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Laser induction hybrid rapid cladding of WC particles reinforced NiCrBSi composite coatings
Applied Surface Science 256 (2010) 4708–4714
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Laser induction hybrid rapid cladding of WC particles reinforced NiCrBSi composite coatings Shengfeng Zhou a,∗ , Xiaoqin Dai b a b
School of Material Science and Engineering, Nanchang Hangkong University, Fenghenan Road 696, Nanchang, Jiangxi 330063, PR China School of Information Engineering, Nanchang Hangkong University, Nanchang, Jiangxi 330063, PR China
a r t i c l e
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Article history: Received 23 December 2009 Received in revised form 24 February 2010 Accepted 24 February 2010 Available online 3 March 2010 Keywords: Laser induction hybrid rapid cladding (LIHRC) Carbides Cast WC particles Individual laser cladding Crack-free
a b s t r a c t In order to investigate the microstructure characteristics and properties of Ni-based WC composite coatings containing a relatively large amount of WC particles by laser induction hybrid rapid cladding (LIHRC) and compare to the individual laser cladding without preheating, Ni60A + 35 wt.% WC composite coatings are deposited on A3 steel plates by LIHRC and the individual laser cladding without preheating. The composite coating produced by the individual laser cladding without preheating exhibits many cracks and pores, while the smooth composite coating without cracks and pores is obtained by LIHRC. Moreover, the cast WC particles take on the similar dissolution characteristics in Ni60A + 35 wt.% WC composite coatings by LIHRC and the individual laser cladding without preheating. Namely, the completely dissolved WC particles interact with Ni-based alloy solvent to precipitate the blocky and herringbone carbides, while the partially dissolved WC particles still preserve the primary lamellar eutectic structure. A few WC particles are split at the interface of WC and W2 C, and then interact with Ni-based alloy solvent to precipitate the lamellar carbides. Compared with the individual laser cladding without preheating, LIHRC has the relatively lower temperature gradient and the relatively higher laser scanning speed. Therefore, LIHRC can produce the crack-free composite coating with relatively higher microhardness and relatively more homogeneous distribution of WC particles and is successfully applied to strengthen the corrugated roller, showing that LIHRC process has a higher efficiency and good cladding quality. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Composite coating generally consists of a metallic matrix and the reinforcements such as non-metallic solid particles, hard metal particles and short fibers. Electro-deposition, thermal spraying and laser cladding are a few important techniques for the fabrication of composite coating [1]. Laser cladding has been developed for its capability of introducing hard particles such as SiC, TiC and WC as reinforcements in the metallic matrix such as Ni-based alloy, Co-based alloy and Fe-based alloy to form the ceramic–metal composite coatings, which have very high hardness and good wear resistance [2–6]. Therefore, the theoretical analysis and experimental verification on laser cladding ceramic–metal composite coatings have received considerable attention during the past decade [7–11]. However, up to now, the technique of laser cladding ceramic–metal composite coatings is not widely applied in industry. The reasons mainly lie in two aspects: the low cladding efficiency and the cracks of cladding layer. The former makes the
∗ Corresponding author. Tel.: +86 791 3863026. E-mail address: [email protected] (S. Zhou). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.078
laser cladding process expensive and the latter limits its application in very high demanding environments. Therefore, many methods have been put forward to eliminate the cracks of cladding layer during the individual laser cladding, such as preheating the substrate [12], adding the rare earth or other metal oxides [13], adopting the optimized parameters [14], the functionally graded coating [15] and the pulsed laser cladding [16]. However, summing up the published results [17–19], it is found that the formation of cracks during laser cladding is very complicated, which not only is related to the thermophysical properties of the substrate and the cladding material, such as the melting point, the elastic modulus and the coefficient of thermal expansion, but also is related to the processing conditions, such as the laser processing parameters, the composition of the metallic matrix, the proportion of carbides in the mixture and the size of the proper ceramic particles. Although the above-mentioned methods can decrease the crack sensitivity of cladding layer, the cladding efficiency does not significantly increase [20]. Obviously, it is difficult to increase the cladding efficiency and eliminate the cracks of cladding layer at the same time. In our previous work [21], the crack-free Ni60A + 20 wt.% WC composite coatings with high microhardness and metallurgical
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bonding to substrate are obtained by LIHRC when the efficiency of LIHRC is increased much four times higher than that of the individual laser cladding without preheating. Moreover, it is found that increasing the addition of WC weight fraction in the coating for enhancing the wear resistance results in the precipitation of complex carbides and the residual stress which leads to the propagation of cracks during the individual laser cladding without preheating [22,23]. What are the characteristics of cracks, microstructure and properties of the composite coating when the weight fraction of WC particles in the coating increases during LIHRC? Therefore, the present paper is to investigate the processing parameters of the critical state, the microstructure and properties of Ni60A + 35 wt.% WC composite coatings by LIHRC and compare to the individual laser cladding without preheating. 2. Experimental procedures Fig. 1. Surface morphology of Ni60A + 35 wt.% WC composite coating by LIHRC.
In the experiment of LIHRC, the substrate used was A3 steel which was machined into the rectangular plates with the dimension of 180 mm × 60 mm × 8 mm. The ceramic phase was the cast WC particles with polygonal morphology, which was composed of the lamellar WC + W2 C eutectic with the size of 20–40 m. The bonding metal used was self-fluxing Ni60A alloy powder with the size of 40–100 m, whose chemical composition was (wt.%) 0.5–0.9 C, 15–18 Cr, 3.0–4.5 B, 3.5–5.5 Si, 13.0–15.0 Fe and Ni in balance. The composite powder composed of 65 wt.% Ni60A and 35 wt.% WC was prepared by granulating method to feed powder automatically. The experimental apparatus used has been described in detail in our previous paper [21]. During LIHRC, the hybrid processing parameters were listed as follows: the laser power was 5 kW, the laser scanning speed, Vs , was in the range of 500–3000 mm/min, the major axis and the minor axis of the elliptical spot were 8 mm and 6 mm, respectively, and the major axis of the elliptical spot was parallel to the direction of the laser scanning speed, the frequency of induction heater was 30–50 kHz and the distance of induction coil from the surface of substrate was varied from 2 to 10 mm, the substrate was preheated to 1173 K by adjusting the power of induction heater, the angle between the powder nozzle and the normal of the substrate was 37◦ , the distance between the tip of the powder nozzle and the surface of substrate was 12 mm, the powder flowing ˙ was varied from 18 to 90 g/min. rate, m, After LIHRC, all specimens were detected by dye penetrant nondestructive testing (DPNDT) to show the presence of any pores and cracks, and their surface morphology was analyzed by optical microscopy. Afterwards, the transverse sections of each specimen were cut, mechanically grounded, polished in diamond paste and double etched: etching with a solution of KOH + K3 Fe(CN)6 in water (5 s), revealing WC and W2 C eutectic structure, followed by etching with a mixed acid of 75 vol.% HCl and 25 vol.% HNO3 (3 s), revealing the microstructure of the bonding metal. The cross-sectional view of the composite coatings was examined by scanning electron microscopy (SEM) and chemical composition was measured by energy dispersive spectrum (EDS). The X-ray diffraction (XRD) was applied to study the phases in Ni60A + 35 wt.% WC composite coating. Microhardness profile along the cross-section of the composite coating was measured by means of HVS-1000 digital microhardness tester using a load of 0.2 kg and a dwelling time of 15 s.
laser specific energy, the cladding width decreases and the little pits increase gradually on the edge of the composite coating, leading to a reduction of the full state; with the decreasing of the powder density, the little pits increase gradually and are interconnected to form the grooves in the center of the composite coating, leading to an unmelted state), the processing parameters of the critical state of Ni60A + 35 wt.% WC composite coating during LIHRC obtained are as follows: the maximum laser scanning speed is 2200 mm/min, the maximum powder flowing rate is 75.6 g/min. When the abovementioned processing parameters are adopted during LIHRC, the surface morphology of Ni60A + 35 wt.% WC composite coating is shown in Fig. 1. It can be seen that the composite coating has a good appearance without discontinuities, cracks or lack of adherence. Fig. 2 shows the transverse morphology of Ni60A + 35 wt.% WC composite coating by LIHRC. The cladding height and the cladding width are 1.06 and 5.92 mm, respectively. The dilution which can be defined mathematically as the ratio of the cladding depth in the substrate to the sum of cladding height and cladding depth in Ref. [25] is 5.2%, which is in the acceptable range of <10%. Moreover, the distribution of WC particles in Ni-based metallic matrix is fairly uniform. 3.2. Microstructure of the composite coating by LIHRC Fig. 3 shows the microstructure of Ni60A + 35 wt.% WC composite coating by LIHRC. The thin bonding line (1–2 m) is observed at the coating–substrate interface, showing that the excellent metallurgical bonding is formed between the composite coating and the substrate. Moreover, with increasing distance from the surface of substrate, the bonding metal which has a typical microstructure characteristic of the rapid solidification presents the planar, dendritic growth and eutectic structure. The X-ray diffraction result of Ni60A + 35 wt.% WC composite coating by LIHRC is shown in Fig. 4. It indicates that the major phases in the composite coating are ␥-(Ni, Fe) with the fcc crystal structure, Ni4 B3 with an orthorhombic crystal structure and M23 C6 (M = W, Ni, Cr, Fe) type carbide with the fcc crystal structure, and that other carbides corresponding peaks have a low intensity. Therefore, Ni60A + 35 wt.% WC composite coating
3. Results 3.1. Macrostructure of the composite coating by LIHRC According to the definition of the critical state of Ni-based WC composite coating during LIHRC [24] (i.e. with the decreasing of the
Fig. 2. Cross-section morphology of Ni60A + 35 wt.% WC composite coating by LIHRC.
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S. Zhou, X. Dai / Applied Surface Science 256 (2010) 4708–4714 Table 1 Chemical composition of the carbides in Ni60A + 35 wt.% WC composite coating by LIHRC. Carbides characteristics
Composition (wt.%) Ni
White particle (A) Blocky carbides (B) Herringbone carbides (C) Lamellar carbides (D)
28.18 17.71 5.36
Cr 6.75 30.01 28.27
Fe
W
C
18.62 2.81 1.25
94.68 40.82 42.98 56.88
5.32 5.63 6.49 8.24
Fig. 3. Microstructure of Ni60A + 35 wt.% WC composite coating by LIHRC.
Fig. 6. Surface morphology of Ni60A + 35 wt.% WC composite coating by the indi˙ = 25 g/min. vidual laser cladding without preheating, Vs = 600 mm/min, m
Fig. 4. X-ray diffraction result of Ni60A + 35 wt.% WC composite coating by LIHRC.
by LIHRC contains a metallic matrix, constituted by Ni-based alloy and in which the carbides such as WC, W2 C, M12 C and M23 C6 are embedded. Fig. 5 shows BSE micrograph of carbides in Ni60A + 35 wt.% WC composite coating by LIHRC. According to the morphological features of the carbide phases, XRD result (Fig. 4) and EDS analysis (Table 1), the white particles with lamellar eutectic structure should be the cast WC (marked A in Fig. 5a), and an alloyed reaction
layer is formed at the edge of WC particles. Moreover, the polygonal morphology of WC particles disappears and their edge angles become smooth, showing that Ni-based metallic matrix has a good consistency and wettability with the cast WC particles. Moreover, the W-rich blocky carbide (marked B in Fig. 5a) containing a high concentration of Ni and a small amount of Fe, the W-rich herringbone carbide (marked C in Fig. 5a) containing a large amount of Cr and a low concentration of Ni, and the W-rich lamellar carbide (marked D in Fig. 5b) containing a large amount of Cr are also precipitated around WC particles. 3.3. Comparison to the composite coating by the individual laser cladding without preheating Fig. 6 shows the surface morphology of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. Although the composite coating has a smooth surface and good
Fig. 5. BSE micrograph of carbides in Ni60A + 35 wt.% WC composite coating by LIHRC.
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˙ = 25 g/min. Fig. 7. Cross-section morphology of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating, Vs = 600 mm/min, m
Fig. 8. X-ray diffraction result of Ni60A + 35 wt.% WC composite coating by the indi˙ = 25 g/min. vidual laser cladding without preheating, Vs = 600 mm/min, m
profile, many cracks perpendicular to the direction of the laser scanning speed pass through the whole composite coating. Fig. 7 shows the transverse morphology of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. The cladding height and the cladding width are 1.25 and 5.82 mm, respectively. Moreover, many pores are observed and WC particles are distributed unevenly on both sides of the composite coating. The cast WC particles are dissolved remarkably in the center and at the top of the composite coating, where only a few WC particles with integrate structure are observed. The X-ray diffraction result of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating is shown in Fig. 8. It can be seen that the bonding metal is composed of ␥-Ni with the fcc crystal structure, Ni3 B and Ni3 B4 with an orthorhombic crystal structure, and that the carbides consist of
W2 C, WC, WC1−x , M12 C, M6 C and M23 C6 (M = W, Ni, Cr, Fe). The former two kinds of carbides have a hexagonal crystal structure, while the latter four kinds of carbides have a cubic crystal structure. Fig. 9 shows the morphology of carbides in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. Due to dissolution of the cast WC particles, the carbides with different shapes which are similar to those in Ni60A + 35 wt.% WC composite coating by LIHRC are precipitated in the center of the composite coating (Fig. 9a), such as the herringbone carbide (marked E), the blocky carbide (marked F) and the lamellar carbide (marked G). Moreover, it can be seen from Fig. 9b that a large amount of WC particles still preserve their primary lamellar eutectic structure, and that their polygonal morphology disappears. It is evident that the cast WC particles take on the different dissolution characteristics in the different regions of the composite coating by the individual laser cladding without preheating. Fig. 10 shows the microhardness profile of Ni60A + 35 wt.% WC composite coatings by LIHRC and the individual laser cladding without preheating. It can be seen that the microhardness of Ni60A + 35 wt.% WC composite coating by LIHRC is higher than that of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. For comparison purpose, the corrugated roller is strengthened by LIHRC and the individual laser cladding without preheating. Generally, the microhardness of the corrugated roller (about 600HV0.2 ) can be increased to 700–800HV0.2 by traditional high frequency quenching. If Ni60A + 35 wt.% WC composite coating is produced by the individual laser cladding without preheating on the corrugate roller, the microhardness of the composite coating is 950–1050HV0.2 , but many cracks appear in the composite coating. Interestingly, although the laser scanning speed and the powder flow rate increases to 2200 mm/min and 75.6 g/min, respectively, a high quality composite coating free of cracks is obtained by LIHRC on
˙ = 25 g/min. (a) The carbides Fig. 9. Morphology of carbides in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating, Vs = 600 mm/min, m in the center of Ni60A + 35 wt.% WC composite coating. (b) The carbides at the edge of Ni60A + 35 wt.% WC composite coating.
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S. Zhou, X. Dai / Applied Surface Science 256 (2010) 4708–4714 Table 2 Chemical composition of the bonding metal in Ni60A + 35 wt.% WC composite coating. Processing method
LIHRC Individual laser cladding without preheating
Fig. 10. Microhardness of Ni60A + 35 wt.% WC composite coating by LIHRC and the individual laser cladding without preheating.
Fig. 11. Ni60A + 35 wt.% WC composite coating produced by LIHRC to strengthen ˙ = 75.6 g/min. the corrugated roller, Vs = 2200 mm/min, m
the corrugate roller (Fig. 11) and the microhardness of composite coating can be increased to 1063–1198HV0.2 . According to the above results, LIHRC has a relatively higher efficiency and better cladding quality in comparison with the individual laser cladding without preheating. 4. Discussion Although the substrate is preheated to 1173 K by induction heater, LIHRC is still a process of the rapid heating and rapid solidification. Based on the theory of the rapid solidification [26], the microstructure characteristics depend on the ratio of the temperature gradient G to the solidification front rate R (i.e. G/R). When the surface of substrate is molten immediately by laser induction hybrid rapid cladding heat resource, the solidification front rate R is approximately zero at the interface of the molten pool and the unmelted substrate, but the temperature gradient G is very high. Consequently, the G/R is infinite and the solidification should occur with a planar front. Afterwards, with increasing distance from the surface of substrate, the solidification front rate R increases rapidly and the temperature gradient G decreases. As a result, the G/R decreases markedly and the planar solid–liquid interface becomes unstable to induce the onset of the dendritic growth and eutectic structure. Due to a low free formation enthalpy of WC (38.5 kJ/mol), WC is apt to dissolve in Ni-based alloy solvent, and interact with Ni-based
Composition (wt.%) Ni
Cr
Fe
W
C
49.39 40.74
15.48 12.48
8.71 7.37
21.99 34.18
4.45 5.23
alloy solvent to precipitate the carbides with different shapes during LIHRC and the individual laser cladding without preheating. This can be explained by Ni–W–C ternary phase diagram, where there exist two regions such as the C-lean and C-rich region [27]. In the C-rich region, there exist three phase fields such as WC + C + ␥ and two phase fields such as C + ␥ , where ␥ is Ni-based supersaturated solid solution containing large amounts of W and C. In the C-lean region, the carbides with different structures can be precipitated, such as 1 , 2 , 3 and W2 C, where 1 and 2 are (Ni, W)6 C and M12 C type carbides, respectively, both have the fcc crystal structure, and 3 is the mixed carbide with a hexagonal crystal structure. Therefore, the dissolution of WC particles results in a large amount of free W and C atoms in the molten pool during LIHRC and the individual laser cladding without preheating. For Ni-based alloy solvent, these W and C atoms can dissolve into the dendrites ␥-Ni to form the supersaturated solid solution, ␥ , during the rapid solidification. Moreover, the interaction between solute atoms in Ni-based alloy such as Ni, Cr, Fe and B occur, forming the eutectic ␥-Ni + Ni2 B + Ni3 B4 in the interdendritic matrix phase, ␥ . For the cast WC particles with relatively large size, they are not dissolved completely during LIHRC or the individual laser cladding without preheating, but their edge angles can be dissolved preferentially. As a result, a C-lean region is formed, and Ni, Cr and Fe atoms in Ni-based alloy solvent diffuse simultaneously into the C-lean region. Subsequently, the concentration gradient formed at the edge of WC particles can induce the phase transformation during the rapid solidification. Consequently, an alloyed reaction layer with a thickness of 1–3 m is generated at the edge of WC particles and the polygonal morphology of WC particles disappears, but the other regions of WC particles still preserve the primary lamellar eutectic structure. For the cast WC particles with relatively small size, they can be dissolved completely during LIHRC or the individual laser cladding without preheating, resulting in a large amount of W and C atoms in the molten pool. These W and C can interact with Ni, Cr and Fe in Ni-based alloy solvent to precipitate the W-rich blocky M6 C or M12 C type carbide containing a high concentration of Ni and a small amount of Fe, and the W-rich herringbone M23 C6 containing a large amount of Cr and a small amount of Ni (Table 1). Moreover, due to the heat impact of laser beam and the capillarity of the molten pool, only a few WC particles are dispersed into many flakes, which interact with Ni, Cr and Fe in Ni-based alloy solvent to form the lamellar W-rich carbide containing a large amount of Cr. The carbide morphology in Ni60A + 35 wt.% WC composite coating by LIHRC (Fig. 5) is compared to that in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating (Fig. 9). It reveals that the amounts of the blocky, lamellar and herringbone carbides precipitated in the composite coating by LIHRC are obviously less than those precipitated in the composite coating by the individual laser cladding without preheating. Moreover, the average W content in Ni-based metallic matrix produced by LIHRC is less than that in Ni-based metallic matrix produced by the individual laser cladding without preheating (Table 2). The above results show that the dissolution level or heat damage degree of WC particles in Ni60A + 35 wt.% WC composite coat-
S. Zhou, X. Dai / Applied Surface Science 256 (2010) 4708–4714
ing by LIHRC is less than that in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. As a result, the high microhardess characteristic of WC particles is preserved during LIHRC. Additionally, the distribution of WC particles in Ni60A + 35 wt.% WC composite coating by LIHRC is more homogeneous than that in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating, showing that WC particles have a better strengthening effect on Ni-based metallic matrix during LIHRC. Therefore, Ni60A + 35 wt.% WC composite coating by LIHRC has a higher microhardness than that by the individual laser cladding without preheating (Fig. 10). Moreover, Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating exhibits many cracks which are approximately perpendicular to the direction of the laser scanning speed. It can be explained by the following reasons. Firstly, the carbides are distributed unevenly in the dense Ni-based metallic matrix, resulting in the interface stress between the carbides and Ni-based metallic matrix [18]. Secondly, the temperature gradient is very high during the individual laser cladding without preheating, leading to the thermal stress in the composite coating [17]. Thirdly, WC particles suffer from the different dissolution level or heat damage degree in the different regions of the composite coating, and then interact with Ni-based alloy solvent to precipitate the carbides with different shapes. The stress can also be induced due to phase transformation. Therefore, the cracking is caused by residual stress during the individual laser cladding without preheating. However, when the weight percentage of WC particles in the composite powder increases to 35 wt.% and the laser scanning speed increases to 2200 mm/min during LIHRC, the crack-free Ni-based WC composite coating with high microhardness is obtained. It can be explained by the following reasons. Firstly, the thermal stress in the composite coating can be calculated as follows [17]: T =
Ec · Es · ts · (˛c − ˛s ) · T (1 − v) · (Es ts + Ec tc )
(1)
where T is the thermal stress, Ec and Es are Young’s modulus of the composite coating and the substrate, respectively, ˛c and ˛s are the coefficient of thermal expansion of the composite coating and the substrate, respectively, tc and ts are the height of the composite coating and the substrate, respectively, v is Poisson’s ratio and T is the difference between the solidification temperature of the molten cladding material and the room temperature. Clearly, when the substrate is preheated to 1173 K, the temperature gradient calculated in Ref. [18] during LIHRC is decreased much nineteen times lower than that during the individual laser cladding without preheating. It can be seen from Eq. (1) that the thermal stress in Ni60A + 35 wt.% composite coating by LIHRC is decreased remarkably. Secondly, the time of the convection and the stirring in the molten pool during laser cladding can be calculated as follows [28]: =
L Vs
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composite coating by the individual laser cladding without preheating. As a result, the residual stress induced during LIHRC is less than that induced during the individual laser cladding without preheating. Therefore, when the laser scanning speed increases to 2200 mm/min during LIHRC, the crack-free Ni60A + 35 wt.% WC composite coating is obtained. 5. Conclusions (1) According to the definition of the critical state of Ni-based WC composite coating during LIHRC, the maximum laser scanning speed (i.e. 2200 mm/min) and the maximum powder feeding rate (i.e. 75.6 g/min) can be obtained when Ni60A + 35 wt.% composite coating is produced by LIHRC. Moreover, the composite coating in which WC particles are distributed homogeneously has a good profile and no cracks. (2) During LIHRC, the bonding line is formed at the interface of the composite coating and the substrate. The bonding metal presents the planar, dendritic growth and eutectic structure which is composted of ␥-Ni + Ni3 B + Ni3 B4 . The carbides consist of WC, W2 C, M12 C and M23 C6 (M = W, Ni, Cr, Fe) are embedded in the dense Ni-based metallic matrix. (3) In Ni60A + 35 wt.% WC composite coating by LIHRC and the individual laser cladding without preheating, the cast WC particles take on three kinds of the dissolution characteristics. Firstly, WC particles with relatively small size are completely dissolved and interact with Ni-based alloy solvent to precipitate the blocky and herringbone carbides. Secondly, WC particles with relatively large size are partially dissolved and still consist of the lamellar eutectic structure, but their primary polygonal morphology disappears. Thirdly, due to the heat impact of laser beam and the capillarity of the molten pool, a few WC particles are dispersed into many flakes, which interact with Ni-based alloy solvent to form the lamellar W-rich carbides. (4) During LIHRC, the increasing of the laser scanning speed can improve the homogeneous distribution of WC particles and decrease the dissolution level of WC particles in Ni60A + 35 wt.% WC composite coating. Moreover, the temperature gradient is also decreased because the substrate is preheated during LIHRC. As a result, the crack-free Ni60A + 35 wt.% WC composite coating can be produced by LIHRC and its microhardness is higher than that of Ni60A + 35 wt.% WC composite coating produced by the individual laser cladding without preheating. Acknowledgements The supports of this work by the National Natural Science Foundation of China (grant no. 50901040), the Aviation Science Foundation of China (grant no. 2009ZE56013) and Jiangxi Province Education Department Foundation of China (grant no. GJJ10507) are gratefully acknowledged.
(2)
where L represents the major axis of the elliptical spot, and Vs is the laser scanning speed. When the laser scanning speed increases to 2200 mm/min, the time of the convection and the stirring in the molten pool during LIHRC is about 0.22 s, which is decreased much three times lower than that during the individual laser cladding without preheating. Therefore, the decreasing of the time of the convection and the stirring in the molten pool during LIHRC can help prevent WC particles from sinking to the bottom of the molten pool. The distribution homogeneity of WC particles is also improved accordingly in the composite coating. Thirdly, the dissolution level or heat damage degree of WC particles in Ni60A + 35 wt.% WC composite coating by LIHRC is less than that in Ni60A + 35 wt.% WC
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laser induction hybrid rapid cladding of WC particles reinforced NiCrBSi composite coatings Shengfeng Zhou a,∗ , Xiaoqin Dai b a b
School of Material Science and Engineering, Nanchang Hangkong University, Fenghenan Road 696, Nanchang, Jiangxi 330063, PR China School of Information Engineering, Nanchang Hangkong University, Nanchang, Jiangxi 330063, PR China
a r t i c l e
i n f o
Article history: Received 23 December 2009 Received in revised form 24 February 2010 Accepted 24 February 2010 Available online 3 March 2010 Keywords: Laser induction hybrid rapid cladding (LIHRC) Carbides Cast WC particles Individual laser cladding Crack-free
a b s t r a c t In order to investigate the microstructure characteristics and properties of Ni-based WC composite coatings containing a relatively large amount of WC particles by laser induction hybrid rapid cladding (LIHRC) and compare to the individual laser cladding without preheating, Ni60A + 35 wt.% WC composite coatings are deposited on A3 steel plates by LIHRC and the individual laser cladding without preheating. The composite coating produced by the individual laser cladding without preheating exhibits many cracks and pores, while the smooth composite coating without cracks and pores is obtained by LIHRC. Moreover, the cast WC particles take on the similar dissolution characteristics in Ni60A + 35 wt.% WC composite coatings by LIHRC and the individual laser cladding without preheating. Namely, the completely dissolved WC particles interact with Ni-based alloy solvent to precipitate the blocky and herringbone carbides, while the partially dissolved WC particles still preserve the primary lamellar eutectic structure. A few WC particles are split at the interface of WC and W2 C, and then interact with Ni-based alloy solvent to precipitate the lamellar carbides. Compared with the individual laser cladding without preheating, LIHRC has the relatively lower temperature gradient and the relatively higher laser scanning speed. Therefore, LIHRC can produce the crack-free composite coating with relatively higher microhardness and relatively more homogeneous distribution of WC particles and is successfully applied to strengthen the corrugated roller, showing that LIHRC process has a higher efficiency and good cladding quality. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Composite coating generally consists of a metallic matrix and the reinforcements such as non-metallic solid particles, hard metal particles and short fibers. Electro-deposition, thermal spraying and laser cladding are a few important techniques for the fabrication of composite coating [1]. Laser cladding has been developed for its capability of introducing hard particles such as SiC, TiC and WC as reinforcements in the metallic matrix such as Ni-based alloy, Co-based alloy and Fe-based alloy to form the ceramic–metal composite coatings, which have very high hardness and good wear resistance [2–6]. Therefore, the theoretical analysis and experimental verification on laser cladding ceramic–metal composite coatings have received considerable attention during the past decade [7–11]. However, up to now, the technique of laser cladding ceramic–metal composite coatings is not widely applied in industry. The reasons mainly lie in two aspects: the low cladding efficiency and the cracks of cladding layer. The former makes the
∗ Corresponding author. Tel.: +86 791 3863026. E-mail address: [email protected] (S. Zhou). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.078
laser cladding process expensive and the latter limits its application in very high demanding environments. Therefore, many methods have been put forward to eliminate the cracks of cladding layer during the individual laser cladding, such as preheating the substrate [12], adding the rare earth or other metal oxides [13], adopting the optimized parameters [14], the functionally graded coating [15] and the pulsed laser cladding [16]. However, summing up the published results [17–19], it is found that the formation of cracks during laser cladding is very complicated, which not only is related to the thermophysical properties of the substrate and the cladding material, such as the melting point, the elastic modulus and the coefficient of thermal expansion, but also is related to the processing conditions, such as the laser processing parameters, the composition of the metallic matrix, the proportion of carbides in the mixture and the size of the proper ceramic particles. Although the above-mentioned methods can decrease the crack sensitivity of cladding layer, the cladding efficiency does not significantly increase [20]. Obviously, it is difficult to increase the cladding efficiency and eliminate the cracks of cladding layer at the same time. In our previous work [21], the crack-free Ni60A + 20 wt.% WC composite coatings with high microhardness and metallurgical
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bonding to substrate are obtained by LIHRC when the efficiency of LIHRC is increased much four times higher than that of the individual laser cladding without preheating. Moreover, it is found that increasing the addition of WC weight fraction in the coating for enhancing the wear resistance results in the precipitation of complex carbides and the residual stress which leads to the propagation of cracks during the individual laser cladding without preheating [22,23]. What are the characteristics of cracks, microstructure and properties of the composite coating when the weight fraction of WC particles in the coating increases during LIHRC? Therefore, the present paper is to investigate the processing parameters of the critical state, the microstructure and properties of Ni60A + 35 wt.% WC composite coatings by LIHRC and compare to the individual laser cladding without preheating. 2. Experimental procedures Fig. 1. Surface morphology of Ni60A + 35 wt.% WC composite coating by LIHRC.
In the experiment of LIHRC, the substrate used was A3 steel which was machined into the rectangular plates with the dimension of 180 mm × 60 mm × 8 mm. The ceramic phase was the cast WC particles with polygonal morphology, which was composed of the lamellar WC + W2 C eutectic with the size of 20–40 m. The bonding metal used was self-fluxing Ni60A alloy powder with the size of 40–100 m, whose chemical composition was (wt.%) 0.5–0.9 C, 15–18 Cr, 3.0–4.5 B, 3.5–5.5 Si, 13.0–15.0 Fe and Ni in balance. The composite powder composed of 65 wt.% Ni60A and 35 wt.% WC was prepared by granulating method to feed powder automatically. The experimental apparatus used has been described in detail in our previous paper [21]. During LIHRC, the hybrid processing parameters were listed as follows: the laser power was 5 kW, the laser scanning speed, Vs , was in the range of 500–3000 mm/min, the major axis and the minor axis of the elliptical spot were 8 mm and 6 mm, respectively, and the major axis of the elliptical spot was parallel to the direction of the laser scanning speed, the frequency of induction heater was 30–50 kHz and the distance of induction coil from the surface of substrate was varied from 2 to 10 mm, the substrate was preheated to 1173 K by adjusting the power of induction heater, the angle between the powder nozzle and the normal of the substrate was 37◦ , the distance between the tip of the powder nozzle and the surface of substrate was 12 mm, the powder flowing ˙ was varied from 18 to 90 g/min. rate, m, After LIHRC, all specimens were detected by dye penetrant nondestructive testing (DPNDT) to show the presence of any pores and cracks, and their surface morphology was analyzed by optical microscopy. Afterwards, the transverse sections of each specimen were cut, mechanically grounded, polished in diamond paste and double etched: etching with a solution of KOH + K3 Fe(CN)6 in water (5 s), revealing WC and W2 C eutectic structure, followed by etching with a mixed acid of 75 vol.% HCl and 25 vol.% HNO3 (3 s), revealing the microstructure of the bonding metal. The cross-sectional view of the composite coatings was examined by scanning electron microscopy (SEM) and chemical composition was measured by energy dispersive spectrum (EDS). The X-ray diffraction (XRD) was applied to study the phases in Ni60A + 35 wt.% WC composite coating. Microhardness profile along the cross-section of the composite coating was measured by means of HVS-1000 digital microhardness tester using a load of 0.2 kg and a dwelling time of 15 s.
laser specific energy, the cladding width decreases and the little pits increase gradually on the edge of the composite coating, leading to a reduction of the full state; with the decreasing of the powder density, the little pits increase gradually and are interconnected to form the grooves in the center of the composite coating, leading to an unmelted state), the processing parameters of the critical state of Ni60A + 35 wt.% WC composite coating during LIHRC obtained are as follows: the maximum laser scanning speed is 2200 mm/min, the maximum powder flowing rate is 75.6 g/min. When the abovementioned processing parameters are adopted during LIHRC, the surface morphology of Ni60A + 35 wt.% WC composite coating is shown in Fig. 1. It can be seen that the composite coating has a good appearance without discontinuities, cracks or lack of adherence. Fig. 2 shows the transverse morphology of Ni60A + 35 wt.% WC composite coating by LIHRC. The cladding height and the cladding width are 1.06 and 5.92 mm, respectively. The dilution which can be defined mathematically as the ratio of the cladding depth in the substrate to the sum of cladding height and cladding depth in Ref. [25] is 5.2%, which is in the acceptable range of <10%. Moreover, the distribution of WC particles in Ni-based metallic matrix is fairly uniform. 3.2. Microstructure of the composite coating by LIHRC Fig. 3 shows the microstructure of Ni60A + 35 wt.% WC composite coating by LIHRC. The thin bonding line (1–2 m) is observed at the coating–substrate interface, showing that the excellent metallurgical bonding is formed between the composite coating and the substrate. Moreover, with increasing distance from the surface of substrate, the bonding metal which has a typical microstructure characteristic of the rapid solidification presents the planar, dendritic growth and eutectic structure. The X-ray diffraction result of Ni60A + 35 wt.% WC composite coating by LIHRC is shown in Fig. 4. It indicates that the major phases in the composite coating are ␥-(Ni, Fe) with the fcc crystal structure, Ni4 B3 with an orthorhombic crystal structure and M23 C6 (M = W, Ni, Cr, Fe) type carbide with the fcc crystal structure, and that other carbides corresponding peaks have a low intensity. Therefore, Ni60A + 35 wt.% WC composite coating
3. Results 3.1. Macrostructure of the composite coating by LIHRC According to the definition of the critical state of Ni-based WC composite coating during LIHRC [24] (i.e. with the decreasing of the
Fig. 2. Cross-section morphology of Ni60A + 35 wt.% WC composite coating by LIHRC.
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S. Zhou, X. Dai / Applied Surface Science 256 (2010) 4708–4714 Table 1 Chemical composition of the carbides in Ni60A + 35 wt.% WC composite coating by LIHRC. Carbides characteristics
Composition (wt.%) Ni
White particle (A) Blocky carbides (B) Herringbone carbides (C) Lamellar carbides (D)
28.18 17.71 5.36
Cr 6.75 30.01 28.27
Fe
W
C
18.62 2.81 1.25
94.68 40.82 42.98 56.88
5.32 5.63 6.49 8.24
Fig. 3. Microstructure of Ni60A + 35 wt.% WC composite coating by LIHRC.
Fig. 6. Surface morphology of Ni60A + 35 wt.% WC composite coating by the indi˙ = 25 g/min. vidual laser cladding without preheating, Vs = 600 mm/min, m
Fig. 4. X-ray diffraction result of Ni60A + 35 wt.% WC composite coating by LIHRC.
by LIHRC contains a metallic matrix, constituted by Ni-based alloy and in which the carbides such as WC, W2 C, M12 C and M23 C6 are embedded. Fig. 5 shows BSE micrograph of carbides in Ni60A + 35 wt.% WC composite coating by LIHRC. According to the morphological features of the carbide phases, XRD result (Fig. 4) and EDS analysis (Table 1), the white particles with lamellar eutectic structure should be the cast WC (marked A in Fig. 5a), and an alloyed reaction
layer is formed at the edge of WC particles. Moreover, the polygonal morphology of WC particles disappears and their edge angles become smooth, showing that Ni-based metallic matrix has a good consistency and wettability with the cast WC particles. Moreover, the W-rich blocky carbide (marked B in Fig. 5a) containing a high concentration of Ni and a small amount of Fe, the W-rich herringbone carbide (marked C in Fig. 5a) containing a large amount of Cr and a low concentration of Ni, and the W-rich lamellar carbide (marked D in Fig. 5b) containing a large amount of Cr are also precipitated around WC particles. 3.3. Comparison to the composite coating by the individual laser cladding without preheating Fig. 6 shows the surface morphology of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. Although the composite coating has a smooth surface and good
Fig. 5. BSE micrograph of carbides in Ni60A + 35 wt.% WC composite coating by LIHRC.
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˙ = 25 g/min. Fig. 7. Cross-section morphology of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating, Vs = 600 mm/min, m
Fig. 8. X-ray diffraction result of Ni60A + 35 wt.% WC composite coating by the indi˙ = 25 g/min. vidual laser cladding without preheating, Vs = 600 mm/min, m
profile, many cracks perpendicular to the direction of the laser scanning speed pass through the whole composite coating. Fig. 7 shows the transverse morphology of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. The cladding height and the cladding width are 1.25 and 5.82 mm, respectively. Moreover, many pores are observed and WC particles are distributed unevenly on both sides of the composite coating. The cast WC particles are dissolved remarkably in the center and at the top of the composite coating, where only a few WC particles with integrate structure are observed. The X-ray diffraction result of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating is shown in Fig. 8. It can be seen that the bonding metal is composed of ␥-Ni with the fcc crystal structure, Ni3 B and Ni3 B4 with an orthorhombic crystal structure, and that the carbides consist of
W2 C, WC, WC1−x , M12 C, M6 C and M23 C6 (M = W, Ni, Cr, Fe). The former two kinds of carbides have a hexagonal crystal structure, while the latter four kinds of carbides have a cubic crystal structure. Fig. 9 shows the morphology of carbides in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. Due to dissolution of the cast WC particles, the carbides with different shapes which are similar to those in Ni60A + 35 wt.% WC composite coating by LIHRC are precipitated in the center of the composite coating (Fig. 9a), such as the herringbone carbide (marked E), the blocky carbide (marked F) and the lamellar carbide (marked G). Moreover, it can be seen from Fig. 9b that a large amount of WC particles still preserve their primary lamellar eutectic structure, and that their polygonal morphology disappears. It is evident that the cast WC particles take on the different dissolution characteristics in the different regions of the composite coating by the individual laser cladding without preheating. Fig. 10 shows the microhardness profile of Ni60A + 35 wt.% WC composite coatings by LIHRC and the individual laser cladding without preheating. It can be seen that the microhardness of Ni60A + 35 wt.% WC composite coating by LIHRC is higher than that of Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. For comparison purpose, the corrugated roller is strengthened by LIHRC and the individual laser cladding without preheating. Generally, the microhardness of the corrugated roller (about 600HV0.2 ) can be increased to 700–800HV0.2 by traditional high frequency quenching. If Ni60A + 35 wt.% WC composite coating is produced by the individual laser cladding without preheating on the corrugate roller, the microhardness of the composite coating is 950–1050HV0.2 , but many cracks appear in the composite coating. Interestingly, although the laser scanning speed and the powder flow rate increases to 2200 mm/min and 75.6 g/min, respectively, a high quality composite coating free of cracks is obtained by LIHRC on
˙ = 25 g/min. (a) The carbides Fig. 9. Morphology of carbides in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating, Vs = 600 mm/min, m in the center of Ni60A + 35 wt.% WC composite coating. (b) The carbides at the edge of Ni60A + 35 wt.% WC composite coating.
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S. Zhou, X. Dai / Applied Surface Science 256 (2010) 4708–4714 Table 2 Chemical composition of the bonding metal in Ni60A + 35 wt.% WC composite coating. Processing method
LIHRC Individual laser cladding without preheating
Fig. 10. Microhardness of Ni60A + 35 wt.% WC composite coating by LIHRC and the individual laser cladding without preheating.
Fig. 11. Ni60A + 35 wt.% WC composite coating produced by LIHRC to strengthen ˙ = 75.6 g/min. the corrugated roller, Vs = 2200 mm/min, m
the corrugate roller (Fig. 11) and the microhardness of composite coating can be increased to 1063–1198HV0.2 . According to the above results, LIHRC has a relatively higher efficiency and better cladding quality in comparison with the individual laser cladding without preheating. 4. Discussion Although the substrate is preheated to 1173 K by induction heater, LIHRC is still a process of the rapid heating and rapid solidification. Based on the theory of the rapid solidification [26], the microstructure characteristics depend on the ratio of the temperature gradient G to the solidification front rate R (i.e. G/R). When the surface of substrate is molten immediately by laser induction hybrid rapid cladding heat resource, the solidification front rate R is approximately zero at the interface of the molten pool and the unmelted substrate, but the temperature gradient G is very high. Consequently, the G/R is infinite and the solidification should occur with a planar front. Afterwards, with increasing distance from the surface of substrate, the solidification front rate R increases rapidly and the temperature gradient G decreases. As a result, the G/R decreases markedly and the planar solid–liquid interface becomes unstable to induce the onset of the dendritic growth and eutectic structure. Due to a low free formation enthalpy of WC (38.5 kJ/mol), WC is apt to dissolve in Ni-based alloy solvent, and interact with Ni-based
Composition (wt.%) Ni
Cr
Fe
W
C
49.39 40.74
15.48 12.48
8.71 7.37
21.99 34.18
4.45 5.23
alloy solvent to precipitate the carbides with different shapes during LIHRC and the individual laser cladding without preheating. This can be explained by Ni–W–C ternary phase diagram, where there exist two regions such as the C-lean and C-rich region [27]. In the C-rich region, there exist three phase fields such as WC + C + ␥ and two phase fields such as C + ␥ , where ␥ is Ni-based supersaturated solid solution containing large amounts of W and C. In the C-lean region, the carbides with different structures can be precipitated, such as 1 , 2 , 3 and W2 C, where 1 and 2 are (Ni, W)6 C and M12 C type carbides, respectively, both have the fcc crystal structure, and 3 is the mixed carbide with a hexagonal crystal structure. Therefore, the dissolution of WC particles results in a large amount of free W and C atoms in the molten pool during LIHRC and the individual laser cladding without preheating. For Ni-based alloy solvent, these W and C atoms can dissolve into the dendrites ␥-Ni to form the supersaturated solid solution, ␥ , during the rapid solidification. Moreover, the interaction between solute atoms in Ni-based alloy such as Ni, Cr, Fe and B occur, forming the eutectic ␥-Ni + Ni2 B + Ni3 B4 in the interdendritic matrix phase, ␥ . For the cast WC particles with relatively large size, they are not dissolved completely during LIHRC or the individual laser cladding without preheating, but their edge angles can be dissolved preferentially. As a result, a C-lean region is formed, and Ni, Cr and Fe atoms in Ni-based alloy solvent diffuse simultaneously into the C-lean region. Subsequently, the concentration gradient formed at the edge of WC particles can induce the phase transformation during the rapid solidification. Consequently, an alloyed reaction layer with a thickness of 1–3 m is generated at the edge of WC particles and the polygonal morphology of WC particles disappears, but the other regions of WC particles still preserve the primary lamellar eutectic structure. For the cast WC particles with relatively small size, they can be dissolved completely during LIHRC or the individual laser cladding without preheating, resulting in a large amount of W and C atoms in the molten pool. These W and C can interact with Ni, Cr and Fe in Ni-based alloy solvent to precipitate the W-rich blocky M6 C or M12 C type carbide containing a high concentration of Ni and a small amount of Fe, and the W-rich herringbone M23 C6 containing a large amount of Cr and a small amount of Ni (Table 1). Moreover, due to the heat impact of laser beam and the capillarity of the molten pool, only a few WC particles are dispersed into many flakes, which interact with Ni, Cr and Fe in Ni-based alloy solvent to form the lamellar W-rich carbide containing a large amount of Cr. The carbide morphology in Ni60A + 35 wt.% WC composite coating by LIHRC (Fig. 5) is compared to that in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating (Fig. 9). It reveals that the amounts of the blocky, lamellar and herringbone carbides precipitated in the composite coating by LIHRC are obviously less than those precipitated in the composite coating by the individual laser cladding without preheating. Moreover, the average W content in Ni-based metallic matrix produced by LIHRC is less than that in Ni-based metallic matrix produced by the individual laser cladding without preheating (Table 2). The above results show that the dissolution level or heat damage degree of WC particles in Ni60A + 35 wt.% WC composite coat-
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ing by LIHRC is less than that in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating. As a result, the high microhardess characteristic of WC particles is preserved during LIHRC. Additionally, the distribution of WC particles in Ni60A + 35 wt.% WC composite coating by LIHRC is more homogeneous than that in Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating, showing that WC particles have a better strengthening effect on Ni-based metallic matrix during LIHRC. Therefore, Ni60A + 35 wt.% WC composite coating by LIHRC has a higher microhardness than that by the individual laser cladding without preheating (Fig. 10). Moreover, Ni60A + 35 wt.% WC composite coating by the individual laser cladding without preheating exhibits many cracks which are approximately perpendicular to the direction of the laser scanning speed. It can be explained by the following reasons. Firstly, the carbides are distributed unevenly in the dense Ni-based metallic matrix, resulting in the interface stress between the carbides and Ni-based metallic matrix [18]. Secondly, the temperature gradient is very high during the individual laser cladding without preheating, leading to the thermal stress in the composite coating [17]. Thirdly, WC particles suffer from the different dissolution level or heat damage degree in the different regions of the composite coating, and then interact with Ni-based alloy solvent to precipitate the carbides with different shapes. The stress can also be induced due to phase transformation. Therefore, the cracking is caused by residual stress during the individual laser cladding without preheating. However, when the weight percentage of WC particles in the composite powder increases to 35 wt.% and the laser scanning speed increases to 2200 mm/min during LIHRC, the crack-free Ni-based WC composite coating with high microhardness is obtained. It can be explained by the following reasons. Firstly, the thermal stress in the composite coating can be calculated as follows [17]: T =
Ec · Es · ts · (˛c − ˛s ) · T (1 − v) · (Es ts + Ec tc )
(1)
where T is the thermal stress, Ec and Es are Young’s modulus of the composite coating and the substrate, respectively, ˛c and ˛s are the coefficient of thermal expansion of the composite coating and the substrate, respectively, tc and ts are the height of the composite coating and the substrate, respectively, v is Poisson’s ratio and T is the difference between the solidification temperature of the molten cladding material and the room temperature. Clearly, when the substrate is preheated to 1173 K, the temperature gradient calculated in Ref. [18] during LIHRC is decreased much nineteen times lower than that during the individual laser cladding without preheating. It can be seen from Eq. (1) that the thermal stress in Ni60A + 35 wt.% composite coating by LIHRC is decreased remarkably. Secondly, the time of the convection and the stirring in the molten pool during laser cladding can be calculated as follows [28]: =
L Vs
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composite coating by the individual laser cladding without preheating. As a result, the residual stress induced during LIHRC is less than that induced during the individual laser cladding without preheating. Therefore, when the laser scanning speed increases to 2200 mm/min during LIHRC, the crack-free Ni60A + 35 wt.% WC composite coating is obtained. 5. Conclusions (1) According to the definition of the critical state of Ni-based WC composite coating during LIHRC, the maximum laser scanning speed (i.e. 2200 mm/min) and the maximum powder feeding rate (i.e. 75.6 g/min) can be obtained when Ni60A + 35 wt.% composite coating is produced by LIHRC. Moreover, the composite coating in which WC particles are distributed homogeneously has a good profile and no cracks. (2) During LIHRC, the bonding line is formed at the interface of the composite coating and the substrate. The bonding metal presents the planar, dendritic growth and eutectic structure which is composted of ␥-Ni + Ni3 B + Ni3 B4 . The carbides consist of WC, W2 C, M12 C and M23 C6 (M = W, Ni, Cr, Fe) are embedded in the dense Ni-based metallic matrix. (3) In Ni60A + 35 wt.% WC composite coating by LIHRC and the individual laser cladding without preheating, the cast WC particles take on three kinds of the dissolution characteristics. Firstly, WC particles with relatively small size are completely dissolved and interact with Ni-based alloy solvent to precipitate the blocky and herringbone carbides. Secondly, WC particles with relatively large size are partially dissolved and still consist of the lamellar eutectic structure, but their primary polygonal morphology disappears. Thirdly, due to the heat impact of laser beam and the capillarity of the molten pool, a few WC particles are dispersed into many flakes, which interact with Ni-based alloy solvent to form the lamellar W-rich carbides. (4) During LIHRC, the increasing of the laser scanning speed can improve the homogeneous distribution of WC particles and decrease the dissolution level of WC particles in Ni60A + 35 wt.% WC composite coating. Moreover, the temperature gradient is also decreased because the substrate is preheated during LIHRC. As a result, the crack-free Ni60A + 35 wt.% WC composite coating can be produced by LIHRC and its microhardness is higher than that of Ni60A + 35 wt.% WC composite coating produced by the individual laser cladding without preheating. Acknowledgements The supports of this work by the National Natural Science Foundation of China (grant no. 50901040), the Aviation Science Foundation of China (grant no. 2009ZE56013) and Jiangxi Province Education Department Foundation of China (grant no. GJJ10507) are gratefully acknowledged.
(2)
where L represents the major axis of the elliptical spot, and Vs is the laser scanning speed. When the laser scanning speed increases to 2200 mm/min, the time of the convection and the stirring in the molten pool during LIHRC is about 0.22 s, which is decreased much three times lower than that during the individual laser cladding without preheating. Therefore, the decreasing of the time of the convection and the stirring in the molten pool during LIHRC can help prevent WC particles from sinking to the bottom of the molten pool. The distribution homogeneity of WC particles is also improved accordingly in the composite coating. Thirdly, the dissolution level or heat damage degree of WC particles in Ni60A + 35 wt.% WC composite coating by LIHRC is less than that in Ni60A + 35 wt.% WC
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