Microstructures of Composite Coatings Fabricated on Ti-6Al-4V by Laser Alloying Technique Y. S.
Tian,†
L. X.
Chen,‡
and C. Z.
Chen*,†
School of Materials Science and Engineering, Shandong UniVersity, Jinan Shandong 250061, P. R. China, and Shandong Electronic DeVices Co. Ltd, Jinan, Shandong 250161, P. R. China
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1509-1513
ReceiVed August 20, 2005; ReVised Manuscript ReceiVed March 22, 2006
ABSTRACT: Composite coatings were fabricated by laser alloying of titanium alloy Ti-6Al-4V with boron and graphite mixed powders in a nitrogen gas environment. XRD spectra indicate that the compounds formed in the coatings are mainly titanium carbides, borides, and nitrides. SEM and EMPA micrographs show that the morphology of the compounds in the top layer of the coatings is different from that in the inner layer. This is attributed primarily to uneven distribution of the alloying elements and different solidification cooling rates of the areas in the same melt pool. The microstructure of the coatings produced by laser alloying with boron and graphite mixed powder is finer than that produced by alloying with boron or graphite powder separately. In addition, yttria has a refining effect on the microstructure of the coatings, and the mechanism of refinement is analyzed. Introduction The properties of high strength-to-weight ratios and corrosion resistance of pure titanium and its alloys make them widely used in aerospace, marine, chemical industries, etc. Nevertheless, they are currently restricted to nontribological applications owing to their poor friction and wear resistance and high tendency to galling. In addition, titanium alloys present limited oxidation resistance at high temperature because of their strong affinity toward oxygen at elevated temperatures in air. To improve the wear and oxidation resistance of pure titanium and its alloys, different technologies have been developed, and among them a surface modification technique is the subject of a number of investigations. However, conventional chemical heat treatments such as nitriding, carburizing, and boronizing have some disadvantages, such as long processing time, thin treated layer, and easy deformation of the workpiece being treated. Spray coatings also have some demerits such as low coating density and limited bond strength between the coating and the substrate. Laser beams, due to their good coherence and directionality, are widely used in the surface modification of many kinds of metals. So, the disadvantages of pure titanium and its alloys can be overcome by laser surface modification treatment on the special surfaces of the workpieces where they suffer in operation. Laser treatment has several advantages over commonly used heat treatment methods, including precise control over the width and depth of processing, the ability to selectively process specific areas of a component, and the ability to process complex parts. Laser nitriding effects have been demonstrated for various materials and for different laser systems. The works of laser nitriding of titanium alloys have been carried out for improving their wear and corrosion resistance by many researchers.1-4 The wear resistance of laser nitrided Ti-6Al-4V alloy is enhanced noticeably under both two-body abrasive and dry sliding wear conditions.5,6 Nevertheless, well-developed dendritic titanium nitrides produced by laser nitriding in pure nitrogen gas environment can easily induce cracks in the coatings (see Figure 17). Mridha and Hu et al.8,9 found in laser surface nitriding of Ti-6Al-4V that the surface cracking is associated with dendrite * To whom correspondence should be addressed. Tel.: +86-53188395991. Fax: +86-531-88392313. E-mail:
[email protected]. † Shandong University. ‡ Shandong Electronic Devices Co. Ltd.
Figure 1. Cracks in the coatings fabricated by laser nitriding of Ti6Al-4V.
density and reducing the dendrite concentration is responsible for the absence of cracks in the laser tracks. On the other hand, titanium carbides have high hardness, modulus, and flexural strength, and therefore are used as a reinforcement phase for alloy tool steels and nickel-based superalloys as well as various wear resistance coatings.10,11 Courant et al.12,13 fabricated composite coatings on the surface of pure titanium by laser irradiation with graphite addition. The test results show that the coatings containing titanium carbides have higher microhardness, lower friction coefficients, and excellent wear resistance compared with the as-received sample. But the coarse dendritic TiC produced by laser carbonizing of pure titanium can decrease the toughness, which limits its application scope. As wellknown, titanium borides have very high hardness and stability at elevated temperature. Test results demonstrate that the coatings containing titanium borides produced by laser surface boronizing can effectively improve the wear resistance and hightemperature stability of the titanium alloy.14 But the friction coefficient of the coatings is still similar to the untreated sample.7 In previous experiments,15 we found that the morphology of the compounds in the coatings produced by laser alloying of titanium alloy with carbon and boron (argon gas shield) is not well-developed dendritic and is of benefit for improving the toughness of the coatings. So, in the present study the laseralloying technique was employed to produce composite coatings on the surface of titanium alloy Ti-6Al-4V in a nitrogen environment. Boron and graphite powders are selected as additions of alloying elements for the purpose of enhancing the properties and reducing the friction coefficient of the coatings. In addition, yttria was added to modify the microstructure of the coatings.
10.1021/cg0504307 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/10/2006
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Figure 2. Cross-section micrograph of sample 1. Table 1. Weight Ratio of the Powders alloying powder
sample 1
sample 2
sample 3
sample 4
sample 5
sample 6
boron (%) graphite (%) Y2O3 (%)
50 50 0
25 25 50
100 0 0
50 0 50
0 100 0
0 50 50
Experimental Procedures The samples of titanium alloy Ti-6Al-4V, of size 10 × 10 × 12 mm, were abraded with SiC grit paper prior to the coating operation. Graphite, boron, and yttria powders, with an average particle size of about 10 µm, blended with diluted poly(vinyl alcohol) solution, were precoated on the surface of the samples with a thickness of approximately 0.5 mm. A 1500 W continuous wave CO2 laser, output power of 1200 W, beam size of 4 mm, and scanning speed of 2.5 mm/s was employed to melt the surface of the samples, and the tracks were 50% overlapped. Nitrogen gas at a pressure of 0.3 MPa was fed through a nozzle that was coaxial with the laser beam acting as both alloying element and protecting the melt pool from oxidation during laser processing. Metallographic samples were prepared using standard mechanical polishing procedures and then etched in a solution of HF, HNO3, and H2O in volume ratio of 2:1:47 to reveal the growth morphologies of compounds. Microstructure was characterized using JXA-8800R EMPA and JXA-840 SEM. The phase composition of the coatings was identified using D/max-RC XRD with Cu KR radiation operated at a voltage of 40 kV, a current of 40 mA, and a scanning rate of 4°/min. The alloying element powders used in the experiment and their weight ratios are shown in Table 1.
3. Results and Discussions Figure 2 is the cross-section micrographs of the sample 1. It is seen that the morphology of the top layer is different from that in the inner layer. The compounds in the top layer are not well-developed dendritic compounds (see Figure 3a); however, granular compounds, of about 3-6 µm, are distributed in the intermediate layer (see Figure 3b), and petal-like compounds formed in the layer near the substrate. Courant et al. have also found granular titanium carbides in their experiment of laser alloying of pure titanium with graphite powder.12,13 This is mainly attributed to the uneven distribution of the alloying elements and the different solidification cooling rates of the areas in the same melt pool. It can be explained as follows. In the present experiment, the melt pool solidifies rapidly (taking about 1-2 s) after the laser beam moves away. So, the melted additions do not have enough time to diffuse fully in the melt despite both the Marangoni convection and the shield gas flow facilitating diffusion. On the other hand, under the irradiation of the laser beam, all the added material particles cannot be melted instantaneously and simultaneously because their size and absorbed heat energy are different from each other. Thus, the diffusion time of the melted particles is different before
Figure 3. Micrographs of sample 1: (a) top layer; (b) intermediate layer; (c) the layer near the substrate.
the melt solidifies. Gyo¨rgy et al.16 reported that the titanium nitrides produced by laser nitriding of titanium alloys are mostly formed in the top layer of the coatings because nitrogen gas cannot penetrate deeply into the melt under the laser processing conditions, which has been confirmed by the XRD spectra (see Figure 4) and TEM diffraction patterns (see Figures 5-7) of the present experiment. It may be another factor for the uneven distribution of the alloying elements. It is well-known that different solidification cooling rates result in different crystal morphologies. Shibkov et al.17 found in their investigation of ice crystals freely growing from
Microstructures of Composite Coatings on Ti-6Al-4V
Crystal Growth & Design, Vol. 6, No. 6, 2006 1511
Figure 6. (a) Inner layer micrograph of sample 2; (b) SAED pattern of TiB, [1h22] zone axis; (c) SAED pattern of TiC, [331] zone axis.
Figure 7. (a) Inner layer micrograph of sample 2; (b) SAED pattern of R-Ti, [100] zone axis; (c) SAED pattern of TiB, [111] zone axis. Figure 4. XRD spectrum of sample 1: (a) top layer; (b) abrading off 0.5 mm from the surface.
Figure 5. (a) Inner layer micrograph of sample 2; (b) SAED pattern of TiC, [110] zone axis; (c) SAED pattern of TiB, [111] zone axis.
supercooled pure water that when supercooling increases from 0.1 °C to about 30 °C, different structures of ice crystals change sequentially from disk, to perturbed disk, to dense-branching morphology occurring due to splitting of the finger tips, to dendrite, to stable needle, to fractal needled branch, to compact needled branch, and finally to platelet. Chen et al.18 also reported that with the solidification cooling rate increasing, the growth morphology of titanium carbides produced by laser surface alloying of γ-TiAl intermetallic with carbon changes from dendritic to cross-petal-like with symmetrical arms, to irregular block or undeveloped dendrite. Because of the substrate’s chill effect, in the same melt pool the areas near the substrate have higher solidification cooling rates than that away from the substrate, and the melt with a higher cooling rate surely has higher supercooling. Therefore, the difference of the cooling rate at the areas in the same melt affects both the distribution of the alloying elements and the morphology of the compounds. Furthermore, the buoyancy forces resulting from the difference
Figure 8. Top layer micrographs of the samples: (a) sample 2; (b) sample 3; (c) sample 4; (d) sample 5; (e) sample 6.
in density between the additions and the melt may be also responsible for the incomplete blend. On the other hand, in a nitrogen environment laser alloying with boron and graphite mixed powder has a beneficial effect on the microstructure of the coatings, which can be identified
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the radius of the critical nucleus is expressed as
r* ) 2γ/∆gv and the critical nucleation work is expressed as
∆G* )
Figure 9. XRD spectrum of the top layer of the sample 2.
by comparing Figure 3a with Figure 8b,d. It can be seen that the coarse stick (see Figure 8b) and well-developed dendritic (see Figure 8d) compounds are formed when alloyed with boron or graphite separately. But when the sample is alloyed with boron and graphite mixed powders, the morphology of the compounds is not well-developed dendritic, as shown in Figure 3a. This may be attributed to the intergrowth of titanium borides and carbides, as shown in Figures 5 and 6, which surely has a restraining effect on the formation of well-developed compounds. In addition, acicular titanium borides insert into the matrix of the sample, as shown in Figure 7, which plays a role in refinement of the microstructure of the coatings. Figure 9 is the XRD spectrum of sample 2. It is seen that although the coatings mainly contain titanium borides, carbides, and nitrides, residual yttria particles still exist. Comparing panel b with c and panel d with e in Figure 8 as well as Figure 3a with Figure 8a, respectively, it can be seen that the microstructure of the coatings without yttria are coarser and more welldeveloped than that with yttria. This means that the addition of Y2O3 can improve the morphology of compounds and refine the coarse microstructure of the coatings. The mechanism of microstructure refinement by the addition of Y2O3 is explained as follows. When the sample is irradiated by a high energy density laser beam, a large proportion of added Y2O3 decomposes and releases atomic Y:
2Y2O3 f 4Y + 3O2 The result of microstructure refinement of the coatings is mainly due to the characteristics of element Y. First, it is surface-active elements and thus reduces the surface tension and the interfacial energy between the crystal nuclei and the melt. The change in Gibb’s free energy (∆G) between the crystalline phase and the surrounding melt results in a driving force that stimulates crystallization. For rapid crystallization ∆G < 0, the energy required to form a spherical nucleus is given by the equation:
4 ∆G ) - πr3∆gv + 4πr2γ 3 where ∆gv is the energy change per unit volume, r is the radius of the nucleus, and γ is the interfacial tension. The first term expresses the difference in the chemical potential between the crystalline phase and the surrounding melt, and the second term expresses the surface energy formation. At the critical state, the free energy formation obeys the condition d(∆G)/dr ) 0. Hence
16 3 πγ (∆gv)2 3
It can be seen that the lower the solid-liquid interfacial tension γ, the lower the Gibbs free energy needed for forming a nucleus. Therefore, element Y can decrease critical nucleation work because of its chemical activity and thus the addition of yttria facilitates the formation of nuclei. Second, during the crystallization, the high-temperature phase β-Ti (body-centered cubic lattice structure) has the same lattice type as Y2O3, which may act as heterogeneous nuclei and thus increase the number of crystal nuclei. Zhang et al.19 have found in synthesized titanium matrix composites that the orientation relationships between TiB and Y2O3 are [1h01]TiB//[021]Y2O3 and (111)TiB/(12h1)Y2O3. Kral et al.20 reported that the orientation relationships between titanium and RE oxides (CeO2, Er2O3 and La2O3) are 〈12h1〉R-Ti//〈11h0〉CeO2, (0001)R-Ti//(111)CeO2; 〈12h10〉R-Ti// 〈11h 〉Er2O3, (0001)R-Ti//(111)Er2O3 and 〈12h 10〉R-Ti//〈11h 0〉La2O3, (0001)R-Ti//(111)La2O3. Therefore, the unmelted Y2O3 particles can supply heterogeneous nucleation sites for the subsequent crystallization of the high-temperature TiB, β-Ti, R-Ti, etc. crystalline phases, and the microstructure of the composite coatings can be further refined. In addition, since the atomic radius of Y is rather large (radius of Y: 0.180 nm; Ti: 0.145 nm21), the existence of Y atoms within the solid solution would surely cause great distortion and mismatch of the lattice, which increase the energy of the system.22 To retain the lowest free energy, an enrichment of Y atoms over a grain boundary, where the atomic arrangement is irregular, would be required. Thus, Y atoms in the coatings distribute mostly over the grain boundary. When the grains grow, the Y atoms over the grain boundaries produce a dragging effect on the movement of the grain boundaries, and the growth of grains is hindered. 4. Conclusion Composite coatings were fabricated by a laser surface alloying technique. The microstructure and phase composition of the coatings are summarized as follows. The different morphologies of the compounds in the same coatings are mainly attributed to the uneven distribution of the alloying elements and the different solidification cooling rates of the areas in the same melt pool. Laser alloying with boron and graphite mixed powders has a restraining effect on the formation of coarse stick and welldeveloped dendritic compounds produced with boron or graphite separately. Because of the special physical and chemical properties of the rare earth elements, yttria has a beneficial effect on the morphology modification of the compounds and on the microstructure refinement of the coatings. XRD spectra confirm that the coatings primarily are hard ceramic compounds that contain titanium carbides, borides, and nitrides, etc. that definitely improve the load-bearing capability of the substrate. References (1) Walker, A.; Folkes, J.; Steen, W.; West, D. R. J. Surf. Eng. 1985, 1, 23.
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Crystal Growth & Design, Vol. 6, No. 6, 2006 1513 (14) Badini, C.; Bianco, M.; Talentino, S.; Guo, X. B.; Gianoglio, C. Appl. Surf. Sci. 1992, 54374. (15) Tian, Y. S.; Chen, C. Z.; Wang, D. Y.; Huo, Q. H.; Lei, T. Q. Surf. ReV. Lett. 2005, 12, 443. (16) Gyo¨rgy, E.; Pe´rez del Pino, A.; Serra, P.; Morenza, J. L. Surf. Coat. Technol. 2003, 173, 265. (17) Shibkov, A. A.; Golovin, Y. I.; Zheltov, M. A.; Korolev, A. A.; Leonov, A. A. Physica A 2003, 319, 65. (18) Chen, Y.; Wang, H. M. Scr. Mater. 2004, 50, 507. (19) Zhang, D.; Geng, K.; Qin, Y.; Lu, W.; Ji, B. J. Alloys Compd. 2005, 392, 282. (20) Kral, M. V.; Hofmeister, W. H.; Wittig, J. E. Scr. Mater. 1997, 36, 157. (21) http://www.crystalstar.org/resourcelink.asp. (22) Wang, K. L.; Zhang, Q. B.; Sun, M. L.; Wei, X. G.; Zhu, Y. M. Appl. Surf. Sci. 2001, 174, 191.
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