PRODUCING NANOCOMPOSITE LAYER ON THE SURFACE OF AS-CAST AZ91 MAGNESIUM ALLOY BY FRICTION STIR PROCESSING

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1 2nd International Conference on Ultrafine Grained & Nanostructured Materials (UFGNSM) International Journal of Modern Physics: Conference Series Vol. 5 (2012) World Scientific Publishing Company DOI: /S PRODUCING NANOCOMPOSITE LAYER ON THE SURFACE OF AS-CAST AZ91 MAGNESIUM ALLOY BY FRICTION STIR PROCESSING P. ASADI School of Mechanical Engineering, College of Engineering, University of Tehran. Tehran, Iran. p.asadi85@gmail.com M. K. BESHARATI GIVI * School of Mechanical Engineering, College of Engineering, University of Tehran. Tehran, Iran. bgivi@ut.ac.ir G. FARAJI School of Mechanical Engineering, College of Engineering, University of Tehran. Tehran, Iran. gfaraji@engmail.ut.ac.ir Friction stir processing (FSP) is an effective tool to produce a surface composite layer with enhanced mechanical properties and modified microstructure of as-cast and sheet metals. In the present work, the mechanical and microstructural properties of as-cast AZ91 magnesium alloy were enhanced by FSP and an AZ91/SiC surface nanocomposite layer has been produced using 30 nm SiC particles. Effect of the FSP pass number on the microstructure, grain size, microhardness, and powder distributing pattern of the surface developed has been investigated. The developed surface nanocomposite layer presents a higher hardness, an ultra fine grain size and a better homogeneity. Results show that, increasing the number of FSP passes enhances distribution of nano-sized SiC particles in the AZ91 matrix, decreases the grain size, and increases the hardness significantly. Also, changing of the tool rotating direction results much uniform distribution of the SiC particles, finer grains, and a little higher hardness. Keywords: FSP; magnesium alloy; surface nanocomposite layer; microhardness; microstructure. 1. Introduction Magnesium is the lightest constructional metal on earth; it is 35% lighter than aluminium, and 78% lighter than steel [1, 2]. Magnesium and its alloys are primarily used in aeronautical and * Corresponding author, address: Kargar shomali St, School of Mechanical Engineering, college of engineering, university of Tehran Tehran, Iran. Po Box: 11155/4563 Tel.:

2 376 P. Asadi, M. K. B. Givi & G. Faraji automobile industry in wide variety of structural characteristics because of their favorable properties [3]. So, it is necessary to investigate very actively properties of individual alloys. However, the use of magnesium alloys has been strongly limited because of their poor formability at near room temperature as a consequence of hcp lattice [4]. Also, the mechanical properties, such as the hardness of the magnesium alloys, are not sufficient to enhance their applications, hence some processes to fabricate magnesium alloys composites have been studied to improve the mechanical properties [1-5]. Recently, friction stir processing has been developed based on friction stir welding for microstructural modification of metals. Also, it is possible to produce surface composite layer by this process as well. Homogenized microstructure with equiaxed fine grains due to FSP causes improvement in ductility, formability, and mechanical strength [6-9]. Very limited works have been done on FSP of magnesium alloys. Also, FSP of AZ91 magnesium alloy is difficult, because of its weak formability and ductility. Also, lack of AZ91 sheet form has limited the working on AZ91. Therefore, few papers have studied the as-cast AZ91 magnesium alloy mechanical enhancing by FSP. So, extensive investigation of FSP for AZ91 magnesium alloy is needed. In the present study, an AZ91/SiC surface nanocomposite layer was produced. Effects of the FSP pass numbers and rotating direction of the FSP tool on the powder distribution pattern, microstructure, and microhardness of the surface developed were investigated. 2. Experimental Details Commercially available SiC powders with 30 nm particle size and 99.8% pure were used in the present work. The material used was an AZ91 as-cast magnesium alloy with the composition of (in wt %): Al, 9.1; Zn, 0.68; Mn, 0.21; Si, 0.085; Cu, ; Ni, 0.001; Fe, ; and Mg, bal. The thickness of the plate was 5 mm. The SiC powders were filled into a groove of mm 2 machined on the AZ91 as-cast plate. Two 2344 hot working steel tools were used in this work. The first tool had a shoulder without any pin. This tool was employed to cover the top of the grooves after implanting the nano-sized SiC particles into the groove, in order to prevent the powders to be scattered during the FSP. The second tool had a square pin with a 4.5 mm diameter ( mm 2 ) and a 1.8 mm length; both of the tools shoulders were 15 mm in diameter. The second tool was inserted into the prepared specimen by the first tool in order to carry out the FSP. The rotational and traverse speeds were 900 rpm and 63 mm/min, respectively. The FSP tool was rotated in both clockwise and counterclockwise directions. The tilt angle was 3. The transverse sections of the FSPed specimens were cut in order to carry out the metallographic and microhardness investigations. The specimen surfaces were prepared by standard metallographic techniques and etched with a solution of 5 ml acetic acid, 6 g picric acid, 10 ml water, 100 ml ethanol, 5 ml HCl, and 7 ml nitric acid for 1-2 s. The microstructural observations of the FSPed zone and the distribution of the SiC particles in stirred zone (SZ) were carried out by optical and scanning electron microscopy (SEM).

3 Nanocomposite Layer on the Surface of AS-Cast AZ91 Magnesium Alloy 377 Microhardness of the specimens was measured in 1 mm distance from the upper surface in the cross section using a Vickers microhardness testing machine. In all tests a load of 200 g was applied for 15 seconds. To acquire the microhardness profiles, the Vickers microhardness test was carried out on the consecutive points with 0.25 mm distance from one another. 3. Results and Discussion 3.1. Macrostructure and microstructure Figure 1a shows the SEM macrograph from the SZ of the specimen produced by two FSP passes without changing the rotating direction of the tool (in both of the FSP passes, the tool rotating direction was clockwise). As can be seen, the friction stirred zone is about the same size as the rotating pin, namely width and depth of 4.5 and 1.8 mm, respectively. The surface composite layer appears to be very well bonded to the magnesium alloy substrate and no defect is visible in the interface. Distribution of the nano-sized SiC particles in the AZ91 matrix is not uniform after two FSP passes and the SZ is divided in two regions. The light region is rich of the SiC powders while the dark region is poor in SiC powders (Figure 1a). In this paper, the name light region is referred to the region with dense distribution and the dark region to the region with the less distribution of the nano-sized SiC particles. Such a nonuniform distribution of powders causes the microstructure of the SZ to become nonuniform after recrystallization. Figures 1b and 1c show the microstructure of the SZ in the light and dark regions, respectively. According to the principle of the Zener limiting grain size, increasing the volume fraction of SiC powders causes a decrease in the grain size of the matrix [10]. In fact, the pining effect resulting from the presence of the nano-sized SiC particles restricts the grain growth during recrystallization. So, the dense distribution (without clinging) of the SiC particles in the light region prevented the grain growth while the grains were allowed to grow in the region with the less distribution of the SiC particles (dark region). Also, the distribution of the nano-sized SiC particles even inside the light region was not uniform in the specimen produced by two FSP passes without changing the tool rotating direction. Figures 1d and 1e show the SEM and optical micrographs of the inhomogeneous microstructure resulted from such nonuniform distribution of the SiC particles. Presence of the regions with flame-like strips above the bottom TMAZ at AS shows the SiC particles distributing pattern developed as a result of the tool pin motion. Figure 2 shows the effect of the FSP pass numbers on distributing pattern of nano-sized SiC particles in the SZ. Figure 2a shows the macro image of the cross section of the specimen produced by one pass FSP. It can be seen that, most of the SiC particles are distributed in a small region (light region) of the SZ. Figures 2b, 2d and 2f show the macro image of the cross section of the specimens produced by two, four, and eight FSP passes respectively in which the tool rotating direction was clockwise in all cases. Figures 2c, 2e and 2g show the macro image of the cross section of the specimens produced by two, four, and eight FSP

4 378 P. Asadi, M. K. B. Givi & G. Faraji passes respectively in which the tool rotating direction was clockwise in odd pass numbers and counterclockwise in even pass numbers. Fig. 1. (a) SEM macrograph from the SZ of the specimen produced by two FSP passes without changing the tool rotating direction, (b)-(e) optical and SEM micrograph of the selected regions in Figure 1a. Increasing the FSP pass numbers caused the expansion of the light region and the enhancing of the powder distribution. The light region in Figure 2c is larger than that of Figure 2b and similarly in Figure 2e is larger than Figure 2d. It shows that, changing the tool rotating direction causes a severely change in material flow which develops the powder distribution and expands the region rich of SiC particles. There is a thin dark band in the brink of the SZ in the specimen produced by eight FSP passes without changing the tool rotating direction (Figure 2f). The light region has discovered all over the stirred zone in the specimen produced by eight FSP passes with changing the tool rotating direction (Figure 2g).

5 Nanocomposite Layer on the Surface of AS-Cast AZ91 Magnesium Alloy 379 Fig. 2. Macro image of the specimen produced by (a) one FSP pass, (b) and (c) two FSP passes, (d) and (e) four FSP passes, and (f) and (g) eight FSP passes. (b), (d), and (f) without changing the tool rotating direction, (c), (e), and (g) with changing the tool rotating direction. Figures 3a and 3b show the optical micrographs of the zone with flame-like strips in the SZ of the specimens produced by two and four FSP passes without changing the tool rotating direction respectively. The flame-like strips in the specimen produced by two FSP passes are clearly illustrated while they are inconspicuous in the specimen produced by four FSP passes and completely vanished in the specimen produced by eight FSP passes. That microstructure was not seen in the specimens FSPed by changing the tool rotating direction even in the specimen produced by two FSP passes. It shows that, changing the tool rotating direction after each FSP passes results in a more uniform microstructure. Also, the difference between the large and small grains of the zone with flame-like strips in the specimen produced by two FSP passes (Figure 3a) is higher than that of the specimen produced by four FSP passes (Figure 3b). Increase in FSP pass numbers from two to four caused an increase in grain size in the zone with flame-like strips due to the dispersion of the powders in a larger area. However, average grain size decreased in whole of the light region.

6 380 P. Asadi, M. K. B. Givi & G. Faraji Fig. 3. Optical micrograph of the region with flame-like strips in the specimen produced by (a) two and (b) four FSP passes without changing the rotating direction. Grain Size (µm) pass 2 passes 4 passes 8 passes Pass Number without changing rotating direction with changing rotating direction Fig. 4. Average grain size in the SZ of the samples produced by different FSP passes with and without changing the tool rotating direction. Increasing the FSP pass numbers decreases the average grain size of stirred zone because: (1) increasing in FSP pass numbers expands the light region having very fine grains due to a good dispersion of nano-sized SiC particles and (2) the grain size reduces in the both light and dark regions due to a new recrystallization resulted from severe plastic deformation of the new FSP pass. Figure 4 shows the average grain size in the SZ of the samples produced by one, two, four, and eight FSP passes with and without changing the tool rotating direction. Figure 5 shows the SEM micrograph of the SZ in the specimens produced by eight FSP passes with and without changing the tool rotating direction. Although, the nano-sized SiC particles were distributed almost all over the SZ in the specimen without changing the rotating direction, its grain size was larger than that of the specimen with changing the rotating direction.

7 Nanocomposite Layer on the Surface of AS-Cast AZ91 Magnesium Alloy 381 Fig. 5. SEM micrograph of the SZ of the specimen produced by eight FSP passes (a) without and (b) with changing the tool rotating direction. 3.2 Microhardness Microhardness profiles of the specimens produced by two and eight FSP passes with and without changing the tool rotating direction are depicted in Figure 6a. The average microhardness value of the as-cast AZ91 was 63 HV. Presence of very fine grains and nanosized SiC particles having high hardness caused an increase in the hardness of the SZ. As it can be seen in Figure 6a the hardness profile of the specimens produced by two FSP passes was divided in two parts due to the nonuniform distribution of the nano-sized SiC particles which was discussed (light and dark regions). Changing the tool rotating direction increased the wide of the region with higher hardness (light region) in the specimens produced by two FSP passes. Finally, a uniform microhardness profile of the SZ was attained in the specimens produced by eight FSP passes due to the uniform microstructure. Microhardness (HV) Distance from centre (mm) (a) 8 passes, changing rotation 8 passes, not changing rotation 2 passes, changing rotation 2 passes, not changing rotation Microhardness (HV) Pass number (b) light region, changing rotation light region, not changing rotation dark region, changing rotation dark region, not changing rotation Fig. 6. (a) Microhadness profile in the cross section of the specimen produced by two and eight FSP passes with and without changing tool rotating direction and (b) effect of FSP pass numbers on the average hardness of the light and dark regions.

8 382 P. Asadi, M. K. B. Givi & G. Faraji Figure 6b shows the average microhardness of the dark and light regions in the specimens produced by one, two, four, and eight FSP passes. Dense presence of the SiC particles yielding very fine grains caused a severe increase in the hardness of the light region up to 115 HV while the hardness of the dark region was increased up to 90 HV (for the specimen produced by one FSP pass). Microhardness value does not exist for the dark region in the specimens produced by eight FSP passes due to elimination of this region. Microhardness of the specimens produced with changing the tool rotating direction is a little higher than that of the specimens produced without changing the tool rotating direction because of having smaller grain size. 4. Conclusions In this work, an AZ91/SiC nanocomposite surface layer produced using 30 nm SiC particles. Effect of the FSP pass numbers on the microstructure, grain size, microhardness, and powder distributing pattern of the developed nanocomposite layer was investigated. Results showed that, increasing the number of FSP passes leads to the homogenized microstructure and the uniform distribution of the nano-sized SiC particles in the AZ91 matrix and causes a decrease in the grain size. Also, changing the tool rotating direction results much uniform distribution of the SiC particles, finer grains, and higher hardness. Hardness and grain size of the developed surface layer were reached to 140 HV and 600 nm respectively by eight FSP passes and changing the tool rotating direction. References 1. Y. Morisada, H. Fujji, T. Nagaoka, M.Fukusumi, Effect of friction stir processing with SiC particles on microstructure and hardness of AZ31, Mater. Sci. Eng. A 433 (2006) B.M. Darras, M.K. Khraisheh, F.K. Abu-Farha, M.A. Omar, Friction stir processing of commercial AZ31 magnesium alloy, Mat. Proc. Tech. 191 (2007) L. Cizek, M. Gregera, L. Pawlicaa, L.A. Dobrzanskib, T. Tanskib, Study of selected properties of magnesium alloy AZ91 after heat treatment and forming, J. Mater. Pro. Tech (2004) P. Cavaliere, P.P. De Marco, Superplastic behaviour of friction stir processed AZ91 magnesium alloy produced by high pressure die cast, J. Mater. Proc. Tech. 184 (2007) W. Woo, H. Choo, "Microstructure, texture and residual stress in a friction-stir-processed AZ31B magnesium alloy", Acta Materialia 56 (2008) R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, Scripta Mater. 42 (2000) R.S. Mishra, M.W. Mahoney, Mater. Sci. Forum 507 (2001) R.S. Mishra, Z.Y. Ma, I. Charit, Mater. Sci. Eng. A 341 (2003) P. Asadi, Ghader Faraji, M. K. Besharati, Producing of AZ91/SiC composite by friction stir processing (FSP), Int. J Adv. Manuf. Technol. DOI /s z. 10. A. Shafiei-Zarghani, S.F. Kashani-Bozorg, A. Zarei-Hanzaki Microstructures and mechanical properties of Al/Al2O3 surface nano-composite layer produced by friction stir processing, Mater. Sci. Eng. A 500 (2009)