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Ni–TiC coating deposited on Ti–6Al–4V substrate by thermal spraying and laser remelting of Ni-clad graphite powder
Materials Letters 61 (2007) 2356 – 2358 www.elsevier.com/locate/matlet
Ni–TiC coating deposited on Ti–6Al–4V substrate by thermal spraying and laser remelting of Ni-clad graphite powder Shengqun Yang, Qingwu Meng ⁎, Lin Geng, Lixin Guo, Lin Wu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. O. Box 433, PR China Received 3 April 2006; accepted 5 September 2006 Available online 22 September 2006
Abstract A process of thermal spraying and laser remelting of a Ni-clad graphite powder to form a coating on Ti–6Al–4V substrate was carried out. A good coating without cracks and pores was obtained. The microstructure of the coating was examined using SEM and EDS. The coating mainly consists of austenitic nickel as matrix and TiC dendrite as reinforcement. During the laser remelting process, a reaction between C and Ti occurred, which lead to an in-situ synthesis of TiC reinforcement in the coating. The microhardness of the coating was measured using a Vickers hardness tester. The average microhardness of the composite coating is HV 1000 and it is two times greater than that of the Ti–6Al–4V substrate. © 2006 Elsevier B.V. All rights reserved. Keywords: Coating; Laser remelt; Thermal spray; Ti–6Al–4V; Ni-clad graphite powder
1. Introduction In order to improve the wear resistance of titanium alloy, laser cladding of ceramic–metal composite coatings on titanium substrate has been proposed [1–4]. In these studies, most of ceramic phases such as TiC [1], WC [2], SiC [3], TiN [4] are directly added into laser clad coatings. However, it has been often found that the ceramic phase is poorly dispersed in the coating if the pre-placed technique of ceramic phase is adopted and that a poor adhesion of reinforced phases to matrix occurs. A possibility to overcome the problem is the in-situ synthesis of ceramic phases in the coating during laser cladding process. As a reaction of C with Ti may lead to the formation of TiC reinforcements at high temperature, in-situ synthesis of TiC in laser clad coating has been reported in a few studies. The precursor powders of Cr3C2 + Ti [5] or C + Ti [6] are pre-placed directly or by means of organic binder onto the substrates in these studies. However, the pre-placed powders have poor adhesion to substrate and are easily splashed off with chemical reaction during laser cladding process. The appropriate method of pre-placed powders is using thermal spraying or plasma spraying. A Ni-clad graphite powder is a kind of composite ⁎ Corresponding author. Tel.: +86 451 86418836; fax: +86 451 86413922. E-mail address: [email protected] (Q. Meng). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.09.011
powder that is used in thermal spraying or plasma spraying. However, Ni-clad graphite powder used in laser cladding coating has never been reported. The present study aims at thermal spraying and laser remelting of a Ni-clad graphite powder to form a coating on Ti–6Al–4V substrate, and to evaluate the microstructure and microhardness of the coating. 2. Experimental details Ti–6Al–4V alloy was used as a substrate material. The substrate material was hot rolled and annealed in the α + β range. The thickness of the substrate samples was 10 mm. The substrate surfaces were cleaned using grit blasting with emery to remove the oxide scale. A Ni-clad graphite powder was used as the pre-placed material. The chemical composition of the powder is 20 wt.% C + 80 wt.% Ni. The particle size of the powder is in the range of 50–80 μm. An acetylene-fuel thermal spray apparatus (UP-1 type) is used to pre-place a Ni-clad graphite powder layer on Ti–6Al– 4V substrate. The thickness of the pre-placed layer was 0.5 mm. Laser remelting process was carried out using a CO2 laser. The laser parameters were: 750 W power, 5 mm/s scanning speed and 3 mm beam diameter. The laser remelting process was carried out without Ar gas blowing in order to estimate whether a good coating can be obtained.
S. Yang et al. / Materials Letters 61 (2007) 2356–2358
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Fig. 1. SEM micrograph showing the morphology of one coating track in transverse cross-section.
Fig. 3. SEM micrograph showing the interface between the coating and the substrate.
The laser remelted samples were polished and etched in a solution containing 30 ml HF + 10 ml HNO3 + 60 ml H2O for about 1 min. The microstructure of the laser remelted coating was examined using a S-4700 type scanning electron microscope (SEM) with an EDAX type energy dispersive spectroscopy (EDS). The microhardness of the coating was measured using a HVS-1000 Vickers hardness tester with a load of 200 g and a loading time of 15 s.
connecting. Because there are mainly Ni, Ti and C elements in the coating, the chemical composition of the reinforcement and the base can be used for identification. EDS results show that the reinforcement consists of 54 at.% Ti, 41 at.% C and 5 at.% other elements, and the base consists of 68 at.% Ni, 23 at.% Ti and 9 at.% other elements. Consequently, the reinforcement must be based on TiC phase and the base of the coating should be austenitic nickel in which a certain amount of Ti and other elements is in solution. TiC dendrite has a length of about 30–50 μm in the first arm and 3–5 μm in the second arm. The diameter of the TiC arm is about 1–2 μm. The micrograph of the interface between the coating and the substrate is shown in Fig. 3. No cracks and pores are present in the interface zone. There is a diluted intergradation with a width of 20– 30 μm in the interface. The diluted zone consists of a layer of planar crystal and a layer of fringe crystal. There is no TiC reinforcement in the zone. The diluted zone reduces the difference of hardness and thermal expansion coefficient between the coating and the substrate, which is expected to enhance the adhesion of the coating to the substrate. TiC is a new phase that must have been in-situ synthesized in the coating. Titanium is a strong carbide forming element, and easily reacts with C. The formation mechanism of TiC reinforcements is analyzed as follows. In the laser remelting process, the nickel of the Ni-clad graphite powder melted and solid graphite existed in the melting pool. The surface of the Ti–6Al–4V substrate near the bottom of the melting
3. Results and discussion The morphology of the laser remelted coating is shown in Fig. 1. The surface of the coating is macroscopically even, but microscopically rough. The width of one coating track is 3.5 mm and the maximal thickness of the coating is 0.6 mm. Considering the thickness of the pre-placed layer, the pre-placed powder was not diluted more than 15– 20% during laser remelting process. The quality of the coating is good and there are no significant cracks, pores and other inclusions in the coating. The microstructure of the coating is shown in Fig. 2. There are many dendrite phases that occur as reinforcements in the coating. These reinforcements are uniformly dispersed in the coating without
Fig. 2. SEM micrograph showing morphology of TiC reinforcements in the coating.
Fig. 4. The microhardness distribution across the coating.
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pool melted and Ti diffused into the melting pool. A reaction between Ti and C then took place according to the following chemical formula Ti + C → TiC. Then many TiC reinforcements could be in-situ synthesized in the coating. When laser beam passed, the melting pool cooled rapidly. TiC phase began to nucleate primarily and grew in dendrite morphology from undercooled liquid. A microhardness profile of the coating is shown in Fig. 4. There is an intergradation of microhardness in the interface between the coating and the substrate. The microhardness is in the range of HV 900– HV 1100 in the coating zone, and in the range of HV 320–HV 360 in the substrate. The average hardness of the coating is approximately HV 1000 and it is two times greater than that of Ti–6Al–4V substrate.
(2) The coating mainly consists of austenitic nickel matrix and TiC dendrite reinforcements. TiC reinforcements are uniformly dispersed in the coating. (3) Ti came from the Ti–6Al–4V substrate and diffused into the melting pool during the laser remelting process. There was a reaction between C and Ti, thereby in-situ synthesizing TiC reinforcements. (4) The average microhardness of the coating is approximately HV 1000 and it is two times greater than that of Ti–6Al–4V substrate. References
4. Conclusions (1) With the deposition of pre-placed Ni-clad graphite powder layer by thermal spraying, a laser remelted coating on Ti–6Al–4V substrate without cracks and pores was obtained.
[1] [2] [3] [4] [5] [6]
R.L. Sun, J.F. Mao, D.Z. Yang, Surf. Coat. Technol. 155 (2002) 203. J.A. Vreeling, V. Ocelik, J.T.M. De Hosson, Acta Mater. 50 (2002) 4913. S. Mridha, T.N. Baker, Surf. Eng. 13 (1997) 233. R.X. Liu, T.Q. Lei, L.X. Guo, Surf. Rev. Lett. 11 (2004) 291. H.C. Man, S. Zhang, F.T. Cheng, et al., Scr. Mater. 44 (2001) 2801. Y. Chen, H.M. Wang, Mater. Lett. 57 (2003) 1233.
Ni–TiC coating deposited on Ti–6Al–4V substrate by thermal spraying and laser remelting of Ni-clad graphite powder Shengqun Yang, Qingwu Meng ⁎, Lin Geng, Lixin Guo, Lin Wu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. O. Box 433, PR China Received 3 April 2006; accepted 5 September 2006 Available online 22 September 2006
Abstract A process of thermal spraying and laser remelting of a Ni-clad graphite powder to form a coating on Ti–6Al–4V substrate was carried out. A good coating without cracks and pores was obtained. The microstructure of the coating was examined using SEM and EDS. The coating mainly consists of austenitic nickel as matrix and TiC dendrite as reinforcement. During the laser remelting process, a reaction between C and Ti occurred, which lead to an in-situ synthesis of TiC reinforcement in the coating. The microhardness of the coating was measured using a Vickers hardness tester. The average microhardness of the composite coating is HV 1000 and it is two times greater than that of the Ti–6Al–4V substrate. © 2006 Elsevier B.V. All rights reserved. Keywords: Coating; Laser remelt; Thermal spray; Ti–6Al–4V; Ni-clad graphite powder
1. Introduction In order to improve the wear resistance of titanium alloy, laser cladding of ceramic–metal composite coatings on titanium substrate has been proposed [1–4]. In these studies, most of ceramic phases such as TiC [1], WC [2], SiC [3], TiN [4] are directly added into laser clad coatings. However, it has been often found that the ceramic phase is poorly dispersed in the coating if the pre-placed technique of ceramic phase is adopted and that a poor adhesion of reinforced phases to matrix occurs. A possibility to overcome the problem is the in-situ synthesis of ceramic phases in the coating during laser cladding process. As a reaction of C with Ti may lead to the formation of TiC reinforcements at high temperature, in-situ synthesis of TiC in laser clad coating has been reported in a few studies. The precursor powders of Cr3C2 + Ti [5] or C + Ti [6] are pre-placed directly or by means of organic binder onto the substrates in these studies. However, the pre-placed powders have poor adhesion to substrate and are easily splashed off with chemical reaction during laser cladding process. The appropriate method of pre-placed powders is using thermal spraying or plasma spraying. A Ni-clad graphite powder is a kind of composite ⁎ Corresponding author. Tel.: +86 451 86418836; fax: +86 451 86413922. E-mail address: [email protected] (Q. Meng). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.09.011
powder that is used in thermal spraying or plasma spraying. However, Ni-clad graphite powder used in laser cladding coating has never been reported. The present study aims at thermal spraying and laser remelting of a Ni-clad graphite powder to form a coating on Ti–6Al–4V substrate, and to evaluate the microstructure and microhardness of the coating. 2. Experimental details Ti–6Al–4V alloy was used as a substrate material. The substrate material was hot rolled and annealed in the α + β range. The thickness of the substrate samples was 10 mm. The substrate surfaces were cleaned using grit blasting with emery to remove the oxide scale. A Ni-clad graphite powder was used as the pre-placed material. The chemical composition of the powder is 20 wt.% C + 80 wt.% Ni. The particle size of the powder is in the range of 50–80 μm. An acetylene-fuel thermal spray apparatus (UP-1 type) is used to pre-place a Ni-clad graphite powder layer on Ti–6Al– 4V substrate. The thickness of the pre-placed layer was 0.5 mm. Laser remelting process was carried out using a CO2 laser. The laser parameters were: 750 W power, 5 mm/s scanning speed and 3 mm beam diameter. The laser remelting process was carried out without Ar gas blowing in order to estimate whether a good coating can be obtained.
S. Yang et al. / Materials Letters 61 (2007) 2356–2358
2357
Fig. 1. SEM micrograph showing the morphology of one coating track in transverse cross-section.
Fig. 3. SEM micrograph showing the interface between the coating and the substrate.
The laser remelted samples were polished and etched in a solution containing 30 ml HF + 10 ml HNO3 + 60 ml H2O for about 1 min. The microstructure of the laser remelted coating was examined using a S-4700 type scanning electron microscope (SEM) with an EDAX type energy dispersive spectroscopy (EDS). The microhardness of the coating was measured using a HVS-1000 Vickers hardness tester with a load of 200 g and a loading time of 15 s.
connecting. Because there are mainly Ni, Ti and C elements in the coating, the chemical composition of the reinforcement and the base can be used for identification. EDS results show that the reinforcement consists of 54 at.% Ti, 41 at.% C and 5 at.% other elements, and the base consists of 68 at.% Ni, 23 at.% Ti and 9 at.% other elements. Consequently, the reinforcement must be based on TiC phase and the base of the coating should be austenitic nickel in which a certain amount of Ti and other elements is in solution. TiC dendrite has a length of about 30–50 μm in the first arm and 3–5 μm in the second arm. The diameter of the TiC arm is about 1–2 μm. The micrograph of the interface between the coating and the substrate is shown in Fig. 3. No cracks and pores are present in the interface zone. There is a diluted intergradation with a width of 20– 30 μm in the interface. The diluted zone consists of a layer of planar crystal and a layer of fringe crystal. There is no TiC reinforcement in the zone. The diluted zone reduces the difference of hardness and thermal expansion coefficient between the coating and the substrate, which is expected to enhance the adhesion of the coating to the substrate. TiC is a new phase that must have been in-situ synthesized in the coating. Titanium is a strong carbide forming element, and easily reacts with C. The formation mechanism of TiC reinforcements is analyzed as follows. In the laser remelting process, the nickel of the Ni-clad graphite powder melted and solid graphite existed in the melting pool. The surface of the Ti–6Al–4V substrate near the bottom of the melting
3. Results and discussion The morphology of the laser remelted coating is shown in Fig. 1. The surface of the coating is macroscopically even, but microscopically rough. The width of one coating track is 3.5 mm and the maximal thickness of the coating is 0.6 mm. Considering the thickness of the pre-placed layer, the pre-placed powder was not diluted more than 15– 20% during laser remelting process. The quality of the coating is good and there are no significant cracks, pores and other inclusions in the coating. The microstructure of the coating is shown in Fig. 2. There are many dendrite phases that occur as reinforcements in the coating. These reinforcements are uniformly dispersed in the coating without
Fig. 2. SEM micrograph showing morphology of TiC reinforcements in the coating.
Fig. 4. The microhardness distribution across the coating.
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pool melted and Ti diffused into the melting pool. A reaction between Ti and C then took place according to the following chemical formula Ti + C → TiC. Then many TiC reinforcements could be in-situ synthesized in the coating. When laser beam passed, the melting pool cooled rapidly. TiC phase began to nucleate primarily and grew in dendrite morphology from undercooled liquid. A microhardness profile of the coating is shown in Fig. 4. There is an intergradation of microhardness in the interface between the coating and the substrate. The microhardness is in the range of HV 900– HV 1100 in the coating zone, and in the range of HV 320–HV 360 in the substrate. The average hardness of the coating is approximately HV 1000 and it is two times greater than that of Ti–6Al–4V substrate.
(2) The coating mainly consists of austenitic nickel matrix and TiC dendrite reinforcements. TiC reinforcements are uniformly dispersed in the coating. (3) Ti came from the Ti–6Al–4V substrate and diffused into the melting pool during the laser remelting process. There was a reaction between C and Ti, thereby in-situ synthesizing TiC reinforcements. (4) The average microhardness of the coating is approximately HV 1000 and it is two times greater than that of Ti–6Al–4V substrate. References
4. Conclusions (1) With the deposition of pre-placed Ni-clad graphite powder layer by thermal spraying, a laser remelted coating on Ti–6Al–4V substrate without cracks and pores was obtained.
[1] [2] [3] [4] [5] [6]
R.L. Sun, J.F. Mao, D.Z. Yang, Surf. Coat. Technol. 155 (2002) 203. J.A. Vreeling, V. Ocelik, J.T.M. De Hosson, Acta Mater. 50 (2002) 4913. S. Mridha, T.N. Baker, Surf. Eng. 13 (1997) 233. R.X. Liu, T.Q. Lei, L.X. Guo, Surf. Rev. Lett. 11 (2004) 291. H.C. Man, S. Zhang, F.T. Cheng, et al., Scr. Mater. 44 (2001) 2801. Y. Chen, H.M. Wang, Mater. Lett. 57 (2003) 1233.