- Email: [email protected]
Al metal matrix composite by laser cladding
Materials Science and Engineering A 447 (2007) 307–313
Investigation on microstructural characterization of in situ TiB/Al metal matrix composite by laser cladding Jiang Xu a,∗ , Zhengyang Li b , Wenhui Zhu a , Zili Liu a , Wenjin Liu b a
b
Department of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, PR China Key Laboratory for Advanced Materials Manufacturing Processing, Mechanical Engineering Department, Tsinghua University, Beijing 10084, PR China Received 1 August 2006; received in revised form 29 September 2006; accepted 20 October 2006
Abstract The aluminum matrix composite (AMC) coating reinforced with TiB was prepared utilizing in situ synthesized technique by laser cladding. Microstructural characterization and dry sliding wear behavior of in situ TiB/Al metal matrix composite were studied by SEM, XRD, TEM and Pin-on-disc friction and wear tester. The phase structure of the composite coating consists of ␣-Al, TiB, Al3 Ti and Al3 Fe. It has been found that the shape of in situ synthesized TiB is mainly taken on micro-magnitude lump and nano-magnitude whisker. Owing to B27 structure of TiB, the TiB has an anisotropy axis of growth, which results in the TiB strip and whisker preferring grown along [0 1 0] direction. It is worth to notice that the novel microstructure inside of TiB is particle and strip Al5 Fe2 phase and definite crystallographic relationship between the Al5 Fe2 phase and TiB has been determined by selected area diffraction pattern. The wear tests results show that the composite coatings can only improve wear resistance at the lower applied load (below 26.7 N), but at higher applied load (26.7–35.6 N) the wear resistance behavior of the coating is worsened due to the fracture and pullout of reinforcement phase. © 2006 Elsevier B.V. All rights reserved. Keywords: In situ synthesized TiB; Microstructure characterization; Wear mechanism; Laser cladding
1. Introduction Due to high strength-to-weight ratio, good mechanical properties, good corrosion resistance, the aluminum and its alloys are attractive engineering materials and widely used in chemical, aeronautical, automotive, food and aerospace industries. However, the poor tribological performance limits the application of aluminum and its alloys in industry [1]. Because the monolithic materials hardly meet the demand, in recent years, researchers pay attention to aluminum matrix composites (AMC), because of their excellent combination of higher specific strength, wear resistance and stiffness, and so on. AMCs find potential applications in automobile components, such as piston, cylinder liner, brake drums, crankshafts, etc. Some of these components are translated into commercial production [2–4]. In general speaking, reinforcing phase exhibit different shape, such as particle, whisker and fiber. The major problem of AMC is focused on the interface characteristic and interface bonding between the
∗
Corresponding author. Fax: +86 25 848 91012. E-mail address: [email protected] (J. Xu).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.057
reinforcing phase and metallic matrix. The favorable bonding can keep the reinforcing phase intact in matrix alloy and provide the higher wear resistance under severe friction condition. For example, Mondal and Das [5] have studied the high stress abrasive wear behavior of aluminum alloy (ADC-12)–SiC particle reinforced composites. The result shows that the wear rate of composite decreases linearly with increasing the amount of SiC, but the wear resistance of composite varies inversely with square of the reinforcement particle size [5]. Utilizing in situ synthesized technique can effectively avoid the physical and chemical incompatibility between the reinforcing phase and matrix. Moreover, selection of suitable reinforcing phase is a key factor for fabricating in situ reinforced MMC. The TiB single crystal has a Young’s modulus of 550 GPa, a coefficient of thermal expansion of 8.6 × 10−6 K−1 and a density of 4.51 [6]. Compared with a number of ceramic reinforcements, TiB has been identified as possessing the most appropriate balance of thermochemical stability, good mechanical properties and thermal expansion [7]. Surface strengthening is an effective way to improve the wear resistance of aluminum alloy [8]. Serbinski reported that Mn–N–S composite coating, which is achieved by hybrid method of gas sulphur nitriding and electroplated
308
J. Xu et al. / Materials Science and Engineering A 447 (2007) 307–313
manganese coating, have high hardness and good tribological properties. Laser cladding is an attractive surface treatment technology which takes possession of advanced features, such as the metallurgical bond between the cladding coating and substrate, high process flexibility, high working speeds and no requirement for post process treatment. There is a growing interest in fabrication of a surface layer of particulate reinforced metal matrix composites on metallic materials. This process involves a massive introduction of hard particle into the wear resisting surface clad layer, such as WC, TiC, SiC, ZrO2 or Al2 O3 ceramics, which are directly injected into the high temperature molten pool created in the surface of substrate by laser beam [8–13]. There is a major drawback in adding the reinforced phase directly into laser cladding coating because of poor wetting behavior between ceramic phase and metal matrix and a large different thermal expansion coefficient between them. In order to solve this problem, the in situ synthesized particles particulate reinforced metal matrix composites have been extensively investigated [14,15]. The present work is aimed at the investigation of microstructural characterization and dry sliding wear behaviors of in situ TiB/Al metal matrix composite coating by laser cladding. The interface and crystallographic relationship of TiB and matrix has been identified with TEM. With observation and analyses of worn surfaces and wear debris, the wear mechanism of laser cladding coating was discussed. 2. Experimental details 2.1. Material and specimen preparation The materials used were Fe coated Boron, pure aluminum and pure Ti powders with a size of around 150 mesh. In order to avoid burn of boron by laser, Fe has been plated on the surface of boron powder. The ratio of Fe to Boron is 4/6 in weight in Fe coated Boron powder. A mixture of 20.19 wt.% Fe coated B, 50.81 wt.%Ti and 20 wt.%Al powder was used as the coating materials for TiB/Al metal matrix composite coating. The various powders were pre-mixed by hand, using a spoon in a glass beaker; then the mixed powders were transported into melt pool by coaxial powder feeding instrument. The substrate material was a 2024 aluminum alloy. The rectangular specimens of 40 mm × 15 mm × 15 mm were cut from alloy plate and were degreased prior to the cladding process and removed any surface contaminants for good adhesion between the coating and substrate. The nominal chemical composition of the alloy in wt.% was: Cu, 3.8–4.9; Mg, 1.2–1.8; Mn, 0.3–0.9; Zn, 0.25; and the balance, Al.
2.3. Microstructure examination Metallographic cross-section of the clad samples was prepared in the plane parallel and perpendicular to the scan direction. The chemical compositions and microstructure of laser-clad coating were analyzed by a LEO-1450 scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (X-EDS). The phase structure identification was conducted by D/Max-RB X-ray diffraction (XRD). The radiation source was Cu K␣, working voltage at 40 kV with a scan rate 15 min−1 . The microhardness measurement was done on HX-200 micro-Vickers machine with a 0.2 kg load and load time was set at 15 s. TEM specimen was prepared according to the standard method including cutting to 0.2 mm thick slices with an electric spark machine along the laser scanning direction, followed by dimpling and finally ion-milling with 4 kV voltage and 0.4 mA current. A JEM-2000FX TEM was used to identify the phase. 2.4. Pin-on-ring friction and wear experiment Dry sliding friction and wear tests without lubricant were performed in a pin-on-ring mode on a Falex-6 model friction and wear-testing machine (Falex corporation, Sugar Grove, Illinois, USA). The pin specimens were machined in the form of cylinders with 4.8 mm diameter and 12.7 mm length. The counterpart discs were made of an annealed 1045 medium carbon, surface hardness of 147HB and surface roughness of Ra = 0.2 m. The applied load was 8.9, 17.8, 26.7 and 35.6 N. The sliding speed was kept constant at 0.24 m/s. The total sliding distance for the test was 150 m. The weight loss during the wear test was measured using a photoelectric balance 1712MP8 with the resolution of ±0.01 mg. 3. Results and discussion 3.1. Phase constituents of in situ synthesis TiB/Al composites coating Under laser radiation, molten pool is formed by physical metallurgy action of the addition mixture powder, the chemical reactions which accord with thermodynamic laws can be taken place as follows: Al + 2B → AlB2
(1)
Ti + 2B → TiB2
(2)
3Al + Ti → Al3 Ti
(3)
2.2. Laser cladding
3Al + Fe → FeAl3
(4)
The laser cladding was carried out with a defocused laser beam of 3 mm diameter using a PRC-3 kW continuous wave CO2 laser processing system. The laser was operated at a TEM01 mode. The laser cladding parameters were: laser power 1.7 kW and traverse scan speed 5 mm/s. A coaxial jet and a side jet of argon gas were used to prevent the surface of sample from oxidation.
Ti + B → TiB
(5)
Ti + TiB2 → 2TiB
(6)
the Gibbs free energy (G) of above-mentioned reactions is all negative, which indicates that all the above reactions are capable of taking place. However, other chemical reactions would eventually affect the products of laser cladding coating. When AlB2
J. Xu et al. / Materials Science and Engineering A 447 (2007) 307–313
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and Ti are heated, the following reaction is thermodynamically possible [16]: AlB2 + 43 Ti → 13 Al3 Ti+TiB2
(7)
When the titanium is in excess, the G of reactions (5) and (6) is less than that of reaction (2), which indicates that the reaction (5) and (6) are apt to take place, namely TiB phase behaves higher stability than that of TiB2 phase throughout the temperature range. Furthermore, the above-mentioned only involve thermodynamics without considering the effect of kinetics. The laser cladding technique belong to the category of rapid solidification, thereby the ultimate produce of laser cladding coating is influenced by reaction kinetics. Fig. 1 shows the XRD pattern of the as-reacted materials. It can be seen that main phase constituents of in situ synthesis TiB/Al composites coating consists of ␣-Al, TiB, Al3 Ti and Al3 Fe. Only the diffraction peaks of TiB are found and TiB2 cannot present in the final synthesized product for Fe–Ti–Al–B system with excess Ti. The XRD result is in a good agreement with the theoretical analysis. 3.2. Microstructure of in situ synthesized TiB/Al composites coating Fig. 2a shows the scanning electron image in the backscattered electron imaging (BEI) mode of the macrostructure of composite coating from cross-section at a scanning velocity of 5 mm/s and 1.7 kW output power. The composition of various
Fig. 1. X-ray diffraction spectra of in situ synthesized TiB/Al composites coating by laser cladding.
microzones in the Fig. 2a is analyzed by energy dispersive Xray (EDS). The white–gray needle-shaped phase (B) and grey needle-like (C) are identified as Al3 Fe and Al3 Ti by EDX. The white lump-like or strip-like phase (A) which is rich in Ti is identified TiB. The TiB appears lump-like or strip-like in micromagnitude size. It can be seen clearly that a zigzag covered layer of TiB contains Al and Fe element. Fig. 2b shows the microstructure of two conjoint particles. The covered layer of two TiB lump
Fig. 2. SEM photographs of microstructure of in situ synthesized TiB/Al composites coating.
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Fig. 3. TEM bright field image of in situ synthesized TiB/Al composites coating (a and b) TiB lump morphology and selected area diffraction patterns; (c) TEM bright field image of TiB whisker; (d–f) microstructure of inside of TiB lump and selected area diffraction patterns.
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is thicker than that of individual TiB lump. The chemical reaction Ti + B → TiB is exothermic reaction and the released heat generated by the reaction can melt the boundary of TiB lump. The Ti from the melted TiB lump reacts with Fe to form compound. Obviously, the more TiB lumps are integrated together, the more quantity of heat is released and thicker covered layer is formed. Fig. 3a shows bright field TEM images of TiB in aluminum matrix composite coating. Fig. 3b shows selected area diffraction patterns in Fig. 3a. TEM observations revealed that TiB lump are indexing to the orthorhombic structure TiB (B27) with lattice parameters a = 0.612 nm, b = 0.306 nm and c = 0.456 nm. It has been confirmed that lump axis is parallel to the [0 1 0] axis of the B27 cell and the growth direction of TiB is [0 1 0]. TiB crystallizes with an FeB type structure with an orthorhombic structure, characterized by zigzag chains of boron atoms parallel to the [0 1 0] direction [17]. This strong anisotropy in crystal structure may be responsible for the observed preferred growth directions and low energy crystal faces [18]. As shown in Fig. 3a and b, the interface of TiB lump and aluminum matrix is very clean and there is not any interface reaction due to the less released quantity of heat. Beside the lump-shape and stripshaped, the nano-magnitude whisker-shaped TiB of the average aspect ratio above 20 with diameter 50 nm have been found. Fig. 3d–f present the novel microstructure of inside of TiB and its selected area diffraction patterns. According to analysis result of selected area diffraction pattern, the inside of microstructure is identified as Al5 Fe2 phase and the definite crystallographic relationship between the Al5 Fe2 phase and TiB has been determined ¯ TiB //[3¯ 1¯ 2]Al Fe (0 1 1)TiB //(0 2 1)Al Fe . The by Fig. 3e: [2 3 3] 5 2 5 2 Al3 Ti, Al3 Fe compound and TiB are formed at the elevated temperature, which evidently increased the viscosity of aluminum melt. Owing to high cooling rates of laser cladding process, inrich Al and Fe element matrix melt is easy to be captured by two close TiB lumps in the viscous melt. The captured member forms new Al5 Fe2 compound phase and sinters the two TiB lumps together (Fig. 3d). The Fig. 2b can also give proof of formation of the covered phase. Because of covered effect, the Al5 Fe2 phase cannot be identified by XRD.
311
Fig. 4. Hardness profiles of in situ synthesized TiB/Al composites coating.
with increasing the load for the in situ synthesized TiB/Al composites coating and 2024 aluminum alloy under the dry sliding wear condition. When the applied load is below 26.7 N, the wear loss rate of in situ synthesized TiB/Al composites coating is considerably lower than that of 2024 aluminum alloy. There is a sharp transition in wear rate (i.e. sudden increase in wear rate), when the applied load changes from 26.7 to 35.6 N, and the wear loss rate of the 2024 Al alloy is inferior to that of in situ synthesized TiB/Al composites coating. In general, wear resistance is proportional to the hardness of material, the presence of the TiB and intermetallic compound increase the hardness of composites coating and act as load bearing member at the lower applied load. However, the hardness is not the exclusive factor to control the wear behaviors of composite coating. Owing to a large strain in the Al matrix at higher applied loads and the difference of mechanical properties (strength and toughness) of TiB, intermetallic compound and Al matrix, the interface of reinforcement and Al matrix becomes weak and fragile. Over a certain applied load, the TiB and intermetallic compound are fallen off and lost their ability to support the load. In this case, the Al matrix contacts the counterfaces directly. The worn sur-
3.3. Microhardness and wear resistance of in situ synthesized TiB/Al composites coating The microhardness profile of in situ synthesized TiB/Al composites coating by laser cladding is shown in Fig. 4. It can be seen that the highest hardness emerges at subsurface and a gradual distribution of hardness is formed in coating after the maximal hardness was achieved. The maximal hardness of synthesized TiB/Al composites coating is 8-fold that of 2024 aluminum alloy matrix. In the composite coating, TiB has the highest hardness, followed by the Al3 Fe, Al3 Ti and the Al matrix has the lowest hardness. The increased hardness is attributed to the reinforcement in composite coating. The wear resistance of the coating was evaluated by measuring the wear weight loss with pin-on-ring mode on a Falex-6 model friction and wear testing machine. Wear losses of the tested materials as a function of applied load were measured and the results are presented in Fig. 5. The wear losses rate increased
Fig. 5. Wear weight loss rate of composite coatings and 2024 Al alloy as function of the normal load at given sliding velocity (0.24 m/s) and wear time of 10 min.
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Fig. 6. The appearance of worn surface: at applied load of 8.9 N (a) 2024 Al alloy and (b) composite coatings; at applied load of 35.6 N (c) 2024 Al alloy; (d and e) composite coatings.
face of composite coating and 2024 aluminum alloy at applied load of 8.9 and 35.6 N is shown in Fig. 6. It is evident from the Fig. 6a that the plough grooves and flakes were found, which confirm that the wear process of 2024 Al alloy is governed by the combining effect of micro-cutting and adhesion mechanism. At high load of 35.6 N, the appearance of worn surface of 2024 Al alloy (Fig. 6c) exhibits mainly adhesion wear characteristic due to detachment of fragment from the aluminum alloy surface. As shown in Fig. 6b, TiB appear on the worn surface and are good bond with Al-based matrix. The major portion of applied load on the specimen is carried by reinforcement and wear of composite coating is controlled by the wear of reinforcement phase. At high load of 35.6 N, interface bonding of reinforcement and matrix is a major factor of wear resistance of composite coating. Because of deformation disaccord between reinforcement phase and matrix, the tip of reinforcement phase gives rise to the stress concentration and cracks are easy to be propagated and expanded. Simultaneously, the softer matrix would wear out faster compared to the reinforcement phase and the
reinforcement phase will be protruded over the worn surface. Owing to the reduction of support by matrix, which results in reinforcement phase flaking off (Fig. 6d). If the effective load on the individual reinforcement phase increase above its flexural strength, the reinforcement phase can get fractured (Fig. 6e). It needs to be further investigated whether the covered Al5 Fe2 in TiB accelerate the fracture of TiB lump. Therefore, the 2024 Al alloy manifests higher wear resistance than that of composite coating at higher applied load. 4. Conclusions 1. The phase constituents of in situ synthesis TiB/Al composites coating consists of ␣-Al, TiB, Al3 Ti and Al3 Fe. TiB2 cannot exist in the final synthesized product for Fe–Ti–Al–B system with excess Ti. 2. Owing to B27 structure of TiB, the TiB has an anisotropy axis of growth, which results in the TiB strip and whisker preferring grown along [0 1 0] direction. The phase inside
J. Xu et al. / Materials Science and Engineering A 447 (2007) 307–313
of TiB is identified as Al5 Fe2 and the definite crystallographic relationship between the Al5 Fe2 phase and TiB ¯ TiB //[3¯ 1¯ 2]Al Fe has been determined by Fig. 3e: [2 3 3] 5 2 (0 1 1)TiB //(0 2 1)Al5 Fe2 . 3. When the applied load is below 26.7 N, the wear loss rate of in situ synthesized TiB/Al composites coating is considerably lower than that of 2024 aluminum alloy. There is a sharp transition in wear rate (i.e. sudden increase in wear rate), when the applied load change from 26.7 to 35.6 N, and the wear loss rate of the 2024 Al alloy is inferior to that of in situ synthesized TiB/Al composites coating. References [1] S. Tomida, K. Nakata, S. Saji, T. Kubo, Surf. Coat. Technol. 174–175 (2001) 585–589. [2] M.H. Korkut, Mater. Sci. Technol. 20 (2004) 73–81. [3] O.M. Suarez, J. Mech. Behav. Mater. 11 (2001) 225. [4] M.A. Martinez, A. Martin, J. Llorca, Scripta Metall. Mater. 28 (1993) 207–212.
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[5] D.P. Mondal, S. Das, Tribol. Int. 6 (2006) 470–478. [6] Z. Fan, et al., J. Mater. Sci. 29 (1994) 1127. [7] T. Saito, T. Furuta, T. Yamaguchi, K. Ogino, in: Y. Bando, L. Kosuge (Eds.), Proceedings of the Conference 1993 Powder Metallurgy World Congress, JPMA-JSPM, Kyoto, 1993, pp. 642–645. [8] U. Yongqiang Yang, H.C. Man, Surf. Coat. Technol. 132 (2000) 130–136. [9] A. Hidouci, J.M. Pelletier, F. Ducoin, D. Dezert, R. El Guerjouma, Surf. Coat. Technol. 23 (2001) 7–23. [10] P.H. Chong, H.C. Man, T.M. Yue, Surf. Coat. Technol. 154 (2002) 268–275. [11] Y. Herrera, I.C. Grigorescu, J. Ramirez, C. Di Rauso, M.H. Staia, Surf. Coat. Technol. 108–109 (1998) 308–311. [12] A. Mchimann, S.F. Dirnfeld, I. Minkoff, Surf. Coat. Technol. 42 (1990) 275–281. [13] T.C. Lei, J.H. Quyang, Y.T. Pei, J. Harbin Inst. Technol. 1 (1995) 90–96. [14] Sen Yang, Minlin Zhong, Wenjin Liu, Mater. Sci. Eng. A 343 (2003) 57– 62. [15] B. Ralph, H.C. Yuen, W.B. Lee, J. Mater. Process. Technol. 63 (1997) 339. [16] S. Gorsse, D.B. Miracle, Acta Mater. 51 (2003) 2427–2442. [17] B.F. Decker, B.F. Kasper, Acta Crystallogr. 7 (1954) 77. [18] M.E. Hyman, C. McCullough, J.J. Valencia, C.G. Levi, R. Mehrabian, Metall. Trans. 20A (1989) 1847.
Investigation on microstructural characterization of in situ TiB/Al metal matrix composite by laser cladding Jiang Xu a,∗ , Zhengyang Li b , Wenhui Zhu a , Zili Liu a , Wenjin Liu b a
b
Department of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, PR China Key Laboratory for Advanced Materials Manufacturing Processing, Mechanical Engineering Department, Tsinghua University, Beijing 10084, PR China Received 1 August 2006; received in revised form 29 September 2006; accepted 20 October 2006
Abstract The aluminum matrix composite (AMC) coating reinforced with TiB was prepared utilizing in situ synthesized technique by laser cladding. Microstructural characterization and dry sliding wear behavior of in situ TiB/Al metal matrix composite were studied by SEM, XRD, TEM and Pin-on-disc friction and wear tester. The phase structure of the composite coating consists of ␣-Al, TiB, Al3 Ti and Al3 Fe. It has been found that the shape of in situ synthesized TiB is mainly taken on micro-magnitude lump and nano-magnitude whisker. Owing to B27 structure of TiB, the TiB has an anisotropy axis of growth, which results in the TiB strip and whisker preferring grown along [0 1 0] direction. It is worth to notice that the novel microstructure inside of TiB is particle and strip Al5 Fe2 phase and definite crystallographic relationship between the Al5 Fe2 phase and TiB has been determined by selected area diffraction pattern. The wear tests results show that the composite coatings can only improve wear resistance at the lower applied load (below 26.7 N), but at higher applied load (26.7–35.6 N) the wear resistance behavior of the coating is worsened due to the fracture and pullout of reinforcement phase. © 2006 Elsevier B.V. All rights reserved. Keywords: In situ synthesized TiB; Microstructure characterization; Wear mechanism; Laser cladding
1. Introduction Due to high strength-to-weight ratio, good mechanical properties, good corrosion resistance, the aluminum and its alloys are attractive engineering materials and widely used in chemical, aeronautical, automotive, food and aerospace industries. However, the poor tribological performance limits the application of aluminum and its alloys in industry [1]. Because the monolithic materials hardly meet the demand, in recent years, researchers pay attention to aluminum matrix composites (AMC), because of their excellent combination of higher specific strength, wear resistance and stiffness, and so on. AMCs find potential applications in automobile components, such as piston, cylinder liner, brake drums, crankshafts, etc. Some of these components are translated into commercial production [2–4]. In general speaking, reinforcing phase exhibit different shape, such as particle, whisker and fiber. The major problem of AMC is focused on the interface characteristic and interface bonding between the
∗
Corresponding author. Fax: +86 25 848 91012. E-mail address: [email protected] (J. Xu).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.057
reinforcing phase and metallic matrix. The favorable bonding can keep the reinforcing phase intact in matrix alloy and provide the higher wear resistance under severe friction condition. For example, Mondal and Das [5] have studied the high stress abrasive wear behavior of aluminum alloy (ADC-12)–SiC particle reinforced composites. The result shows that the wear rate of composite decreases linearly with increasing the amount of SiC, but the wear resistance of composite varies inversely with square of the reinforcement particle size [5]. Utilizing in situ synthesized technique can effectively avoid the physical and chemical incompatibility between the reinforcing phase and matrix. Moreover, selection of suitable reinforcing phase is a key factor for fabricating in situ reinforced MMC. The TiB single crystal has a Young’s modulus of 550 GPa, a coefficient of thermal expansion of 8.6 × 10−6 K−1 and a density of 4.51 [6]. Compared with a number of ceramic reinforcements, TiB has been identified as possessing the most appropriate balance of thermochemical stability, good mechanical properties and thermal expansion [7]. Surface strengthening is an effective way to improve the wear resistance of aluminum alloy [8]. Serbinski reported that Mn–N–S composite coating, which is achieved by hybrid method of gas sulphur nitriding and electroplated
308
J. Xu et al. / Materials Science and Engineering A 447 (2007) 307–313
manganese coating, have high hardness and good tribological properties. Laser cladding is an attractive surface treatment technology which takes possession of advanced features, such as the metallurgical bond between the cladding coating and substrate, high process flexibility, high working speeds and no requirement for post process treatment. There is a growing interest in fabrication of a surface layer of particulate reinforced metal matrix composites on metallic materials. This process involves a massive introduction of hard particle into the wear resisting surface clad layer, such as WC, TiC, SiC, ZrO2 or Al2 O3 ceramics, which are directly injected into the high temperature molten pool created in the surface of substrate by laser beam [8–13]. There is a major drawback in adding the reinforced phase directly into laser cladding coating because of poor wetting behavior between ceramic phase and metal matrix and a large different thermal expansion coefficient between them. In order to solve this problem, the in situ synthesized particles particulate reinforced metal matrix composites have been extensively investigated [14,15]. The present work is aimed at the investigation of microstructural characterization and dry sliding wear behaviors of in situ TiB/Al metal matrix composite coating by laser cladding. The interface and crystallographic relationship of TiB and matrix has been identified with TEM. With observation and analyses of worn surfaces and wear debris, the wear mechanism of laser cladding coating was discussed. 2. Experimental details 2.1. Material and specimen preparation The materials used were Fe coated Boron, pure aluminum and pure Ti powders with a size of around 150 mesh. In order to avoid burn of boron by laser, Fe has been plated on the surface of boron powder. The ratio of Fe to Boron is 4/6 in weight in Fe coated Boron powder. A mixture of 20.19 wt.% Fe coated B, 50.81 wt.%Ti and 20 wt.%Al powder was used as the coating materials for TiB/Al metal matrix composite coating. The various powders were pre-mixed by hand, using a spoon in a glass beaker; then the mixed powders were transported into melt pool by coaxial powder feeding instrument. The substrate material was a 2024 aluminum alloy. The rectangular specimens of 40 mm × 15 mm × 15 mm were cut from alloy plate and were degreased prior to the cladding process and removed any surface contaminants for good adhesion between the coating and substrate. The nominal chemical composition of the alloy in wt.% was: Cu, 3.8–4.9; Mg, 1.2–1.8; Mn, 0.3–0.9; Zn, 0.25; and the balance, Al.
2.3. Microstructure examination Metallographic cross-section of the clad samples was prepared in the plane parallel and perpendicular to the scan direction. The chemical compositions and microstructure of laser-clad coating were analyzed by a LEO-1450 scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (X-EDS). The phase structure identification was conducted by D/Max-RB X-ray diffraction (XRD). The radiation source was Cu K␣, working voltage at 40 kV with a scan rate 15 min−1 . The microhardness measurement was done on HX-200 micro-Vickers machine with a 0.2 kg load and load time was set at 15 s. TEM specimen was prepared according to the standard method including cutting to 0.2 mm thick slices with an electric spark machine along the laser scanning direction, followed by dimpling and finally ion-milling with 4 kV voltage and 0.4 mA current. A JEM-2000FX TEM was used to identify the phase. 2.4. Pin-on-ring friction and wear experiment Dry sliding friction and wear tests without lubricant were performed in a pin-on-ring mode on a Falex-6 model friction and wear-testing machine (Falex corporation, Sugar Grove, Illinois, USA). The pin specimens were machined in the form of cylinders with 4.8 mm diameter and 12.7 mm length. The counterpart discs were made of an annealed 1045 medium carbon, surface hardness of 147HB and surface roughness of Ra = 0.2 m. The applied load was 8.9, 17.8, 26.7 and 35.6 N. The sliding speed was kept constant at 0.24 m/s. The total sliding distance for the test was 150 m. The weight loss during the wear test was measured using a photoelectric balance 1712MP8 with the resolution of ±0.01 mg. 3. Results and discussion 3.1. Phase constituents of in situ synthesis TiB/Al composites coating Under laser radiation, molten pool is formed by physical metallurgy action of the addition mixture powder, the chemical reactions which accord with thermodynamic laws can be taken place as follows: Al + 2B → AlB2
(1)
Ti + 2B → TiB2
(2)
3Al + Ti → Al3 Ti
(3)
2.2. Laser cladding
3Al + Fe → FeAl3
(4)
The laser cladding was carried out with a defocused laser beam of 3 mm diameter using a PRC-3 kW continuous wave CO2 laser processing system. The laser was operated at a TEM01 mode. The laser cladding parameters were: laser power 1.7 kW and traverse scan speed 5 mm/s. A coaxial jet and a side jet of argon gas were used to prevent the surface of sample from oxidation.
Ti + B → TiB
(5)
Ti + TiB2 → 2TiB
(6)
the Gibbs free energy (G) of above-mentioned reactions is all negative, which indicates that all the above reactions are capable of taking place. However, other chemical reactions would eventually affect the products of laser cladding coating. When AlB2
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and Ti are heated, the following reaction is thermodynamically possible [16]: AlB2 + 43 Ti → 13 Al3 Ti+TiB2
(7)
When the titanium is in excess, the G of reactions (5) and (6) is less than that of reaction (2), which indicates that the reaction (5) and (6) are apt to take place, namely TiB phase behaves higher stability than that of TiB2 phase throughout the temperature range. Furthermore, the above-mentioned only involve thermodynamics without considering the effect of kinetics. The laser cladding technique belong to the category of rapid solidification, thereby the ultimate produce of laser cladding coating is influenced by reaction kinetics. Fig. 1 shows the XRD pattern of the as-reacted materials. It can be seen that main phase constituents of in situ synthesis TiB/Al composites coating consists of ␣-Al, TiB, Al3 Ti and Al3 Fe. Only the diffraction peaks of TiB are found and TiB2 cannot present in the final synthesized product for Fe–Ti–Al–B system with excess Ti. The XRD result is in a good agreement with the theoretical analysis. 3.2. Microstructure of in situ synthesized TiB/Al composites coating Fig. 2a shows the scanning electron image in the backscattered electron imaging (BEI) mode of the macrostructure of composite coating from cross-section at a scanning velocity of 5 mm/s and 1.7 kW output power. The composition of various
Fig. 1. X-ray diffraction spectra of in situ synthesized TiB/Al composites coating by laser cladding.
microzones in the Fig. 2a is analyzed by energy dispersive Xray (EDS). The white–gray needle-shaped phase (B) and grey needle-like (C) are identified as Al3 Fe and Al3 Ti by EDX. The white lump-like or strip-like phase (A) which is rich in Ti is identified TiB. The TiB appears lump-like or strip-like in micromagnitude size. It can be seen clearly that a zigzag covered layer of TiB contains Al and Fe element. Fig. 2b shows the microstructure of two conjoint particles. The covered layer of two TiB lump
Fig. 2. SEM photographs of microstructure of in situ synthesized TiB/Al composites coating.
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Fig. 3. TEM bright field image of in situ synthesized TiB/Al composites coating (a and b) TiB lump morphology and selected area diffraction patterns; (c) TEM bright field image of TiB whisker; (d–f) microstructure of inside of TiB lump and selected area diffraction patterns.
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is thicker than that of individual TiB lump. The chemical reaction Ti + B → TiB is exothermic reaction and the released heat generated by the reaction can melt the boundary of TiB lump. The Ti from the melted TiB lump reacts with Fe to form compound. Obviously, the more TiB lumps are integrated together, the more quantity of heat is released and thicker covered layer is formed. Fig. 3a shows bright field TEM images of TiB in aluminum matrix composite coating. Fig. 3b shows selected area diffraction patterns in Fig. 3a. TEM observations revealed that TiB lump are indexing to the orthorhombic structure TiB (B27) with lattice parameters a = 0.612 nm, b = 0.306 nm and c = 0.456 nm. It has been confirmed that lump axis is parallel to the [0 1 0] axis of the B27 cell and the growth direction of TiB is [0 1 0]. TiB crystallizes with an FeB type structure with an orthorhombic structure, characterized by zigzag chains of boron atoms parallel to the [0 1 0] direction [17]. This strong anisotropy in crystal structure may be responsible for the observed preferred growth directions and low energy crystal faces [18]. As shown in Fig. 3a and b, the interface of TiB lump and aluminum matrix is very clean and there is not any interface reaction due to the less released quantity of heat. Beside the lump-shape and stripshaped, the nano-magnitude whisker-shaped TiB of the average aspect ratio above 20 with diameter 50 nm have been found. Fig. 3d–f present the novel microstructure of inside of TiB and its selected area diffraction patterns. According to analysis result of selected area diffraction pattern, the inside of microstructure is identified as Al5 Fe2 phase and the definite crystallographic relationship between the Al5 Fe2 phase and TiB has been determined ¯ TiB //[3¯ 1¯ 2]Al Fe (0 1 1)TiB //(0 2 1)Al Fe . The by Fig. 3e: [2 3 3] 5 2 5 2 Al3 Ti, Al3 Fe compound and TiB are formed at the elevated temperature, which evidently increased the viscosity of aluminum melt. Owing to high cooling rates of laser cladding process, inrich Al and Fe element matrix melt is easy to be captured by two close TiB lumps in the viscous melt. The captured member forms new Al5 Fe2 compound phase and sinters the two TiB lumps together (Fig. 3d). The Fig. 2b can also give proof of formation of the covered phase. Because of covered effect, the Al5 Fe2 phase cannot be identified by XRD.
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Fig. 4. Hardness profiles of in situ synthesized TiB/Al composites coating.
with increasing the load for the in situ synthesized TiB/Al composites coating and 2024 aluminum alloy under the dry sliding wear condition. When the applied load is below 26.7 N, the wear loss rate of in situ synthesized TiB/Al composites coating is considerably lower than that of 2024 aluminum alloy. There is a sharp transition in wear rate (i.e. sudden increase in wear rate), when the applied load changes from 26.7 to 35.6 N, and the wear loss rate of the 2024 Al alloy is inferior to that of in situ synthesized TiB/Al composites coating. In general, wear resistance is proportional to the hardness of material, the presence of the TiB and intermetallic compound increase the hardness of composites coating and act as load bearing member at the lower applied load. However, the hardness is not the exclusive factor to control the wear behaviors of composite coating. Owing to a large strain in the Al matrix at higher applied loads and the difference of mechanical properties (strength and toughness) of TiB, intermetallic compound and Al matrix, the interface of reinforcement and Al matrix becomes weak and fragile. Over a certain applied load, the TiB and intermetallic compound are fallen off and lost their ability to support the load. In this case, the Al matrix contacts the counterfaces directly. The worn sur-
3.3. Microhardness and wear resistance of in situ synthesized TiB/Al composites coating The microhardness profile of in situ synthesized TiB/Al composites coating by laser cladding is shown in Fig. 4. It can be seen that the highest hardness emerges at subsurface and a gradual distribution of hardness is formed in coating after the maximal hardness was achieved. The maximal hardness of synthesized TiB/Al composites coating is 8-fold that of 2024 aluminum alloy matrix. In the composite coating, TiB has the highest hardness, followed by the Al3 Fe, Al3 Ti and the Al matrix has the lowest hardness. The increased hardness is attributed to the reinforcement in composite coating. The wear resistance of the coating was evaluated by measuring the wear weight loss with pin-on-ring mode on a Falex-6 model friction and wear testing machine. Wear losses of the tested materials as a function of applied load were measured and the results are presented in Fig. 5. The wear losses rate increased
Fig. 5. Wear weight loss rate of composite coatings and 2024 Al alloy as function of the normal load at given sliding velocity (0.24 m/s) and wear time of 10 min.
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Fig. 6. The appearance of worn surface: at applied load of 8.9 N (a) 2024 Al alloy and (b) composite coatings; at applied load of 35.6 N (c) 2024 Al alloy; (d and e) composite coatings.
face of composite coating and 2024 aluminum alloy at applied load of 8.9 and 35.6 N is shown in Fig. 6. It is evident from the Fig. 6a that the plough grooves and flakes were found, which confirm that the wear process of 2024 Al alloy is governed by the combining effect of micro-cutting and adhesion mechanism. At high load of 35.6 N, the appearance of worn surface of 2024 Al alloy (Fig. 6c) exhibits mainly adhesion wear characteristic due to detachment of fragment from the aluminum alloy surface. As shown in Fig. 6b, TiB appear on the worn surface and are good bond with Al-based matrix. The major portion of applied load on the specimen is carried by reinforcement and wear of composite coating is controlled by the wear of reinforcement phase. At high load of 35.6 N, interface bonding of reinforcement and matrix is a major factor of wear resistance of composite coating. Because of deformation disaccord between reinforcement phase and matrix, the tip of reinforcement phase gives rise to the stress concentration and cracks are easy to be propagated and expanded. Simultaneously, the softer matrix would wear out faster compared to the reinforcement phase and the
reinforcement phase will be protruded over the worn surface. Owing to the reduction of support by matrix, which results in reinforcement phase flaking off (Fig. 6d). If the effective load on the individual reinforcement phase increase above its flexural strength, the reinforcement phase can get fractured (Fig. 6e). It needs to be further investigated whether the covered Al5 Fe2 in TiB accelerate the fracture of TiB lump. Therefore, the 2024 Al alloy manifests higher wear resistance than that of composite coating at higher applied load. 4. Conclusions 1. The phase constituents of in situ synthesis TiB/Al composites coating consists of ␣-Al, TiB, Al3 Ti and Al3 Fe. TiB2 cannot exist in the final synthesized product for Fe–Ti–Al–B system with excess Ti. 2. Owing to B27 structure of TiB, the TiB has an anisotropy axis of growth, which results in the TiB strip and whisker preferring grown along [0 1 0] direction. The phase inside
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of TiB is identified as Al5 Fe2 and the definite crystallographic relationship between the Al5 Fe2 phase and TiB ¯ TiB //[3¯ 1¯ 2]Al Fe has been determined by Fig. 3e: [2 3 3] 5 2 (0 1 1)TiB //(0 2 1)Al5 Fe2 . 3. When the applied load is below 26.7 N, the wear loss rate of in situ synthesized TiB/Al composites coating is considerably lower than that of 2024 aluminum alloy. There is a sharp transition in wear rate (i.e. sudden increase in wear rate), when the applied load change from 26.7 to 35.6 N, and the wear loss rate of the 2024 Al alloy is inferior to that of in situ synthesized TiB/Al composites coating. References [1] S. Tomida, K. Nakata, S. Saji, T. Kubo, Surf. Coat. Technol. 174–175 (2001) 585–589. [2] M.H. Korkut, Mater. Sci. Technol. 20 (2004) 73–81. [3] O.M. Suarez, J. Mech. Behav. Mater. 11 (2001) 225. [4] M.A. Martinez, A. Martin, J. Llorca, Scripta Metall. Mater. 28 (1993) 207–212.
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