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Coaxial laser cladding of Al2O3-13%TiO2 powders on Ti-6Al-4 V alloy
Surface & Coatings Technology 228 (2013) S452–S455
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Coaxial laser cladding of Al2O3-13%TiO2 powders on Ti-6Al-4 V alloy Yunxiao Chen, Dongjiang Wu ⁎, Guangyi Ma, Weifeng Lu, Dongming Guo Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian, Liaoning 116024, China
a r t i c l e
i n f o
Available online 18 May 2012 Keywords: Coaxial laser cladding Ti alloy Al2O3-13wt. %TiO2 Microstructure Electron microprobe
a b s t r a c t Spray-dried sintered Al2O3-13wt. %TiO2 powders were directly coated on Ti-6Al-4 V alloy by coaxial laser cladding. The microstructure, elements distribution and phase analysis of the cladding sample were studied by optical microscopy, EPMA, SEM and XRD methods. The results indicated that the coatings with fine metallurgical bonding to the substrate comprised major stable α-Al2O3 and demonstrated three different regions of solidification microstructures. One region, at the bottom of the molten pool, contained mainly Ti enrichment columnar grains and randomly scattered ceramic particles, named Transition Zone. The second region, within the 0.1 mm scope above the transition zone, contained mainly fine equiaxed Al2O3 grains. The third region, at the upper zone of the molten pool, showed coarse equiaxed and cellular Al2O3 grains with obvious segregation of Ti and V elements. The mechanisms of microstructure formation, elements migration and segregation in the experiment were discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Titanium alloys have been widely used in aerospace, energy and chemistry applications for their high specific strength, corrosion resistance and good mechanical properties, while the alloys also suffer from low hardness and poor tribological behaviors [1,2]. It is necessary to provide surface protection of the related components [3]. Fortunately laser technologies provide excellent approaches to material processing and laser cladding is an optimum approach which has been used effectively to create various coatings [3–6]. And oxide ceramics are of special interest because of their low specific weight, low thermal conductivity, and good tribological properties [7]. Al2O3-TiO2 powders have been widely used in the plasma sprayed coatings for a wide variety of applications that require resistance to wear, erosion and corrosion [8–10]. And the addition of TiO2 was proved helpful to enhance the toughness and wear resistance compared to Al2O3 coatings [9]. But laser cladding of Al2O3-TiO2 powders has rarely been reported in open literatures [7,11]. Nowotny et al. [7] summarized the surface protection of light metals using one-step laser cladding with oxide ceramics, including the process of Al2O3TiO2 coatings on the Ti-6Al-4 V alloy, however, a high-melting metallic bond coat was used between the Al2O3-TiO2 coatings and the titanium alloy substrate, and the detailed interaction between ceramic coatings and the Ti-6Al-4Valloy was not analyzed as well as the microstructure formation mechanism. Cui et al. [11], who used laser cladding technique to produce Al-Si matrix Al2O3-TiO2 coatings on AZ31B magnesium alloy, reported fine metallurgical bonding at the ⁎ Corresponding author at: No.2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province, P. R. C., 116024. Tel.: + 86 411 84707625; fax: + 86 411 84707625. E-mail address: [email protected] (D. Wu). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.05.027
coating-substrate interface and great improvement in wear resistance of the alloy. The Al-Si powders were employed to ameliorate the bonding problem of ceramic and magnesium alloy. However the low expansion coefficient of titanium and its alloys makes them much easier to use in combination with ceramics than most other metals. For this reason, directly clad Al2O3-13wt. %TiO2 coatings on the Ti-6Al-4 V alloy by coaxial laser cladding was carried out in this paper. The unique microstructure and bonding character of the coatings were investigated. The mechanisms of phase segregation and elements migration were also discussed. 2. Experimental details A 6 mm thick Ti-6Al-4 V alloy plate, with the nominal alloying element composition of 6 wt. %Al and 4 wt. %V was used as the substrate. The commercial spray-dried sintered Al2O3-13wt. %TiO2 powders with 45–90 μm in size and spherical in shape were used as the cladding materials. Before the cladding process, the substrate samples were ground with sand paper and then cleaned with alcohol. During the coaxial cladding process, the powders are delivered by high purity argon, which is also used as the coaxial shielding gas, with a mass flow rate of 1.78 g/min. A CW Nd:YAG laser with average power of 1 kW was used as the heat source. The laser power and scanning speed used in the experiment were 235 W and 300 mm/min, respectively. The beam spot diameter on the substrate was 1.38 mm. The cladding samples were cut vertically to the laser scanning direction. After they are ground and polished, the samples were etched using a mixture of deionized H2O, HNO3 and HF with the volume ratio: 48H2O:1HF:1HNO3. The microstructure of the layer's crosssection and substrate-coating interface was characterized by optical microscope (Olympus MX40F, Japan) and SEM (JEOL JSM-6360LV,
Y. Chen et al. / Surface & Coatings Technology 228 (2013) S452–S455
Japan). The EPMA (Shimadzu EPMA-1600, Japan) method was adopted to analyze the element distribution characteristic of the cross-section. The phase constituents of the feedstock powders and the coatings were examined by XRD (D/max-Ultima, Japan). 3. Results and discussion 3.1. Microstructures During solidification, the temperature gradient (G) at the solid/ liquid interface and the growth rate of the solid/liquid interface (R) are two important factors. The ratio of G/R determines the mode of solidification while the product of GR (average cooling rate) governs the size of the solidification structure [12,13]. With the decrease value of G/R, the plan front solidification, cellular structure, columnar structure and equiaxed structure tend to be formed in turn. The scale of the dendritic/grain structure is inversely proportional to the solidification cooling rate. The morphology of the cross-section is shown in Fig. 1. As seen in Fig. 1(a), the interface between the substrate and the coating is crackfree and follows an uneven path due to the uneven energy distribution of the laser. The cross-section demonstrates three different regions of solidification microstructures as labeled in Fig. 1(b). Fig. 2 is the high magnification images of the three regions. Region (I), at the bottom of the molten pool, presents directional solidification columnar structures (Fig. 2(a)) because of the G vertical to the liquid-solid interface at the bottom of the molten pool is the largest. If the distance to the surface decreases, the G decreases while the R increases. Moreover at the thin surface of the coating, air convection also takes part in the cooling process. So the formation of fine equiaxed grains at the thin surface of the coating, and coarse equiaxed grains at the middle of the coating could be predicted. However in this experiment, fine equiaxed grain zone (region (II)) as shown in Fig. 2 (b) is formed within the 0.1 mm scope above the region (I), while coarse equiaxed and cellular grains are presented in region (III)(Fig. 2(c)). This can be explained by the special character of ceramic thermal conductivity. In ceramics, thermal conductivity decreases rapidly with increasing temperature. During the solidification of region (III), the heat conduct process is very slow due to the high temperature of the ceramics at the middle of the coating. However, Region (II) is close to the transition zone that includes lots of substrate metals which leads to a faster cooling process than that of region (III). Hence the special morphology is formed.
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3.2. X-ray diffraction analysis It is believed that during laser process with high cooling rate, the high-temperature metastable γ-Al2O3 phase can be maintained at room temperature [14]. But as the XRD pattern of the clad sample (Fig. 3) shows, the cladding layer contains major stable α-Al2O3 (JCPDS NO.741081, JCPDS NO.50-1496), as well as some brookite TiO2 and Al2TiO5, and no obvious γ-Al2O3 phase is detected. In [14], J.A. Vreeling et al. reported that a cooling rate with the value of 400 K/s is high enough to form γ-Al2O3 phase. And the amount of γ-Al2O3 is mainly determined by the actual cooling rate between 1200 K and 800 K, in which γ-Al2O3 is formed. When the cooling rate is too high (1.6 × 10 4 K/s), almost no γ-Al2O3 peaks are detected. Considering the process condition in this experiment, the cooling rate (∂T/∂t) can be estimated by Rosenthal solution [15]: 2
∂T=∂t ¼ −2πκ ðV=P Þ ð△T Þ
ð1Þ
Where k is the thermal conductivity, V is the scanning speed, P is the absorbed laser power, and △T is the range of temperature variation during cooling. In the calculation, it is assumed that △T is the melting temperature of Al2O3 (2327 K). Table 1 presents the calculated result of cooling rate for Al2O3 under the laser cladding condition of this study. The result shows that the maximum cooling rate can reach to 2.07 × 10 4 K/s. While in the calculation, the important cooling effect of the argon gas is not considered. So the actual cooling rate during the solidification should be higher. Under this condition, the cooling process is too rapid to form enough γ-Al2O3. Consequently, no obvious γ-Al2O3 peaks were found. 3.3. Electron microprobe analysis Table 2 shows the EPMA results corresponding to a typical black particle and a columnar region marked as A and B in Fig. 2(a), and a grain body and an inter-grain region marked as C and D in Fig. 2(c), respectively. It is observed that element Al is enriched in the dark regions (A, C), while large proportion of Ti accumulates in the bright regions (B, D) with a significant proportion of V element from the substrate. And the regions all contain a large amount of element O. According to the atomic proportions of Al, Ti, O and the XRD results, it is believed that the regions like A and C are composite of major Al2O3 and small proportion of TiO2, while the regions like B and D are composite of TiO2 and substrate metals. The interaction of
Fig. 1. Typical cross section of the clad samples.
Y. Chen et al. / Surface & Coatings Technology 228 (2013) S452–S455
Fig. 3. XRD results of the clad layer.
Fig. 2. Magnified view to the region labeled “I” “II” “III” in Fig. 1(b), respectively.
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ceramics and substrate metals in the transition zone (region (I)) is helpful to improve the bonding quality between the ceramic coating and the substrate. Fig. 4 shows the secondary electron image of Fig. 1(b) and the elemental mapping of Ti and V corresponding to it. It can be seen that Ti and V show uniform distribution in region (I) and (II). However obvious segregation of Ti and V is presented in region (III). During the solidification, Al2O3 with high melting point is prone to first nucleate and solidify due to a high supercooling. As a result, the TiO2 and substrate metals are pushed to the inter-grain regions by the advancing solidification front. And the dilution of the coating with the substrate element V can be associated with the convection in the melt pool and solid-state diffusion [4]. But this cannot explain why so many bright structures with serious accumulation of Ti and V are only present in region (III). During the coaxial cladding, part of the powders is heated to molten status before reaching to the fusion surface of the substrate. The impact effect of the molten drop to the fusion surface of the substrate may take an important role in the element distribution during the solidification process of the coating.
4. Conclusion Directly clad Al2O3-13wt. %TiO2 powders on Ti-6Al-4 V alloy was conducted successfully by coaxial laser cladding technology. The main conclusions of the study are as follows: 1. The coating demonstrates three different regions of solidification microstructures from the bottom to the top surface of the molten pool. Region (I) consists of major Ti enrichment columnar grains and randomly scattered ceramic particles. Region (II) contains mainly fine equiaxed Al2O3 grains, while region (III) shows coarse equiaxed and cellular Al2O3 grains with obvious segregation of Ti and V elements. 2. The bright structures in region (III) present serious accumulation of Ti and V. And this is close to the impact effect of the ceramic molten drop to the fusion surface. 3. The cooling rate of the coating in the solidification process is too high to form enough γ-Al2O3. The ceramic phases are major stable α-Al2O3 as well as some brookite TiO2 and Al2TiO5. 4. Crack-free coatings with fine metallurgical bonding to the substrate were obtained.
Table 1 Parameters used in the calculations and estimated cooling rate. k (W/m·K) [16]
6.3–28.7
V (m/s) P (W) △ T (K) -∂T/∂t (K/s)
0.005 235 2327 4.55 × 103− 2.07 × 104
Y. Chen et al. / Surface & Coatings Technology 228 (2013) S452–S455
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Table 2 EPMA of the areas marked in Fig. 2.
Point Point Point Point
A B C D
Al
Ti
O
V
32.851 1.718 35.406 3.031
6.374 52.899 4.662 43.312
60.704 45.545 59.868 52.937
0.071 1.837 0.064 0.719
Fig. 4. The SE image of the cross section region and the elemental mapping of Ti and V corresponding to it.
Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 51175061) and National Basic Research Program of China (973 Program 2009CB724307).
[6] [7] [8] [9] [10] [11]
References [12] [1] Y.S. Tian, C.Z. Chen, S.T. Li, Q.H. Huo, Appl. Surf. Sci. 242 (2005) 177. [2] T. Moskalewicza, B. Wendlerb, S. Zimowskic, B. Dubiel, A. Czyrska-Filemonowicz, Surf. Coat. Technol. 205 (2010) 2668. [3] Li Jianing, Chen Chuanzhong, Zong Lei, Int. J. Refract. Met. Hard Mater. 29 (2011) 49. [4] M. Alhammad, S. Esmaeili, E. Toyserkani, Surf. Coat. Technol. 203 (2008) 1. [5] Zhang Kernin, Zou Jianxin, Li Jun, Trans. Nonferrous Met. Soc. China 20 (2010) 2192.
[13] [14] [15] [16]
T.C. Chong, M.H. Hong, L.P. Shi, Laser Photonics Rev. 4 (2010) 123. S. Nowotny, A. Richter, K. Tangermann, J. Therm. Spray Technol. 8 (1999) 258. E.H. Jordan, M. Gell, Y.H. Sohn, Mater. Sci. Eng., A 301 (2001) 80. N. Dejang, A. Watcharapasorn, S. Wirojupatump, P. Niranatlumpong, S. Jiansirisomboon, Surf. Coat. Technol. 204 (2010) 1651. E. Sánchez, E. Bannier, V. Cantavella, M.D. Salvador, E. Klyatskina, J. Morgiel, J. Grzonka, A.R. Boccaccini, J. Therm. Spray Technol. 17 (2008) 329. Zeqin Cui, Wenxian Wang, Hongliang Wu, Xu Bingshe, Chin. J. Lasers 38 (2011) 060320–060321. Vamsi Krishna Balla, Susmita Bose, Amit Bandyopadhyay, Int. J. Appl. Ceram. Technol. 5 (2008) 234. Sindo Kon, Welding Metallurgy, Second ed. Wiley Interscience, New Jersey, 2003. J.A. Vreeling, Y.T. Pei, B. Wind, V. Ocelĺk, J.Th.M. De Hosson, Scr. Mater. 44 (2001) 643. F. Audebert, R. Colaço, R. Vilar, H. Sirkin, Scr. Mater. 48 (2003) 281. Yu. Zhou, Ceramic Materials, Second ed. Science Press, Beijing, 2004.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Coaxial laser cladding of Al2O3-13%TiO2 powders on Ti-6Al-4 V alloy Yunxiao Chen, Dongjiang Wu ⁎, Guangyi Ma, Weifeng Lu, Dongming Guo Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian, Liaoning 116024, China
a r t i c l e
i n f o
Available online 18 May 2012 Keywords: Coaxial laser cladding Ti alloy Al2O3-13wt. %TiO2 Microstructure Electron microprobe
a b s t r a c t Spray-dried sintered Al2O3-13wt. %TiO2 powders were directly coated on Ti-6Al-4 V alloy by coaxial laser cladding. The microstructure, elements distribution and phase analysis of the cladding sample were studied by optical microscopy, EPMA, SEM and XRD methods. The results indicated that the coatings with fine metallurgical bonding to the substrate comprised major stable α-Al2O3 and demonstrated three different regions of solidification microstructures. One region, at the bottom of the molten pool, contained mainly Ti enrichment columnar grains and randomly scattered ceramic particles, named Transition Zone. The second region, within the 0.1 mm scope above the transition zone, contained mainly fine equiaxed Al2O3 grains. The third region, at the upper zone of the molten pool, showed coarse equiaxed and cellular Al2O3 grains with obvious segregation of Ti and V elements. The mechanisms of microstructure formation, elements migration and segregation in the experiment were discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Titanium alloys have been widely used in aerospace, energy and chemistry applications for their high specific strength, corrosion resistance and good mechanical properties, while the alloys also suffer from low hardness and poor tribological behaviors [1,2]. It is necessary to provide surface protection of the related components [3]. Fortunately laser technologies provide excellent approaches to material processing and laser cladding is an optimum approach which has been used effectively to create various coatings [3–6]. And oxide ceramics are of special interest because of their low specific weight, low thermal conductivity, and good tribological properties [7]. Al2O3-TiO2 powders have been widely used in the plasma sprayed coatings for a wide variety of applications that require resistance to wear, erosion and corrosion [8–10]. And the addition of TiO2 was proved helpful to enhance the toughness and wear resistance compared to Al2O3 coatings [9]. But laser cladding of Al2O3-TiO2 powders has rarely been reported in open literatures [7,11]. Nowotny et al. [7] summarized the surface protection of light metals using one-step laser cladding with oxide ceramics, including the process of Al2O3TiO2 coatings on the Ti-6Al-4 V alloy, however, a high-melting metallic bond coat was used between the Al2O3-TiO2 coatings and the titanium alloy substrate, and the detailed interaction between ceramic coatings and the Ti-6Al-4Valloy was not analyzed as well as the microstructure formation mechanism. Cui et al. [11], who used laser cladding technique to produce Al-Si matrix Al2O3-TiO2 coatings on AZ31B magnesium alloy, reported fine metallurgical bonding at the ⁎ Corresponding author at: No.2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province, P. R. C., 116024. Tel.: + 86 411 84707625; fax: + 86 411 84707625. E-mail address: [email protected] (D. Wu). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.05.027
coating-substrate interface and great improvement in wear resistance of the alloy. The Al-Si powders were employed to ameliorate the bonding problem of ceramic and magnesium alloy. However the low expansion coefficient of titanium and its alloys makes them much easier to use in combination with ceramics than most other metals. For this reason, directly clad Al2O3-13wt. %TiO2 coatings on the Ti-6Al-4 V alloy by coaxial laser cladding was carried out in this paper. The unique microstructure and bonding character of the coatings were investigated. The mechanisms of phase segregation and elements migration were also discussed. 2. Experimental details A 6 mm thick Ti-6Al-4 V alloy plate, with the nominal alloying element composition of 6 wt. %Al and 4 wt. %V was used as the substrate. The commercial spray-dried sintered Al2O3-13wt. %TiO2 powders with 45–90 μm in size and spherical in shape were used as the cladding materials. Before the cladding process, the substrate samples were ground with sand paper and then cleaned with alcohol. During the coaxial cladding process, the powders are delivered by high purity argon, which is also used as the coaxial shielding gas, with a mass flow rate of 1.78 g/min. A CW Nd:YAG laser with average power of 1 kW was used as the heat source. The laser power and scanning speed used in the experiment were 235 W and 300 mm/min, respectively. The beam spot diameter on the substrate was 1.38 mm. The cladding samples were cut vertically to the laser scanning direction. After they are ground and polished, the samples were etched using a mixture of deionized H2O, HNO3 and HF with the volume ratio: 48H2O:1HF:1HNO3. The microstructure of the layer's crosssection and substrate-coating interface was characterized by optical microscope (Olympus MX40F, Japan) and SEM (JEOL JSM-6360LV,
Y. Chen et al. / Surface & Coatings Technology 228 (2013) S452–S455
Japan). The EPMA (Shimadzu EPMA-1600, Japan) method was adopted to analyze the element distribution characteristic of the cross-section. The phase constituents of the feedstock powders and the coatings were examined by XRD (D/max-Ultima, Japan). 3. Results and discussion 3.1. Microstructures During solidification, the temperature gradient (G) at the solid/ liquid interface and the growth rate of the solid/liquid interface (R) are two important factors. The ratio of G/R determines the mode of solidification while the product of GR (average cooling rate) governs the size of the solidification structure [12,13]. With the decrease value of G/R, the plan front solidification, cellular structure, columnar structure and equiaxed structure tend to be formed in turn. The scale of the dendritic/grain structure is inversely proportional to the solidification cooling rate. The morphology of the cross-section is shown in Fig. 1. As seen in Fig. 1(a), the interface between the substrate and the coating is crackfree and follows an uneven path due to the uneven energy distribution of the laser. The cross-section demonstrates three different regions of solidification microstructures as labeled in Fig. 1(b). Fig. 2 is the high magnification images of the three regions. Region (I), at the bottom of the molten pool, presents directional solidification columnar structures (Fig. 2(a)) because of the G vertical to the liquid-solid interface at the bottom of the molten pool is the largest. If the distance to the surface decreases, the G decreases while the R increases. Moreover at the thin surface of the coating, air convection also takes part in the cooling process. So the formation of fine equiaxed grains at the thin surface of the coating, and coarse equiaxed grains at the middle of the coating could be predicted. However in this experiment, fine equiaxed grain zone (region (II)) as shown in Fig. 2 (b) is formed within the 0.1 mm scope above the region (I), while coarse equiaxed and cellular grains are presented in region (III)(Fig. 2(c)). This can be explained by the special character of ceramic thermal conductivity. In ceramics, thermal conductivity decreases rapidly with increasing temperature. During the solidification of region (III), the heat conduct process is very slow due to the high temperature of the ceramics at the middle of the coating. However, Region (II) is close to the transition zone that includes lots of substrate metals which leads to a faster cooling process than that of region (III). Hence the special morphology is formed.
S453
3.2. X-ray diffraction analysis It is believed that during laser process with high cooling rate, the high-temperature metastable γ-Al2O3 phase can be maintained at room temperature [14]. But as the XRD pattern of the clad sample (Fig. 3) shows, the cladding layer contains major stable α-Al2O3 (JCPDS NO.741081, JCPDS NO.50-1496), as well as some brookite TiO2 and Al2TiO5, and no obvious γ-Al2O3 phase is detected. In [14], J.A. Vreeling et al. reported that a cooling rate with the value of 400 K/s is high enough to form γ-Al2O3 phase. And the amount of γ-Al2O3 is mainly determined by the actual cooling rate between 1200 K and 800 K, in which γ-Al2O3 is formed. When the cooling rate is too high (1.6 × 10 4 K/s), almost no γ-Al2O3 peaks are detected. Considering the process condition in this experiment, the cooling rate (∂T/∂t) can be estimated by Rosenthal solution [15]: 2
∂T=∂t ¼ −2πκ ðV=P Þ ð△T Þ
ð1Þ
Where k is the thermal conductivity, V is the scanning speed, P is the absorbed laser power, and △T is the range of temperature variation during cooling. In the calculation, it is assumed that △T is the melting temperature of Al2O3 (2327 K). Table 1 presents the calculated result of cooling rate for Al2O3 under the laser cladding condition of this study. The result shows that the maximum cooling rate can reach to 2.07 × 10 4 K/s. While in the calculation, the important cooling effect of the argon gas is not considered. So the actual cooling rate during the solidification should be higher. Under this condition, the cooling process is too rapid to form enough γ-Al2O3. Consequently, no obvious γ-Al2O3 peaks were found. 3.3. Electron microprobe analysis Table 2 shows the EPMA results corresponding to a typical black particle and a columnar region marked as A and B in Fig. 2(a), and a grain body and an inter-grain region marked as C and D in Fig. 2(c), respectively. It is observed that element Al is enriched in the dark regions (A, C), while large proportion of Ti accumulates in the bright regions (B, D) with a significant proportion of V element from the substrate. And the regions all contain a large amount of element O. According to the atomic proportions of Al, Ti, O and the XRD results, it is believed that the regions like A and C are composite of major Al2O3 and small proportion of TiO2, while the regions like B and D are composite of TiO2 and substrate metals. The interaction of
Fig. 1. Typical cross section of the clad samples.
Y. Chen et al. / Surface & Coatings Technology 228 (2013) S452–S455
Fig. 3. XRD results of the clad layer.
Fig. 2. Magnified view to the region labeled “I” “II” “III” in Fig. 1(b), respectively.
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ceramics and substrate metals in the transition zone (region (I)) is helpful to improve the bonding quality between the ceramic coating and the substrate. Fig. 4 shows the secondary electron image of Fig. 1(b) and the elemental mapping of Ti and V corresponding to it. It can be seen that Ti and V show uniform distribution in region (I) and (II). However obvious segregation of Ti and V is presented in region (III). During the solidification, Al2O3 with high melting point is prone to first nucleate and solidify due to a high supercooling. As a result, the TiO2 and substrate metals are pushed to the inter-grain regions by the advancing solidification front. And the dilution of the coating with the substrate element V can be associated with the convection in the melt pool and solid-state diffusion [4]. But this cannot explain why so many bright structures with serious accumulation of Ti and V are only present in region (III). During the coaxial cladding, part of the powders is heated to molten status before reaching to the fusion surface of the substrate. The impact effect of the molten drop to the fusion surface of the substrate may take an important role in the element distribution during the solidification process of the coating.
4. Conclusion Directly clad Al2O3-13wt. %TiO2 powders on Ti-6Al-4 V alloy was conducted successfully by coaxial laser cladding technology. The main conclusions of the study are as follows: 1. The coating demonstrates three different regions of solidification microstructures from the bottom to the top surface of the molten pool. Region (I) consists of major Ti enrichment columnar grains and randomly scattered ceramic particles. Region (II) contains mainly fine equiaxed Al2O3 grains, while region (III) shows coarse equiaxed and cellular Al2O3 grains with obvious segregation of Ti and V elements. 2. The bright structures in region (III) present serious accumulation of Ti and V. And this is close to the impact effect of the ceramic molten drop to the fusion surface. 3. The cooling rate of the coating in the solidification process is too high to form enough γ-Al2O3. The ceramic phases are major stable α-Al2O3 as well as some brookite TiO2 and Al2TiO5. 4. Crack-free coatings with fine metallurgical bonding to the substrate were obtained.
Table 1 Parameters used in the calculations and estimated cooling rate. k (W/m·K) [16]
6.3–28.7
V (m/s) P (W) △ T (K) -∂T/∂t (K/s)
0.005 235 2327 4.55 × 103− 2.07 × 104
Y. Chen et al. / Surface & Coatings Technology 228 (2013) S452–S455
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Table 2 EPMA of the areas marked in Fig. 2.
Point Point Point Point
A B C D
Al
Ti
O
V
32.851 1.718 35.406 3.031
6.374 52.899 4.662 43.312
60.704 45.545 59.868 52.937
0.071 1.837 0.064 0.719
Fig. 4. The SE image of the cross section region and the elemental mapping of Ti and V corresponding to it.
Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 51175061) and National Basic Research Program of China (973 Program 2009CB724307).
[6] [7] [8] [9] [10] [11]
References [12] [1] Y.S. Tian, C.Z. Chen, S.T. Li, Q.H. Huo, Appl. Surf. Sci. 242 (2005) 177. [2] T. Moskalewicza, B. Wendlerb, S. Zimowskic, B. Dubiel, A. Czyrska-Filemonowicz, Surf. Coat. Technol. 205 (2010) 2668. [3] Li Jianing, Chen Chuanzhong, Zong Lei, Int. J. Refract. Met. Hard Mater. 29 (2011) 49. [4] M. Alhammad, S. Esmaeili, E. Toyserkani, Surf. Coat. Technol. 203 (2008) 1. [5] Zhang Kernin, Zou Jianxin, Li Jun, Trans. Nonferrous Met. Soc. China 20 (2010) 2192.
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