Effect of process parameters on the formation of Fe-Al intermetallic coating fabricated by mechanical alloying

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 21, December 2014, pp Effect of process parameters on the formation of Fe-Al intermetallic coating fabricated by mechanical alloying A Canakci*, F Erdemir, T Varol & S Ozkaya Department of Metallurgical and Materials Engineering, Engineering Faculty, Karadeniz Technical University, Trabzon, Turkey Received 18 December 2013; accepted 27 June 2014 Fe-Al intermetallic coatings have relatively high corrosion resistance, superior electrical resistivity and low thermal conductivity at elevated temperatures under oxidizing atmosphere. In this study, the Fe-Al coatings are fabricated on the steel substrate by mechanical alloying method. The steel substrate surface is hardened and activated as a result of the highenergy impact of balls. Formation of such intermetallic phase increase the local temperature and Fe-Al intermetallic phases are formed in the coating layer. The effects of milling speed, particle size and milling time on thickness of Fe-Al coatings in a planetary ball-mill are investigated by Taguchi method to determine the optimal conditions. Coating thickness is employed as the response to evaluate the effect of the coating parameters. It is found that an increase in the applied milling time and milling speed significantly affected the coating thickness and densification level of the deposited coatings while the particle size is found to be statistically insignificant. The results indicated that the milling time has the most significant contribution (81%) on the coating process while the milling speed time has a lower effect (19 %). Keywords: Mechanical alloying, Coating, Taguchi method, Intermetallics Low-carbon steel with limited compact-scale formation on the steel surface plays an important role in structural applications due to its lower cost than high-alloy steels. However, it cannot be used for corrosion resistance applications, particularly in oxidizing environments at high temperatures 1. The corrosion is one of the most significant material damage. The development of surface properties are commonly used to prevent corrosion. Surface coating is an effective method for surface modification through the formation of an alloyed layer on the substrate material 2,3. The low-carbon steel part and alloys can be coated with zinc, tungsten or zinc nickel using a chromatation treatment 4,5. The surface coating of Fe based alloys with aluminum is also a commonly-used method 6,7. The mechanical alloying technique is a new method used for coating of lowcarbon steel materials. The main advantages of mechanical alloying are low-cost, controllable coating thickness, good bonding between coating materials and substrate, simplicity and prevention of adverse chemical reactions. Mechanical alloying (MA) is a process based on solid state reactions. It takes place between the fresh powder surfaces of reactant materials at room *Corresponding author ( aykut@ktu.edu.tr) temperature. Consequently, it can be used to produce alloys and compounds that are difficult or impossible to obtain by conventional melting and casting techniques 8. MA is a new research area with potential industrial applications for producing coatings on lowcarbon steel. Chemical interactions between substrate surface and powder occur by the impact effect and rapid temperature rise which then lead to formation of intermetallics 9. Various intermetallics of Fe-Al can be produced by MA. Fe-Al compounds are commonly preferred in structural and coating applications at elevated temperatures and in hostile environments 10,11. There are some difficulties in fabricating coatings using conventional coating techniques because of considerable differences in the physical properties of the substrate and coating materials such as their melting points, bonding properties, and low deposition rates. Development of the MA method requires a better understanding of the relationships between the coating characteristics and process variables. However, to the author s knowledge, there is no study on the optimization of coating parameters for Al coating on low-carbon steel substrates by MA through Taguchi method. In this study, the particle size, milling speed and milling time were selected as the controllable factors and the optimum conditions were determined.

2 596 INDIAN J. ENG. MATER. SCI., DECEMBER 2014 Experimental Procedure Materials and methods Al 2024 powders were coated on steel substrate by means of mechanical alloying. The starting materials were 6 g Al 2024 powders and the low carbon steel as a cubic bulk sample (12 mm 12 mm 3 mm). The Al2024 powders with three different average particle sizes of 23, 60 and 155 µm were used as the raw coating materials. The coating process was carried out in a planetary ball-mill (Fritsch Gmbh, model Pulverisette Premium line 7 ) for 10 h using a tungsten carbide bowl and balls (10 mm in diameter) at room temperature. The rotation rates were selected as 200, 300 and 400 rpm while the ball to powder weight ratio of 10:1 was kept constant in all the experiments. The particle size of the initial and milled Al 2024 powder was quantified by a laser particle size analyzer (Malvern, model Mastersizer Hydro 2000 ). The measurement of coating thickness and morphological investigation of coated steel substrates were performed by a Zeiss Evo LS10 scanning electron microscope (SEM). The phase analysis of coated samples was performed by X-ray diffraction (Rigaku Corporation, Japan) using CuKα radiation (1.5405Å) operating at 30 ma and 40 kv. The XRD patterns were recorded in the 2Ѳ range of o (step size 0.02 o and time per step 1 s). Experimental design Taguchi design (L 9 (3 3 ) orthogonal array) was employed to assess the effect of parameters on the coating thickness of the coated steel samples. The experiments were carried out randomly. The significant parameters and levels of coating experiments were given in Table 1. Taguchi method requires fewer experiments in order to study all levels of all input parameters, and filters out some effects due to statistical variation. So, this design can also determine the experimental condition having the least variability as the optimum condition. The variability can be expressed by signal-to-noise (S/N) ratio. Signal-to-noise (S/N) ratios are recommended for use in the optimization of process parameters within a Taguchi design. In this study, S/N ratios were calculated using a larger is better approach 12. S/N = -10 log [ (y i 2 )/n] (1) Where y i are the response data of mean particle size and n is the number of replicates. The Taguchi L 9 (3 3 ) array used in this study with the mean particle size results and signal-to-noise (S/N) ratios of the experiments are given in Table 2. Results and Discussion Formation mechanism of Fe-Al coatings The planetary ball mill owes its name to the planetlike movement of the grinding vials which are arranged on a rotating support disk. Centrifugal forces are thus produced by the vials rotating around their own axes and by the support disk orbiting around the central axis. The forces are then exerted on the grinding balls and materials to be ground. Since the vials and the supporting disk rotate in opposite directions, the centrifugal forces alternately change their directions. Under this condition, the grinding Table 1 Parameters and levels of coating experiments Parameters Level 1 Level 2 Level 3 A- particle size (µm) fine(23 µm) medium(60 µm) coarse(155 µm) B- milling time (h) C- milling speed (rpm) Table 2 Taguchi L 9 design and mean coating thickness of the experiments Experimental Number A Particle size B Milling time C Milling speed Coating thickness S/N ratio 1 fine fine fine medium medium medium course course course

3 CANAKCI et al.: Fe-Al INTERMETALLIC COATING 597 balls are primarily carried up the inner surface of the vials and then run down the inside wall, producing a so-called friction effect on the inner wall. Subsequently, the grinding balls and the materials being ground are propelled off the inner wall and travel freely across the inner chamber of the vial, colliding against the opposite inside wall. In this situation, another new mechanism, a so-called impact effect, is developed during the interaction. The energy developed through the impacts is several times higher than that for conventional ball mills As stated above, two different kinds of action mechanisms, i.e., friction effect and impact effect, act alternately on the vial and its contents. In particular, the impact effect during milling causes the powder materials and the grinding balls to lift off and then travel close to the inner wall of the vial. It is assumed that the high Al 2024 content coating can be mechanically alloyed on carbon steel substrate by means of the following three steps, as schematically illustrated in Fig. 1. (i) Because of repeated collisions between the milling balls and the inner wall, the steel substrate surface suffered a large degree of plastic deformation and, thus, became activated. A fraction of the Al2024 powder particles, consequently, easily transferred and adhered to the activated substrate surface. (ii) Further ball-substrate-ball collisions elevated the impact effect significantly. More and more adhered Al2024 particles were hammered together on the highly activated low-carbon steel surface, owing to the repeated action of cold welding. The present coating layer, meanwhile, became denser and smoother due to the incident fraction effect. Additionally, the impact effect initiated the diffusion of Fe atoms from the substrate to the interior of the coating. (iii) With prolonged milling time, sufficient interdiffusion and alloying between individual Al and Fe elements occurred. Meanwhile, more Al 2024 particles were flattened, cold welded, and deposited on the newly present and activated layer due to continuous ball collisions, leading to outward growth and thickening of the high Al content coating. Similar observations have also been reported earlier by other researchers 17,18. The optimization of processing conditions for a successful preparation of the mechanically alloyed coating revealed that a proper increase in the milling time and the disc rotation speed favored an improvement in the thickness, surface smoothness, densification level, and micro structural homogeneity of the prepared coatings. It is known that the faster the mill speeds or the longer the milling persists, the higher would be the energy input into the powder and grinding balls. As a consequence, higher centrifugal forces are generated within the milling system, thereby intensifying the strength of impact and friction effects. The elevated impact effect can Fig. 1 Schematic of formation mechanism of Fe-Al coatings during mechanical alloying

4 598 INDIAN J. ENG. MATER. SCI., DECEMBER 2014 contribute to the following two aspects. First, the inner substrate tends to undergo a higher rate of plastic deformation, thereby increasing its surface activity significantly. The Al 2024 powder particles, accordingly, can adhere to such an activated surface much more easily. Figure 2a shows that backscattered SEM image of coating layer on the steel substrate and EDS results of marked areas after 6 h of milling. According to the Fe-Al binary diagram, five types of intermetallics (Fe 3 Al, FeAl, FeAl 2, Fe 2 Al 5 and FeAl 3 ) might occur in Fe-Al binary alloys. Aluminum-rich compounds (Fe 2 Al 5, FeAl 3 and FeAl) can be obtained in carbon steel substrates using different techniques. In this study, Fe-Al intermetallics were observed in the coating layer produced by mechanical alloying (Fig. 2a). Combining the EDS results with the Fe-Al binary system, it can be inferred that (A) region was rich in FeAl 2, (B) region rich in FeAl, (C) region rich in Fe 3 Al, (D) region rich in Fe 2 Al 5, (E) region rich in Fe and (F) region rich in Al. This was consistent with the XRD results (Fig. 3). Second aspect that the elevated impact effect can contribute was that the Al 2024 powder particles were repeatedly flattened and cold-welded to each other, growing outwards from the activated substrate in a layer-by-layer manner (Fig. 2b). Accompanied by the enhanced friction effect, the laminated structure experiences a sufficient densification, yielding a dense, uniform, and metallurgically bonded coating on the inner surface (Fig. 2c). On the other hand, a higher mechanical energy input leads to a larger macroscopic temperature rise of the vial. In this situation, the local temperature within the vial may reach a high value 16. This favors the diffusion of Fe atoms from the substrate into the interior of the coating 17, thereby promoting homogenization and alloying of Fe and Al elements (Fig. 2b). Fig. 2 Back scattered electron SEM image of the coatings prepared at 300 rpm rotation speed: (a) surface image of coating layer for 6 h, (b) surface image of coating layer for 10 h and (c) cross-sectional microstructure for 10 h Fig. 3 X-ray patterns of the coating surface 30 µm from the steel substrate surface

5 CANAKCI et al.: Fe-Al INTERMETALLIC COATING 599 Table 3 Results of ANOVA for the parameters investigated Source of variation Df Sum of squares P-value Percent (%) A-particle size (µm) B- milling time (h) C-milling speed (rpm) Error Total Fig. 4 Plots for the effects of the main parameters of the mechanical milling: (A) particle size (µm), (B) milling time (h) and (C) milling speed (rpm) Optimization of process parameters The analysis of variance (ANOVA) was used to determine the significant factors. The results were presented in Table 3. According to the ANOVA, the milling time and milling speed were found to be statistically significant at a confidence level of 95% (α=0.05) with contributions of 81% and 19%, respectively. Figure 4 shows the main effects of the parameters for each level of the parameters. The effect of particle size of ductile powders (parameter A) was found to be negligible on the coating thickness due to ductile-ductile milling. The Al2024 powders were deposited on the steel surface by ball-substrate-ball collisions. During the milling process, some powders didn t deposit due to cold welding on the steel surface, and these powders remained outside of the coating process. Increasing the milling time (parameter B) positively contributed on the coating thickness. The steel surface underwent very little change at 1 h and the majority of particles weren t cold weld on the steel substrate or ball surface at this stage of the process. For a short milling time (1 h) the particles were not subjected to an adequate compressive force or kinetic energy input, and therefore, only a minimal coating layer formed on the steel substrate surface 18,21. It was observed that an increase in the milling time led to an increase in the coating layer thickness. The formation of coating layer could be attributed to the deposition ability of the powder in the cold welded particles region. By increasing the milling time, this ability of the powders increased due to repeated action of cold welding on the steel surface. After 10 h (Fig. 2a), the coating became denser at a comparatively early stage of the coating process, indicating that the increase in the milling time increased the flattened particle regions. Notably, almost the entire surface was coated with the Al2024 powder after 10 h of milling (Fig. 2b). The milling speed (parameter C) had a significant influence on the process since higher milling speed provides energy in the rotation movement of the mill. The impacts with higher energy between balls and ball milling bowl improved the deformation and fracture of Al2024 powder particle and hence the coating thickness. On the other hand, the decrease in the average coating thickness at low milling speed can be attributed to lower energy imparted on the substrate by the impacting balls. It was observed that at low milling speed of 200 rpm average coating thickness dramatically decreased. The coating thickness of the Al 2024 powder was considerably increased with the higher milling speed. This increase can be related to the fact that the powder particles trapped between the ball and substrate became coldwelded to the substrate surface 18. Cold welding and fracturing are two opposing phenomena involved in the milling process 19,20. Moreover, with the increase in the rotation speed, substrate surface was subjected to higher energy ball impacts and contributed an increase in coating thickness. However, the speed of 300 rpm can be the threshold for critical speed to stick on steel substrate of ductile Al2024 powder because no change in coating thickness was observed after 300 rpm. Moreover, milled ductile powders were sticked on and/or embedded in flattened hills (Fig. 2a). These mechanisms can explain the negligible change in coating thickness from 300 to 400 rpm. It was found that the best conditions to achieve high coating thickness were A1B3C2. X-ray diffraction analysis The surface microstructure was refined by ball-substrate-ball collisions in the mechanical alloying process 20. Ball-powder-ball and ball-substrate-ball collisions, which caused a large amount of structural defects 19,21 and high local temperatures at the collision surface, resulted in severe plastic deformation of the

6 600 INDIAN J. ENG. MATER. SCI., DECEMBER 2014 substrate and the milled powders 22. Notably, the temperature at the collision surface may have risen above the melting point of Al (660 C); therefore, the reaction between Fe and Al could occur, and Fe-Al intermetallic compounds were formed. The reaction between the low-carbon steel substrate and the Al2024 powder can be examined by XRD. Figure 4 shows XRD spectra of the coating surface after heat treatment at 600 C. The XRD patterns confirmed the presence of both elemental Fe and Al and compounds such as Fe 3 Al, Fe 2 Al 5, FeAl 2 and FeAl. The formation of new phases was observed at distances from the steel surface of up to 30 µm. Conclusions The following conclusions can be drawn from this study: (i) To obtain the highest coating thickness, 10 mm ball diameter, ball-to-powder weight ratio of 10:1, 300 rpm milling speed, the milling time of 10 h and coarse powder should be selected. (ii) The milling time was detected as the most effective parameter than the others on coating thickness. The milling time and speed were found to be statistically significant parameters on the coating process while the contributions were calculated to be 81% and 19%, respectively. (iii) Increasing the milling time from 1 h to 10 h provided an improvement in coating thickness and morphology. After 300 rpm, negligible change in coating thickness was observed. (iv) The XRD patterns confirmed the formation of intermetallics of Fe 3 Al, Fe 2 Al 5, FeAl 2 and FeAl. Acknowledgements The authors are grateful to the Research Foundation of Karadeniz Technical University for the financial support (Project No: KTÜ-BAP ). The authors also would like to thank the Gundogdu Exotherm Service for kindly providing the Al 2024 powders. References 1 Jeng W C & Ming C S, Surf Coat Technol, 200 (2006) Korkut M H & Gok M S, Surf Eng, 25 (2009) Zhou Z, Xie Fei & Hu Jing, Surf Coat Technol, 203 (2008) Praveen B M, Venkatesha T V & Naik Y A, Synth React Inorg, Met-Org Nano-Met Chem, 37 (2007) Araujo P D, Steyer P, Millet J P, Damond E, Stauder B & Jacquot P, Surf Eng, 19 (2003) Jiang J, Wang Yi, Zhong Qingdong, Zhou Qiongyu & Zhang Lei, Surf Coat Technol, 206 (2011) Mao S W, et al., Surf Coat Technol, 205 (2010) Varol T & Canakci A, Philos Mag Lett, 2013, DOI: / Thein M A, et al., J Mater Process Technol, 209 (2009) Kim K D, et al., J Mater Process Technol, 202 (2008) Takacs L & Torosyan A R, J Alloys Compd, 434 (2007) Park K S, et al., Chem Eng Res Des, 89 (2011) Canakci A, et al., Micro Nano Lett, 9 (2014) Varol T & Canakci A, Met Mater Int, 19 (2013) Varol T, et al., Composites: Part B, 54 (2013) Suryanarayana C, Progr Mater Sci, 46 (2001) Gu D & Shen Y, Appl Surf Sci, 256 (2009) Li B, et al., Mater Des, 35 (2012) Romankov S, et al., Int J Refract Met Hard Mater, 27 (2009) Nouri A, et al., Mater Sci Eng C, 31 (2011) Lee H S, et al., J Alloys Compd, 478 (2009) Zhan Z L, et al., Intermetallics, 14 (2006)