- Email: [email protected]
Effect of Cr additions on the microstructure and corrosion resistance of Diode laser clad CuAl10 coating
Surface & Coatings Technology 381 (2020) 125215
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of Cr additions on the microstructure and corrosion resistance of Diode laser clad CuAl10 coating
T
⁎
Jiaoxi Yanga, , Feiyu Wua, Bing Baia, Gaosheng Wanga, Lei Yanga, Shengfeng Zhoub, Jianbo Leib a b
Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, PR China Laser Technology Institute, Tianjin Polytechnic University, Tianjin 300387, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser cladding Aluminum bronze Cr additions Corrosion resistance
The effect of Cr additions on the microstructure and corrosion resistance of CuAl10 coating deposited by laser cladding was investigated. Laser cladding was performed using a 4000 W Diode laser under the same conditions, and the premixed powders with different weight ratios of CuAl10 to Cr were fed synchronously by TWIN10C powder feeder and D70 coaxial nozzle. The microstructures were observed under a scanning electron microscope equipped with an energy dispersive X-ray spectrometer (EDS) for chemical analysis, and phase analysis was performed with an X-ray diffractometer (XRD). The electrochemical corrosion behaviors of cladding layers were evaluated. The results showed that the CuAl10 coating has a good metallurgical bonding with the 17-4PH substrate. The addition of Cr can improve the corrosion resistance of CuAl10 coatings, and the passivation ability of the coatings increases with increasing Cr contents. The CuAl10 coating without Cr addition include α, γ2, and K phase; however, the addition of Cr leads to an increase of β′ phase and Cr and a decrease of γ2 and K phase in the CuAl10 coating.
1. Introduction
[8]. In the process of manufacturing new structural parts by laser cladding aluminum bronze material on the surface of iron base alloy, a certain amount of iron diffuses from the matrix into the coating [9], which will reduce the corrosion resistance of the coating. A possible way to solve this problem is by adding alloying elements to the alloy materials. Adding chromium element can improve the corrosion resistance of materials. In the similar research of adding Cr to improve the properties of materials, Zhang et al. [10] studied the effect of Cr on the corrosion resistance of cold-rolled copper alloy, and the results showed that the corrosion resistance of CueCr alloy increased with the increase of Cr contents. Liu et al. [11] found that (Cu47Zr11Ti34Ni8)99.5M0.5 (M = 0, Cr, Mo and W) bulk metallic glasses had an excellent corrosion resistance with the presence of a small amount of Cr. Jeon et al. [12] found that the addition of Cr could prevent the localized corrosion behavior of Cu-6%Ni-4%Sn-x% alloys caused by decreased Sn-rich precipitates in these alloys. These precipitates induced galvanic corrosion due to the difference in chemical composition. As for Cu-based alloy coatings deposited by laser cladding, the effect of different iron and nickel contents on wear resistance of aluminum bronze coatings was investigated by X. P. Tao [13]. This highlights the need for an understanding of the effects and underlying mechanism of the alloying elements added on the corrosion resistance of the coating.
Aluminum bronze is a Cu-based alloy with Al as the major alloying element, and it is the material of choice for a wide variety of applications due to its excellent physical, mechanical and tribological properties [1], such as high strength, thermal conductivity and excellent resistance to wear and corrosion. It is particularly suitable for the corrosion-resistant parts working in the marine environment, such as propellers, valves, and pipes [2,3], as it is resistant to corrosion by seawater and can prevent colonization by marine organisms. Given the potential importance of these parts, it is not surprising that the surface failure of these parts caused by wear and corrosion can lead to substantial economic losses and disastrous consequences [4]. Obviously, it would be desirable to further improve the corrosion resistance of CueAl alloys. Several technologies have been developed to produce coatings with improved surface properties, including thermal spraying [5] and laser cladding [6]. Laser cladding is an advanced surface modification technology by which a powdered material is melted and consolidated on the substrate by use of a high energy-density laser beam as the heat source [7]. The coating has a metallurgical bonding with the substrate, and its composition and properties can be totally different from the substrate. It has become an established method for surface strengthening and additive manufacturing of high-value parts and components ⁎
Corresponding author. E-mail address: [email protected] (J. Yang).
https://doi.org/10.1016/j.surfcoat.2019.125215 Received 19 September 2019; Received in revised form 26 November 2019; Accepted 29 November 2019 Available online 30 November 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
reference electrode, and platinum wire electrode as the auxiliary electrode, and the scanning speed was 0.33 mV/s. The test was carried out in the electrolyte of 3.5% NaCl solution.
High power diode laser (HPDL) based cladding is found to be an economical process for additive manufacturing valuable parts. Nowadays, HPDL is successfully used in industry as a versatile heat source for the cladding process [14]. To our knowledge, there are few studies investigating the effect of the addition of Cr on the microstructures and corrosion resistance of CueAl coating deposited by high power Diode laser cladding. The alloy composition can be optimized quickly and economically by adding different content of Cr into powder material to prepare coatings. It is also an effective method to explore the influence of alloy elements on the microstructure and properties of coating materials. To address this problem, the effect of different amounts of Cr (0%, 1.5%, 4.5%, 7.5%) on the microstructure and corrosion resistance of CuAl10 coating is investigated in this research, and then the mechanism responsible for the substantial improvement in the corrosion resistance of the CuAl10 coatings is discussed.
3. Experimental results and discussion 3.1. Microstructure of laser cladding coatings The SEM micrographs of the coatings containing 0 wt%, 1.5 wt%, 4.5 wt%, and 7.5 wt% Cr are shown in Fig. 3. The aspect ratio (ratio of height and width) of one clad track is 42%–50%, and the coatings have a good metallurgical bonding with the substrate, with a geometrical dilution ratio of 3%–5%. The microstructure of Cr-1# aluminum bronze coating is significantly different from that of Cr addition coatings (Cr2# to Cr-4#), which is attributed to the change of the physical characteristics of the molten pool caused by the addition of Cr under the action of laser. According to the formula of Marangoni flow [15], Cr addition reduces the viscosity of liquid metal and the convection of molten pool，resulting in the less elements diffusion at the interface. The different areas of Cr-1# coating show a certain parallelism among them (like bands), which is due to the significant convection of liquid metal, forming a typical metallurgical characteristic of dynamic convection melting pool in the direction of laser scanning. A close comparison of these SEM micrographs shows that the addition of Cr leads to an increasing number of polygonal particles. The EDS results in Fig. 4 confirm that these particles are partially melted Cr particles containing about 96.42 wt% Cr and 3.58 wt% Al. It is thus concluded that with the addition of Cr, the small-sized Cr particles are completely melted, whereas the larger-sized particles were partially melted and became embedded in the coating during the laser cladding process. In order to study the microstructure evolution of Cr addition coatings, the morphology of different coating samples was compared and analyzed. Fig. 5 shows the SEM micrographs of the CueAl coatings near the interface. The fine dendritic crystals were formed during the laser cladding process with the rapid cooling rate and large supercooling. In addition, the convection in the laser-induced molten pool can also increase the nucleation rate. It is clear that with the Cr content increases, the large dendrites on the bottom of the coating are reduced in both number and size, whereas the small dendrites or rod-shaped microstructures begin to appear and increase in number. Fig. 6 shows the microstructures of the interface between CueAl coating and 17-4PH substrate of Cr-1# sample. The EDS analysis shows that the Fe content is approximately as high as 74.17 at.% in the interface region marked by a cross in Fig. 6, indicating a substantial diffusion of Fe from the substrate into the coating. However, as noted in Table 2, such diffusion is greatly restricted in Cr-2#, Cr-3#, and Cr-4# coatings. It thus can be concluded that the addition of Cr changes the
2. Experimental materials and method Laser cladding was performed using a TruDiode 4006 Diode fiber laser (Supplier: TRUMPF, wavelength 920–1040 nm, beam quality 30 mm·mrad), a double-hopper powder feeder (Sultzer Metco TWIN10C) and a D70 coaxial nozzle (Supplier: TRUMPF, focal length 300 mm, automatic adjustment of circular laser beam from 1 mm to 5 mm), as shown in Fig. 1. The aluminum-bronze (CuAl10) spherical powders (chemical composition was about 89.06 wt% Cu, 9.70 wt% Al, 1.14 wt% Fe, 0.10 others; particle size: 20–120 μm) mixed with pure Cr powders (purity: 99.95%; particle size: 10–100 μm) were deposited on a Ф100mm × 150 mm substrate of 17-4PH stainless steel, as shown in Fig. 2. The composite powders with different weight ratios of pure Cr particles to CuAl10 powder were pre-mixed and dried prior to laser cladding, and the setting of laser cladding parameters is based on previous research, as shown in Table 1. The powders were fed synchronously by TWIN10C powder feeder and D70 coaxial nozzle, with argon as the carrier and shielding gas. The metallographic samples were sectioned from the coatings by linear cutting machine. The cross-section of each coating was ground in turn with 320#, 800#, 1500#, 2000# abrasive papers, polished, etched with 100 ml distilled water containing 1 g FeCl3 and 20 ml HCl for 50–60 s, and ultrasonic cleaned in ethyl alcohol. The microstructures of the samples were observed under a scanning electron microscope (LEO1450 SEM) equipped with an energy dispersive X-ray spectrometer (EDS) for chemical analysis. Phase analysis was performed on an X-ray diffractometer (XRD, D8 ADVANCE). The polarization curve is measured using the CHI660D electrochemical workstation. The test samples were processed into round bar with a diameter of 2 mm. The samples were encapsulated and the test surface was polished. A three-electrode system was used, with the sample as the working electrode, saturated calomel electrode as the
Fig. 1. Laser additive equipment and D70 cladding head. 2
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 2. Laser clad aluminum bronze coatings on 17-4PH steel substrate.
probably formed by the β phase during rapid cooling. It is deduced that the addition of Cr leads to a reduction of dendrites near the fusion line of the coating, which is due to restricted diffusion of Fe in this case, and resulted in the nucleation reduction. However, an increasing number of flower- or rod-shaped FeeCr phases precipitated with increasing Cr content. Yet, there is no obvious regional distribution of FeeCr phase in the coating, which is related to the reduced convection in the molten pool after the addition of Cr. More spheroidal particles are present at the midsection and distributed non-uniformly. Some larger Cr particles were not completely melted, which hindering the effective convection of the molten pool and leading to a non-uniform distribution of the Fe-rich spheroidal particles. The formation of Fe-rich spherical particles is attributed to the liquid phase separation prior to solidification [16].
properties of the melts, resulting in less diffusion of Fe from the substrate into the laser clad coating. Besides, the unmelted Cr particles in the material will increase the resistance of convection, and further reducing the convection of molten pool and element diffusion at the interface. A line-scan was conducted across the fusion line from the coating to the 17-4PH substrate of Cr-3# sample, and the results are shown in Fig. 7. It shows that Al and Cu decreased, whereas Fe increased from the coating to the Fe-based alloy. But the distribution of Cr element appeared a peak which showed its precipitation at the interface. Cr content is significantly higher than that of 17-4PH matrix, so the interface is the area of Cr element segregation. The reason for this phenomenon is that Cr and iron-based alloy have good wettability and tend to form FeCr solid solution. Overall, there existed a gradient transition region in the interface, which was indicative of certain mutual dissolution and diffusion between the coating material and the Fe-based alloy. In addition, because of the metallurgical bonding between chromium particles and copper alloys, particle detachment was very difficult to occur under the service conditions. Fig. 8 shows that the laser clad CuAl10 (Cr-1#) coating is mainly composed of α solid solution and a small amount of non-equilibrium dendritic α + γ2 phase. The dendrites (chemical composition was about 17.37 wt% Al, 46.69 wt% Fe, and 35.94 wt% Cu) are formed mainly by the nucleation and growth of Fe in the supercooling molten pool. The dendritic microstructures are clearly observed near the substrate, and the equiaxed crystals of about 2–10 μm. The spherical morphology is dominant at the midsection of the coating, which is formed primarily by the diffusion of Fe from the substrate into the coating. The gray particle marked by A in Fig. 8(b) is rich in Fe, the matrix marked by B is Cu-rich α phase, and the black needle-shaped particles dispersedly distributed in the coating are AlFe3 (K). A significant microstructural change occurs with the addition of Cr. As a result, more unmelted Cr particles appear in the coating, and spherical, rod-shaped, and dendritic microstructures are on the increase. Table 3 shows that with the addition of Cr, Fe is partly bonded with Cr and forms solid solution with Cu and Al, and precipitate with spherical, flower-shaped, or particles with irregular morphology, as indicated by C, D, E in Fig. 9, respectively. In the gray region marked by F, the atomic ratio of Cu to Al is 3:1, which should be a solid solution based on Cu3Al. The EDS analysis reveals that it is metastable β′ phase
3.2. Phase analysis of the cladding coatings Fig. 10 shows that the main phases in the Cr-1# coating include αCu, α-Fe, AlFe3, and Cu9Al4, whereas those of the Cr-3# coating include α-Cu, Cubic-Cr, and Cu3Al. This is consistent with the results of morphology and composition analysis. The α phase is Cu-based substitutional solid solution; the β phase is a Cu3Al-based solid solution with a body-centered cubic structure, which can be converted to metastable β′ phase under rapid cooling; the γ2 phase is a Cu9Al4 electronic compound based solid solution with a complex cubic lattice; and the K phase is an intermetallic compound, AlFe3. The metallurgical and phase change process of CueAl alloy during the laser cladding was analyzed. The results show that the Cr-1# coating contains approximately 20.36 at.% Al. β phase is formed first at high temperature, and transformed into α and γ2 phase by the eutectoid reaction with decreasing temperature. A high Fe content in the coating can inhibit the formation and netting of γ2 phase and refine the gains. As the Cr-1# coating contains more than 4% Fe, it is not completely in solid solution of α phase, but precipitated as dendritic or spheroidal particles. With the addition of Cr, Fe is not precipitated as α-Fe, but bonded with Cr to form a solid solution. It is evident that the Cr addition has a significant effect on the microstructures of the coatings, which is characterized by the increase of β′ phase and decrease of γ2 phase, presence of unmelted Cr particles, and a significant reduction of the α-Fe and K phase. This is because the
Table 1 Powder composition and laser cladding parameters. Sample No.
Cr (wt%)
CuAl10 (wt%)
Laser power (W)
Laser beam diameter (mm)
Scanning speed (mm/min)
Powder feeding rate (g/min)
Overlapping ratio
Cr-1# Cr-2# Cr-3# Cr-4#
0 1.5 4.5 7.5
100 98.5 95.5 92.5
1500
3.5
300
16
50%
3
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 3. SEM micrographs of the CuAl10 coatings with different Cr contents: (a) Cr-1#; (b) Cr-2#; (c) Cr-3#; (d) Cr-4#.
means of Faraday's Law of Electrolysis. It is known that the smaller the corrosion current density, the better the corrosion resistance. Fig. 11 and Table 4 show that the addition of Cr leads to a substantial reduction in the corrosion current density and an enhancement of passivation, indicating that a passive state can be easily achieved with the addition of Cr, and that the passivating film with a lower dissolution rate has a better corrosion resistance. The polarization curves of Cr-1# and Cr-2# coatings coincide to a large extent, indicating that the passivation current decreases and the passivation increases with increasing Cr content. It is also observed that the corrosion potential increases dramatically in Cr-4# coating. An overall consideration of corrosion current density, corrosion potential, and passivation ability allows us to conclude that the addition of Cr can effectively improve the corrosion resistance of CuAl10 coatings.
diffusion of Fe from the substrate into the coating is restricted, and because Fe which has diffused into the coating is precipitated in solid solution with melted Cr. However, it is not indicated in the XRD patterns due to its low content. 3.3. Corrosion resistance of the cladding coatings The polarization curve measurements were performed using the CHI660D electrochemical workstation, and the corrosion resistance of each sample in 3.5% NaCl solution is assessed. The polarization curves for the different samples are shown in Fig. 11. The electrochemical results obtained by Tafel curve extrapolation are shown in Table 4. Corrosion current density is used to determine the corrosion rate by
(a)
(b) Fig. 4. SEM micrograph of the Cr particle (a) and EDS analysis. 4
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 5. Micrographs of the coating near the interface: (a) Cr-1#; (b) Cr-2#; (c) Cr-3#; (d) Cr-4#.
Fig. 6. Micrograph of the interface of Cr-1# sample.
Fig. 7. Element distributions from the CuAl10 coating to the 17-4PH substrate in Cr-3# sample.
Table 2 Chemical composition for different samples. Sample
Cr-1# Cr-2# Cr-3# Cr-4#
Al
Cr
wt%
at.%
9.94 ± 0.05 11.35 ± 0.06 11.73 ± 0.06 9.38 ± 0.05
20.36 23.05 23.61 19.19
± ± ± ±
0.10 0.12 0.12 0.10
Fe
Cu
wt%
at.%
wt%
at.%
wt%
0 0.98 ± 0.01 3.77 ± 0.02 7.48 ± 0.04
0 1.03 ± 0.01 3.94 ± 0.02 7.94 ± 0.04
11.13 ± 0.06 2.96 ± 0.02 1.99 ± 0.01 5.50 ± 0.03
11.02 ± 0.06 2.90 ± 0.01 1.94 ± 0.01 5.43 ± 0.03
78.92 84.71 82.51 77.64
5
at.% ± ± ± ±
0.40 0.42 0.41 0.39
68.62 73.02 70.52 67.43
± ± ± ±
0.34 0.37 0.35 0.34
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 8. SEM micrographs of Cr-1# coating. (a) Dendritic microstructures at the bottom. (b) Spherical and needle-shaped microstructures at the midsection.
Table 3 Chemical composition in different regions of the CuAl alloy. Region
A B C D E F
Al
Cr
Fe
Cu
wt%
at.%
wt%
at.%
wt%
at.%
wt%
at.%
8.62 ± 0.04 10.15 ± 0.05 4.91 ± 0.03 9.06 ± 0.05 5.56 ± 0.03 10.88 ± 0.05
16.75 ± 0.08 20.92 ± 0.10 9.46 ± 0.08 17.54 ± 0.09 10.68 ± 0.05 22.08 ± 0.11
0 0 38.11 ± 0.19 21.48 ± 0.10 37.77 ± 0.19 3.75 ± 0.02
0 0 38.11 ± 0.19 21.59 ± 0.11 37.63 ± 0.19 3.95 ± 0.02
69.50 ± 0.30 4.10 ± 0.02 51.25 ± 0.26 33.04 ± 0.17 48.88 ± 0.24 3.19 ± 0.02
65.22 ± 0.32 4.08 ± 0.02 47.73 ± 0.24 30.92 ± 0.16 45.35 ± 0.23 3.13 ± 0.02
21.87 ± 0.11 85.74 ± 0.43 5.73 ± 0.03 36.42 ± 0.18 7.78 ± 0.04 82.18 ± 0.41
18.04 ± 0.09 75.00 ± 0.38 4.69 ± 0.02 29.95 ± 0.15 6.34 ± 0.03 70.84 ± 0.35
Fig. 9. Typical microstructures of laser clad coating with Cr addition (Cr-3# sample).
Cu + Cl− → CuCl + e E(SHE) = 0.124 V
3.4. Corrosion mechanism 3.4.1. Electrochemical process In the corrosion experiments, a white flocculent precipitate (Al (OH)3) was formed on the surface of the Cr-1# sample in the NaCl solution. The solution itself becomes yellow, due to the presence of Fe oxides. A large amount of white precipitate and a small amount of yellow precipitate were observed on the bottom of the electrolytic chamber. The element with more negative electrode potential is more likely to lose electrons, thus the preferential corrosion of Al and Fe is mainly caused by the potential difference between Cu, Al, and Fe. The preferential electrode reaction at the anode is judged by the electrode potential and ions at the solution electrode. The anodic polarization of the Cr-1# coating and corresponding standard electrode potential are:
Al → Al3 + + 3e E(SHE) = –1.67 V Fe → Fe 2 + + 2e E(SHE) = –0.441 V
(3)
The cathodic polarization is:
1/2O2 + H2 O + 2e → 2OH−
(4)
When Cr is added, Fe is mostly bonded with Cr and forms solid solution with Cu and Al, and some Cr particles are not melted. There is only a slight difference in the density between Cr and CuAl10 coatings (7.14 and 7.60 g/cm3, respectively), Cr near the surface also participates in the anodic polarization:
Cr → Cr 3 + + 3e E(SHE) = –0.71 V
(5)
Then ions at the anode undergo further reaction and oxidation in the Cl− environment: (1)
(2)
6
CuCl + Cl− → CuCl2−
(6)
CuCl2− + H2 O → Cu (OH )2 ↓ + 2HCl + e
(7)
Cr 3 + + 3H2 O → Cr (OH )3 ↓ + 3H+
(8)
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 10. XRD patterns (a) and partially enlarged patterns (b) of Cr-1# and Cr-3# samples.
For the Cr-1# sample, the anodic polarization of a small fraction of Al and Fe initially occurs at a high rate, and then Cu participates in the reaction and forms Cu(OH)2, which is consistent with the active-passive transition charted in Fig. 11. A protective film is formed on the coating surface, which can prevent electrons and ions from reaching the alloy and effectively reduces the corrosion rate of the alloy, and these passivating phenomena can be found in the polarization curves [17]. It is evident that the formation of Cr(OH)3 contributes to the improved corrosion resistance. However, as only a small amount of Cr is added, it is impossible to form a dense protective film on the surface to inhibit further growth of corrosion processes. Accordingly, Al(OH)3, Fe (OH)3 and Cu(OH)2 will continue to be formed, and these corrosion products are cubic lattice of p-type semiconductor in which Cu2+, Cr3+, O2− can penetrate each other, therefore the passivating film becomes less effective in preventing the penetration of ions. Cl ions can displace O ions from the corrosion products and produce some new holes in solutions containing Cl ions, which may reduce the protective effect of the film and accelerate the corrosion rate [18]. However, Cr can be embedded in the vacancies and dislocations of the film to block the diffusion channel for ions, making it difficult for ions to diffuse into the corrosion interface, improving the corrosion resistance of the coating.
Fig. 11. Polarization curves of the laser clad CuAl10 coatings with different Cr additions.
Al3 + + 3H2 O → Al (OH )3 ↓ + 3H+ Fe 2 +
→
Fe3 +
+e
Fe3 + + 3H2 O → Fe (OH )3 ↓ + 3H+
(9) 3.4.2. The effect of phase on the corrosion resistance The γ2 phase acts as the anode and the α phase is the cathode, which provides the channel for the corrosion of eutectic structures. The
(10) (11) 7
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Gaosheng Wang: Laser cladding experiment. Lei Yang: Microstructure analysis. Shengfeng Zhou: Microstructure analysis, paper polish. Jianbo Lei: Metallurgical mechanism analysis.
Table 4 Electrochemical test results. Sample
Ecorr (mV)
Cr-1# Cr-2# Cr-3# Cr-4#
−345 −375 −345 −293
± ± ± ±
3 4 3 2
Icorr (10−6 A·cm−2)
Rp (104 Ω·cm2)
2.014 1.522 1.354 1.846
2.653 2.949 3.649 2.957
± ± ± ±
0.021 0.016 0.014 0.019
± ± ± ±
0.028 0.032 0.037 0.030
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
diffusion of Fe from the substrate into the coating can inhibit the formation and netting of γ2 phase and refine the grains. However, it is suggested that a large amount of Fe can reduce corrosion current, and thus have a negative effect on the corrosion resistance. The addition of Cr results in an increase of β´ phase and decrease of K and γ2 phase. Both K and γ2 phases are Al-rich phases, and as a result, the corrosion resistance is greatly improved with the addition of Cr. It can also effectively reduce the magnitude of the stress associated with the dislocation pile-up at the grain boundaries and delay the formation of cracks, thus it plays a role in grain refinement. The addition of Cr increases the passivation ability, and as a result the corrosion resistance increases. However, it is also noted that the corrosion current density is slightly reduced in the Cr-4# sample, which is related to the non-uniform distribution of the unmelted Cr particles and the increased diffusion of Fe from the substrate into the cladding coatings. In general, the corrosion resistance of the coatings with different Cr content has been improved. Cr-4# has a high Cr addition, one part is melted into the copper alloy, the other part exists in the coating in a semi-molten state. Cr completely melted into the copper alloy improves the corrosion resistance of the coating. Although Cr has high corrosion resistance, Cr particles and the coating constitute the primary battery, which to some extent affects the improvement of corrosion resistance.
Acknowledgments The research was supported by the National Natural Science Foundation of China (50971004) and the National High-tech Research and Development Program (2011CB606305-3), China. References [1] B.S. Yilbas, A. Matthews, A. Leyland, C. Karatas, S.S. Akhtar, B.J. Abdul Aleem, Laser surface modification treatment of aluminum bronze with B4C, Appl. Surf. Sci. 263 (2012) 804–809. [2] Qin Luo, Wu Zhong, Zhenbo Qin, Lei Liu, Hu Wenbin, Surface modification of nickel-aluminum bronze alloy with gradient Ni-Cu solid solution coating via thermal diffusion, Surf. Coat. Technol. 309 (2017) 106–113. [3] R.T. Byrnes, S.P. Lynch, An unusual failure of a nickel-aluminium bronze (NAB) hydraulic valve, Eng. Fail. Anal. 49 (2015) 122–136. [4] R.P. Chen, Z.Q. Liang, W.W. Zhang, D.T. Zhang, Z.Q. Luo, Y.Y. Li, Effect of heat treatment on microstructure and properties of hot-extruded nickel-aluminum bronze, Trans. Nonferrous Metals Soc. China 17 (2007) 1254–1258. [5] Ruben Winkler, Erik Saborowski, Gerd Paczkowski, Thomas Grund, Thomas Lampke, Characterization of thermally sprayed copper and numerically supported residual stress determination by the incremental hole-drilling method, Surf. Coat. Technol. 371 (2019) 255–261. [6] Jari Tuominen, Marc Kaubisch, Sebastian Thieme, Jonne Nakki, Steffen Nowotny, Petri Vuoristo, Laser strip cladding for large area metal deposition, Addit. Manuf. 27 (2019) 208–216. [7] R.A. Rahman Rashid, K.A. Nazari, C. Barr, S. Palanisamy, N. Orchowski, N. Matthews, M.S. Dargusch, Effect of laser reheat post-treatment on the microstructural characteristics of laser-cladded ultra-high strength steel, Surf. Coat. Technol. 372 (2019) 93–102. [8] Benjamin Bax, Rohan Rajput, Richard Kellet, Martin Reisacher, Systematic evaluation of process parameter maps for laser cladding and directed energy deposition, Addit. Manuf. 21 (2018) 487–494. [9] O.G. Devojno, E. Feldshtein, M.A. Kardapolava, N.I. Lutsko, On the formation features, structure, microhardness and tribological behavior of single tracks and coating layers formed by laser cladding of Al-Fe powder bronze, Surf. Coat. Technol. 358 (2019) 195–206. [10] Y.N. Zhang, J.L. Zi, M.S. Zheng, C.Z. Xu, Influence of Cr micro-addition on the corrosion resistance of copper alloy, Rare Metal Mater. Eng. 36 (3) (2007) 49–52 (in Chinese). [11] B. Liu, L. Liu, The effect of microalloying on thermal stability and corrosion resistance of Cu-based bulk metallic glasses, Mater. Sci. Eng. A 415 (2006) 286–290. [12] W.S. Jeon, C.C. Shur, J.G. Kim, S.Z. Han, Y.S. Kim, Effect of Cr on the corrosion resistance of Cu–6Ni–4Sn alloys, J. Alloys Compd. 455 (2008) 358–363. [13] X.P. Tao, S. Zhang, C.H. Zhang, C.L. Wu, J. Chen, Adil O. Abdullah, Effect of Fe and Ni contents on microstructure and wear resistance of aluminum bronze coatings on 316 stainless steel by laser cladding, Surf. Coat. Technol. 342 (2018) 76–84. [14] Mahmoud Z. Ibrahim, Ahmed A.D. Sarhan, T.Y. Kuo, Farazila Yusof, M. Hamdi, Characterization and hardness enhancement of amorphous Fe-based metallic glass laser cladded on nickel-free stainless steel for biomedical implant application, Mater. Chem. Phys. 235 (2019) 121745. [15] Bo Li, Lei Zhang, Yi Xu, Zhiyuan Liu, Bo Qian, Fuzhen Xuan, Selective laser melting of CoCrFeNiMn high entropy alloy powder modified with nano-TiN particles for additive manufacturing and strength enhancement: process, particle behavior and effects, Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.068. [16] Soundarapandian Santhanakrishnan, Fanrong Kong, Radovan Kovacevic, An experimentally based thermo-kinetic hardening model for high power direct diode laser cladding, J. Mater. Process. Technol. 211 (2011) 1247–1259. [17] M. Scendo, J. Uznanska, The effect of ionic liquids on the corrosion inhibition of copper in acidic chloride solutions, Int. J. Corros. 2011 (2011) 1–13. [18] Z.Q. Cao, J. Bian, R. Xue, W.H. Liu, Electrochemical corrosion behavior of Cu-40Ni20Cr alloys with different grain sizes in solutions containing chloride ions, Trans. Nonferrous Metals Soc. China 17 (2007) 1236–1241.
4. Conclusions This research investigated the effect of different amounts of Cr (0 wt %, 1.5 wt%, 4.5 wt%, 7.5 wt%) on the microstructures and corrosion resistance of CuAl10 coating deposited on 17-4PH stainless steel substrate by Diode laser cladding. Several conclusions can be drawn from the research: (1) The CuAl10 coating has an excellent metallurgical bonding with the substrate. The addition of Cr changes the physical properties of the melts, resulting in less diffusion of Fe from the substrate into the cladding coating. (2) Metallurgical reaction in the molten pool is complex due to the inherent characteristics of laser cladding, such as rapid heating and cooling. As a result, the principal phases of CuAl10 coating without Cr addition include α, γ2, and K phase, whereas that with Cr addition include α, Cr particles, and β′ phase. (3) Cr can effectively improve the corrosion resistance of CuAl10 coatings, and the passivation ability of the coatings appears to increase with increasing Cr contents. This is because Cr can promote the formation of the passivating film and reduce the diffusion of Fe from the substrate into the coatings, so that the formation of γ2 and K phase is reduced. Author contributions Jiaoxi Yang: Experimental scheme and material design, paper writing. Feiyu Wu: Laser cladding experiment, paper writing. Bing Bai: Electrochemical test and analysis.
8
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of Cr additions on the microstructure and corrosion resistance of Diode laser clad CuAl10 coating
T
⁎
Jiaoxi Yanga, , Feiyu Wua, Bing Baia, Gaosheng Wanga, Lei Yanga, Shengfeng Zhoub, Jianbo Leib a b
Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, PR China Laser Technology Institute, Tianjin Polytechnic University, Tianjin 300387, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser cladding Aluminum bronze Cr additions Corrosion resistance
The effect of Cr additions on the microstructure and corrosion resistance of CuAl10 coating deposited by laser cladding was investigated. Laser cladding was performed using a 4000 W Diode laser under the same conditions, and the premixed powders with different weight ratios of CuAl10 to Cr were fed synchronously by TWIN10C powder feeder and D70 coaxial nozzle. The microstructures were observed under a scanning electron microscope equipped with an energy dispersive X-ray spectrometer (EDS) for chemical analysis, and phase analysis was performed with an X-ray diffractometer (XRD). The electrochemical corrosion behaviors of cladding layers were evaluated. The results showed that the CuAl10 coating has a good metallurgical bonding with the 17-4PH substrate. The addition of Cr can improve the corrosion resistance of CuAl10 coatings, and the passivation ability of the coatings increases with increasing Cr contents. The CuAl10 coating without Cr addition include α, γ2, and K phase; however, the addition of Cr leads to an increase of β′ phase and Cr and a decrease of γ2 and K phase in the CuAl10 coating.
1. Introduction
[8]. In the process of manufacturing new structural parts by laser cladding aluminum bronze material on the surface of iron base alloy, a certain amount of iron diffuses from the matrix into the coating [9], which will reduce the corrosion resistance of the coating. A possible way to solve this problem is by adding alloying elements to the alloy materials. Adding chromium element can improve the corrosion resistance of materials. In the similar research of adding Cr to improve the properties of materials, Zhang et al. [10] studied the effect of Cr on the corrosion resistance of cold-rolled copper alloy, and the results showed that the corrosion resistance of CueCr alloy increased with the increase of Cr contents. Liu et al. [11] found that (Cu47Zr11Ti34Ni8)99.5M0.5 (M = 0, Cr, Mo and W) bulk metallic glasses had an excellent corrosion resistance with the presence of a small amount of Cr. Jeon et al. [12] found that the addition of Cr could prevent the localized corrosion behavior of Cu-6%Ni-4%Sn-x% alloys caused by decreased Sn-rich precipitates in these alloys. These precipitates induced galvanic corrosion due to the difference in chemical composition. As for Cu-based alloy coatings deposited by laser cladding, the effect of different iron and nickel contents on wear resistance of aluminum bronze coatings was investigated by X. P. Tao [13]. This highlights the need for an understanding of the effects and underlying mechanism of the alloying elements added on the corrosion resistance of the coating.
Aluminum bronze is a Cu-based alloy with Al as the major alloying element, and it is the material of choice for a wide variety of applications due to its excellent physical, mechanical and tribological properties [1], such as high strength, thermal conductivity and excellent resistance to wear and corrosion. It is particularly suitable for the corrosion-resistant parts working in the marine environment, such as propellers, valves, and pipes [2,3], as it is resistant to corrosion by seawater and can prevent colonization by marine organisms. Given the potential importance of these parts, it is not surprising that the surface failure of these parts caused by wear and corrosion can lead to substantial economic losses and disastrous consequences [4]. Obviously, it would be desirable to further improve the corrosion resistance of CueAl alloys. Several technologies have been developed to produce coatings with improved surface properties, including thermal spraying [5] and laser cladding [6]. Laser cladding is an advanced surface modification technology by which a powdered material is melted and consolidated on the substrate by use of a high energy-density laser beam as the heat source [7]. The coating has a metallurgical bonding with the substrate, and its composition and properties can be totally different from the substrate. It has become an established method for surface strengthening and additive manufacturing of high-value parts and components ⁎
Corresponding author. E-mail address: [email protected] (J. Yang).
https://doi.org/10.1016/j.surfcoat.2019.125215 Received 19 September 2019; Received in revised form 26 November 2019; Accepted 29 November 2019 Available online 30 November 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
reference electrode, and platinum wire electrode as the auxiliary electrode, and the scanning speed was 0.33 mV/s. The test was carried out in the electrolyte of 3.5% NaCl solution.
High power diode laser (HPDL) based cladding is found to be an economical process for additive manufacturing valuable parts. Nowadays, HPDL is successfully used in industry as a versatile heat source for the cladding process [14]. To our knowledge, there are few studies investigating the effect of the addition of Cr on the microstructures and corrosion resistance of CueAl coating deposited by high power Diode laser cladding. The alloy composition can be optimized quickly and economically by adding different content of Cr into powder material to prepare coatings. It is also an effective method to explore the influence of alloy elements on the microstructure and properties of coating materials. To address this problem, the effect of different amounts of Cr (0%, 1.5%, 4.5%, 7.5%) on the microstructure and corrosion resistance of CuAl10 coating is investigated in this research, and then the mechanism responsible for the substantial improvement in the corrosion resistance of the CuAl10 coatings is discussed.
3. Experimental results and discussion 3.1. Microstructure of laser cladding coatings The SEM micrographs of the coatings containing 0 wt%, 1.5 wt%, 4.5 wt%, and 7.5 wt% Cr are shown in Fig. 3. The aspect ratio (ratio of height and width) of one clad track is 42%–50%, and the coatings have a good metallurgical bonding with the substrate, with a geometrical dilution ratio of 3%–5%. The microstructure of Cr-1# aluminum bronze coating is significantly different from that of Cr addition coatings (Cr2# to Cr-4#), which is attributed to the change of the physical characteristics of the molten pool caused by the addition of Cr under the action of laser. According to the formula of Marangoni flow [15], Cr addition reduces the viscosity of liquid metal and the convection of molten pool，resulting in the less elements diffusion at the interface. The different areas of Cr-1# coating show a certain parallelism among them (like bands), which is due to the significant convection of liquid metal, forming a typical metallurgical characteristic of dynamic convection melting pool in the direction of laser scanning. A close comparison of these SEM micrographs shows that the addition of Cr leads to an increasing number of polygonal particles. The EDS results in Fig. 4 confirm that these particles are partially melted Cr particles containing about 96.42 wt% Cr and 3.58 wt% Al. It is thus concluded that with the addition of Cr, the small-sized Cr particles are completely melted, whereas the larger-sized particles were partially melted and became embedded in the coating during the laser cladding process. In order to study the microstructure evolution of Cr addition coatings, the morphology of different coating samples was compared and analyzed. Fig. 5 shows the SEM micrographs of the CueAl coatings near the interface. The fine dendritic crystals were formed during the laser cladding process with the rapid cooling rate and large supercooling. In addition, the convection in the laser-induced molten pool can also increase the nucleation rate. It is clear that with the Cr content increases, the large dendrites on the bottom of the coating are reduced in both number and size, whereas the small dendrites or rod-shaped microstructures begin to appear and increase in number. Fig. 6 shows the microstructures of the interface between CueAl coating and 17-4PH substrate of Cr-1# sample. The EDS analysis shows that the Fe content is approximately as high as 74.17 at.% in the interface region marked by a cross in Fig. 6, indicating a substantial diffusion of Fe from the substrate into the coating. However, as noted in Table 2, such diffusion is greatly restricted in Cr-2#, Cr-3#, and Cr-4# coatings. It thus can be concluded that the addition of Cr changes the
2. Experimental materials and method Laser cladding was performed using a TruDiode 4006 Diode fiber laser (Supplier: TRUMPF, wavelength 920–1040 nm, beam quality 30 mm·mrad), a double-hopper powder feeder (Sultzer Metco TWIN10C) and a D70 coaxial nozzle (Supplier: TRUMPF, focal length 300 mm, automatic adjustment of circular laser beam from 1 mm to 5 mm), as shown in Fig. 1. The aluminum-bronze (CuAl10) spherical powders (chemical composition was about 89.06 wt% Cu, 9.70 wt% Al, 1.14 wt% Fe, 0.10 others; particle size: 20–120 μm) mixed with pure Cr powders (purity: 99.95%; particle size: 10–100 μm) were deposited on a Ф100mm × 150 mm substrate of 17-4PH stainless steel, as shown in Fig. 2. The composite powders with different weight ratios of pure Cr particles to CuAl10 powder were pre-mixed and dried prior to laser cladding, and the setting of laser cladding parameters is based on previous research, as shown in Table 1. The powders were fed synchronously by TWIN10C powder feeder and D70 coaxial nozzle, with argon as the carrier and shielding gas. The metallographic samples were sectioned from the coatings by linear cutting machine. The cross-section of each coating was ground in turn with 320#, 800#, 1500#, 2000# abrasive papers, polished, etched with 100 ml distilled water containing 1 g FeCl3 and 20 ml HCl for 50–60 s, and ultrasonic cleaned in ethyl alcohol. The microstructures of the samples were observed under a scanning electron microscope (LEO1450 SEM) equipped with an energy dispersive X-ray spectrometer (EDS) for chemical analysis. Phase analysis was performed on an X-ray diffractometer (XRD, D8 ADVANCE). The polarization curve is measured using the CHI660D electrochemical workstation. The test samples were processed into round bar with a diameter of 2 mm. The samples were encapsulated and the test surface was polished. A three-electrode system was used, with the sample as the working electrode, saturated calomel electrode as the
Fig. 1. Laser additive equipment and D70 cladding head. 2
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 2. Laser clad aluminum bronze coatings on 17-4PH steel substrate.
probably formed by the β phase during rapid cooling. It is deduced that the addition of Cr leads to a reduction of dendrites near the fusion line of the coating, which is due to restricted diffusion of Fe in this case, and resulted in the nucleation reduction. However, an increasing number of flower- or rod-shaped FeeCr phases precipitated with increasing Cr content. Yet, there is no obvious regional distribution of FeeCr phase in the coating, which is related to the reduced convection in the molten pool after the addition of Cr. More spheroidal particles are present at the midsection and distributed non-uniformly. Some larger Cr particles were not completely melted, which hindering the effective convection of the molten pool and leading to a non-uniform distribution of the Fe-rich spheroidal particles. The formation of Fe-rich spherical particles is attributed to the liquid phase separation prior to solidification [16].
properties of the melts, resulting in less diffusion of Fe from the substrate into the laser clad coating. Besides, the unmelted Cr particles in the material will increase the resistance of convection, and further reducing the convection of molten pool and element diffusion at the interface. A line-scan was conducted across the fusion line from the coating to the 17-4PH substrate of Cr-3# sample, and the results are shown in Fig. 7. It shows that Al and Cu decreased, whereas Fe increased from the coating to the Fe-based alloy. But the distribution of Cr element appeared a peak which showed its precipitation at the interface. Cr content is significantly higher than that of 17-4PH matrix, so the interface is the area of Cr element segregation. The reason for this phenomenon is that Cr and iron-based alloy have good wettability and tend to form FeCr solid solution. Overall, there existed a gradient transition region in the interface, which was indicative of certain mutual dissolution and diffusion between the coating material and the Fe-based alloy. In addition, because of the metallurgical bonding between chromium particles and copper alloys, particle detachment was very difficult to occur under the service conditions. Fig. 8 shows that the laser clad CuAl10 (Cr-1#) coating is mainly composed of α solid solution and a small amount of non-equilibrium dendritic α + γ2 phase. The dendrites (chemical composition was about 17.37 wt% Al, 46.69 wt% Fe, and 35.94 wt% Cu) are formed mainly by the nucleation and growth of Fe in the supercooling molten pool. The dendritic microstructures are clearly observed near the substrate, and the equiaxed crystals of about 2–10 μm. The spherical morphology is dominant at the midsection of the coating, which is formed primarily by the diffusion of Fe from the substrate into the coating. The gray particle marked by A in Fig. 8(b) is rich in Fe, the matrix marked by B is Cu-rich α phase, and the black needle-shaped particles dispersedly distributed in the coating are AlFe3 (K). A significant microstructural change occurs with the addition of Cr. As a result, more unmelted Cr particles appear in the coating, and spherical, rod-shaped, and dendritic microstructures are on the increase. Table 3 shows that with the addition of Cr, Fe is partly bonded with Cr and forms solid solution with Cu and Al, and precipitate with spherical, flower-shaped, or particles with irregular morphology, as indicated by C, D, E in Fig. 9, respectively. In the gray region marked by F, the atomic ratio of Cu to Al is 3:1, which should be a solid solution based on Cu3Al. The EDS analysis reveals that it is metastable β′ phase
3.2. Phase analysis of the cladding coatings Fig. 10 shows that the main phases in the Cr-1# coating include αCu, α-Fe, AlFe3, and Cu9Al4, whereas those of the Cr-3# coating include α-Cu, Cubic-Cr, and Cu3Al. This is consistent with the results of morphology and composition analysis. The α phase is Cu-based substitutional solid solution; the β phase is a Cu3Al-based solid solution with a body-centered cubic structure, which can be converted to metastable β′ phase under rapid cooling; the γ2 phase is a Cu9Al4 electronic compound based solid solution with a complex cubic lattice; and the K phase is an intermetallic compound, AlFe3. The metallurgical and phase change process of CueAl alloy during the laser cladding was analyzed. The results show that the Cr-1# coating contains approximately 20.36 at.% Al. β phase is formed first at high temperature, and transformed into α and γ2 phase by the eutectoid reaction with decreasing temperature. A high Fe content in the coating can inhibit the formation and netting of γ2 phase and refine the gains. As the Cr-1# coating contains more than 4% Fe, it is not completely in solid solution of α phase, but precipitated as dendritic or spheroidal particles. With the addition of Cr, Fe is not precipitated as α-Fe, but bonded with Cr to form a solid solution. It is evident that the Cr addition has a significant effect on the microstructures of the coatings, which is characterized by the increase of β′ phase and decrease of γ2 phase, presence of unmelted Cr particles, and a significant reduction of the α-Fe and K phase. This is because the
Table 1 Powder composition and laser cladding parameters. Sample No.
Cr (wt%)
CuAl10 (wt%)
Laser power (W)
Laser beam diameter (mm)
Scanning speed (mm/min)
Powder feeding rate (g/min)
Overlapping ratio
Cr-1# Cr-2# Cr-3# Cr-4#
0 1.5 4.5 7.5
100 98.5 95.5 92.5
1500
3.5
300
16
50%
3
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 3. SEM micrographs of the CuAl10 coatings with different Cr contents: (a) Cr-1#; (b) Cr-2#; (c) Cr-3#; (d) Cr-4#.
means of Faraday's Law of Electrolysis. It is known that the smaller the corrosion current density, the better the corrosion resistance. Fig. 11 and Table 4 show that the addition of Cr leads to a substantial reduction in the corrosion current density and an enhancement of passivation, indicating that a passive state can be easily achieved with the addition of Cr, and that the passivating film with a lower dissolution rate has a better corrosion resistance. The polarization curves of Cr-1# and Cr-2# coatings coincide to a large extent, indicating that the passivation current decreases and the passivation increases with increasing Cr content. It is also observed that the corrosion potential increases dramatically in Cr-4# coating. An overall consideration of corrosion current density, corrosion potential, and passivation ability allows us to conclude that the addition of Cr can effectively improve the corrosion resistance of CuAl10 coatings.
diffusion of Fe from the substrate into the coating is restricted, and because Fe which has diffused into the coating is precipitated in solid solution with melted Cr. However, it is not indicated in the XRD patterns due to its low content. 3.3. Corrosion resistance of the cladding coatings The polarization curve measurements were performed using the CHI660D electrochemical workstation, and the corrosion resistance of each sample in 3.5% NaCl solution is assessed. The polarization curves for the different samples are shown in Fig. 11. The electrochemical results obtained by Tafel curve extrapolation are shown in Table 4. Corrosion current density is used to determine the corrosion rate by
(a)
(b) Fig. 4. SEM micrograph of the Cr particle (a) and EDS analysis. 4
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 5. Micrographs of the coating near the interface: (a) Cr-1#; (b) Cr-2#; (c) Cr-3#; (d) Cr-4#.
Fig. 6. Micrograph of the interface of Cr-1# sample.
Fig. 7. Element distributions from the CuAl10 coating to the 17-4PH substrate in Cr-3# sample.
Table 2 Chemical composition for different samples. Sample
Cr-1# Cr-2# Cr-3# Cr-4#
Al
Cr
wt%
at.%
9.94 ± 0.05 11.35 ± 0.06 11.73 ± 0.06 9.38 ± 0.05
20.36 23.05 23.61 19.19
± ± ± ±
0.10 0.12 0.12 0.10
Fe
Cu
wt%
at.%
wt%
at.%
wt%
0 0.98 ± 0.01 3.77 ± 0.02 7.48 ± 0.04
0 1.03 ± 0.01 3.94 ± 0.02 7.94 ± 0.04
11.13 ± 0.06 2.96 ± 0.02 1.99 ± 0.01 5.50 ± 0.03
11.02 ± 0.06 2.90 ± 0.01 1.94 ± 0.01 5.43 ± 0.03
78.92 84.71 82.51 77.64
5
at.% ± ± ± ±
0.40 0.42 0.41 0.39
68.62 73.02 70.52 67.43
± ± ± ±
0.34 0.37 0.35 0.34
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 8. SEM micrographs of Cr-1# coating. (a) Dendritic microstructures at the bottom. (b) Spherical and needle-shaped microstructures at the midsection.
Table 3 Chemical composition in different regions of the CuAl alloy. Region
A B C D E F
Al
Cr
Fe
Cu
wt%
at.%
wt%
at.%
wt%
at.%
wt%
at.%
8.62 ± 0.04 10.15 ± 0.05 4.91 ± 0.03 9.06 ± 0.05 5.56 ± 0.03 10.88 ± 0.05
16.75 ± 0.08 20.92 ± 0.10 9.46 ± 0.08 17.54 ± 0.09 10.68 ± 0.05 22.08 ± 0.11
0 0 38.11 ± 0.19 21.48 ± 0.10 37.77 ± 0.19 3.75 ± 0.02
0 0 38.11 ± 0.19 21.59 ± 0.11 37.63 ± 0.19 3.95 ± 0.02
69.50 ± 0.30 4.10 ± 0.02 51.25 ± 0.26 33.04 ± 0.17 48.88 ± 0.24 3.19 ± 0.02
65.22 ± 0.32 4.08 ± 0.02 47.73 ± 0.24 30.92 ± 0.16 45.35 ± 0.23 3.13 ± 0.02
21.87 ± 0.11 85.74 ± 0.43 5.73 ± 0.03 36.42 ± 0.18 7.78 ± 0.04 82.18 ± 0.41
18.04 ± 0.09 75.00 ± 0.38 4.69 ± 0.02 29.95 ± 0.15 6.34 ± 0.03 70.84 ± 0.35
Fig. 9. Typical microstructures of laser clad coating with Cr addition (Cr-3# sample).
Cu + Cl− → CuCl + e E(SHE) = 0.124 V
3.4. Corrosion mechanism 3.4.1. Electrochemical process In the corrosion experiments, a white flocculent precipitate (Al (OH)3) was formed on the surface of the Cr-1# sample in the NaCl solution. The solution itself becomes yellow, due to the presence of Fe oxides. A large amount of white precipitate and a small amount of yellow precipitate were observed on the bottom of the electrolytic chamber. The element with more negative electrode potential is more likely to lose electrons, thus the preferential corrosion of Al and Fe is mainly caused by the potential difference between Cu, Al, and Fe. The preferential electrode reaction at the anode is judged by the electrode potential and ions at the solution electrode. The anodic polarization of the Cr-1# coating and corresponding standard electrode potential are:
Al → Al3 + + 3e E(SHE) = –1.67 V Fe → Fe 2 + + 2e E(SHE) = –0.441 V
(3)
The cathodic polarization is:
1/2O2 + H2 O + 2e → 2OH−
(4)
When Cr is added, Fe is mostly bonded with Cr and forms solid solution with Cu and Al, and some Cr particles are not melted. There is only a slight difference in the density between Cr and CuAl10 coatings (7.14 and 7.60 g/cm3, respectively), Cr near the surface also participates in the anodic polarization:
Cr → Cr 3 + + 3e E(SHE) = –0.71 V
(5)
Then ions at the anode undergo further reaction and oxidation in the Cl− environment: (1)
(2)
6
CuCl + Cl− → CuCl2−
(6)
CuCl2− + H2 O → Cu (OH )2 ↓ + 2HCl + e
(7)
Cr 3 + + 3H2 O → Cr (OH )3 ↓ + 3H+
(8)
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Fig. 10. XRD patterns (a) and partially enlarged patterns (b) of Cr-1# and Cr-3# samples.
For the Cr-1# sample, the anodic polarization of a small fraction of Al and Fe initially occurs at a high rate, and then Cu participates in the reaction and forms Cu(OH)2, which is consistent with the active-passive transition charted in Fig. 11. A protective film is formed on the coating surface, which can prevent electrons and ions from reaching the alloy and effectively reduces the corrosion rate of the alloy, and these passivating phenomena can be found in the polarization curves [17]. It is evident that the formation of Cr(OH)3 contributes to the improved corrosion resistance. However, as only a small amount of Cr is added, it is impossible to form a dense protective film on the surface to inhibit further growth of corrosion processes. Accordingly, Al(OH)3, Fe (OH)3 and Cu(OH)2 will continue to be formed, and these corrosion products are cubic lattice of p-type semiconductor in which Cu2+, Cr3+, O2− can penetrate each other, therefore the passivating film becomes less effective in preventing the penetration of ions. Cl ions can displace O ions from the corrosion products and produce some new holes in solutions containing Cl ions, which may reduce the protective effect of the film and accelerate the corrosion rate [18]. However, Cr can be embedded in the vacancies and dislocations of the film to block the diffusion channel for ions, making it difficult for ions to diffuse into the corrosion interface, improving the corrosion resistance of the coating.
Fig. 11. Polarization curves of the laser clad CuAl10 coatings with different Cr additions.
Al3 + + 3H2 O → Al (OH )3 ↓ + 3H+ Fe 2 +
→
Fe3 +
+e
Fe3 + + 3H2 O → Fe (OH )3 ↓ + 3H+
(9) 3.4.2. The effect of phase on the corrosion resistance The γ2 phase acts as the anode and the α phase is the cathode, which provides the channel for the corrosion of eutectic structures. The
(10) (11) 7
Surface & Coatings Technology 381 (2020) 125215
J. Yang, et al.
Gaosheng Wang: Laser cladding experiment. Lei Yang: Microstructure analysis. Shengfeng Zhou: Microstructure analysis, paper polish. Jianbo Lei: Metallurgical mechanism analysis.
Table 4 Electrochemical test results. Sample
Ecorr (mV)
Cr-1# Cr-2# Cr-3# Cr-4#
−345 −375 −345 −293
± ± ± ±
3 4 3 2
Icorr (10−6 A·cm−2)
Rp (104 Ω·cm2)
2.014 1.522 1.354 1.846
2.653 2.949 3.649 2.957
± ± ± ±
0.021 0.016 0.014 0.019
± ± ± ±
0.028 0.032 0.037 0.030
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
diffusion of Fe from the substrate into the coating can inhibit the formation and netting of γ2 phase and refine the grains. However, it is suggested that a large amount of Fe can reduce corrosion current, and thus have a negative effect on the corrosion resistance. The addition of Cr results in an increase of β´ phase and decrease of K and γ2 phase. Both K and γ2 phases are Al-rich phases, and as a result, the corrosion resistance is greatly improved with the addition of Cr. It can also effectively reduce the magnitude of the stress associated with the dislocation pile-up at the grain boundaries and delay the formation of cracks, thus it plays a role in grain refinement. The addition of Cr increases the passivation ability, and as a result the corrosion resistance increases. However, it is also noted that the corrosion current density is slightly reduced in the Cr-4# sample, which is related to the non-uniform distribution of the unmelted Cr particles and the increased diffusion of Fe from the substrate into the cladding coatings. In general, the corrosion resistance of the coatings with different Cr content has been improved. Cr-4# has a high Cr addition, one part is melted into the copper alloy, the other part exists in the coating in a semi-molten state. Cr completely melted into the copper alloy improves the corrosion resistance of the coating. Although Cr has high corrosion resistance, Cr particles and the coating constitute the primary battery, which to some extent affects the improvement of corrosion resistance.
Acknowledgments The research was supported by the National Natural Science Foundation of China (50971004) and the National High-tech Research and Development Program (2011CB606305-3), China. References [1] B.S. Yilbas, A. Matthews, A. Leyland, C. Karatas, S.S. Akhtar, B.J. Abdul Aleem, Laser surface modification treatment of aluminum bronze with B4C, Appl. Surf. Sci. 263 (2012) 804–809. [2] Qin Luo, Wu Zhong, Zhenbo Qin, Lei Liu, Hu Wenbin, Surface modification of nickel-aluminum bronze alloy with gradient Ni-Cu solid solution coating via thermal diffusion, Surf. Coat. Technol. 309 (2017) 106–113. [3] R.T. Byrnes, S.P. Lynch, An unusual failure of a nickel-aluminium bronze (NAB) hydraulic valve, Eng. Fail. Anal. 49 (2015) 122–136. [4] R.P. Chen, Z.Q. Liang, W.W. Zhang, D.T. Zhang, Z.Q. Luo, Y.Y. Li, Effect of heat treatment on microstructure and properties of hot-extruded nickel-aluminum bronze, Trans. Nonferrous Metals Soc. China 17 (2007) 1254–1258. [5] Ruben Winkler, Erik Saborowski, Gerd Paczkowski, Thomas Grund, Thomas Lampke, Characterization of thermally sprayed copper and numerically supported residual stress determination by the incremental hole-drilling method, Surf. Coat. Technol. 371 (2019) 255–261. [6] Jari Tuominen, Marc Kaubisch, Sebastian Thieme, Jonne Nakki, Steffen Nowotny, Petri Vuoristo, Laser strip cladding for large area metal deposition, Addit. Manuf. 27 (2019) 208–216. [7] R.A. Rahman Rashid, K.A. Nazari, C. Barr, S. Palanisamy, N. Orchowski, N. Matthews, M.S. Dargusch, Effect of laser reheat post-treatment on the microstructural characteristics of laser-cladded ultra-high strength steel, Surf. Coat. Technol. 372 (2019) 93–102. [8] Benjamin Bax, Rohan Rajput, Richard Kellet, Martin Reisacher, Systematic evaluation of process parameter maps for laser cladding and directed energy deposition, Addit. Manuf. 21 (2018) 487–494. [9] O.G. Devojno, E. Feldshtein, M.A. Kardapolava, N.I. Lutsko, On the formation features, structure, microhardness and tribological behavior of single tracks and coating layers formed by laser cladding of Al-Fe powder bronze, Surf. Coat. Technol. 358 (2019) 195–206. [10] Y.N. Zhang, J.L. Zi, M.S. Zheng, C.Z. Xu, Influence of Cr micro-addition on the corrosion resistance of copper alloy, Rare Metal Mater. Eng. 36 (3) (2007) 49–52 (in Chinese). [11] B. Liu, L. Liu, The effect of microalloying on thermal stability and corrosion resistance of Cu-based bulk metallic glasses, Mater. Sci. Eng. A 415 (2006) 286–290. [12] W.S. Jeon, C.C. Shur, J.G. Kim, S.Z. Han, Y.S. Kim, Effect of Cr on the corrosion resistance of Cu–6Ni–4Sn alloys, J. Alloys Compd. 455 (2008) 358–363. [13] X.P. Tao, S. Zhang, C.H. Zhang, C.L. Wu, J. Chen, Adil O. Abdullah, Effect of Fe and Ni contents on microstructure and wear resistance of aluminum bronze coatings on 316 stainless steel by laser cladding, Surf. Coat. Technol. 342 (2018) 76–84. [14] Mahmoud Z. Ibrahim, Ahmed A.D. Sarhan, T.Y. Kuo, Farazila Yusof, M. Hamdi, Characterization and hardness enhancement of amorphous Fe-based metallic glass laser cladded on nickel-free stainless steel for biomedical implant application, Mater. Chem. Phys. 235 (2019) 121745. [15] Bo Li, Lei Zhang, Yi Xu, Zhiyuan Liu, Bo Qian, Fuzhen Xuan, Selective laser melting of CoCrFeNiMn high entropy alloy powder modified with nano-TiN particles for additive manufacturing and strength enhancement: process, particle behavior and effects, Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.068. [16] Soundarapandian Santhanakrishnan, Fanrong Kong, Radovan Kovacevic, An experimentally based thermo-kinetic hardening model for high power direct diode laser cladding, J. Mater. Process. Technol. 211 (2011) 1247–1259. [17] M. Scendo, J. Uznanska, The effect of ionic liquids on the corrosion inhibition of copper in acidic chloride solutions, Int. J. Corros. 2011 (2011) 1–13. [18] Z.Q. Cao, J. Bian, R. Xue, W.H. Liu, Electrochemical corrosion behavior of Cu-40Ni20Cr alloys with different grain sizes in solutions containing chloride ions, Trans. Nonferrous Metals Soc. China 17 (2007) 1236–1241.
4. Conclusions This research investigated the effect of different amounts of Cr (0 wt %, 1.5 wt%, 4.5 wt%, 7.5 wt%) on the microstructures and corrosion resistance of CuAl10 coating deposited on 17-4PH stainless steel substrate by Diode laser cladding. Several conclusions can be drawn from the research: (1) The CuAl10 coating has an excellent metallurgical bonding with the substrate. The addition of Cr changes the physical properties of the melts, resulting in less diffusion of Fe from the substrate into the cladding coating. (2) Metallurgical reaction in the molten pool is complex due to the inherent characteristics of laser cladding, such as rapid heating and cooling. As a result, the principal phases of CuAl10 coating without Cr addition include α, γ2, and K phase, whereas that with Cr addition include α, Cr particles, and β′ phase. (3) Cr can effectively improve the corrosion resistance of CuAl10 coatings, and the passivation ability of the coatings appears to increase with increasing Cr contents. This is because Cr can promote the formation of the passivating film and reduce the diffusion of Fe from the substrate into the coatings, so that the formation of γ2 and K phase is reduced. Author contributions Jiaoxi Yang: Experimental scheme and material design, paper writing. Feiyu Wu: Laser cladding experiment, paper writing. Bing Bai: Electrochemical test and analysis.
8