Formation of Nano Microstructured Aluminum Alloy Film Using Thermal Spray Gun with Ultra Rapid Cooling

(1)Materials Transactions, Vol. 57, No. 4 (2016) pp. 488 to 493 ©2016 The Japan Institute of Metals and Materials. Formation of Nano-Microstructured Aluminum Alloy Film Using Thermal Spray Gun with Ultra Rapid Cooling Yoshio Shin1, Yasuhiro Ohmori2, Toshiharu Morimoto1, Takashi Kumai2 and Akira Yanagida3 1. Amorphous Technology Section, Nakayama Amorphous, Osaka 551–0022, Japan Surface Treatment Division, Yoshikawa Kogyo, Himeji 671–1241, Japan 3 Mechanical Engineering, Tokyo Denki University, Tokyo 120–8551, Japan 2. A new thermal spray gun for nitrogen gas cooling and mist cooling has been developed. Aluminum-magnesium alloy films prepared by nitrogen gas cooling using the thermal spray gun had a partial nano microstructure because residual strains induced by rapid cooling acted as solid nuclei and a small amount of the amorphous phase became crystalline as a result of the heat of the thermal spray. Furthermore, the nano-microstructured aluminum magnesium films exhibited high corrosion resistance. Moreover, aluminum-magnesium alloy films containing a small amount of titanium prepared by mist gas cooling exhibited higher corrosion resistance than those without titanium. We consider that the aluminum -titanium compound acted as solid nuclei during ultra rapid solidification and that a strong passive layer was formed by mist gas cooling. [doi:10.2320/matertrans.M2015392] (Received October 20, 2015; Accepted January 20, 2016; Published March 25, 2016) Keywords: thermal spray, nano-microstructure, ultra rapid cooling, aluminum-magnesium alloy, titanium. 1. Introduction. 2. Amorphous Alloy Thermal Spraying Technique. Several studies on rapidly quenched metals fabricated by melt-spraying techniques have been reported1–4). When melted alloys are rapidly solidified to form a ribbon shape at cooling rates of about 105 and 106 K/s by a single roller technique, the solid solubility increases and a nano structure is formed. For example, the microstructure of aluminum containing titanium solidified using a single roller was examined5–7). Furthermore, new high specific strength aluminum alloys for use in lighter, more functional automobile components have been developed by optimization of the chemical composition and the rapid solidification technique8–10). Ultra fine grained aluminum alloy produced by severe plastic deformation shows high corrosion resistance11). It has been clearly shown that the potential for the pitting corrosion of Al and Al-Mg alloys is reduced by equal-channel angular pressing (ECAP), indicating that this process improves the resistance to pitting corrosion12). Using the accumulative roll-bonding process (ARB) and the mechanical alloying process (MA), it is expected that nano structured aluminum alloys with high corrosion resistance can be fabricated13,14). Steel structures are easily corroded in coastal regions because the chloride ions in seawater accelerate corrosion. By acting as a sacrificial anode, aluminum and zinc alloys can be used to protect steel structures from corrosion. Thermal spraying is a convenient method for producing coating films on metal substrates that involves rapid quenching15–18). Thus, the thermal spraying of aluminum magnesium alloys has been recently introduced to coat marine structures because both aluminum hydroxide and magnesium hydroxide have strong corrosion resistance19,20). In this study, using a new thermal spray gun that enables nitrogen gas cooling21,22) and mist gas cooling23), an aluminum magnesium films with and without a small amount of titanium were fabricated and their anti corrosion resistance was evaluated.. Crystal solids are fabricated using a conventional thermal spraying gun because it has low cooling ability. However, our new thermal spray gun has a cooling rate of more than 106 K/s. Figure 1 shows a schematic illustration of the world s first compact amorphous alloy thermal spray gun. To control the temperature of the particles and the substrate using an outer layer of cooling nitrogen gas, a double cylindrical nozzle was developed and attached at the front of the thermal spraying gun. First, an alloy powder is melted in a gas with a temperature of about 2800 K. Then, the melted powder is cooled ultra rapidly at a rate of more than 106 K/s, resulting in the formation of an amorphous alloy film on the substrate. The oxidization of the melted powder is prevented by introducing the outer cooling nitrogen gas, and the melted powder is reduced by carbon monoxide gas. Furthermore, owing to the super cooled region of the amorphous alloy, a fine microstructure is obtained24,25). Figure 2 shows a scanning electron microscope (SEM) image of a 74.3Ni15.2Cr9.7P0.8B mass% thermally sprayed film, uniform droplets proved that the thermally sprayed film was amorphous. Figure 3 shows the differential scanning calorimetry (DSC) result as one example for the 74.3Ni15.2Cr9.7P0.8B mass% thermally sprayed film. The DSC conditions were a sample weight of 20 mg, temperature heating ratio of 20 K/min, an argon gas flow of 40 × 10−3 m3/min and an argon gas pressure of 0.5 MPa. The. Fig. 1 Schematic illustration of the world s first compact amorphous alloy thermal spray gun..

(2) Formation of Nano-Microstructured Aluminum Alloy Film Using Thermal Spray Gun with Ultra Rapid Cooling. 489. Fig. 4 EBSD analysis results of conventional and new 95Al5Mg mass% films.. gun with ultra rapid cooling. Beforehand, the steel substrate was blasted by alumina grits to a surface roughness of Ra 3.0 μm. A reducing flame was used during thermal spraying, where the ratio of acetylene gas to oxygen was 6 to 5. The thickness of the aluminum magnesium alloy thermally sprayed films was about 250 μm. We also prepared films using the thermal spraying gun without a double cylindrical nozzle. Furthermore, for comparison with 95Al5Mg films, industrial pure iron (hereafter, pure Fe) was also thermally sprayed. Fig. 2 SEM image of a 74.3Ni15.2Cr9.7P0.8B mass% amorphous alloy film.. Fig. 3 DSC analysis result of a 74.3Ni15.2Cr9.7P0.8B mass% amorphous alloy film.. glass transition temperature and crystallization temperature were thus determined26). 3. Aluminum Magnesium Alloy Thermally Sprayed Film Produced with Ultra Rapid Cooling 3.1 Experimental procedure An aluminum magnesium alloy powder with a chemical composition of 95Al5Mg mass% (hereafter 95Al5Mg) was thermally sprayed on a steel substrate using the thermal spray. 3.2 Microstructure analysis The percentage of oxygen in the films was measured to estimate their degree of oxidation. The percentage of oxygen was determined to be 0.17% for 95Al5Mg film obtained using the new thermal spraying gun ( new film ) and 0.24% for 95Al5Mg coating obtained by conventional thermal spraying ( conventional film ), indicating that the oxidation was suppressed in the film by the external nitrogen gas. Figure 4 shows the results of electron back scatter diffraction (EBSD) analysis for the two 95Al5Mg films in the cross section. Although strains accumulated as a result of rapid cooling, the grains of the new 95Al5Mg film were finer than those of the conventional 95Al5Mg film. Next, to investigate the grain size in the thermally sprayed films, cross sectional scanning electron microscope (SEM) images of the new 95Al5Mg film are shown in Fig. 5. There were fine grains in parts of the films. Figure 6 shows cross sectional electron microscope (TEM) images of the new 95Al5Mg film. The left image in Fig. 6 was obtained with the full range of vision of the TEM and the right image is an enlargement of the left image. Around the high grain boundaries of the micro size grains, fine nano size grains were observed in some areas. Figure 7 shows the results of X-ray analysis of the conventional and new 95Al5Mg films, where the anode material was copper, voltage was 40 kV, electric current was 200 mA and measurement angles was from 5 to 90 in measured step of 0.02 and the scanning speed was 4 /min. The peaks of the new 95Al5Mg film were slightly lower than those of the conventional 95Al5Mg film and the peak angles of the new 95Al5Mg film were lower than those of the conventional 95Al5Mg film. We consider that a partially amorphous phase was produced as a result of thermal spraying with rapid cooling27,28). Then, ultra fine grains of aluminum magnesium alloy were generated from the amorphous phase owing to the recuperative heat.

(3) 490. Y. Shin, Y. Ohmori, T. Morimoto, T. Kumai and A. Yanagida. Fig. 8 X-ray analysis results of conventional and new Fe films.. Fig. 5 SEM image of the new 95Al5Mg mass% film.. Fig. 9 DSC results of conventional and new 95Al5Mg mass% films.. Fig. 6 TEM images of new 95Al5Mg mass% film. The left image was obtained with the full range of vision of the TEM and the right image is an enlargement of the left image. The left image was the full range of vision and the right image was enlargement of the left one.. Fig. 7 X-ray analysis results of conventional and new 95Al5Mg mass% films.. provided by spraying29,30). Figure 8 shows the results of X-ray analysis of Fe films obtained using the new thermal spraying gun ( new film ) and by conventional thermal spraying ( conventional film ). The peak of the new film was lower and broader than that of the conventional film. Figures 7 and 8 suggested that it was more difficult to form the amorphous phase in the aluminum alloy than in the iron alloy31). The new and conventional thermally sprayed films of aluminum magnesium alloy were examined by DSC to investigate the stored energy and the precipitation from the supersaturated solid solution as shown in Fig. 9. The DSC diagrams for a heating rate of 20 K/min show a sequence of peaks whose intensity depends on the accumulated strain and microstructure. That is to say, an exothermal peak is observed that corresponds to the release of stored energy during recrystallization32–34). We consider that fine coherent precipitates of Al3Mg in the new film impede the migration of dislocations, increase the recovery temperature, and delay recrystallization. However, the recuperative heating time was very short and not sufficient to precipitate Al3Mg. Thus, it is also possible that accumulated strain impeded the recovery and recrys-.

(4) Formation of Nano-Microstructured Aluminum Alloy Film Using Thermal Spray Gun with Ultra Rapid Cooling. 491. Fig. 10 Electrochemical polarization measurements of conventional and new 95Al5Mg mass% thermally sprayed films.. 35–37). tallization. .. 3.3 Anti-corrosion test of thermally sprayed films Electrochemical polarization measurements in 5% NaCl aqueous solution were carried out to estimate the corrosion rate and the resistance to pitting corrosion of aluminum alloy thermally sprayed films38,39). The corrosion rate quantitatively agrees with that obtained by extrapolating the cathodic current density to the corrosion potential40). Figure 10 shows the results of electrochemical polarization measurements of the new and conventional 95Al5Mg thermally sprayed films. Because the natural potential of the new film was higher than that of the conventional film and the anticorrosion current was reduced, the corrosion resistance of the new film was higher than that of the conventional film. Therefore, thermal spraying with ultra-rapid cooling can suppress the diffusion of oxygen in the film and is expected to prolong the corrosion life of steel in salt water. Then, the electrochemical characteristics of aluminum alloys in acidified chloride solution were determined to investigate the validity of the copper accelerated salt spray (hereafter, CASS) test for evaluating atmospheric corrosion resistance41). Thermally sprayed coatings for rust prevention are generally evaluated by the CASS test as shown in Fig. 11. The new 95Al5Mg alloy film coating on a steel substrate showed higher corrosion resistance than the conventional 95Al5Mg film coating on a steel substrate. The results of the corrosion test were consistent with those of the electrochemical polarization test. 4. Aluminum Magnesium Thermally Sprayed Films with Additional Titanium 4.1 Experimental procedure After changing the nitrogen gas to a mist gas to improve the cooling ability, 94.4Al5Mg0.6Ti mass% alloy (hereafter, 94.4Al5Mg0.6Ti) was thermally sprayed. The conditions of thermal spraying were the same as those in sections 3.1.. Fig. 11 CASS test results of conventional and new 95Al5Mg mass% films on steel substrates.. Fig. 12 EBSD analysis results of a nitrogen gas cooled and a mist gas cooled 94.4Al5Mg0.6Ti mass% thermally sprayed films.. 4.2 Microstructure analysis Figure 12 shows the results of EBSD analysis of the nitrogen gas cooled and mist gas cooled 94.4Al5Mg0.6Ti films obtained from a cross section. Because strains accumulated as a result of rapid cooling, the grains of the two 94.4Al5Mg0. 6Ti films were not clearly observed. Thus, to investigate the grain size in the thermally sprayed films, cross sectional SEM images of the two 94.4Al5Mg0.6Ti films are shown in Fig. 13. We considered that color changes of SEM images depended on crystallite orientations in the two 94.4Ti5Mg0.6Ti films. The grain size of these films was less than that of the aluminum magnesium alloy films. In particular, the grain size of the aluminum magnesium alloy films containing a small amount of titanium subjected to mist gas cooling was the smallest observed in this study. Figure 14 shows the results of DSC analysis of the 94.4Al5Mg0.6Ti films. The small peak at about 300 C means that Al3Ti was precipitated. Figure 15 shows the results of X-ray analysis of the 94.4Al5Mg0.6Ti films. The peaks of the 94.4Al5Mg0.6Ti nitrogen cooled film were slightly lower than those of the 94.4Al5Mg0.6Ti mist cooled film and the peak angles of the 94.4Al5Mg0.6Ti mist.

(5) 492. Y. Shin, Y. Ohmori, T. Morimoto, T. Kumai and A. Yanagida. Fig. 13 SEM images of aluminum magnesium alloys containing a small amount of titanium films with nitrogen gas cooling and mist cooling.. Fig. 14 DSC analysis results of 94.4Al5Mg0.6Ti mass% films subjected to nitrogen gas cooling and mist cooling.. Fig. 16 Electrochemical polarization measurements of 94.4Al5Mg0.6Ti mass% films subjected to nitrogen gas cooling and mist gas cooling.. Fig. 17 CASS test results of 94.4Al5Mg0.6Ti mass% films subjected to nitrogen gas cooling and mist cooling.. cooling film were lower than those of the Al5Mg0.6Ti nitrogen cooled film.. Fig. 15 X-ray analysis results of 94.4Al5Mg0.6Ti mass% films subjected to nitrogen gas cooling and mist cooling.. 4.3 Anticorrosion test of thermally sprayed coatings Electrochemical polarization measurement in 5% NaCl aqueous solution and determination of the electrochemical characteristics by the CASS test were carried out to evaluate the atmospheric corrosion resistance, the results of which are respectively shown in Figs. 16 and 17. The new 94.4Al5Mg0. 6Ti alloy films showed higher corrosion resistance than the new 95Al5Mg films discussed in the previous section, and the new 94.4Al5Mg0.6Ti mist cooled film had the highest corrosion resistance. Further research indicated that our aluminum-magnesium-titanium films also had microstructures and superior passive layers. Also, in a continuous salt-spraying test, they exhibited a lower corrosion rate than conventional coatings. The mechanism of the grain refining effect in this.

(6) Formation of Nano-Microstructured Aluminum Alloy Film Using Thermal Spray Gun with Ultra Rapid Cooling 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) Fig. 18 Steel bridge with new aluminum magnesium thermally sprayed coating.. 21) 22). alloy is considered to be as follows. The new 94.4Al5Mg0. 6Ti films have a dendritic core structure and a metastable TiAl3 phase with a preferential growth direction and tangled dislocations are concentrated around the core structure42). 5. Conclusion Thermal spraying is a rapid cooling method used to fabricate films with a fine microstructure. Using our newly developed thermal spray gun with ultra rapid cooling, we formed new 95Al5Mg and 94.4Al5Mg0.6Ti coating films with a sacrificial protection effect to prevent the corrosion of marine structures. It was found that the addition of a small amount of titanium improved the anti corrosion ability. Such aluminum-magnesium alloy film coatings can be applied to marine structures such as the external parts of offshore wind power plants and large steel bridges. Our aluminum magnesium alloy spraying technique is also expected to reduce the cost of maintaining infrastructure not only in Japan but also worldwide. Figure 18 shows a steel bridge with a new aluminum magnesium thermally sprayed coating. REFERENCES 1) H. Shingu and K. Kobayashi: J. JILM 31 (1981) 491–496. 2) A. Kamio, H. Tezuka, T. Sato, T.T. Long and T. Takahashi: J. JILM 36 (1986) 72–80.. 23) 24) 25) 26) 27) 28) 29) 30) 31) 32) 33) 34) 35) 36) 37) 38) 39) 40) 41) 42). 493. E. Vogt and G. Frommeyer: Z. Metallkde 78 (1987) 262–267. I. Ohnaka: J. JILM 39 (1989) 514–523. T. Ohashi and R. Ichikawa: J. JILM 27 (1977) 105–112. S. Hori, H. Tai and Y. Narita: J. JILM 32 (1982) 596–603. S. Hori, S. Saji and A. Takehara: J. JILM 31 (1981) 793–797. H. Adachi, K. Osamura and J. Kusui: J. JILM 54 (2004) 69–74. H. Kimura, K. Sasamori and A. Inoue: J. Jpn. Soc. Powder Powder Metall. 56 (2009) 697–708. H. Matsuoka and T. Maeshima: J. JILM 60 (2010) 585–589. I.J. Son, H. Nakano, S. Oue, S. Kobayashi, H. Fukushima and Z. Horita: Mater. Trans. 47 (2006) 1163–1169. H. Nakano and I.J. Son: J. JILM 62 (2012) 412–418. K. Kashihara, Y. Komi, D. Terada and N. Tsuji: Mater. Trans. 56 (2015) 803–807. M. Fukumoto, M. Yamasaki, M. Nie and T. Yasui: Japan Weld. Soc. 24 (2006) 87–92. P.H. Shingu, K. Shimomura and R. Ozaki:Trans. JIM 20 (1979) 33–35. K. Murakami and T. Okamoto: Casting Forging & Heat Treatment 2 (1988) 5–10. K. Murakami: Thermal Spray 35 (1998) 300–306. K. Murakami: Thermal Spray 40 (2003) 18–23. Y. Koga: J. Japan Thermal Spray Soc. 41 (2004) 109–112. K. Fujikawa, Y. Kojima and Y. Koga: Proc. 97th Conference of Japan Thermal Spray Society, (2013) pp. 49–50. M. Komaki, T. Mimura, Y. Kusumoto, R. Kurahasi, M. Kouzaki and T. Yamasaki: Mater. Trans. 51 (2010) 1581–1585. M. Komaki, T. Mimura, R. Kurahashi, M. Kouzaki and T. Yamasaki: Mater. Trans. 52 (2011) 474–480. T. Kumai, Y. Nishiura, Y. Ohmori, Y. Shin and T. Morimoto: Proc. Int. Thermal Spray Conference, (2015) pp. 241–242. M. Komaki, T. Mimura, R. Kurahashi, H. Odahara, K. Amiya, Y. Saotome and T. Yamasak: Mater. Trans. 53 (2012) 681–689. M. Komaki, T. Mimura, S. Tsuji, K. Amiya, Y. Saotome and T. Yamasaki: Mater. Trans. 53 (2012) 2151–2159. A. Inoue: Materials Science and Engineering of Bulk Metallic Glasses, (CMC Publishing, 2008) pp. 341–343. H. Era: Tsukuru FUJIICO Technical Report 20 (2012) 12–16. C. Lee and J. Kim: J. Thermal Spray Technol. 24 (2015) 592–610. M. Yamada, T. Yamasaki and Y. Yokoyama: J. Japan Inst. Met. Mater. 78 (2014) 90–97. K. Takenaka, A.D. Setyawan, Y. Zhang, P. Sharma, N. Nishiyama and A. Makino: Mater. Trans. 56 (2015) 372–376. A. Inoue, T. Zhang, K. Kita and T. Matsumoto: Mater. Trans., JIM 30 (1989) 870–877. M. Verdier, I. Groma, L. Flandin, J. Lendvai, Y. Brechet and P. Guyot: Scr. Mater. 37 (1997) 449–454. M. Voncina, P. Mrvar, F. Zupanic and J. Medeved: RMZ-Mater. Geoenviron. 54 (2007) 457–470. M.S. Kaiser: Iranian. J. Mater. Sci. Eng. 10 (2013) 1–11. T. Nobuki, J-C Crivello and T. Kuji: Mater. Trans. 49 (2008) 2679– 2685. T. Nobuki and T. Kuji: J. Japam Inst. Metals 72 (2008) 236–243. Y.H. Kim, A. Inoue and T. Matsumoto: Mater. Trans., JIM 32 (1991) 599–608. M. Kato and Y. Nakamura: Seisan Kenkyu 5 (1953) 25–29. K. Tohma and Y. Takeuchi: J. JILM 29 (1979) 232–239. R. Suzuki, T. Shibata and H. Nagasaka: J. JILM 30 (1980) 679–683. K. Tohma and Y. Takeuchi: J. JILM 33 (1983) 457–465. Micrstructure and Characteristic of Aluminum, ed. by A. Kamio, (The Japan Institute of Light Metals, Japan, 1991) pp. 346–350..

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