Available online at ScienceDirect. Procedia Materials Science 4 (2014 ) 85 89

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1 Available online at ScienceDirect Procedia Materials Science 4 (2014 ) th International Conference on Porous Metals and Metallic Foams, Metfoam 2013 Fabrication of porous aluminum alloys with aligned unidirectional pores by dipping pipes into liquid and semi-solid base metals Tatsuro Hayashida, Shinsuke Suzuki*, Junichi Ichikawa, Ryuji Toyoyama Faculty of Science and Engineering, Waseda University, Tokyo , Japan Abstract The new fabrication method to fabricate porous aluminum alloys with aligned unidirectional pores was invented by dipping pipes into semi-solid slurry. Pure aluminum pipes were dipped into Al-4mass% Cu liquid and semi-solid slurry of base metal in an electric furnace. The base metal and dipped pipes were cooled and solidified together in the air. The pipes were bonded to the base metal and the original shape of the pipes was maintained by using the semi-solid slurry of the base metal. However, pipes are usually ruptured and buried in the liquid base metal during the process. On the other hand, it is possible to avoid the rupturing of pipes by using the semi-solid slurry because thermal damage of pipes can be reduced by an increase in solid fraction of the base metal. The analysis results of Electron Probe MicroAnalyser showed that Cu diffused into grain boundaries of pipes, which decreased the melting temperature of surface of pipes. As a result, the α phase melted partly and then the pipes were bonded with the base metal metallurgically. This dipping method can fabricate porous Al-Cu alloys with aligned unidirectional pores with porosity of 33% when nineteen pipes are used Published The Authors. by Elsevier Published Ltd. This by Elsevier is an open Ltd. access article under the CC BY-NC-ND license ( Peer-review under responsibility of Scientific Committee of North Carolina State University. Peer-review under responsibility of Scientific Committee of North Carolina State University Keywords: Aluminum; Aluminum-copper alloys; Casting; Unidirectional pore; Semi-solid slurry. 1. Introduction Porous aluminum alloys with aligned unidirectional pores have been receiving a lot of attention as structural materials because they have the superior mechanical properties (Ichikawa et al. (2013), Hyun and Nakajima (2001), Hyun et al. (2003)). For example, the specific strength does not decrease in the longitudinal direction of the pore * Corresponding author. Tel.: ; fax: address: suzuki-s@waseda.jp Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of Scientific Committee of North Carolina State University doi: /j.mspro

2 86 Tatsuro Hayashida et al. / Procedia Materials Science 4 ( 2014 ) even when increasing their porosity, because of lack of stress concentration. These kinds of porous metals are usually fabricated with solubility gap of hydrogen between the solid and the liquid using unidirectional solidification processes (Nakajima (2007) and Shapovalov (1998)). However, much simpler fabrication method is strongly desired. Thus, we invented a new simple fabrication method in which pure aluminum pipes are dipped into a base metal of aluminum alloy melt (Ichikawa et al. (2012)) or semi-solid slurry. Fig. 1 shows a schematic illustration of the procedure steps. The fabrication method for porous aluminum alloys with aligned unidirectional pores can be realized without large and expensive equipment because liquid and semi-solid slurry are available only with an electric furnace. So the pore shape, size, position and porosity can be decided by the dimension of the pipes and the initial position of the pipes easily and simply. Therefore, the method of dipping pipes into the base metals is easier and simpler than conventional ones. In this study, we investigated possibility of fabrication to establish this new fabrication method and discussed their bonding mechanisms between the pipe and base metal. 2. Experimental Seven or nineteen pure aluminum pipes with internal diameter of 3mm and wall thickness of 0.5mm and base metal ingots of Al-4mass% Cu were prepared. In addition, base metal ingots for semi-solid slurry were prepared by a cooling slope (Fig. 2) (Motegi and Piao (2005), Haga and Kapranos (2002)). Firstly, the number of pipes was decided on seven in order to investigate the possibility of fabrication by using semi-solid slurry as a basic research. Secondly, nineteen pipes were used to fabricate porous aluminum alloys with a porosity higher than 30% and with a size large enough to evaluate the characteristics. The base metal melt was flown on the cooling slope (copper, coated by boron nitride (BN)) and solidified in a mold (stainless steel, coated by BN). The base metal ingots of 100g were set in a crucible. Next, the crucible was set on an apparatus for pipe dipping experiment in an electric furnace. The base metal ingot on the apparatus was heated at 656 (liquid), 647, and 645 C (semi-solid slurry) for 1.5-2h in the electric furnace. The solid fraction of semi-solid slurry was 30 or 50% from Al-Cu phase diagram, respectively. After heating up, seven pure aluminum pipes were dipped into the liquid or semi-solid slurry of the base metal. Without holding time in the electric furnace, the crucible was taken out from the electric furnace and cooled down in the air. The solidified specimens were cut and polished perpendicularly to the longitudinal direction of the pipe. These cross-sections were observed by an optical microscope (NIKON, ECLIPSE MA200). The porosity of fabricated specimens was calculated by Eq. 1. p = A p / A 100 (1) Here A is defined as the hexagonal area (Fig. 3) and A p was the area of pores in A. The A and A p were measured by image analyzer WinROOF TM (Mitani Corporation). The Cu content in the vicinity of the boundary between the pipe and the base metal was measured by Electron Probe Micro Analyser (EPMA, JEOL Ltd., JXA8100, acceleration voltage 15 kv). 3. Results and discussion Fig. 4 shows the porous Al-Cu alloy with aligned unidirectional pores fabricated by dipping pipes into the semisolid slurry with solid fraction of 30%. The porosity p is 33%. Fig. 5 shows the cross-section of the specimens with solid fraction of 0, 30 and 50%. The porosity p of these specimens was 25% (Fig. 5 (a)-(c)). Fig. 5 (a) shows the sample without rupturing and burying pipes. However, the pipes were usually ruptured and buried in the liquid base metal (Fig. 5 (a )). The occurrence of rupture and burying of pipes was a statistical problem due to the inhomogeneous distribution of the thin oxide films on the surface of the pipes. Fig. 5 (c) shows that the position of pipes is moved randomly from the initial position by using the base metal with solid fraction of 50%. This is due to the buckling of pipes by relative high flow stress of the semi-solid slurry during dipping pipes. On the other hand, neither rupturing nor buckling of pipes occurs when the solid fraction is 30%. As a result, the rupturing and burying of pipes can be avoided by using the semi-solid slurry with solid fraction

3 Tatsuro Hayashida et al. / Procedia Materials Science 4 ( 2014 ) of 30%. Furthermore, it may be possible to avoid the buckling of pipe by inserting a columnar support into pipe during fabrication process. melt (Al-Cu) primary α cooling slope (Cu) BN coating mold (SUS304) Fig. 2. Schematic illustration of preparation of base metal for semi-solid slurry by cooling slope. hexagonal area = A A p Fig. 1. Schematic illustration of experimental procedure. Fig. 3. Schematic illustration of the cross section of fabricated porous aluminum alloy (7 pipes). Fig. 4. Porous aluminum alloy with unidirectional through pores fabricated by dipping pipes in semi-solid slurry.

4 88 Tatsuro Hayashida et al. / Procedia Materials Science 4 ( 2014 ) (a) pore f s =0% (a ) pipe base metal the flowing of the base metal f s =30% f s =50% (b) (c) 5mm misalignment of pores f s : solid fraction Fig. 5. Cross sectional views of the specimens by the experiment using pipes at different solid fraction. Solid fraction (a) Microstructure by the optical microscope Mapping image of Cu by EPMA 0% (b) pipe base metal grain boundary 30% initial position of the outer wall 200μm Fig. 6. The microstructures and the mapping images of Cu content (> 2mass% ) by the EPMA analyses in the vicinity of the boundary between the pipe and the base metal. Fig. 6 shows the microstructures and the mapping images of Cu content higher than 2 mass% by the EPMA analyses in the vicinity of the boundary between the pipe and the base metal with solid fraction of 0% (Fig. 6 (a)) and 30% (Fig. 6 (b)). The microstructures show that the pipes and the liquid or semi-solid slurry bond metallurgically at both solid fractions because no significant gaps are found. As a result, the fabrication of porous

5 Tatsuro Hayashida et al. / Procedia Materials Science 4 ( 2014 ) Al-Cu alloys with aligned unidirectional pores is possible by dipping aligned pipes into the semi-solid slurry with solid fraction of 30%. The mapping images of Cu by EPMA analyses in Fig. 6 show that Cu diffused from the base metal into the pipe along the grain boundaries in both cases. The Cu diffusion decreases the liquidus temperature of a phase in the vicinity of the grain boundary of the pipe, which results in melting the grain boundary preferentially and bonding the pipes with the base metal metallurgically. 4. CONCLUSION The new simple fabrication method by dipping aluminum pipes into a semi-solid base metal of Al-Cu alloys was developed and experimented in order to fabricate porous aluminum alloys which have aligned unidirectional pores. The following conclusions were obtained. (1) Porous Al-Cu alloys which have aligned unidirectional pores are fabricated by dipping aligned pipes into the semi-solid slurry with appropriate solid fraction. (2) The appropriate solid fraction is 30% in this study because neither rupturing nor buckling of pipes occurs. (3) Diffusion of Cu from the base metal into grain boundary of pipe decreases the liquidus temperature of a phase in the vicinity of the grain boundary of the pipe. Therefore, the grain boundary of the pipe melts preferentially and partly, which bonds the pipes with the base metal metallurgically. Acknowledgements The pure aluminum ingots used in this study were provided from The Light Metal Educational Foundation, Inc. This study was carried out as Japan Science and Technology Agency, Special Coordination Funds for Promoting Science and Technology Waseda Institute for Advanced Study Tenure Track Program and Waseda University Grant for Special Research Projects (2011A-103), Grant-in-Aid from the Light Metal Educational Foundation, and Hitachi Metals - Materials Science Foundation, and Kimura Chuzosho, Co. Ltd,. The authors are deeply grateful to Mr. M.Fujiwara and Mr. Y.Seo (Kagami Memorial Research Institute for Materials Science and Technology, Waseda Univ.) for the technical support of the EPMA analyses. References Haga, T., Kapranos, P., Billetless Simple Thixoforming Process. Materials Processing Technology , Hyun, S. K. Nakajima, H., Anisotropic Compressive Properties of Porous Copper produced by Unidirectional Solidification. Materials Science and Engineering A 304, Hyun, S. K., Murakami, K., Nakajima, H., Anisotropic Mechanical Properties of Porous Copper Fabricated by Unidirectional Solidification. Materials Science and Engineering A 299, Ichikawa, J., Hayashida, T., Suzuki, S., Compressive Properties of Porous Aluminum Alloy Fabricated by Joining Pipes and Melt through Continuous Casting. Materials Science Forum 761, Ichikawa, J., Suzuki, S., Hayashida, T., Yahara, R., Nakae, H., Fabrication of Porous Aluminum Alloy with Aligned Unidirectional Pores by Dipping Pipes in Base Metal Melt. Materials Transactions 53, Motegi, T., Piao, L., Semisolid Continuous Casting of 5052 Aluminum Alloy. Proceedings of Mechanical Engineering Congregation. 1, Nakajima, H., Fabrication, Properties and Application of Porous metals with Directional pores. Progress in Materials Science 52, Shapovalov, V. I., Formation of Ordered Gas-Solid Structures via Solidification in Metal-Hydrogen Systems. MRS Symposium 1998 Proceedings, 521,