quality relies heavily on the skills of the operator. Post-processing is always expensive and time consuming [23]. In recent years, laser-induced breakdown spectroscopy (LIBS) has become one of the online analytical and diagnostic techniques [24]. This technique based on laser ablation of the sample and the analysis by optical emission spectroscopy of the plasma formed. Some advantages of the LIBS technique have demonstrated its unique versatility, such as: rapid and real-time analysis, allowing fast contact-less analysis of almost any type of material, in situ analysis with no sample preparation required, and its relatively low cost [25–27]. Lednev et al. [28] used EDS and LIBS techniques profiling of major components in the multilayer wear resistant coatings, a good correlation between EDS element mapping and LIBS data was established with minor difference explained by different sampling depth. Varela et al. [29] analyzed the characterization of laser claddings based on hard facing alloys and partial dilution of some WC spheres in the coating by using LIBS technique. The behavior of hardness can be explained by LIBS maps which evidenced the partial dilution of some WC spheres in the coating. Song et al. [30] used operating parameter conditioned support vector regression (SVR) method to improve the composition prediction performance during laser process. The SVR methods showed a stable and accurate performance. Previous reports found that the segregation of copper was due to the enthalpy of mixing between copper and the other main elements were more positive [31,32]. The melting of aluminum matrix resulted in Al enrichment within the coating during the laser processing [33]. In this study, we decrease the atomic percentage of aluminum and copper, then deposit Al0.5FeCu0.7NiCoCr high-entropy alloy coatings on aluminum by laser cladding. The effect of laser processing parameters on the phase formation, substrate dilution and microstructure was investigated by a combination of XRD, LIBS and EDS techniques. 2. Experimental procedure The 5083 aluminum with dimensions of 60 mm 20 mm 10 mm was selected as the substrate material, and then cleaned with acetone to remove surface dirt and oil. The Al, Cr, Fe, Ni, Cu and Co elemental powders had a purity of 99.5% with a size range of 44– 149 um (100 / +325 mesh) as cladding materials. Alloy powder mixed with the aid of a ball mill for 2 h in an argon atmosphere. The mixed powders were then placed onto the surface of aluminum alloy, using ethyl alcohol as binders, and then dried in an oven at 80 °C for 8 h. The thickness of the power was approximately 0.8 mm. Laser cladding was carried out using a Nd: YAG laser of 1.06 um wavelength, and laser beam diameter of 2 mm. With a series of optimization trail runs, the processing parameters were: laser power was 1.1 kW, scanning speed were 270 mm/min, 360 mm/min, 450 mm/min and 630 mm/min. High purity argon gas at a flow rate 10 L/min was used to prevent oxidation. The fiber optic spectrometer was based on the AvaBench-75 optical platform designed, the ambient temperature of the spectrometer was minimal. The wavelength was ranging from 200 to 950 nm, and integration time 100 ms. After the laser cladding, metallographic samples were sectioned perpendicular to the scanning track with a Wire cut Electrical Discharge Machining (WEDM) machine. The structural features of HEA alloy samples were examined by a EMPYGREAN X-ray Diffraction analysis system (XRD) with Cu Ka radiation at 40 kV and 30
mA. XRD patterns were taken at 2h angles from 20° to 90° at a scanning rate 4°/min. HEA coatings morphology and microstructure were analyzed by scanning electron microscope (SEM, JSM6510F) equipped with energy dispersive spectroscopy (EDS). The microhardness was measured by MH-60 Vickers Hardness Tester with a load of 200 g and a duration time of 10 s.
3. Results and discussion 3.1. XRD analysis Fig. 1 shows the patterns of the Al0.5FeCu0.7NiCoCr highentropy alloy coating with different scanning speeds. Phase analysis revealed the existence of Al peak (fcc1) and HEA phase (fcc2, bcc). In the present study, the Gibbs free energy is used to calculate the formation of high-entropy and can be expressed as:
DGmix ¼ DHmix T DSmix
ð1Þ
where 4Gmix means mixing Gibb’s free energy, 4Hmix is the mixing enthalpy, T is the absolute temperature, and 4Smix is the entropy of mixing. As we know that the 4Hmix demonstrate the tendency for ordering or cluster, and the 4Smix at a given temperature indicated the tendency for the formation of disordered solid solutions (DSS) [34]. Furthermore, several investigations proposed that the solid solution formation can be determined based on these parameters including atomic size difference (d), electronegativity difference (4v) and valence electron concentration (VEC) [35], which can be expressed as:
DHmix ¼
n X
Xij ci cj
ð2Þ
n X ci lnðcj Þ
ð3Þ
i¼1;i–j
DSmix ¼ R
i¼1
Tm ¼
n X ci ðT m Þi
ð4Þ
i¼1
Fig. 1. XRD patterns of the HEA coating.
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C. Ni et al. / Optics and Laser Technology 105 (2018) 257–263 Table 1 Parameters of d, 4Hmix, 4Smix, Tm, VEC, 4v and X for Al0.5FeCu0.7NiCoCr HEA coatings. Alloys
d (%)
4Hmix (kJ/mol)
4Smix (JK1mol1)
Tm (K)
X
VEC
4v
Al0.5FeCu0.7NiCoCr
3.74
3.23
14.296
1456.16
6.44
8.12
0.11
Fig. 2. LIBS spectrum of Al0.5FeCu0.7NiCoCr high-entropy alloy coating: (a) at different scanning speeds; (b) in wide range (400–550 nm); (c) in wide range (600–850 nm).
X¼
T m DSmix jDHmix j
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Xn r i 2 c d¼ i 1 i¼1 r qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Xn 2 Dv ¼ c ðvi vÞ i¼1 i VEC ¼
n X C i ðVECÞi i¼1
ð5Þ
ð6Þ ð7Þ ð8Þ
where Xij ¼ 4DHmix menas the regular solution interaction parameij ter between the ith and jth elements, ci and cj is the atomic percentage of the ith and jth component, R (=8.314 JK1mol1) indicates P the gas constant, and ni¼1 ci r i is the average atomic radius and ri is defined as the atomic radius. Tm is related to the melting point P of the ith component of alloy. r ¼ ni¼1 ci r i is the average atomic P radius and ri is the atomic radius. Moreover, v ¼ ni¼1 ci vi means the average electronegativity and vi demonstrates the Pauling electronegativity for the ith component. (VEC)i is the VEC for individual elements.
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3.2. Spectrum analysis
Fig. 3. The intensity of Al-II ranges from 422 nm to 423 nm.
Zhang et al. [36] pointed out that the solid solution was easily formed and stable in the region: 15 4Hmix 5 kJ/mol and 1 d 6. Guo et al. [37] found that when 22 4Hmix 7 kJ/mol, 0 d 8.5, and 11 4Smix 19.5 JK1mol1, the solid solution tend to form. They emphasized that the critical parameter was the atomic size difference (d) which determined the formation of the solid solution. Fang et al. [38] proposed that the large value of 4v was aiding the formation of solid solution. Guo and Ng. [39] described that the value of VEC between 6.87 and 8.0 contribute to the co-exist of bcc and fcc phase. The formation of bcc phase will be more stable at VEC < 6.87, and when VEC 8.0, sole fcc phase will form. Yang et al. [40] concluded that when X 1.1 and d 6.6%, the solid solution and bulk metallic glasses tend to form. According to Eqs. (2–8), the calculated results showed in Table 1, where the formulae used in the calculation were from Ref [41,42]. The value of both X and d for the studied coatings were meet formation rules of solid solutions. However, the VEC was 8.12 which mean that fcc phases are more stable. The parameters of X, d, 4v and VEC were used to estimate solid solution formation ability in multi-component alloy systems by using vacuum melting method, while the solidification temperature must be considered in laser high-entropy alloy [43]. The laser rapid cooling rate could facilitate the formation of simple structures, and effectively prohibits the formation of undesired intermetallic compounds. The presence of Al phases may attribute to the melting of Al substrate during the processing and dissolve into the molten pool which causes composition segregation from the ideal composition.
The laser induced plasma emitted spectrums were measured in a wide spectral range (400–850 nm) for different parameter coating layers in Fig. 2. Because the excessive number of elements in the cladding layer, the plasma peaks observed in the spectrum were much larger. It is found that the scanning velocity has little effect on the solid solution phase of the cladding layer by XRD analysis as shown in Fig. 1 and the peak did not change significantly in Fig. 2a. The whole spectrum is between 400 and 800 nm, and the spectral peak distribution from 400 nm to 600 nm is relatively dense. Fig. 2(b) and (c) are the calibration of the spectrum of the plasma spectrum. Each element of Al, Cu, Co, Cr, Ni, Fe in the cladding layer can be found in the map. Therefore, only Fe, Cr, Cu, Al elements in the form of II valent ions, and the peaks of Fe, Cr and Cu in the spectrum are more. The ionization of Ar was found at 588 nm and 590 nm, the peak value was very large, which indicated that the process of Ar gas was sufficient to protect the cladding. Compared with AlFeCuNiCoCr high-entropy alloys prepared by arc melting and casting method [44–46], the fcc phase of Al element which corresponds to Al-II in the spectrum, and Al-II is produced by the aluminum alloy substrate. Fig. 3 shows the intensity of Al-II during 422–423 nm under different scanning speeds. With the scanning speed increasing, the intensity of Al-II decreasing that means the dilution of aluminum declined. Therefore, the intensity of Al-II can indirectly reflect the atomic weight of the Al element in the substrate. Fig. 4 shows the distribution of layer depth and Al-II spectral intensity at different speed, respectively. Because the segment is the thermal radiation spectrum, the intensity of the spectrum is the corresponding peak value minus the bottom corresponding baseline value. It can be seen that the spectral intensity of Al-II decreases gradually with the increase of the speed, and the fluctuation range of Al-II shows a downward trend. We may find that the spectral intensity of Al-II is substantially equivalent to that of cladding depth. This also exhibits that the fluctuation of the spectral intensity of Al-II reflects the deep change of the cladding to a certain extent. 3.3. Microstructure Fig. 5 shows the cross-sectional microstructure of a single-track Al0.5FeCu0.7NiCoCr alloy coating at different scanning speeds. We may see that when scanning speed is 630 mm/min, the clad layer exhibits a good adhesion between the deposited material and the
Fig. 4. The influence of scanning speeds on layer depth and spectral intensity.
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Fig. 5. Corrosion-sectional topographies (SEM image) of the HEA coatings: (a) 270 mm/min, (b) 360 mm/min, (c) 450 mm/min, (d) 630 mm/min.
Fig. 6. EDS analysis of characteristic clad areas: (a) 270 mm/min, (b) 360 mm/min, (c) 450 mm/min, (d) 630 mm/min.
Fig. 7. SEM images of the Al0.5FeCu0.7NiCoCr high-entropy alloy coating at 630 mm/min: (a) top area, (b) central area, (c) interface area.
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Fig. 8. EDS maps of the Al0.5FeCu0.7NiCoCr high-entropy alloy coating top area at 630 mm/min.
For mapping the compositional distribution of top area at 630 mm/min, EDS maps analysis of every element is presented in Fig. 8. The qualitative segregation characteristics can be visualized. It can be found that Al element segregated preferentially to the gray area, while Co, Ni and Cu elements preference to the bright regions, as shown in Fig. 8d–f. The distribution of Fe and Cr elements is relatively homogeneous in these two regions and is lower to the average concentration, in Fig. 8b–c. This indicates a good agreement between the XRD and LIBS analysis. 3.4. Microhardness
Fig. 9. Microhardness of Al0.5FeCu0.7NiCoCr high-entropy alloy coatings at different scanning speeds.
substrate, almost crack-free and pores-free over the surface and beneath. SEM - EDS line scan on the clad center area revealed the elemental composition of HEA coating from the substrate to coatings presented in Fig. 6. We may see that the molar ratio of elements distinct from the initial. The content of Al is much higher than other elements. The substrate is partially melted during the laser processing, a certain amount of Al enters the inside of the bath. Moreover, Al is easily oxidized to a low density Of Al2O3, in the molten pool, internal liquid metal convection stirring, will float on the surface of the pool in the form of scum discharge, which is caused by the coating in the high content of Al. The typical microstructure of the cladding coating is illustrated in Fig. 7. Due to the temperature gradient, the microstructure of the coating from the top to the interface change from equiaxed grains to columnar grains (Fig. 7a–c). As can be seen that the formation of coating with the phases of gray and bright contrast in various proportions. The gray phase is displayed near the central and interface area of the coating. This phenomenon could attribute to the melting of Al substrate during the process, which leads to Al enrichment within the coating, and finally giving rise to gray contrast.
Fig. 9 shows the microhardness distribution from the surface of the coatings to the matrix. The average thickness of Al0.5FeCu0.7NiCoCr high-entropy alloy coating was about 0.8 mm. As we can see that the microhardness of the coatings was approximately 750 HV0.2, which was about 8 times as the substrate 5083 aluminum alloy. There might be three reasons contribute to the high microhardness of the HEA coating as follows: First, the effect of laser rapid heating and rapid cooling result in the generation of grain refinement in the microstructure, and increased solubility limitation in HEA coating [47,48]. Second, the different sizes of atomic lead to large lattice strain, high-density of dislocation and solid solution effect in the Al0.5FeCu0.7NiCoCr coating [49]. Furthermore, the atomic radius of Al is much larger than other five elements that increase the lattice crystal distortion and enhance the effect of the solid solution. Third, as we known that the bcc structure owing higher hardness than fcc, the addition of Al came from substrate dilution that resulted in the content of bcc structure increase [50–52]. 4. Conclusion (1) Crack- and porosity-free Al0.5FeCu0.7NiCoCr high-entropy alloy coatings were deposited on 5083 aluminum substrates by using the optimized processing parameters. (2) The crystal structures of Al0.5FeCu0.7NiCoCr high-entropy alloy coatings were composed of fcc + bcc and Al phases. Apart from 4v, d and VEC parameters, laser rapid cooling rate played an important role in phase forming of solid solution structure.
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(3) All elements of the Al0.5FeCu0.7NiCoCr high-entropy alloy can be detected by LIBS measurements. The spectral intensity change of Al-II reflected the depth variety of the cladding to a certain extent. (4) The hardness of the Al0.5FeCu0.7NiCoCr high-entropy alloy coatings was much higher than that of the 5083 aluminum substrate. The average hardness values of HEA coating reach 750HV0.2, which was approximately 8 times larger than the substrate.
References [1] Z. Yang, A. Wang, Z. Weng, D. Xiong, B. Ye, X. Qi, Porosity elimination and heat treatment of diode laser-clad homogeneous coating on cast aluminum-copper alloy, Surf. Coat. Technol. 321 (2017) 26–35. [2] R.S. Rajamure, H.D. Vora, S.G. Srinivasan, N.B. Dahotre, Laser alloyed Al-W coatings on aluminum for enhanced corrosion resistance, Appl. Surf. Sci. 328 (2015) 205–214. [3] R.S. Rajamure, H.D. Vora, N. Gupta, S. Karewar, S.G. Srinivasan, N.B. Dahotre, Laser surface alloying of molybdenum on aluminum for enhanced wear resistance, Surf. Coat. Technol. 258 (2014) 337–342. [4] D-Y. Lin, N-N. Zhang, B. He, G-W. Zhang, Y. Zhang, D-Y. Li, Tribological properties of FeCoCrNiAlBx high-entropy alloys coating prepared by laser cladding, J. Iron Steel Res. 24 (2017) 184–189. [5] C. D’Amato, J. Buhagiar, J.C. Betts, Tribological characteristics of an A356 aluminium alloy laser surface alloyed with nickel and Ni–Ti–C, Appl. Surf. Sci. 313 (2014) 720–729. [6] L. Dubourg, D. Ursescu, F. Hlawka, A. Cornet, Laser cladding of MMC coatings on aluminium substrate: influence of composition and microstructure on mechanical properties, Wear 258 (2005) 1745–1754. [7] J. Xu, W. Liu, Y. Kan, M. Zhong, Microstructure and wear properties of laser cladding Ti–Al–Fe–B coatings on AA2024 aluminum alloy, Mater. Des. 27 (2006) 405–410. [8] S. Nath, S. Pityana, J. Dutta Majumdar, Laser surface alloying of aluminium with WC+Co+NiCr for improved wear resistance, Surf. Coat. Technol. 206 (2012) 3333–3341. [9] D. Ravnikar, N.B. Dahotre, J. Grum, Laser coating of aluminum alloy EN AW 6082–T651 with TiB2 and TiC: microstructure and mechanical properties, Appl. Surf. Sci. 282 (2013) 914–922. [10] B.S. Murty, J.W. Yeh, S. Ranganathan, High Entropy Alloys, 2014, 171–190. [11] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructures and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1–93. [12] C.L. Wu, S. Zhang, C.H. Zhang, J. Chen, S.Y. Dong, Phase evolution characteristics and corrosion behavior of FeCoCrAlCu-X 0.5 coatings on cp Cu by laser high-entropy alloying, Opt. Laser Technol. 94 (2017) 68–71. [13] B. Ren, Z.X. Liu, D.M. Li, L. Shi, B. Cai, M.X. Wang, Corrosion behavior of CuCrFeNiMn high entropy alloy system in 1 M sulfuric acid solution, Mater. Corros. (2011), n/a-n/a. [14] Y.L. Chou, Y.C. Wang, J.W. Yeh, H.C. Shih, Pitting corrosion of the high-entropy alloy Co1.5CrFeNi1.5Ti0.5Mo0.1 in chloride-containing sulphate solutions, Corros. Sci. 52 (2010) 3481–3491. [15] F.Y. Shu, S. Liu, H.Y. Zhao, W.X. He, S.H. Sui, J. Zhang, P. He, B.S. Xu, Structure and high-temperature property of amorphous composite coating synthesized by laser cladding FeCrCoNiSiB high-entropy alloy powder, J. Alloys Compd. 731 (2018) 662–666. [16] C.-M. Lin, H.-L. Tsai, Evolution of microstructure, hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy, Intermetallics 19 (2011) 288– 294. [17] Y. Shi, B. Yang, X. Xie, J. Brechtl, K.A. Dahmen, P.K. Liaw, Corrosion of Al x CoCrFeNi high-entropy alloys: Al-content and potential scan-rate dependent pitting behavior, Corros. Sci. 119 (2017) 33–45. [18] Q. Ye, K. Feng, Z. Li, F. Lu, R. Li, J. Huang, Y. Wu, Microstructure and corrosion properties of CrMnFeCoNi high entropy alloy coating, Appl. Surf. Sci. 396 (2017) 1420–1426. [19] F.Y. Shu, L. Wu, H.Y. Zhao, S.H. Sui, L. Zhou, J. Zhang, W.X. He, P. He, B.S. Xu, Microstructure and high-temperature wear mechanism of laser cladded CoCrBFeNiSi high-entropy alloy amorphous coating, Mater. Lett. 211 (2018) 235–238. [20] S. Zhang, C.L. Wu, C.H. Zhang, M. Guan, J.Z. Tan, Laser surface alloying of FeCoCrAlNi high-entropy alloy on 304 stainless steel to enhance corrosion and cavitation erosion resistance, Opt. Laser Technol. 84 (2016) 23–31. [21] Y. Shon, S.S. Joshi, S. Katakam, R. Shanker Rajamure, N.B. Dahotre, Laser additive synthesis of high entropy alloy coating on aluminum: corrosion behavior, Mater. Lett. 142 (2015) 122–125. [22] G. Meng, T.M. Yue, X. Lin, H. Yang, H. Xie, X. Ding, Laser surface forming of AlCoCrCuFeNi particle reinforced AZ91D matrix composites, Opt. Laser Technol. 70 (2015) 119–127. [23] J.T. Hofman, B. Pathiraj, J. van Dijk, D.F. de Lange, J. Meijer, A camera based feedback control strategy for the laser cladding process, J. Mater. Process. Technol. 212 (2012) 2455–2462.
263
[24] C. Aragón, V. Madurga, J.A. Aguilera, Application of laser-induced breakdown spectroscopy to the analysis of the composition of thin films produced by pulsed laser deposition, Appl. Surf. Sci. 197 (2002) 217–223. [25] A. Alkhawwam, K. Alnama, A. Jazmati, M.D. Zidan, Characterization of laser induced tantalum plasma by spatio-temporal resolved optical emission spectroscopy, Optik – Int. J. Light Electron Optics 125 (2014) 2327–2331. [26] L. Song, J. Mazumder, Real time Cr measurement using optical emission spectroscopy during direct metal deposition process, IEEE Sens. J. 12 (2012) 958–964. [27] L. Song, J. Mazumder, Identification of phase transformation using optical emission spectroscopy for direct metal deposition process, SPIE LASE2012, pp. 82390G-82390G-82399. [28] V.N. Lednev, P.A. Sdvizhenskii, M.N. Filippov, M.Y. Grishin, V.A. Filichkina, A.Y. Stavertiy, R.S. Tretyakov, A.F. Bunkin, S.M. Pershin, Elemental profiling of laser cladded multilayer coatings by laser induced breakdown spectroscopy and energy dispersive X-ray spectroscopy, Appl. Surf. Sci. 416 (2017) 302–307. [29] J.A. Varela, J.M. Amado, M.J. Tobar, M.P. Mateo, A. Yañez, G. Nicolas, Characterization of hard coatings produced by laser cladding using laserinduced breakdown spectroscopy technique, Appl. Surf. Sci. 336 (2015) 396– 400. [30] L. Song, W. Huang, X. Han, J. Mazumder, Real-time composition monitoring using support vector regression of laser-induced plasma for laser additive manufacturing, IEEE Trans. Ind. Electron. 64 (2017) 633–642. [31] Y.F. Ye, Q. Wang, Y.L. Zhao, Q.F. He, J. Lu, Y. Yang, Elemental segregation in solid-solution high-entropy alloys: experiments and modeling, J. Alloys Compd. 681 (2016) 167–174. [32] G.H. Meng, X. Lin, H. Xie, T.M. Yue, X. Ding, L. Sun, M. Qi, The effect of Cu rejection in laser forming of AlCoCrCuFeNi/Mg composite coating, Mater. Des. 108 (2016) 157–167. [33] S. Katakam, S.S. Joshi, S. Mridha, S. Mukherjee, N.B. Dahotre, Laser assisted high entropy alloy coating on aluminum: microstructural evolution, J. Appl. Phys. 116 (2014) 104906. [34] C.L. Wu, S. Zhang, C.H. Zhang, H. Zhang, S.Y. Dong, Phase evolution and cavitation erosion-corrosion behavior of FeCoCrAlNiTix high entropy alloy coatings on 304 stainless steel by laser surface alloying, J. Alloys Compd. 698 (2017) 761–770. [35] A.K. Singh, A. Subramaniam, On the formation of disordered solid solutions in multi-component alloys, J. Alloys Compd. 587 (2014) 113–119. [36] Y. Zhang, Y.J. Zhou, J.P. Lin, G.L. Chen, P.K. Liaw, Solid-solution phase formation rules for multi-component alloys, Adv. Eng. Mater. 10 (2008) 534–538. [37] S. Guo, C.T. Liu, Phase stability in high entropy alloys: formation of solidsolution phase or amorphous phase, Prog. Nat. Sci.: Mater. Int. 21 (2011) 433– 446. [38] S. Fang, X. Xiao, L. Xia, W. Li, Y. Dong, Relationship between the widths of supercooled liquid regions and bond parameters of Mg-based bulk metallic glasses, J. Non-Cryst. Solids 321 (2003) 120–125. [39] S. Guo, C. Ng, J. Lu, C.T. Liu, Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys, J. Appl. Phys. 109 (2011) 103505. [40] X. Yang, Y. Zhang, Prediction of high-entropy stabilized solid-solution in multicomponent alloys, Mater. Chem. Phys. 132 (2012) 233–238. [41] A. Takeuchi, A. Inoue, Classification of bulk metallic glasses by atomic size difference heat of mixing and period of constituent elements and its application to characterization of the main alloying element, Mater. Trans. 46 (2005) 2817–2829. [42] T.B. Massalski, Comments concerning some features of phase diagrams and phase transformations, Materia Japan 51 (2010) 583–596. [43] C.L. Wu, S. Zhang, C.H. Zhang, H. Zhang, S.Y. Dong, Phase evolution and properties in laser surface alloying of FeCoCrAlCuNi x high-entropy alloy on copper substrate, Surf. Coat. Technol. 315 (2017) 368–376. [44] J.-W. Yeh, S.-Y. Chang, Y.-D. Hong, S.-K. Chen, S.-J. Lin, Anomalous decrease in X-ray diffraction intensities of Cu–Ni–Al–Co–Cr–Fe–Si alloy systems with multi-principal elements, Mater. Chem. Phys. 103 (2007) 41–46. [45] H.-P. Chou, Y.-S. Chang, S.-K. Chen, J.-W. Yeh, Microstructure, thermophysical and electrical properties in AlxCoCrFeNi (0 x 2) high-entropy alloys, Mater. Sci. Eng. B 163 (2009) 184–189. [46] C.-C. Tung, J.-W. Yeh, T.-T. Shun, S.-K. Chen, Y.-S. Huang, H.-C. Chen, On the elemental effect of AlCoCrCuFeNi high-entropy alloy system, Mater. Lett. 61 (2007) 1–5. [47] S. Zhang, C.L. Wu, J.Z. Yi, C.H. Zhang, Synthesis and characterization of FeCoCrAlCu high-entropy alloy coating by laser surface alloying, Surf. Coat. Technol. 262 (2015) 64–69. [48] M. Zhang, X. Zhou, X. Yu, J. Li, Synthesis and characterization of refractory TiZrNbWMo high-entropy alloy coating by laser cladding, Surf. Coat. Technol. 311 (2017) 321–329. [49] H. Zhang, Y.-Z. He, Y. Pan, L.-Z. Pei, Phase selection, microstructure and properties of laser rapidly solidified FeCoNiCrAl2Si coating, Intermetallics 19 (2011) 1130–1135. [50] X. Ye, M. Ma, Y. Cao, W. Liu, X. Ye, Y. Gu, The property research on high-entropy alloy AlxFeCoNiCuCr coating by laser cladding, Phys. Procedia 12 (2011) 303– 312. [51] X.-W. Qiu, C.-G. Liu, Microstructure and properties of Al2CrFeCoCuTiNix highentropy alloys prepared by laser cladding, J. Alloys Compd. 553 (2013) 216– 220. [52] H. Zhang, Y. Pan, Y.-Z. He, Synthesis and characterization of FeCoNiCrCu highentropy alloy coating by laser cladding, Mater. Des. 32 (2011) 1910–1915.
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Full length article
Characterization of Al0.5FeCu0.7NiCoCr high-entropy alloy coating on aluminum alloy by laser cladding Cong Ni, Yan Shi ⇑, Jia Liu, Genzhe Huang School of Electromechanical Engineering, Changchun University of Science and Technology, Changchun 130022, Jilin, China Engineering Research Center of Laser Processing for Universities of Jilin Province, Changchun 130022, Jilin, China National Base of International Science and Technology Cooperation in Optics, Changchun 130022, Jilin, China
a r t i c l e
i n f o
Article history: Received 3 December 2017 Received in revised form 24 January 2018 Accepted 26 January 2018
Keywords: High-entropy alloy coatings Laser induced breakdown spectroscopy Substrate dilution Microhardness
a b s t r a c t Al0.5FeCu0.7NiCoCr high-entropy alloy (HEA) coatings were synthesized on aluminum by laser cladding, aiming at enhancing surface properties. Samples were characterized by using scanning electron microscopy with spectroscopy (SEM/EDS), X-ray diffraction, laser induced breakdown spectroscopy (LIBS), microhardness. The results showed that the HEA coatings exhibited good metallurgical bonding to the matrix by using optimized laser processing parameters. The HEA coatings were composed of fcc + bcc phases. All the composed elements can be noted in mapping through the calibration of the plasma, and the plasma of the collected Al confirmed that come from substrate dilution. The intensity change of Al-II reflected the depth variety of the cladding layer. The microstructure of clad layer was consisted of dendrite. The microhardness of HEA layer reached 750HV0.2 that was about 8 times larger than that of the substrate. Ó 2018 Published by Elsevier Ltd.
1. Introduction Aluminum and its alloys have been widely used in the fields of aircraft, household, transportation and military, because of its high intensity, excellent lightweight, high forming quality, and good thermal conductivity [1,2]. However, the low hardness, easy oxidation, poor corrosion and wear resistance limit its wide application. Moreover, it is not feasible to enhance their mechanical properties via conventional solid-state heat treatments and phase transformations, because they have not allotropic transformations [3]. Laser cladding is one of the important surface modification techniques in today’s industry due to its several advantages such as high velocity of heating and cooling, low dilution, small heat affected zone and allows the precise adaptation of surface properties [4]. Laser cladding on aluminum and its alloys have been widely investigated. The materials systems that have been studied, including: molybdenum [3], tungsten [2], Ni-Ti-C [5], MMC [6], TiAl-Fe-B [7], WC-Co-NiCr [8], TiB2-TiC-Al [9]. However, these researches were only aimed at improving wear or corrosion resistance through coating materials. The concept of high-entropy alloy (HEA) was presented by Taiwan scholar Yeh and his colleague that break through the tradi-
⇑ Corresponding author. E-mail address: [email protected] (Y. Shi). https://doi.org/10.1016/j.optlastec.2018.01.058 0030-3992/Ó 2018 Published by Elsevier Ltd.
tional alloy design framework, which was on the base of one or two major alloy element [10–12]. With the effects of the lattice distortion, sluggish diffusion, cocktail effect, and high mixing entropy, the HEA easily form simple solid-solution phases and nanostructures during solidification, rather than intermetallic or other complicated phases [13–15]. Owing to these characterizations, the HEAs usually possess several properties, such as high hardness, super corrosion resistance and better wearing resistance [16–19]. Previous HEAs studies focus on arc melting and casting, while the size and costs of HEAs restricted its usage. So high entropy alloys also considered attractive coating materials for enhancing surface behavior. With high temperature and rapid cooling rate during laser processing, the HEA phase was easy to be synthesized, leading to the enthusiasm of study HEA coatings by laser processing. Zhang et al. [20] deposited FeCoCrAlNi highentropy alloy coating on 304 stainless steel by laser surface alloying. The parameters of mixing entropy (DSmix), mixing enthalpy (DHmix), atom-size difference (d) and valence electron concentration (VEC) play a major role in the formation of simple solid solutions. Shon et al. [21] synthesized AlFeCrCoNi HEA alloy on aluminum by laser cladding through using a pre-coating process. They controlled over dilution by combinations double layered coating and higher energy input, while the coating still existed cracks. Meng et al. [22] fabricated AlCoCrCuFeNi high-entropy alloy on the AZ91D surface by using the LMI techniques and the wear resistance was significantly improved. The quality of a
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cladding layer mainly depends on several parameters including laser power, laser beam size, scanning speed, and powder feed rate, which has an influence on the temperature of the clad interaction zone. Moreover, laser cladding is an open-loop technique, which means the quality relies heavily on the skills of the operator. Post-processing is always expensive and time consuming [23]. In recent years, laser-induced breakdown spectroscopy (LIBS) has become one of the online analytical and diagnostic techniques [24]. This technique based on laser ablation of the sample and the analysis by optical emission spectroscopy of the plasma formed. Some advantages of the LIBS technique have demonstrated its unique versatility, such as: rapid and real-time analysis, allowing fast contact-less analysis of almost any type of material, in situ analysis with no sample preparation required, and its relatively low cost [25–27]. Lednev et al. [28] used EDS and LIBS techniques profiling of major components in the multilayer wear resistant coatings, a good correlation between EDS element mapping and LIBS data was established with minor difference explained by different sampling depth. Varela et al. [29] analyzed the characterization of laser claddings based on hard facing alloys and partial dilution of some WC spheres in the coating by using LIBS technique. The behavior of hardness can be explained by LIBS maps which evidenced the partial dilution of some WC spheres in the coating. Song et al. [30] used operating parameter conditioned support vector regression (SVR) method to improve the composition prediction performance during laser process. The SVR methods showed a stable and accurate performance. Previous reports found that the segregation of copper was due to the enthalpy of mixing between copper and the other main elements were more positive [31,32]. The melting of aluminum matrix resulted in Al enrichment within the coating during the laser processing [33]. In this study, we decrease the atomic percentage of aluminum and copper, then deposit Al0.5FeCu0.7NiCoCr high-entropy alloy coatings on aluminum by laser cladding. The effect of laser processing parameters on the phase formation, substrate dilution and microstructure was investigated by a combination of XRD, LIBS and EDS techniques. 2. Experimental procedure The 5083 aluminum with dimensions of 60 mm 20 mm 10 mm was selected as the substrate material, and then cleaned with acetone to remove surface dirt and oil. The Al, Cr, Fe, Ni, Cu and Co elemental powders had a purity of 99.5% with a size range of 44– 149 um (100 / +325 mesh) as cladding materials. Alloy powder mixed with the aid of a ball mill for 2 h in an argon atmosphere. The mixed powders were then placed onto the surface of aluminum alloy, using ethyl alcohol as binders, and then dried in an oven at 80 °C for 8 h. The thickness of the power was approximately 0.8 mm. Laser cladding was carried out using a Nd: YAG laser of 1.06 um wavelength, and laser beam diameter of 2 mm. With a series of optimization trail runs, the processing parameters were: laser power was 1.1 kW, scanning speed were 270 mm/min, 360 mm/min, 450 mm/min and 630 mm/min. High purity argon gas at a flow rate 10 L/min was used to prevent oxidation. The fiber optic spectrometer was based on the AvaBench-75 optical platform designed, the ambient temperature of the spectrometer was minimal. The wavelength was ranging from 200 to 950 nm, and integration time 100 ms. After the laser cladding, metallographic samples were sectioned perpendicular to the scanning track with a Wire cut Electrical Discharge Machining (WEDM) machine. The structural features of HEA alloy samples were examined by a EMPYGREAN X-ray Diffraction analysis system (XRD) with Cu Ka radiation at 40 kV and 30
mA. XRD patterns were taken at 2h angles from 20° to 90° at a scanning rate 4°/min. HEA coatings morphology and microstructure were analyzed by scanning electron microscope (SEM, JSM6510F) equipped with energy dispersive spectroscopy (EDS). The microhardness was measured by MH-60 Vickers Hardness Tester with a load of 200 g and a duration time of 10 s.
3. Results and discussion 3.1. XRD analysis Fig. 1 shows the patterns of the Al0.5FeCu0.7NiCoCr highentropy alloy coating with different scanning speeds. Phase analysis revealed the existence of Al peak (fcc1) and HEA phase (fcc2, bcc). In the present study, the Gibbs free energy is used to calculate the formation of high-entropy and can be expressed as:
DGmix ¼ DHmix T DSmix
ð1Þ
where 4Gmix means mixing Gibb’s free energy, 4Hmix is the mixing enthalpy, T is the absolute temperature, and 4Smix is the entropy of mixing. As we know that the 4Hmix demonstrate the tendency for ordering or cluster, and the 4Smix at a given temperature indicated the tendency for the formation of disordered solid solutions (DSS) [34]. Furthermore, several investigations proposed that the solid solution formation can be determined based on these parameters including atomic size difference (d), electronegativity difference (4v) and valence electron concentration (VEC) [35], which can be expressed as:
DHmix ¼
n X
Xij ci cj
ð2Þ
n X ci lnðcj Þ
ð3Þ
i¼1;i–j
DSmix ¼ R
i¼1
Tm ¼
n X ci ðT m Þi
ð4Þ
i¼1
Fig. 1. XRD patterns of the HEA coating.
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d (%)
4Hmix (kJ/mol)
4Smix (JK1mol1)
Tm (K)
X
VEC
4v
Al0.5FeCu0.7NiCoCr
3.74
3.23
14.296
1456.16
6.44
8.12
0.11
Fig. 2. LIBS spectrum of Al0.5FeCu0.7NiCoCr high-entropy alloy coating: (a) at different scanning speeds; (b) in wide range (400–550 nm); (c) in wide range (600–850 nm).
X¼
T m DSmix jDHmix j
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Xn r i 2 c d¼ i 1 i¼1 r qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Xn 2 Dv ¼ c ðvi vÞ i¼1 i VEC ¼
n X C i ðVECÞi i¼1
ð5Þ
ð6Þ ð7Þ ð8Þ
where Xij ¼ 4DHmix menas the regular solution interaction parameij ter between the ith and jth elements, ci and cj is the atomic percentage of the ith and jth component, R (=8.314 JK1mol1) indicates P the gas constant, and ni¼1 ci r i is the average atomic radius and ri is defined as the atomic radius. Tm is related to the melting point P of the ith component of alloy. r ¼ ni¼1 ci r i is the average atomic P radius and ri is the atomic radius. Moreover, v ¼ ni¼1 ci vi means the average electronegativity and vi demonstrates the Pauling electronegativity for the ith component. (VEC)i is the VEC for individual elements.
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3.2. Spectrum analysis
Fig. 3. The intensity of Al-II ranges from 422 nm to 423 nm.
Zhang et al. [36] pointed out that the solid solution was easily formed and stable in the region: 15 4Hmix 5 kJ/mol and 1 d 6. Guo et al. [37] found that when 22 4Hmix 7 kJ/mol, 0 d 8.5, and 11 4Smix 19.5 JK1mol1, the solid solution tend to form. They emphasized that the critical parameter was the atomic size difference (d) which determined the formation of the solid solution. Fang et al. [38] proposed that the large value of 4v was aiding the formation of solid solution. Guo and Ng. [39] described that the value of VEC between 6.87 and 8.0 contribute to the co-exist of bcc and fcc phase. The formation of bcc phase will be more stable at VEC < 6.87, and when VEC 8.0, sole fcc phase will form. Yang et al. [40] concluded that when X 1.1 and d 6.6%, the solid solution and bulk metallic glasses tend to form. According to Eqs. (2–8), the calculated results showed in Table 1, where the formulae used in the calculation were from Ref [41,42]. The value of both X and d for the studied coatings were meet formation rules of solid solutions. However, the VEC was 8.12 which mean that fcc phases are more stable. The parameters of X, d, 4v and VEC were used to estimate solid solution formation ability in multi-component alloy systems by using vacuum melting method, while the solidification temperature must be considered in laser high-entropy alloy [43]. The laser rapid cooling rate could facilitate the formation of simple structures, and effectively prohibits the formation of undesired intermetallic compounds. The presence of Al phases may attribute to the melting of Al substrate during the processing and dissolve into the molten pool which causes composition segregation from the ideal composition.
The laser induced plasma emitted spectrums were measured in a wide spectral range (400–850 nm) for different parameter coating layers in Fig. 2. Because the excessive number of elements in the cladding layer, the plasma peaks observed in the spectrum were much larger. It is found that the scanning velocity has little effect on the solid solution phase of the cladding layer by XRD analysis as shown in Fig. 1 and the peak did not change significantly in Fig. 2a. The whole spectrum is between 400 and 800 nm, and the spectral peak distribution from 400 nm to 600 nm is relatively dense. Fig. 2(b) and (c) are the calibration of the spectrum of the plasma spectrum. Each element of Al, Cu, Co, Cr, Ni, Fe in the cladding layer can be found in the map. Therefore, only Fe, Cr, Cu, Al elements in the form of II valent ions, and the peaks of Fe, Cr and Cu in the spectrum are more. The ionization of Ar was found at 588 nm and 590 nm, the peak value was very large, which indicated that the process of Ar gas was sufficient to protect the cladding. Compared with AlFeCuNiCoCr high-entropy alloys prepared by arc melting and casting method [44–46], the fcc phase of Al element which corresponds to Al-II in the spectrum, and Al-II is produced by the aluminum alloy substrate. Fig. 3 shows the intensity of Al-II during 422–423 nm under different scanning speeds. With the scanning speed increasing, the intensity of Al-II decreasing that means the dilution of aluminum declined. Therefore, the intensity of Al-II can indirectly reflect the atomic weight of the Al element in the substrate. Fig. 4 shows the distribution of layer depth and Al-II spectral intensity at different speed, respectively. Because the segment is the thermal radiation spectrum, the intensity of the spectrum is the corresponding peak value minus the bottom corresponding baseline value. It can be seen that the spectral intensity of Al-II decreases gradually with the increase of the speed, and the fluctuation range of Al-II shows a downward trend. We may find that the spectral intensity of Al-II is substantially equivalent to that of cladding depth. This also exhibits that the fluctuation of the spectral intensity of Al-II reflects the deep change of the cladding to a certain extent. 3.3. Microstructure Fig. 5 shows the cross-sectional microstructure of a single-track Al0.5FeCu0.7NiCoCr alloy coating at different scanning speeds. We may see that when scanning speed is 630 mm/min, the clad layer exhibits a good adhesion between the deposited material and the
Fig. 4. The influence of scanning speeds on layer depth and spectral intensity.
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Fig. 5. Corrosion-sectional topographies (SEM image) of the HEA coatings: (a) 270 mm/min, (b) 360 mm/min, (c) 450 mm/min, (d) 630 mm/min.
Fig. 6. EDS analysis of characteristic clad areas: (a) 270 mm/min, (b) 360 mm/min, (c) 450 mm/min, (d) 630 mm/min.
Fig. 7. SEM images of the Al0.5FeCu0.7NiCoCr high-entropy alloy coating at 630 mm/min: (a) top area, (b) central area, (c) interface area.
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Fig. 8. EDS maps of the Al0.5FeCu0.7NiCoCr high-entropy alloy coating top area at 630 mm/min.
For mapping the compositional distribution of top area at 630 mm/min, EDS maps analysis of every element is presented in Fig. 8. The qualitative segregation characteristics can be visualized. It can be found that Al element segregated preferentially to the gray area, while Co, Ni and Cu elements preference to the bright regions, as shown in Fig. 8d–f. The distribution of Fe and Cr elements is relatively homogeneous in these two regions and is lower to the average concentration, in Fig. 8b–c. This indicates a good agreement between the XRD and LIBS analysis. 3.4. Microhardness
Fig. 9. Microhardness of Al0.5FeCu0.7NiCoCr high-entropy alloy coatings at different scanning speeds.
substrate, almost crack-free and pores-free over the surface and beneath. SEM - EDS line scan on the clad center area revealed the elemental composition of HEA coating from the substrate to coatings presented in Fig. 6. We may see that the molar ratio of elements distinct from the initial. The content of Al is much higher than other elements. The substrate is partially melted during the laser processing, a certain amount of Al enters the inside of the bath. Moreover, Al is easily oxidized to a low density Of Al2O3, in the molten pool, internal liquid metal convection stirring, will float on the surface of the pool in the form of scum discharge, which is caused by the coating in the high content of Al. The typical microstructure of the cladding coating is illustrated in Fig. 7. Due to the temperature gradient, the microstructure of the coating from the top to the interface change from equiaxed grains to columnar grains (Fig. 7a–c). As can be seen that the formation of coating with the phases of gray and bright contrast in various proportions. The gray phase is displayed near the central and interface area of the coating. This phenomenon could attribute to the melting of Al substrate during the process, which leads to Al enrichment within the coating, and finally giving rise to gray contrast.
Fig. 9 shows the microhardness distribution from the surface of the coatings to the matrix. The average thickness of Al0.5FeCu0.7NiCoCr high-entropy alloy coating was about 0.8 mm. As we can see that the microhardness of the coatings was approximately 750 HV0.2, which was about 8 times as the substrate 5083 aluminum alloy. There might be three reasons contribute to the high microhardness of the HEA coating as follows: First, the effect of laser rapid heating and rapid cooling result in the generation of grain refinement in the microstructure, and increased solubility limitation in HEA coating [47,48]. Second, the different sizes of atomic lead to large lattice strain, high-density of dislocation and solid solution effect in the Al0.5FeCu0.7NiCoCr coating [49]. Furthermore, the atomic radius of Al is much larger than other five elements that increase the lattice crystal distortion and enhance the effect of the solid solution. Third, as we known that the bcc structure owing higher hardness than fcc, the addition of Al came from substrate dilution that resulted in the content of bcc structure increase [50–52]. 4. Conclusion (1) Crack- and porosity-free Al0.5FeCu0.7NiCoCr high-entropy alloy coatings were deposited on 5083 aluminum substrates by using the optimized processing parameters. (2) The crystal structures of Al0.5FeCu0.7NiCoCr high-entropy alloy coatings were composed of fcc + bcc and Al phases. Apart from 4v, d and VEC parameters, laser rapid cooling rate played an important role in phase forming of solid solution structure.
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(3) All elements of the Al0.5FeCu0.7NiCoCr high-entropy alloy can be detected by LIBS measurements. The spectral intensity change of Al-II reflected the depth variety of the cladding to a certain extent. (4) The hardness of the Al0.5FeCu0.7NiCoCr high-entropy alloy coatings was much higher than that of the 5083 aluminum substrate. The average hardness values of HEA coating reach 750HV0.2, which was approximately 8 times larger than the substrate.
References [1] Z. Yang, A. Wang, Z. Weng, D. Xiong, B. Ye, X. Qi, Porosity elimination and heat treatment of diode laser-clad homogeneous coating on cast aluminum-copper alloy, Surf. Coat. Technol. 321 (2017) 26–35. [2] R.S. Rajamure, H.D. Vora, S.G. Srinivasan, N.B. Dahotre, Laser alloyed Al-W coatings on aluminum for enhanced corrosion resistance, Appl. Surf. Sci. 328 (2015) 205–214. [3] R.S. Rajamure, H.D. Vora, N. Gupta, S. Karewar, S.G. Srinivasan, N.B. Dahotre, Laser surface alloying of molybdenum on aluminum for enhanced wear resistance, Surf. Coat. Technol. 258 (2014) 337–342. [4] D-Y. Lin, N-N. Zhang, B. He, G-W. Zhang, Y. Zhang, D-Y. Li, Tribological properties of FeCoCrNiAlBx high-entropy alloys coating prepared by laser cladding, J. Iron Steel Res. 24 (2017) 184–189. [5] C. D’Amato, J. Buhagiar, J.C. Betts, Tribological characteristics of an A356 aluminium alloy laser surface alloyed with nickel and Ni–Ti–C, Appl. Surf. Sci. 313 (2014) 720–729. [6] L. Dubourg, D. Ursescu, F. Hlawka, A. Cornet, Laser cladding of MMC coatings on aluminium substrate: influence of composition and microstructure on mechanical properties, Wear 258 (2005) 1745–1754. [7] J. Xu, W. Liu, Y. Kan, M. Zhong, Microstructure and wear properties of laser cladding Ti–Al–Fe–B coatings on AA2024 aluminum alloy, Mater. Des. 27 (2006) 405–410. [8] S. Nath, S. Pityana, J. Dutta Majumdar, Laser surface alloying of aluminium with WC+Co+NiCr for improved wear resistance, Surf. Coat. Technol. 206 (2012) 3333–3341. [9] D. Ravnikar, N.B. Dahotre, J. Grum, Laser coating of aluminum alloy EN AW 6082–T651 with TiB2 and TiC: microstructure and mechanical properties, Appl. Surf. Sci. 282 (2013) 914–922. [10] B.S. Murty, J.W. Yeh, S. Ranganathan, High Entropy Alloys, 2014, 171–190. [11] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructures and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1–93. [12] C.L. Wu, S. Zhang, C.H. Zhang, J. Chen, S.Y. Dong, Phase evolution characteristics and corrosion behavior of FeCoCrAlCu-X 0.5 coatings on cp Cu by laser high-entropy alloying, Opt. Laser Technol. 94 (2017) 68–71. [13] B. Ren, Z.X. Liu, D.M. Li, L. Shi, B. Cai, M.X. Wang, Corrosion behavior of CuCrFeNiMn high entropy alloy system in 1 M sulfuric acid solution, Mater. Corros. (2011), n/a-n/a. [14] Y.L. Chou, Y.C. Wang, J.W. Yeh, H.C. Shih, Pitting corrosion of the high-entropy alloy Co1.5CrFeNi1.5Ti0.5Mo0.1 in chloride-containing sulphate solutions, Corros. Sci. 52 (2010) 3481–3491. [15] F.Y. Shu, S. Liu, H.Y. Zhao, W.X. He, S.H. Sui, J. Zhang, P. He, B.S. Xu, Structure and high-temperature property of amorphous composite coating synthesized by laser cladding FeCrCoNiSiB high-entropy alloy powder, J. Alloys Compd. 731 (2018) 662–666. [16] C.-M. Lin, H.-L. Tsai, Evolution of microstructure, hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy, Intermetallics 19 (2011) 288– 294. [17] Y. Shi, B. Yang, X. Xie, J. Brechtl, K.A. Dahmen, P.K. Liaw, Corrosion of Al x CoCrFeNi high-entropy alloys: Al-content and potential scan-rate dependent pitting behavior, Corros. Sci. 119 (2017) 33–45. [18] Q. Ye, K. Feng, Z. Li, F. Lu, R. Li, J. Huang, Y. Wu, Microstructure and corrosion properties of CrMnFeCoNi high entropy alloy coating, Appl. Surf. Sci. 396 (2017) 1420–1426. [19] F.Y. Shu, L. Wu, H.Y. Zhao, S.H. Sui, L. Zhou, J. Zhang, W.X. He, P. He, B.S. Xu, Microstructure and high-temperature wear mechanism of laser cladded CoCrBFeNiSi high-entropy alloy amorphous coating, Mater. Lett. 211 (2018) 235–238. [20] S. Zhang, C.L. Wu, C.H. Zhang, M. Guan, J.Z. Tan, Laser surface alloying of FeCoCrAlNi high-entropy alloy on 304 stainless steel to enhance corrosion and cavitation erosion resistance, Opt. Laser Technol. 84 (2016) 23–31. [21] Y. Shon, S.S. Joshi, S. Katakam, R. Shanker Rajamure, N.B. Dahotre, Laser additive synthesis of high entropy alloy coating on aluminum: corrosion behavior, Mater. Lett. 142 (2015) 122–125. [22] G. Meng, T.M. Yue, X. Lin, H. Yang, H. Xie, X. Ding, Laser surface forming of AlCoCrCuFeNi particle reinforced AZ91D matrix composites, Opt. Laser Technol. 70 (2015) 119–127. [23] J.T. Hofman, B. Pathiraj, J. van Dijk, D.F. de Lange, J. Meijer, A camera based feedback control strategy for the laser cladding process, J. Mater. Process. Technol. 212 (2012) 2455–2462.
263
[24] C. Aragón, V. Madurga, J.A. Aguilera, Application of laser-induced breakdown spectroscopy to the analysis of the composition of thin films produced by pulsed laser deposition, Appl. Surf. Sci. 197 (2002) 217–223. [25] A. Alkhawwam, K. Alnama, A. Jazmati, M.D. Zidan, Characterization of laser induced tantalum plasma by spatio-temporal resolved optical emission spectroscopy, Optik – Int. J. Light Electron Optics 125 (2014) 2327–2331. [26] L. Song, J. Mazumder, Real time Cr measurement using optical emission spectroscopy during direct metal deposition process, IEEE Sens. J. 12 (2012) 958–964. [27] L. Song, J. Mazumder, Identification of phase transformation using optical emission spectroscopy for direct metal deposition process, SPIE LASE2012, pp. 82390G-82390G-82399. [28] V.N. Lednev, P.A. Sdvizhenskii, M.N. Filippov, M.Y. Grishin, V.A. Filichkina, A.Y. Stavertiy, R.S. Tretyakov, A.F. Bunkin, S.M. Pershin, Elemental profiling of laser cladded multilayer coatings by laser induced breakdown spectroscopy and energy dispersive X-ray spectroscopy, Appl. Surf. Sci. 416 (2017) 302–307. [29] J.A. Varela, J.M. Amado, M.J. Tobar, M.P. Mateo, A. Yañez, G. Nicolas, Characterization of hard coatings produced by laser cladding using laserinduced breakdown spectroscopy technique, Appl. Surf. Sci. 336 (2015) 396– 400. [30] L. Song, W. Huang, X. Han, J. Mazumder, Real-time composition monitoring using support vector regression of laser-induced plasma for laser additive manufacturing, IEEE Trans. Ind. Electron. 64 (2017) 633–642. [31] Y.F. Ye, Q. Wang, Y.L. Zhao, Q.F. He, J. Lu, Y. Yang, Elemental segregation in solid-solution high-entropy alloys: experiments and modeling, J. Alloys Compd. 681 (2016) 167–174. [32] G.H. Meng, X. Lin, H. Xie, T.M. Yue, X. Ding, L. Sun, M. Qi, The effect of Cu rejection in laser forming of AlCoCrCuFeNi/Mg composite coating, Mater. Des. 108 (2016) 157–167. [33] S. Katakam, S.S. Joshi, S. Mridha, S. Mukherjee, N.B. Dahotre, Laser assisted high entropy alloy coating on aluminum: microstructural evolution, J. Appl. Phys. 116 (2014) 104906. [34] C.L. Wu, S. Zhang, C.H. Zhang, H. Zhang, S.Y. Dong, Phase evolution and cavitation erosion-corrosion behavior of FeCoCrAlNiTix high entropy alloy coatings on 304 stainless steel by laser surface alloying, J. Alloys Compd. 698 (2017) 761–770. [35] A.K. Singh, A. Subramaniam, On the formation of disordered solid solutions in multi-component alloys, J. Alloys Compd. 587 (2014) 113–119. [36] Y. Zhang, Y.J. Zhou, J.P. Lin, G.L. Chen, P.K. Liaw, Solid-solution phase formation rules for multi-component alloys, Adv. Eng. Mater. 10 (2008) 534–538. [37] S. Guo, C.T. Liu, Phase stability in high entropy alloys: formation of solidsolution phase or amorphous phase, Prog. Nat. Sci.: Mater. Int. 21 (2011) 433– 446. [38] S. Fang, X. Xiao, L. Xia, W. Li, Y. Dong, Relationship between the widths of supercooled liquid regions and bond parameters of Mg-based bulk metallic glasses, J. Non-Cryst. Solids 321 (2003) 120–125. [39] S. Guo, C. Ng, J. Lu, C.T. Liu, Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys, J. Appl. Phys. 109 (2011) 103505. [40] X. Yang, Y. Zhang, Prediction of high-entropy stabilized solid-solution in multicomponent alloys, Mater. Chem. Phys. 132 (2012) 233–238. [41] A. Takeuchi, A. Inoue, Classification of bulk metallic glasses by atomic size difference heat of mixing and period of constituent elements and its application to characterization of the main alloying element, Mater. Trans. 46 (2005) 2817–2829. [42] T.B. Massalski, Comments concerning some features of phase diagrams and phase transformations, Materia Japan 51 (2010) 583–596. [43] C.L. Wu, S. Zhang, C.H. Zhang, H. Zhang, S.Y. Dong, Phase evolution and properties in laser surface alloying of FeCoCrAlCuNi x high-entropy alloy on copper substrate, Surf. Coat. Technol. 315 (2017) 368–376. [44] J.-W. Yeh, S.-Y. Chang, Y.-D. Hong, S.-K. Chen, S.-J. Lin, Anomalous decrease in X-ray diffraction intensities of Cu–Ni–Al–Co–Cr–Fe–Si alloy systems with multi-principal elements, Mater. Chem. Phys. 103 (2007) 41–46. [45] H.-P. Chou, Y.-S. Chang, S.-K. Chen, J.-W. Yeh, Microstructure, thermophysical and electrical properties in AlxCoCrFeNi (0 x 2) high-entropy alloys, Mater. Sci. Eng. B 163 (2009) 184–189. [46] C.-C. Tung, J.-W. Yeh, T.-T. Shun, S.-K. Chen, Y.-S. Huang, H.-C. Chen, On the elemental effect of AlCoCrCuFeNi high-entropy alloy system, Mater. Lett. 61 (2007) 1–5. [47] S. Zhang, C.L. Wu, J.Z. Yi, C.H. Zhang, Synthesis and characterization of FeCoCrAlCu high-entropy alloy coating by laser surface alloying, Surf. Coat. Technol. 262 (2015) 64–69. [48] M. Zhang, X. Zhou, X. Yu, J. Li, Synthesis and characterization of refractory TiZrNbWMo high-entropy alloy coating by laser cladding, Surf. Coat. Technol. 311 (2017) 321–329. [49] H. Zhang, Y.-Z. He, Y. Pan, L.-Z. Pei, Phase selection, microstructure and properties of laser rapidly solidified FeCoNiCrAl2Si coating, Intermetallics 19 (2011) 1130–1135. [50] X. Ye, M. Ma, Y. Cao, W. Liu, X. Ye, Y. Gu, The property research on high-entropy alloy AlxFeCoNiCuCr coating by laser cladding, Phys. Procedia 12 (2011) 303– 312. [51] X.-W. Qiu, C.-G. Liu, Microstructure and properties of Al2CrFeCoCuTiNix highentropy alloys prepared by laser cladding, J. Alloys Compd. 553 (2013) 216– 220. [52] H. Zhang, Y. Pan, Y.-Z. He, Synthesis and characterization of FeCoNiCrCu highentropy alloy coating by laser cladding, Mater. Des. 32 (2011) 1910–1915.