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A study of Ni-based WC composite coatings by laser induction hybrid rapid cladding with elliptical spot
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Applied Surface Science 254 (2008) 3110–3119 www.elsevier.com/locate/apsusc
A study of Ni-based WC composite coatings by laser induction hybrid rapid cladding with elliptical spot Shengfeng Zhou *, Yongjun Huang, Xiaoyan Zeng Division of Laser Science and Technology, Wuhan National Laboratory for Optoelectronics, School of Optoelectronics Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China Received 5 September 2007; received in revised form 18 September 2007; accepted 24 October 2007 Available online 30 October 2007
Abstract Ni-based WC composite coatings by laser induction hybrid rapid cladding (LIHRC) with elliptical spot were investigated. Results indicate that the efficiency using the elliptical spot of 6 mm 4 mm (the major and minor axis of laser beam are 6 mm and 4 mm, respectively, the major axis is parallel to the direction of laser scanning) is higher than that using the elliptical spot of 4 mm 6 mm (the major axis is perpendicular to the direction of laser scanning). The precipitated carbides with the blocky and bar-like shape indicate that WC particles suffer from the heat damage of ‘‘the disintegration pattern + the growth pattern’’, whichever elliptical spot is used at low laser scanning speed. However, at high laser scanning speed, the blocky carbides are only formed if the elliptical spot of 6 mm 4 mm is adopted, showing that WC particles present the heat damage of ‘‘the disintegration pattern’’, whereas the fine carbides are precipitated when the elliptical spot of 4 mm 6 mm is used, showing that WC particles take on the heat damage of ‘‘the radiation pattern’’. Especially, the efficiency of LIHRC is increased much four times higher than that of the general laser cladding and crack-free ceramic-metal coatings can be obtained. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser cladding; Composite coatings; Laser induction hybrid rapid cladding (LIHRC); Elliptical spot; Cracks
1. Introduction Compared to other carbides with high temperature oxidation resistance and wear resistance such as SiC, TiC, TiB2 and Mo2Si, WC combines many favorable properties such as high hardness, certain plasticity and good wettability with the bonding metal [1–3]. Thus, WC are widely used for producing ceramic-metal composite coatings on the substrate by conventional hardfacing techniques, such as thermal spraying, plasma spraying and arc welding, to improve wear resistance. Thermal spraying had some disadvantages, such as the composite coatings with porosities, the mechanical bonding between the composite coating and the substrate. Although the arc welding deposition had a very high efficiency, the dilution of the composite coatings was so high due to excessive energy input that the substrate was severely deformed and WC particles largely dissolved to decrease the comprehensive mechanical
* Corresponding author. Tel.: +86 27 87541780; fax: +86 27 87541423. E-mail address: [email protected] (S. Zhou). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.10.062
property. In recent years, laser cladding technique has been developed and can avoid these disadvantages of the conventional surface techniques mentioned above by choosing optimal parameters. Therefore, there is a wide application potential for the surface repairing of the key components and the fields of the three-dimensional (3D) rapid manufacturing [4–6]. However, WC easily dissolved to precipitate the carbides with different shapes and crystal structures during laser cladding due to a low free formation enthalpy of 38.5 kJ/mol [7]. Przybylowicz and Kusinski [8] investigated the structure of laser cladding tungsten carbide composite coatings and demonstrated that the longer the laser beam–composite material interaction time, the higher level of the dissolution of WC in metal matrix and the lower hardness of the composite coatings were to be expected. Moreover, the dissolution degree of the primary WC particles also depended on their size and weight percentage in the composite coatings, thus the processing conditions should be chosen to keep the dissolution of WC particles at minimum. Acker et al. [9] also investigated the influence of WC particle size and distribution on the wear resistance of laser cladding Ni-based WC coatings and
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indicated that the wear resistance reached a maximum for the highest concentration (50 vol.% WC) and the carbide size had no clear effects on the wear resistance. However, laser cladding technique had some inherent characteristics such as rapid heating and rapid solidification, the residual stress induced during laser cladding could lead to crack formation in the cladding layer. Moreover, the low rate of powder deposition increased the processing cost for cladding of large areas on the substrate [10]. Many methods have been put forward to reduce the crack sensitivity of cladding layer, e.g. preheating the substrate [11], controlling content of alloy elements [12], the gradient coating [13], optimizing processing parameters [14] and addition of the rare-earth element [15]. Usually, preheating the components in furnace or with gas flame was adopted as an effective method to decrease the temperature gradient between the cladding layer and the substrate, then crack induced by the residual stress in the cladding layer could be prevented from occurring [16]. However, this kind of preheating the components had some drawbacks, such as the heat damage of the important components, low cladding efficiency and worsening the work conditions. In order to overcome these disadvantages, laser induction hybrid rapid cladding (LIHRC) has been put forward by us in a precious paper, the microstructure characteristics of Ni-based WC composite coatings by LIHRC with elliptical spot were investigated and the ceramic-metal composite coatings without porosities and cracks were obtained [17]. Furthermore, the use of elliptical spot could lead to higher energy densities in the moving beam and, for a better welding efficiency, to an increase in the interaction time between the beam and the sample [18,19]. However, there are still some important problems to be solved for LIHRC, for example, why was the major axis of laser beam parallel to the direction of laser scanning speed adopted? Why were the ceramic-metal composite coatings detected free of porosities and cracks at high laser scanning speed? Regarding these problems, the
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Table 1 Chemical composition (in wt.%) of bonding metal Ni60A alloy powder C Si B Cr Fe Ni
0.5–0.9 3.5–5.5 3.0–4.5 15–18 14.3 Balance
emphasis of the present paper is, when the two different elliptical spot were adopted, to analyze the macrostructure of composite coatings, the dissolution characteristics of WC particles and the reasons that Ni-based WC composite coatings were free of porosities and cracks. 2. Experimental procedures The substrate material was A3 steel with the dimension of 120 mm 50 mm 8 mm. The surface of substrate was treated by grinding wheel and acetone. Self-fluxing Ni60A alloy powder was used as the bonding metal with an average particle size of 65 mm and the chemical composition was listed in Table 1. WC powder with an average particle size of 38 mm was used as the ceramic phase and the chemical composition was the mixtures of W2C + WC eutectic. Because there existed a significant difference in particle size between self-fluxing Ni60A and WC particles, the composite powder was composed of a mixture of 80 wt.% Ni60A and 20 wt.% WC particles, which was prepared to flow freely in order to feed powders automatically by our own laboratory granulating method. The schematic of LIHRC setup is shown in Fig. 1. The experiments of LIHRC were carried out using a Rofin-TR050 5 kW continuous wave CO2 laser. The laser beam was adjusted into the elliptical spot with an optical adjustment device. The major axis of the elliptical spot was 6 mm and the minor axis was 4 mm. Here, the elliptical spot of 6 mm 4 mm
Fig. 1. Schematic of LIHRC setup and elliptical laser spot: (a) LIHRC setup, (b) elliptical spot of 6 mm 4 mm and (c) elliptical spot of 4 mm 6 mm.
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represented that the major axis was parallel to the direction of laser scanning speed as shown in Fig. 1b. The elliptical spot of 4 mm 6 mm indicated that the major axis was perpendicular to the direction of laser scanning speed as shown in Fig. 1c. The induction heating equipment used was the 100 kW high frequency induction heater of integrated module. Its operation frequency was 30 kHz and power varied from 10 to 50 kW. The average temperature of the preheated substrate was set at 1073 K by adjusting the inductor power. The feeding powder equipment was the HGL off-axial auto-feeding powder. A shielding of Ar gas was used to blow the composite powder into the molten pool to produce the oxide-free coatings. The flow rate of composite powder varied from 52.24 to 92.46 g/min. The angle between the powder nozzle and substrate was set at 558 and distance was set at 10 mm. After LIHRC, the specimens were cut along cross-section, mechanically grounded, polished in diamond paste and chemically etched in a mixed acid consisting of 75 vol.% HCl and 25 vol.% HNO3 to reveal the microstructure which was examined under an optical microscope and an environmental scanning electron microscope (ESEM). Dye penetrant testing was used to detect the cracks of the ceramic-metal composite coatings. Energy dispersive X-ray (EDX) analysis was performed to measure the average composition of the composite coatings. Microhardness of ceramic-metal composite coatings was tested using HVS-1000 microhardness tester with a load of 1.96 N and a dwelling time of 15 s. 3. Results and discussion 3.1. Macrostructure characteristic of composite coatings There are many processing parameters for laser cladding, such as laser power, laser scanning speed and laser spot, etc. Laser specific energy was often used to comprehensively describe the effects of laser processing parameters on the quality of cladding layer [20]. When the elliptical spot was adopted to carry out LIHRC, laser specific energy was calculated as follows [19]: E ¼ 60
P V sD
(1)
where P is the laser power (W), Vs represents the laser scanning speed (mm/min) and D should be considered as the dimension perpendicular to the direction of laser scanning speed (mm). The relationship between laser specific energy and cladding height is shown in Fig. 2. It indicated that, for the different elliptical spot and the same laser scanning speed, the change of cladding height with increasing laser specific energy had the same propensity, namely, the cladding height increased with increasing laser specific energy. For the same elliptical spot and the different laser scanning speed, the cladding height decreased with increasing laser scanning speed. Therefore, the greater the laser specific energy was, the more the laser would melt the powder in unit time, namely, the higher the efficiency of composite powder was, and then the higher the composite coating was. Such a change of cladding height has
Fig. 2. Effect of laser specific energy on cladding height.
been reported by Wu et al. [21]. Moreover, it can be seen from Fig. 2 that, at the same laser scanning speed, the minimum laser specific energy, which was required for a good metallurgical bonding between the composite coating and the substrate, and the maximum laser specific energy using the elliptical spot of 6 mm 4 mm, which was limited for available maximum laser power, were both higher compared to those using the elliptical spot of 4 mm 6 mm. Furthermore, the adjusting range of laser specific energy using the elliptical spot of 6 mm 4 mm was wider than that using the elliptical spot of 4 mm 6 mm, which could be explained by following reasons. Before the laser beam reached the substrate, it passed through the composite powder which could attenuate the laser energy, the residual laser energy required was enough to melt the surface of the substrate resulting in forming the molten pool. Thus, the metallurgical bonding between the composite coating and the substrate could be generated [22]. For instance, when the laser scanning speed was 3000 mm/min, the minimum laser power using the elliptical spot of 6 mm 4 mm (i.e. Pmin = 3.3 kW), which required for a good metallurgical bonding between the composite coating and the substrate, was lower than that using the elliptical spot of 4 mm 6 mm (i.e. Pmin = 3.7 kW). According to Eq. (1), the minimum laser specific energy using the elliptical spot of 6 mm 4 mm was higher than that using the elliptical spot of 4 mm 6 mm as listed in Table 2. Therefore, more laser energy in unit time was simultaneously absorbed by the substrate and the composite powder, and then the molten pool on the surface of the substrate was easily formed. As a result, the melted alloy elements could diffuse each other between the composite coating and the substrate during LIHRC, which contributed to produce the good metallurgical bonding between the composite coating and the substrate. At the different laser scanning speed and elliptical spot, as long as the laser power was higher than the minimum laser power, namely, the laser specific energy was higher than the minimum laser specific energy, the metallurgical bonding between the composite coating and the substrate could be generate during LIHRC. In addition, the maximum laser specific energy was mainly determined by the laser power output (i.e. the maximum laser power was 5 kW) and the
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Table 2 The parameters of LIHRC with two different elliptical laser spots Elliptical laser spot size
Laser scanning velocity, Vs (mm/min)
Minimum laser power, Pmin (kW)
Minimum laser specific energy, Emin (J/mm2)
Maximum laser power, Pmax (kW)
Maximum laser specific energy, Emax (J/mm2)
6 mm 4 mm
1200 3000
3.0 3.3
37.5 16.5
5 5
62.5 25
4 mm 6 mm
1200 3000
3.5 3.7
29.2 12.3
5 5
41.7 16.7
elliptical spot. Because the dimension perpendicular to the direction of laser scanning speed for the elliptical spot of 6 mm 4 mm was smaller than that for the elliptical spot of 4 mm 6 mm, the maximum laser specific energy using the elliptical spot of 6 mm 4 mm was higher than that using the elliptical spot of 4 mm 6 mm during LIHRC (Table 2). Based on the above analyzed results, namely, the minimum laser power using the elliptical spot of 6 mm 4 mm, which was required for the metallurgical bonding between the composite coating and the substrate, was lower than that using the elliptical spot of 4 mm 6 mm, the adjusting range of laser power using the elliptical spot of 6 mm 4 mm was wider than that using the elliptical spot of 4 mm 6 mm when the laser power was adjusted between the minimum laser power and the maximum laser power. For instance, in order to obtain the metallurgical bonding between the composite coating and the substrate, when the laser scanning speed was 1200 mm/min, the laser power using the elliptical spot of 6 mm 4 mm was varied from 3 to 5 kW, whereas the laser power using the elliptical spot of 4 mm 6 mm was varied from 3.5 to 5 kW. Correspondingly, the laser specific energy using the elliptical spot of 6 mm 4 mm could be adjusted from 37.5 to 62.5 J/ mm2, while the laser specific energy using the elliptical spot of 4 mm 6 mm could be adjusted from 29.2 to 41.7 J/mm2 (Table 2). Therefore, the adjusting range of laser specific energy using the elliptical spot of 6 mm 4 mm was wider than that using the elliptical spot of 4 mm 6 mm. The contact angle of the composite coating was calculated as follows [23]:
2H u ¼ 180 2 arctan W
elliptical spot kept constant, the dimension of the molten pool on the surface of the substrate was slightly increased, correspondingly, the cladding width somewhat increased during LIHRC. Moreover, the powder particles cloud attenuated the laser power, which caused that the dimension perpendicular the direction of laser scanning speed for the molten pool was always smaller than that for the elliptical spot. Therefore, the cladding width, W, was only determined by the dimension of elliptical spot which was perpendicular to the direction of laser scanning speed, and had little relation to the laser specific energy. Based on the above propensity of change with the laser specific energy for the cladding height and the cladding width, the contact angles between the composite coating and the substrate using the different elliptical spot decreased with increasing laser specific energy, which were all in an acceptable range of >1308 to be overlapped without defects [24]. Furthermore, when the elliptical spot of 6 mm 4 mm as described in Fig. 1b was adopted and laser scanning speed was increased to 3000 mm/min during LIHRC, the maximum rate of powder deposition, required for a good metallurgical bonding between the substrate and the composite coating with a good profile, was 86.79 g/min. However, for the elliptical spot of 4 mm 6 mm as shown in Fig. 1c, when laser scanning speed was 3000 mm/min, the maximum rate of powder deposition was 55.81 g/min. Therefore, the rate of powder deposition using the elliptical spot of 6 mm 4 mm was approximately increased one point six times higher than that using the elliptical spot of 4 mm 6 mm, which could be explained by following reasons. The higher the rate of powder deposition was, the higher the attenuated rate of the laser power
(2)
where W is the cladding width (mm) and H is the cladding height (mm). The effect of laser specific energy on the cladding width and the contact angle is shown in Fig. 3. It can be seen that the cladding width slightly increased with increasing laser specific energy and gradually approached the dimension perpendicular to the direction of laser scanning speed. Moreover, the contact angle decreased with increasing laser specific energy, the contact angle using the elliptical spot of 4 mm 6 mm was larger than that using the elliptical spot of 6 mm 4 mm. The change characteristics of the cladding width and the contact angle could be explained by following reasons. Although the laser power absorbed by the substrate and the composite powder increased with increasing laser specific energy when the flow rate of composite powder and the
Fig. 3. Effect of laser specific energy on cladding width and contact angle for the different elliptical spot.
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for the composite powder particles cloud was. Thus, the laser power which was absorbed by the composite powder increased, but the residual laser power which was absorbed by the substrate decreased, which easily resulted in the metallurgical bonding not to be generated well between the composite coating and the substrate. Therefore, the laser specific energy need be increased if the rate of powder deposition was to be increased. When the laser power and the laser scanning speed kept constant, it can be seen from Table 2 that the laser specific energy using the elliptical spot of 6 mm 4 mm was higher than that using the elliptical spot of 4 mm 6 mm. Thus, a relatively larger amount of the composite powder using the elliptical spot of 6 mm 4 mm could be cladded on the substrate in unit time compared to that using the elliptical spot of 4 mm 6 mm, i.e. the rate of powder deposition was increased. This was the main reason that the rate of powder deposition was relatively higher when the elliptical spot of 6 mm 4 mm was adopted during LIHRC. 3.2. Microstructure characteristics and microhardness distribution For the elliptical spot of 6 mm 4 mm, when laser scanning speed was 1200 mm/min, the microstructure of Ni-based WC composite coating by LIHRC is shown in Fig. 4. It can be seen from Fig. 4a that a planar growth near the bonding line, relatively coarse columnar dendrites and eutectics were observed in the composite coating. Fig. 4b shows the precipitated carbides due to dissolution of WC particles which were severely suffered from the heat damage. Due to the partial dissolution of some WC particles with large size, the blocky carbides were precipitated around WC particles, which showed that WC particles were characterized by the heat damage of ‘‘the disintegration pattern’’. The results of EDX analysis revealed that the blocky carbides contained approximately 8.42% Ni, 6.34% Cr, 15.3% Fe, 67.76% W and 2.18% C (wt.%), indicating that the blocky carbides were rich in W. However, due to the complete dissolution of other WC particles with small size, the bar-like carbides were reformed far away from WC particles with relatively larger size, which showed that WC particles were characterized by the heat damage of ‘‘the growth
pattern’’. The results of EDX analysis demonstrated that the bar-like carbides contained 10.27% Ni, 51.5% Cr, 6.25% Fe, 15.6% W and 16.38% C (wt.%), indicating that the bar-like carbides were mainly rich in Cr. Furthermore, when laser scanning speed was increased to 3000 mm/min, the microstructure of Ni-based WC composite coating by LIHRC is shown in Fig. 5. It can be seen that WC particles with small size were partially dissolved to precipitate the blocky carbides around WC particles and the bar-like carbides were not observed in the composite coating, which indicated that WC particles were only characterized by the heat damage of ‘‘the disintegration pattern’’ (Fig. 5a). The results of EDX analysis showed that the blocky carbides contained 7.58% Ni, 6.24% Cr, 9.26% Fe, 70.1% W and 6.82% C (wt.%), showing that the blocky carbides were also rich in W. Moreover, the larger carbide with blocky shape, containing 90.26% W and 9.74% C by EDX analysis, kept the integrality of structure. It was clear that the larger carbide with blocky shape was WC particle (Fig. 5b). For the elliptical spot of 4 mm 6 mm, when laser scanning speed was 1200 mm/min, the microstructure of Ni-based WC composite coating by LIHRC is shown in Fig. 6. It can be seen from Fig. 6a that g-nickel had firstly planar growth and then formed fine columnar dendrites and eutectics. Due to dissolution of WC particles, the blocky and bar-like carbides were precipitated in the composite coating (Fig. 6b). WC particles were characterized by the heat damage of ‘‘the disintegration pattern + the growth pattern’’, indicating that the heat damage of WC particles using the elliptical spot of 4 mm 6 mm had the same characteristics as that using the elliptical spot of 6 mm 4 mm did. EDX analysis indicated that the blocky carbides contained 7.8% Ni, 5.96% Cr, 13.6% Fe, 70.61% W and 2.03% C (wt.%), whereas the bar-like carbides contained 8.76% Ni, 56.05% Cr, 6.13% Fe, 13.8% W and 15.26% C (wt.%), indicating that the bar-like carbides contained a relatively larger amount of Cr and Ni, relatively smaller amount of W and Fe compared to the blocky carbides. When laser scanning speed was increased to 3000 mm/min, the microstructure of Ni-based WC composite coating by LIHRC is shown in Fig. 7. It can be seen from Fig. 7a that, due to the partial dissolution of WC particles, the fine carbides were
Fig. 4. Microstructure of Ni-based WC composite coatings by LIHRC with elliptical spot of 6 mm 4 mm, Vs = 1200 mm/min: (a) interface zone and (b) carbides characteristics.
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Fig. 5. Microstructure of Ni-based WC composite coatings by LIHRC with elliptical spot of 6 mm 4 mm, Vs = 3000 mm/min: (a) carbides characteristics and (b) WC characteristics.
precipitated around WC particles. Moreover, the blocky and bar-like carbides were not observed in the composite coating. Thus, WC particles were characterized by the heat damage of ‘‘the radiation pattern’’. EDX analysis demonstrated that those fine carbides contained 6.24% Ni, 4.52% Cr, 2.08% Fe, 82.04% W and 5.12% C (wt.%). Furthermore, the size of the precipitated carbides using the elliptical spot of 4 mm 6 mm was smaller than those using the elliptical spot of 6 mm 4 mm. According to the above results, the dissolution characteristics of WC particles may be deduced as follows. At low laser scanning speed, laser specific energy was relatively higher (Table 2), WC particles severely suffered from the heat damage. As a result, relatively larger amounts of Ni, Cr and Fe elements in the bonding metal, Ni60A, diffused into WC particles and W elements in WC particles diffused into the liquid Ni-based alloy. Thus, the blocky carbides were precipitated in the composite coating, indicating that WC particles with relatively larger size were characterized by the heat damage of ‘‘the disintegration pattern’’. Moreover, WC particles with relatively smaller size were completely dissolved to reform the bar-like carbides with higher concentration of Cr, showing that WC particles were characterized by the heat damage of the growth pattern (Figs. 4b and 6b). With increasing laser scanning speed, laser specific energy decreased, resulting in decreasing the heat
damage of WC particles. Thus, relatively smaller amounts of Ni, Cr and Fe elements in the bonding metal dissolved into WC particles with relatively smaller size, while WC particles with large size kept the integrality of structure, whose edge only suffered from the heat damage (Fig. 5b). With further decreasing laser specific energy, the fine carbides were precipitated due to dissolution of WC particles (Fig. 7a and b). Then, WC particles were characterized by the heat damage of ‘‘the radiation pattern’’. Whether the elliptical spot of 4 mm 6 mm was used or the elliptical spot of 6 mm 4 mm was adopted, the precipitated carbides had a common characteristic, namely, the concentration of Ni, Cr and Fe in the precipitated carbides with different shapes decreased and the amounts of W increased with a decrease of laser specific energy, which indicated that the dissolution of WC particles reduced. As a result, WC particles kept the characteristic of high hardness, the microhardness of the composite coatings was increased (Fig. 8). Moreover, with decreasing laser specific energy, the characteristic of heat damage for WC particles went through a transition of ‘‘the disintegration pattern + the growth pattern’’ to ‘‘the disintegration pattern’’ and ‘‘the radiation pattern’’. Therefore, the higher laser specific energy produced a significant dissolution of WC particles in the composite coatings, the lower microhardness of the composite coatings was obtained.
Fig. 6. Microstructure of Ni-based WC composite coatings by LIHRC with elliptical spot of 4 mm 6 mm, Vs = 1200 mm/min: (a) interface zone and (b) carbides characteristics.
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Fig. 7. Microstructure of Ni-based WC composite coatings by LIHRC with elliptical spot of 6 mm 4 mm, Vs = 3000 mm/min: (a) carbides characteristics and (b) WC characteristics.
3.3. Comparison to the general laser cladding As well known, ceramic-metal composite coatings were sensitive to porosities and cracks which retarded the wide application of laser cladding technique in industry. Fig. 9 shows the structure of Ni-based WC composite coating by the general laser cladding. It can be seen that there existed many porosities and cracks in Ni-based WC composite coatings by the general laser cladding. The typical distribution characteristic of cracks was perpendicular to the direction of laser scanning speed and traversed the whole composite coating (Fig. 9a). That was because the density of WC particles (16.5 g/cm3) was higher than that of Ni60A alloy (7.53 g/cm3) and the violent stirring and convection were driven by thermocapillarity [25], WC particles easily sank at the bottom of the composite coatings to decrease the toughness of bonding metal. As a result, cracks were induced to form in the composite coatings. Furthermore, WC particles sinking at the bottom of the molten pool and the precipitated carbides had an effect of hindrance on the moving of air bubbles in the molten pool. Thus, the moving resistance of air bubbles rising to surface had an increase and air bubbles were captured to form the porosities in the composite coatings (Fig. 9b). XRD analysis (Fig. 10) indicated that the composite coating was
Fig. 8. Microhardness distribution of Ni-based WC composite coatings by LIHRC with elliptical spot.
composed of W2C, WC, WC1x, M3C, M6C and M23C6 phases (M = Ni, Cr, Fe, W). Obviously, because the distribution of WC particles was fairly inhomogeneous and the carbides with various shapes were precipitated, especially, the bar-like carbides with small radius of curvature were formed. Then the structure stress and the phase transformation stress were possibly induced to increase the cracks sensitivity of the composite coating (Fig. 9c).Furthermore, when the laser power was 5 kW, the laser scanning speed of the general laser cladding was only as fast as 720 mm/min and the rate of powder deposition was 22.6 g/min for the elliptical spot of 6 mm 4 mm. However, in relation to LIHRC, when the same laser power and elliptical spot of 6 mm 4 mm were used, the laser scanning speed was increased to 3000 mm/min and the rate of powder deposition was 86.79 g/min. Moreover, a good metallurgical bonding between the composite coating and the substrate can be generated where the composite coating had a smooth surface and good profile without porosities and cracks (Fig. 11). During the general laser cladding, excessive heating resulted in the dissolution of WC particles to precipitate carbon as graphite [10]: 2WC ! W2 C þ C
(3)
The graphite could react with atmospheric oxygen and form CO and CO2, which were to be captured to form porosities during the rapid solidification as shown in Fig. 9b. Therefore, the dissolution of WC particles was required to have a decrease in order to reduce the porosities in the composite coatings. Then the precipitated carbon concentration was also decreased. EDX analysis indicated that the average carbon content was 2.96% (Vs = 3000 mm/min) for LIHRC, but the average carbon content was 6.13% (Vs = 720 mm/min) for the general laser cladding. Since the laser scanning speed was increased to 3000 mm/ min, the laser specific energy was markedly decreased, resulting in a decrease of dissolution of WC particles. Therefore, the produced CO and CO2 forming porosities in the composite coatings were decreased due to combination reaction between C and O2 in the air during LIHRC. This was the main reason that Ni-based WC composite coatings by LIHRC detected were free of porosities. Furthermore, the basic reason why the composite coatings detected were free of cracks was that the temperature gradient
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Fig. 9. Structure of Ni-based WC composite coatings by the general laser cladding with elliptical spot of 6 mm 4 mm, P = 5 kW, Vs = 720 mm/min: (a) cracks characteristic, (b) porosities characteristic and (c) carbides characteristic.
between the composite coating and the substrate was decreased during LIHRC, correspondingly, the produced thermal stress, due to high temperature gradient between the composite coating and the substrate, was decreased. Thus, the crack sensitivity was reduced. For a simple qualitative analysis, the temperature gradient during laser scanning was calculated as follows [26]: G¼
Fig. 10. X-ray diffraction result of Ni-based WC composite coatings by the general laser cladding.
2pKðT T 0 Þ2 hP
(4)
where G is the temperature gradient between the composite coating and the substrate (K/mm), T the liquidus temperature of the Ni-based alloy (K), T0 the preheating temperature of the substrate (K), h represents the laser absorption coefficient, P the laser power (W) and K is the thermal conductivity of the material (W m1 K1). At a laser power of 5 kW, when the substrate was not preheated, the temperature gradient G was
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Fig. 11. Structure of Ni-based WC composite coatings by LIHRC with elliptical spot of 6 mm 4 mm, P = 5 kW, Vs = 3000 mm/min: (a) single-pass profile and (b) cross-section characteristic.
1.69 103 K/mm (K = 82.9 W m1 K1, T = 1573 K, T0 = 300 K and h = 10%). If the average preheated temperature of the substrate was 1073 K, the temperature gradient G was 130.2 K/mm (T0 = 1073 K and h = 20%). For the thermal stress in Ni-based WC composite coatings, it could induce to form cracks and was described in the following [27]: s th ¼
Ec DaDT 1 vc
(5)
where Ec and vc are the elastic modulus and the Poisson ratio, respectively, Da represents the difference in coefficient of thermal expansion between the composite coating and the substrate and DT is the difference between the solidification temperature of the molten cladding material and the room temperature. Based on the above calculated results, the temperature gradient between the composite coating and the substrate during LIHRC was decreased much twelve times lower than that during the general laser cladding. It can be seen from Eq. (5) that the thermal stress in Ni-based WC composite coatings was markedly decreased. Therefore, Ni-based WC composite coatings without porosities and cracks were obtained by LIHRC. Furthermore, the laser scanning speed and the rate of powder deposition were respectively increased more four times and three point eight times higher than those of the general laser cladding. Due to very high laser scanning speed for LIHRC, the time of the stirring and convection in the molten pool was evidently decreased. For a simple calculation, the time of the stirring and convection in the molten pool could be described as the interaction time of substrate and composite powder with laser beam, which was given as follows [19,28]: t¼
L Vs
(6)
where L represents the spot dimension in the longitudinal direction, namely, the spot dimension which is parallel to the direction of the laser scanning speed. For the general laser cladding, when the elliptical spot of 6 mm 4 mm was adopted and the laser scanning speed was 720 mm/min, the time of the stirring and convection in the molten pool, t, was 0.5 s. However, for LIHRC, when the same elliptical spot was
used and laser scanning speed was increased to 3000 mm/min, t was 0.12 s. The time of the stirring and convection in the molten pool during LIHRC was reduced much four times lower than that during the general laser cladding. Thus, the distribution of WC particles across the whole sectional area was rather uniform (Fig. 11b), that was because the stirring and convection in the molten pool had no enough time to contribute WC particles to sink at the bottom of the composite coating. 4. Conclusions The adjusting range of laser specific energy using the elliptical spot of 6 mm 4 mm was wider compared to that using the elliptical spot of 4 mm 6 mm, the maximum rate of powder deposition was about one point six times higher than that using the elliptical spot of 4 mm 6 mm. For high laser specific energy, the blocky and bar-like carbides were precipitated in the composite coating due to dissolution of WC particles, which took on the heat damage of ‘‘the disintegration pattern + the growth pattern’’. With decreasing laser specific energy, the blocky carbides were only precipitated in the composite coating, indicating that WC particles suffered from the heat damage of ‘‘the disintegration pattern’’. With further decreasing laser specific energy, the fine carbides were reform around WC particles, which presented the heat damage of ‘‘the radiation pattern’’. Therefore, the higher laser specific energy produced a significant dissolution of WC particles in the composite coatings, the lower microhardness of the composite coatings was obtained. Due to the introduction of induction heater, the laser scanning speed and the rate of powder deposition were markedly increased, the laser scanning speed and the rate of powder deposition by LIHRC were, respectively, increased much four times and there point eight times higher than those by the general laser cladding. Moreover, the time of stirring and convection in the molten pool during LIHRC were reduced to keep WC particles from sinking at the bottom of the composite coatings. Thus, the distribution of WC particles across the whole sectional area was rather uniform and a good metallurgical bonding between the composite coating and the substrate was produced where the composite coating had a smooth surface and good profile without porosities and cracks.
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Applied Surface Science 254 (2008) 3110–3119 www.elsevier.com/locate/apsusc
A study of Ni-based WC composite coatings by laser induction hybrid rapid cladding with elliptical spot Shengfeng Zhou *, Yongjun Huang, Xiaoyan Zeng Division of Laser Science and Technology, Wuhan National Laboratory for Optoelectronics, School of Optoelectronics Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China Received 5 September 2007; received in revised form 18 September 2007; accepted 24 October 2007 Available online 30 October 2007
Abstract Ni-based WC composite coatings by laser induction hybrid rapid cladding (LIHRC) with elliptical spot were investigated. Results indicate that the efficiency using the elliptical spot of 6 mm 4 mm (the major and minor axis of laser beam are 6 mm and 4 mm, respectively, the major axis is parallel to the direction of laser scanning) is higher than that using the elliptical spot of 4 mm 6 mm (the major axis is perpendicular to the direction of laser scanning). The precipitated carbides with the blocky and bar-like shape indicate that WC particles suffer from the heat damage of ‘‘the disintegration pattern + the growth pattern’’, whichever elliptical spot is used at low laser scanning speed. However, at high laser scanning speed, the blocky carbides are only formed if the elliptical spot of 6 mm 4 mm is adopted, showing that WC particles present the heat damage of ‘‘the disintegration pattern’’, whereas the fine carbides are precipitated when the elliptical spot of 4 mm 6 mm is used, showing that WC particles take on the heat damage of ‘‘the radiation pattern’’. Especially, the efficiency of LIHRC is increased much four times higher than that of the general laser cladding and crack-free ceramic-metal coatings can be obtained. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser cladding; Composite coatings; Laser induction hybrid rapid cladding (LIHRC); Elliptical spot; Cracks
1. Introduction Compared to other carbides with high temperature oxidation resistance and wear resistance such as SiC, TiC, TiB2 and Mo2Si, WC combines many favorable properties such as high hardness, certain plasticity and good wettability with the bonding metal [1–3]. Thus, WC are widely used for producing ceramic-metal composite coatings on the substrate by conventional hardfacing techniques, such as thermal spraying, plasma spraying and arc welding, to improve wear resistance. Thermal spraying had some disadvantages, such as the composite coatings with porosities, the mechanical bonding between the composite coating and the substrate. Although the arc welding deposition had a very high efficiency, the dilution of the composite coatings was so high due to excessive energy input that the substrate was severely deformed and WC particles largely dissolved to decrease the comprehensive mechanical
* Corresponding author. Tel.: +86 27 87541780; fax: +86 27 87541423. E-mail address: [email protected] (S. Zhou). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.10.062
property. In recent years, laser cladding technique has been developed and can avoid these disadvantages of the conventional surface techniques mentioned above by choosing optimal parameters. Therefore, there is a wide application potential for the surface repairing of the key components and the fields of the three-dimensional (3D) rapid manufacturing [4–6]. However, WC easily dissolved to precipitate the carbides with different shapes and crystal structures during laser cladding due to a low free formation enthalpy of 38.5 kJ/mol [7]. Przybylowicz and Kusinski [8] investigated the structure of laser cladding tungsten carbide composite coatings and demonstrated that the longer the laser beam–composite material interaction time, the higher level of the dissolution of WC in metal matrix and the lower hardness of the composite coatings were to be expected. Moreover, the dissolution degree of the primary WC particles also depended on their size and weight percentage in the composite coatings, thus the processing conditions should be chosen to keep the dissolution of WC particles at minimum. Acker et al. [9] also investigated the influence of WC particle size and distribution on the wear resistance of laser cladding Ni-based WC coatings and
S. Zhou et al. / Applied Surface Science 254 (2008) 3110–3119
indicated that the wear resistance reached a maximum for the highest concentration (50 vol.% WC) and the carbide size had no clear effects on the wear resistance. However, laser cladding technique had some inherent characteristics such as rapid heating and rapid solidification, the residual stress induced during laser cladding could lead to crack formation in the cladding layer. Moreover, the low rate of powder deposition increased the processing cost for cladding of large areas on the substrate [10]. Many methods have been put forward to reduce the crack sensitivity of cladding layer, e.g. preheating the substrate [11], controlling content of alloy elements [12], the gradient coating [13], optimizing processing parameters [14] and addition of the rare-earth element [15]. Usually, preheating the components in furnace or with gas flame was adopted as an effective method to decrease the temperature gradient between the cladding layer and the substrate, then crack induced by the residual stress in the cladding layer could be prevented from occurring [16]. However, this kind of preheating the components had some drawbacks, such as the heat damage of the important components, low cladding efficiency and worsening the work conditions. In order to overcome these disadvantages, laser induction hybrid rapid cladding (LIHRC) has been put forward by us in a precious paper, the microstructure characteristics of Ni-based WC composite coatings by LIHRC with elliptical spot were investigated and the ceramic-metal composite coatings without porosities and cracks were obtained [17]. Furthermore, the use of elliptical spot could lead to higher energy densities in the moving beam and, for a better welding efficiency, to an increase in the interaction time between the beam and the sample [18,19]. However, there are still some important problems to be solved for LIHRC, for example, why was the major axis of laser beam parallel to the direction of laser scanning speed adopted? Why were the ceramic-metal composite coatings detected free of porosities and cracks at high laser scanning speed? Regarding these problems, the
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Table 1 Chemical composition (in wt.%) of bonding metal Ni60A alloy powder C Si B Cr Fe Ni
0.5–0.9 3.5–5.5 3.0–4.5 15–18 14.3 Balance
emphasis of the present paper is, when the two different elliptical spot were adopted, to analyze the macrostructure of composite coatings, the dissolution characteristics of WC particles and the reasons that Ni-based WC composite coatings were free of porosities and cracks. 2. Experimental procedures The substrate material was A3 steel with the dimension of 120 mm 50 mm 8 mm. The surface of substrate was treated by grinding wheel and acetone. Self-fluxing Ni60A alloy powder was used as the bonding metal with an average particle size of 65 mm and the chemical composition was listed in Table 1. WC powder with an average particle size of 38 mm was used as the ceramic phase and the chemical composition was the mixtures of W2C + WC eutectic. Because there existed a significant difference in particle size between self-fluxing Ni60A and WC particles, the composite powder was composed of a mixture of 80 wt.% Ni60A and 20 wt.% WC particles, which was prepared to flow freely in order to feed powders automatically by our own laboratory granulating method. The schematic of LIHRC setup is shown in Fig. 1. The experiments of LIHRC were carried out using a Rofin-TR050 5 kW continuous wave CO2 laser. The laser beam was adjusted into the elliptical spot with an optical adjustment device. The major axis of the elliptical spot was 6 mm and the minor axis was 4 mm. Here, the elliptical spot of 6 mm 4 mm
Fig. 1. Schematic of LIHRC setup and elliptical laser spot: (a) LIHRC setup, (b) elliptical spot of 6 mm 4 mm and (c) elliptical spot of 4 mm 6 mm.
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represented that the major axis was parallel to the direction of laser scanning speed as shown in Fig. 1b. The elliptical spot of 4 mm 6 mm indicated that the major axis was perpendicular to the direction of laser scanning speed as shown in Fig. 1c. The induction heating equipment used was the 100 kW high frequency induction heater of integrated module. Its operation frequency was 30 kHz and power varied from 10 to 50 kW. The average temperature of the preheated substrate was set at 1073 K by adjusting the inductor power. The feeding powder equipment was the HGL off-axial auto-feeding powder. A shielding of Ar gas was used to blow the composite powder into the molten pool to produce the oxide-free coatings. The flow rate of composite powder varied from 52.24 to 92.46 g/min. The angle between the powder nozzle and substrate was set at 558 and distance was set at 10 mm. After LIHRC, the specimens were cut along cross-section, mechanically grounded, polished in diamond paste and chemically etched in a mixed acid consisting of 75 vol.% HCl and 25 vol.% HNO3 to reveal the microstructure which was examined under an optical microscope and an environmental scanning electron microscope (ESEM). Dye penetrant testing was used to detect the cracks of the ceramic-metal composite coatings. Energy dispersive X-ray (EDX) analysis was performed to measure the average composition of the composite coatings. Microhardness of ceramic-metal composite coatings was tested using HVS-1000 microhardness tester with a load of 1.96 N and a dwelling time of 15 s. 3. Results and discussion 3.1. Macrostructure characteristic of composite coatings There are many processing parameters for laser cladding, such as laser power, laser scanning speed and laser spot, etc. Laser specific energy was often used to comprehensively describe the effects of laser processing parameters on the quality of cladding layer [20]. When the elliptical spot was adopted to carry out LIHRC, laser specific energy was calculated as follows [19]: E ¼ 60
P V sD
(1)
where P is the laser power (W), Vs represents the laser scanning speed (mm/min) and D should be considered as the dimension perpendicular to the direction of laser scanning speed (mm). The relationship between laser specific energy and cladding height is shown in Fig. 2. It indicated that, for the different elliptical spot and the same laser scanning speed, the change of cladding height with increasing laser specific energy had the same propensity, namely, the cladding height increased with increasing laser specific energy. For the same elliptical spot and the different laser scanning speed, the cladding height decreased with increasing laser scanning speed. Therefore, the greater the laser specific energy was, the more the laser would melt the powder in unit time, namely, the higher the efficiency of composite powder was, and then the higher the composite coating was. Such a change of cladding height has
Fig. 2. Effect of laser specific energy on cladding height.
been reported by Wu et al. [21]. Moreover, it can be seen from Fig. 2 that, at the same laser scanning speed, the minimum laser specific energy, which was required for a good metallurgical bonding between the composite coating and the substrate, and the maximum laser specific energy using the elliptical spot of 6 mm 4 mm, which was limited for available maximum laser power, were both higher compared to those using the elliptical spot of 4 mm 6 mm. Furthermore, the adjusting range of laser specific energy using the elliptical spot of 6 mm 4 mm was wider than that using the elliptical spot of 4 mm 6 mm, which could be explained by following reasons. Before the laser beam reached the substrate, it passed through the composite powder which could attenuate the laser energy, the residual laser energy required was enough to melt the surface of the substrate resulting in forming the molten pool. Thus, the metallurgical bonding between the composite coating and the substrate could be generated [22]. For instance, when the laser scanning speed was 3000 mm/min, the minimum laser power using the elliptical spot of 6 mm 4 mm (i.e. Pmin = 3.3 kW), which required for a good metallurgical bonding between the composite coating and the substrate, was lower than that using the elliptical spot of 4 mm 6 mm (i.e. Pmin = 3.7 kW). According to Eq. (1), the minimum laser specific energy using the elliptical spot of 6 mm 4 mm was higher than that using the elliptical spot of 4 mm 6 mm as listed in Table 2. Therefore, more laser energy in unit time was simultaneously absorbed by the substrate and the composite powder, and then the molten pool on the surface of the substrate was easily formed. As a result, the melted alloy elements could diffuse each other between the composite coating and the substrate during LIHRC, which contributed to produce the good metallurgical bonding between the composite coating and the substrate. At the different laser scanning speed and elliptical spot, as long as the laser power was higher than the minimum laser power, namely, the laser specific energy was higher than the minimum laser specific energy, the metallurgical bonding between the composite coating and the substrate could be generate during LIHRC. In addition, the maximum laser specific energy was mainly determined by the laser power output (i.e. the maximum laser power was 5 kW) and the
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Table 2 The parameters of LIHRC with two different elliptical laser spots Elliptical laser spot size
Laser scanning velocity, Vs (mm/min)
Minimum laser power, Pmin (kW)
Minimum laser specific energy, Emin (J/mm2)
Maximum laser power, Pmax (kW)
Maximum laser specific energy, Emax (J/mm2)
6 mm 4 mm
1200 3000
3.0 3.3
37.5 16.5
5 5
62.5 25
4 mm 6 mm
1200 3000
3.5 3.7
29.2 12.3
5 5
41.7 16.7
elliptical spot. Because the dimension perpendicular to the direction of laser scanning speed for the elliptical spot of 6 mm 4 mm was smaller than that for the elliptical spot of 4 mm 6 mm, the maximum laser specific energy using the elliptical spot of 6 mm 4 mm was higher than that using the elliptical spot of 4 mm 6 mm during LIHRC (Table 2). Based on the above analyzed results, namely, the minimum laser power using the elliptical spot of 6 mm 4 mm, which was required for the metallurgical bonding between the composite coating and the substrate, was lower than that using the elliptical spot of 4 mm 6 mm, the adjusting range of laser power using the elliptical spot of 6 mm 4 mm was wider than that using the elliptical spot of 4 mm 6 mm when the laser power was adjusted between the minimum laser power and the maximum laser power. For instance, in order to obtain the metallurgical bonding between the composite coating and the substrate, when the laser scanning speed was 1200 mm/min, the laser power using the elliptical spot of 6 mm 4 mm was varied from 3 to 5 kW, whereas the laser power using the elliptical spot of 4 mm 6 mm was varied from 3.5 to 5 kW. Correspondingly, the laser specific energy using the elliptical spot of 6 mm 4 mm could be adjusted from 37.5 to 62.5 J/ mm2, while the laser specific energy using the elliptical spot of 4 mm 6 mm could be adjusted from 29.2 to 41.7 J/mm2 (Table 2). Therefore, the adjusting range of laser specific energy using the elliptical spot of 6 mm 4 mm was wider than that using the elliptical spot of 4 mm 6 mm. The contact angle of the composite coating was calculated as follows [23]:
2H u ¼ 180 2 arctan W
elliptical spot kept constant, the dimension of the molten pool on the surface of the substrate was slightly increased, correspondingly, the cladding width somewhat increased during LIHRC. Moreover, the powder particles cloud attenuated the laser power, which caused that the dimension perpendicular the direction of laser scanning speed for the molten pool was always smaller than that for the elliptical spot. Therefore, the cladding width, W, was only determined by the dimension of elliptical spot which was perpendicular to the direction of laser scanning speed, and had little relation to the laser specific energy. Based on the above propensity of change with the laser specific energy for the cladding height and the cladding width, the contact angles between the composite coating and the substrate using the different elliptical spot decreased with increasing laser specific energy, which were all in an acceptable range of >1308 to be overlapped without defects [24]. Furthermore, when the elliptical spot of 6 mm 4 mm as described in Fig. 1b was adopted and laser scanning speed was increased to 3000 mm/min during LIHRC, the maximum rate of powder deposition, required for a good metallurgical bonding between the substrate and the composite coating with a good profile, was 86.79 g/min. However, for the elliptical spot of 4 mm 6 mm as shown in Fig. 1c, when laser scanning speed was 3000 mm/min, the maximum rate of powder deposition was 55.81 g/min. Therefore, the rate of powder deposition using the elliptical spot of 6 mm 4 mm was approximately increased one point six times higher than that using the elliptical spot of 4 mm 6 mm, which could be explained by following reasons. The higher the rate of powder deposition was, the higher the attenuated rate of the laser power
(2)
where W is the cladding width (mm) and H is the cladding height (mm). The effect of laser specific energy on the cladding width and the contact angle is shown in Fig. 3. It can be seen that the cladding width slightly increased with increasing laser specific energy and gradually approached the dimension perpendicular to the direction of laser scanning speed. Moreover, the contact angle decreased with increasing laser specific energy, the contact angle using the elliptical spot of 4 mm 6 mm was larger than that using the elliptical spot of 6 mm 4 mm. The change characteristics of the cladding width and the contact angle could be explained by following reasons. Although the laser power absorbed by the substrate and the composite powder increased with increasing laser specific energy when the flow rate of composite powder and the
Fig. 3. Effect of laser specific energy on cladding width and contact angle for the different elliptical spot.
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for the composite powder particles cloud was. Thus, the laser power which was absorbed by the composite powder increased, but the residual laser power which was absorbed by the substrate decreased, which easily resulted in the metallurgical bonding not to be generated well between the composite coating and the substrate. Therefore, the laser specific energy need be increased if the rate of powder deposition was to be increased. When the laser power and the laser scanning speed kept constant, it can be seen from Table 2 that the laser specific energy using the elliptical spot of 6 mm 4 mm was higher than that using the elliptical spot of 4 mm 6 mm. Thus, a relatively larger amount of the composite powder using the elliptical spot of 6 mm 4 mm could be cladded on the substrate in unit time compared to that using the elliptical spot of 4 mm 6 mm, i.e. the rate of powder deposition was increased. This was the main reason that the rate of powder deposition was relatively higher when the elliptical spot of 6 mm 4 mm was adopted during LIHRC. 3.2. Microstructure characteristics and microhardness distribution For the elliptical spot of 6 mm 4 mm, when laser scanning speed was 1200 mm/min, the microstructure of Ni-based WC composite coating by LIHRC is shown in Fig. 4. It can be seen from Fig. 4a that a planar growth near the bonding line, relatively coarse columnar dendrites and eutectics were observed in the composite coating. Fig. 4b shows the precipitated carbides due to dissolution of WC particles which were severely suffered from the heat damage. Due to the partial dissolution of some WC particles with large size, the blocky carbides were precipitated around WC particles, which showed that WC particles were characterized by the heat damage of ‘‘the disintegration pattern’’. The results of EDX analysis revealed that the blocky carbides contained approximately 8.42% Ni, 6.34% Cr, 15.3% Fe, 67.76% W and 2.18% C (wt.%), indicating that the blocky carbides were rich in W. However, due to the complete dissolution of other WC particles with small size, the bar-like carbides were reformed far away from WC particles with relatively larger size, which showed that WC particles were characterized by the heat damage of ‘‘the growth
pattern’’. The results of EDX analysis demonstrated that the bar-like carbides contained 10.27% Ni, 51.5% Cr, 6.25% Fe, 15.6% W and 16.38% C (wt.%), indicating that the bar-like carbides were mainly rich in Cr. Furthermore, when laser scanning speed was increased to 3000 mm/min, the microstructure of Ni-based WC composite coating by LIHRC is shown in Fig. 5. It can be seen that WC particles with small size were partially dissolved to precipitate the blocky carbides around WC particles and the bar-like carbides were not observed in the composite coating, which indicated that WC particles were only characterized by the heat damage of ‘‘the disintegration pattern’’ (Fig. 5a). The results of EDX analysis showed that the blocky carbides contained 7.58% Ni, 6.24% Cr, 9.26% Fe, 70.1% W and 6.82% C (wt.%), showing that the blocky carbides were also rich in W. Moreover, the larger carbide with blocky shape, containing 90.26% W and 9.74% C by EDX analysis, kept the integrality of structure. It was clear that the larger carbide with blocky shape was WC particle (Fig. 5b). For the elliptical spot of 4 mm 6 mm, when laser scanning speed was 1200 mm/min, the microstructure of Ni-based WC composite coating by LIHRC is shown in Fig. 6. It can be seen from Fig. 6a that g-nickel had firstly planar growth and then formed fine columnar dendrites and eutectics. Due to dissolution of WC particles, the blocky and bar-like carbides were precipitated in the composite coating (Fig. 6b). WC particles were characterized by the heat damage of ‘‘the disintegration pattern + the growth pattern’’, indicating that the heat damage of WC particles using the elliptical spot of 4 mm 6 mm had the same characteristics as that using the elliptical spot of 6 mm 4 mm did. EDX analysis indicated that the blocky carbides contained 7.8% Ni, 5.96% Cr, 13.6% Fe, 70.61% W and 2.03% C (wt.%), whereas the bar-like carbides contained 8.76% Ni, 56.05% Cr, 6.13% Fe, 13.8% W and 15.26% C (wt.%), indicating that the bar-like carbides contained a relatively larger amount of Cr and Ni, relatively smaller amount of W and Fe compared to the blocky carbides. When laser scanning speed was increased to 3000 mm/min, the microstructure of Ni-based WC composite coating by LIHRC is shown in Fig. 7. It can be seen from Fig. 7a that, due to the partial dissolution of WC particles, the fine carbides were
Fig. 4. Microstructure of Ni-based WC composite coatings by LIHRC with elliptical spot of 6 mm 4 mm, Vs = 1200 mm/min: (a) interface zone and (b) carbides characteristics.
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Fig. 5. Microstructure of Ni-based WC composite coatings by LIHRC with elliptical spot of 6 mm 4 mm, Vs = 3000 mm/min: (a) carbides characteristics and (b) WC characteristics.
precipitated around WC particles. Moreover, the blocky and bar-like carbides were not observed in the composite coating. Thus, WC particles were characterized by the heat damage of ‘‘the radiation pattern’’. EDX analysis demonstrated that those fine carbides contained 6.24% Ni, 4.52% Cr, 2.08% Fe, 82.04% W and 5.12% C (wt.%). Furthermore, the size of the precipitated carbides using the elliptical spot of 4 mm 6 mm was smaller than those using the elliptical spot of 6 mm 4 mm. According to the above results, the dissolution characteristics of WC particles may be deduced as follows. At low laser scanning speed, laser specific energy was relatively higher (Table 2), WC particles severely suffered from the heat damage. As a result, relatively larger amounts of Ni, Cr and Fe elements in the bonding metal, Ni60A, diffused into WC particles and W elements in WC particles diffused into the liquid Ni-based alloy. Thus, the blocky carbides were precipitated in the composite coating, indicating that WC particles with relatively larger size were characterized by the heat damage of ‘‘the disintegration pattern’’. Moreover, WC particles with relatively smaller size were completely dissolved to reform the bar-like carbides with higher concentration of Cr, showing that WC particles were characterized by the heat damage of the growth pattern (Figs. 4b and 6b). With increasing laser scanning speed, laser specific energy decreased, resulting in decreasing the heat
damage of WC particles. Thus, relatively smaller amounts of Ni, Cr and Fe elements in the bonding metal dissolved into WC particles with relatively smaller size, while WC particles with large size kept the integrality of structure, whose edge only suffered from the heat damage (Fig. 5b). With further decreasing laser specific energy, the fine carbides were precipitated due to dissolution of WC particles (Fig. 7a and b). Then, WC particles were characterized by the heat damage of ‘‘the radiation pattern’’. Whether the elliptical spot of 4 mm 6 mm was used or the elliptical spot of 6 mm 4 mm was adopted, the precipitated carbides had a common characteristic, namely, the concentration of Ni, Cr and Fe in the precipitated carbides with different shapes decreased and the amounts of W increased with a decrease of laser specific energy, which indicated that the dissolution of WC particles reduced. As a result, WC particles kept the characteristic of high hardness, the microhardness of the composite coatings was increased (Fig. 8). Moreover, with decreasing laser specific energy, the characteristic of heat damage for WC particles went through a transition of ‘‘the disintegration pattern + the growth pattern’’ to ‘‘the disintegration pattern’’ and ‘‘the radiation pattern’’. Therefore, the higher laser specific energy produced a significant dissolution of WC particles in the composite coatings, the lower microhardness of the composite coatings was obtained.
Fig. 6. Microstructure of Ni-based WC composite coatings by LIHRC with elliptical spot of 4 mm 6 mm, Vs = 1200 mm/min: (a) interface zone and (b) carbides characteristics.
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Fig. 7. Microstructure of Ni-based WC composite coatings by LIHRC with elliptical spot of 6 mm 4 mm, Vs = 3000 mm/min: (a) carbides characteristics and (b) WC characteristics.
3.3. Comparison to the general laser cladding As well known, ceramic-metal composite coatings were sensitive to porosities and cracks which retarded the wide application of laser cladding technique in industry. Fig. 9 shows the structure of Ni-based WC composite coating by the general laser cladding. It can be seen that there existed many porosities and cracks in Ni-based WC composite coatings by the general laser cladding. The typical distribution characteristic of cracks was perpendicular to the direction of laser scanning speed and traversed the whole composite coating (Fig. 9a). That was because the density of WC particles (16.5 g/cm3) was higher than that of Ni60A alloy (7.53 g/cm3) and the violent stirring and convection were driven by thermocapillarity [25], WC particles easily sank at the bottom of the composite coatings to decrease the toughness of bonding metal. As a result, cracks were induced to form in the composite coatings. Furthermore, WC particles sinking at the bottom of the molten pool and the precipitated carbides had an effect of hindrance on the moving of air bubbles in the molten pool. Thus, the moving resistance of air bubbles rising to surface had an increase and air bubbles were captured to form the porosities in the composite coatings (Fig. 9b). XRD analysis (Fig. 10) indicated that the composite coating was
Fig. 8. Microhardness distribution of Ni-based WC composite coatings by LIHRC with elliptical spot.
composed of W2C, WC, WC1x, M3C, M6C and M23C6 phases (M = Ni, Cr, Fe, W). Obviously, because the distribution of WC particles was fairly inhomogeneous and the carbides with various shapes were precipitated, especially, the bar-like carbides with small radius of curvature were formed. Then the structure stress and the phase transformation stress were possibly induced to increase the cracks sensitivity of the composite coating (Fig. 9c).Furthermore, when the laser power was 5 kW, the laser scanning speed of the general laser cladding was only as fast as 720 mm/min and the rate of powder deposition was 22.6 g/min for the elliptical spot of 6 mm 4 mm. However, in relation to LIHRC, when the same laser power and elliptical spot of 6 mm 4 mm were used, the laser scanning speed was increased to 3000 mm/min and the rate of powder deposition was 86.79 g/min. Moreover, a good metallurgical bonding between the composite coating and the substrate can be generated where the composite coating had a smooth surface and good profile without porosities and cracks (Fig. 11). During the general laser cladding, excessive heating resulted in the dissolution of WC particles to precipitate carbon as graphite [10]: 2WC ! W2 C þ C
(3)
The graphite could react with atmospheric oxygen and form CO and CO2, which were to be captured to form porosities during the rapid solidification as shown in Fig. 9b. Therefore, the dissolution of WC particles was required to have a decrease in order to reduce the porosities in the composite coatings. Then the precipitated carbon concentration was also decreased. EDX analysis indicated that the average carbon content was 2.96% (Vs = 3000 mm/min) for LIHRC, but the average carbon content was 6.13% (Vs = 720 mm/min) for the general laser cladding. Since the laser scanning speed was increased to 3000 mm/ min, the laser specific energy was markedly decreased, resulting in a decrease of dissolution of WC particles. Therefore, the produced CO and CO2 forming porosities in the composite coatings were decreased due to combination reaction between C and O2 in the air during LIHRC. This was the main reason that Ni-based WC composite coatings by LIHRC detected were free of porosities. Furthermore, the basic reason why the composite coatings detected were free of cracks was that the temperature gradient
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Fig. 9. Structure of Ni-based WC composite coatings by the general laser cladding with elliptical spot of 6 mm 4 mm, P = 5 kW, Vs = 720 mm/min: (a) cracks characteristic, (b) porosities characteristic and (c) carbides characteristic.
between the composite coating and the substrate was decreased during LIHRC, correspondingly, the produced thermal stress, due to high temperature gradient between the composite coating and the substrate, was decreased. Thus, the crack sensitivity was reduced. For a simple qualitative analysis, the temperature gradient during laser scanning was calculated as follows [26]: G¼
Fig. 10. X-ray diffraction result of Ni-based WC composite coatings by the general laser cladding.
2pKðT T 0 Þ2 hP
(4)
where G is the temperature gradient between the composite coating and the substrate (K/mm), T the liquidus temperature of the Ni-based alloy (K), T0 the preheating temperature of the substrate (K), h represents the laser absorption coefficient, P the laser power (W) and K is the thermal conductivity of the material (W m1 K1). At a laser power of 5 kW, when the substrate was not preheated, the temperature gradient G was
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Fig. 11. Structure of Ni-based WC composite coatings by LIHRC with elliptical spot of 6 mm 4 mm, P = 5 kW, Vs = 3000 mm/min: (a) single-pass profile and (b) cross-section characteristic.
1.69 103 K/mm (K = 82.9 W m1 K1, T = 1573 K, T0 = 300 K and h = 10%). If the average preheated temperature of the substrate was 1073 K, the temperature gradient G was 130.2 K/mm (T0 = 1073 K and h = 20%). For the thermal stress in Ni-based WC composite coatings, it could induce to form cracks and was described in the following [27]: s th ¼
Ec DaDT 1 vc
(5)
where Ec and vc are the elastic modulus and the Poisson ratio, respectively, Da represents the difference in coefficient of thermal expansion between the composite coating and the substrate and DT is the difference between the solidification temperature of the molten cladding material and the room temperature. Based on the above calculated results, the temperature gradient between the composite coating and the substrate during LIHRC was decreased much twelve times lower than that during the general laser cladding. It can be seen from Eq. (5) that the thermal stress in Ni-based WC composite coatings was markedly decreased. Therefore, Ni-based WC composite coatings without porosities and cracks were obtained by LIHRC. Furthermore, the laser scanning speed and the rate of powder deposition were respectively increased more four times and three point eight times higher than those of the general laser cladding. Due to very high laser scanning speed for LIHRC, the time of the stirring and convection in the molten pool was evidently decreased. For a simple calculation, the time of the stirring and convection in the molten pool could be described as the interaction time of substrate and composite powder with laser beam, which was given as follows [19,28]: t¼
L Vs
(6)
where L represents the spot dimension in the longitudinal direction, namely, the spot dimension which is parallel to the direction of the laser scanning speed. For the general laser cladding, when the elliptical spot of 6 mm 4 mm was adopted and the laser scanning speed was 720 mm/min, the time of the stirring and convection in the molten pool, t, was 0.5 s. However, for LIHRC, when the same elliptical spot was
used and laser scanning speed was increased to 3000 mm/min, t was 0.12 s. The time of the stirring and convection in the molten pool during LIHRC was reduced much four times lower than that during the general laser cladding. Thus, the distribution of WC particles across the whole sectional area was rather uniform (Fig. 11b), that was because the stirring and convection in the molten pool had no enough time to contribute WC particles to sink at the bottom of the composite coating. 4. Conclusions The adjusting range of laser specific energy using the elliptical spot of 6 mm 4 mm was wider compared to that using the elliptical spot of 4 mm 6 mm, the maximum rate of powder deposition was about one point six times higher than that using the elliptical spot of 4 mm 6 mm. For high laser specific energy, the blocky and bar-like carbides were precipitated in the composite coating due to dissolution of WC particles, which took on the heat damage of ‘‘the disintegration pattern + the growth pattern’’. With decreasing laser specific energy, the blocky carbides were only precipitated in the composite coating, indicating that WC particles suffered from the heat damage of ‘‘the disintegration pattern’’. With further decreasing laser specific energy, the fine carbides were reform around WC particles, which presented the heat damage of ‘‘the radiation pattern’’. Therefore, the higher laser specific energy produced a significant dissolution of WC particles in the composite coatings, the lower microhardness of the composite coatings was obtained. Due to the introduction of induction heater, the laser scanning speed and the rate of powder deposition were markedly increased, the laser scanning speed and the rate of powder deposition by LIHRC were, respectively, increased much four times and there point eight times higher than those by the general laser cladding. Moreover, the time of stirring and convection in the molten pool during LIHRC were reduced to keep WC particles from sinking at the bottom of the composite coatings. Thus, the distribution of WC particles across the whole sectional area was rather uniform and a good metallurgical bonding between the composite coating and the substrate was produced where the composite coating had a smooth surface and good profile without porosities and cracks.
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