Analysis of Manufacturing Processes for Metal Fiber Reinforced Aluminum Alloy Composite Fabricated by Low-Pressure Casting

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1 Materials Transactions, Vol. 47, No. 4 (2006) pp. 227 to 23 #2006 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Analysis of Manufacturing Processes for Metal Fiber Reinforced Aluminum Alloy Composite Fabricated by Low-Pressure Casting Yong Bum Choi ;2, Kazuhiro Matsugi ;2, Gen Sasaki ;2, Kazushi Arita 3 and Osamu Yanagisawa ;2 Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima , Japan 2 Hiroshima Prefectural Institute of Industrial Science and Technology, Higashi-Hiroshima , Japan 3 Testing & Research Gr. Development Dept. Kolbenschmidt K.K, Higashi-Hiroshima , Japan A metal-fiber-reinforced aluminum alloy composite was fabricated through low-pressure casting and its optimum process conditions were determined. The direct finite difference method (DFDM) based on Darcy s law was used to calculate the pressure distribution inside the preform, and then the porosity inside the composite fabricated through low-pressure casting at 0:4{0:8 MPa was estimated. The relationship between the porosity and the pressure distribution inside the preform was also estimated. The porosity of the composites fabricated under the applied pressures of 0.4 MPa, 0.6 MPa, 0.7 MPa and 0.8 MPa was estimated to be 0.%, 4%, 7% and 0%, respectively, at the pressure acceleration time of s. Hence, the composite without pores was fabricated under the pressure of 0.8 MPa in applied pressure. (Received November 25, 2005; Accepted February 5, 2006; Published April 5, 2006) Keywords: direct finite difference method (DFDM), infiltration, preform, pressure distribution, metal matrix composites (MMC). Introduction Aluminum alloy composite castings under low-pressure process has the advantage of being semi-automatic and costeffective while ensuring good casting quality and higher yield.,2) It is believed that low-pressure casting costs less than squeeze casting because high-compression equipment is unnecessary, and this process provides better quality than squeeze casting. Squeeze casting leads to the deformation of preforms due to the high pressure. Hence, the shapes of the preforms used in squeeze casting must be kept simple compared with those of the preforms used in low-pressure casting. However, despite its many advantages, low-pressure casting is not yet fully appreciated. The main problem is the lack of understanding of this process. As the die design and casting operation have not been properly incorporated into the machine to optimize the process, imperfect infiltration inside the preform can occur. 3,4) Recently, the technique of low-pressure casting has been applied especially to the preparation of aluminum metal matrix composite (AlMMC) for automobile parts because of its high performance and easy handling. Oda et al. have developed an AlMMC diesel-engine piston fabricated through low-pressure casting. 5) However, this composite usually has low mechanical properties due to its high porosity, In order to improve these properties, it is necessary to optimize of the process parameters in order to decrease the porosity. The purpose of this study was to fabricate a composite without pores through low-pressure casting. For this purpose, the relationship between the pressure and the porosity, as well as the pressure-porosity curve, under low-pressure casting was obtained. Then, the pressure distribution in the preform was calculated by the direct finite difference method (DFDM) based on Darcy s law. 6) The pressure distribution calculated by the DFDM was compared to the porosity of the composite. The porosity of the composite fabricated through low-pressure casting at MPa was estimated by optical microscopy and scanning electron microscopy (SEM). 2. Experimental Procedure 2. Porosity in low-pressure casting A schematic diagram of the typical low-pressure casting equipment is shown in Fig. (a). A low-pressure casting machine usually includes a pressurized mold, a compressor, a vacuum pump and an air vent. The process of low-pressure casting is shown in Fig. (b). After pouring molten aluminum alloy into the mold, a reducing pressure of 5 MPa applied at the air vent. The purpose of the reducing pressure was to remove the air inside the preform. Then, a pressure of 0.4 MPa was applied on the molten alloy for infiltration. The pressure acceleration time was s, 2 s and 5 s, respectively. The pressure acceleration time is the time needed for the maximum plunger pressure to reach 0.4 MPa, as shown Fig. (b). The preheating temperature of the preform was 673 K. The preform was set in the metal mold. The temperature of the mold was approximately 523 K. Molten aluminum alloy with a temperature of 023 K was poured into the mold. Metal fiber (75%Fe-20%Cr-5%Si, NHK SPRING Co. Ltd., Japan) was used as reinforcement. The diameter of the fiber was 40 mm and the volume fraction of the fiber was 20%. A336.0 aluminum alloy (Al-3% Si-.5%Ni-.3%Cu-.3%Mg) was used as the molten metal infiltrating the preform. The porosity of the metal-fiber reinforced aluminum alloy composite was estimated by optical microscopy and SEM. 2.2 Model of the pressure distribution in the preform In order to estimate the relationship between the porosity and the pressure inside the preform, a simulation model of the pressure distribution is needed. A three-dimensional Cartesian coordinate system was used to investigate the distribution of pressure in the preform. Figure 2 illustrates the initial conditions and the boundary conditions applied to composites for piston head parts. In the initial condition, the applied pressure to the molten alloy by the plunger was 0.4 MPa, and the pressure at the air vent was 0. MPa. The distance between the mesh points was mm.

2 228 Y. B. Choi, K. Matsugi, G. Sasaki, K. Arita and O. Yanagisawa (b) Pressure [MPa] Appling pressure 0.4MPa 0.MPa Pressure acceleration time s,2s and 5s 5MPa Time [sec] Injection (7s) Reducing pressure (5s) Fig. Schematic diagram (a) and process (b) of a typical low-pressure casting used in this study. Fig. 3 Darcy s flow by Direct Finite Difference Method (DFDM). Fig. 2 Schematic drawing of low pressure casting system used in this study. Unit is given by mm. The very small element of the DFDM using Darcy s law is expressed in Fig. 3. The eq. () of Darcy s law can be obtained from Fig. 3 if the permeability (K) and the balanced velocity of the fluid (U a,d ) between node a and node d is considered. The governing equation is based on Darcy s flow for pressure distribution analysis, which can be expressed as following: ðp d P a ÞS ad " ¼ " K U a,dd ad S ad gs ad "ðz d Z a Þ; where K is the permeability (mm 2 ), is the coefficient of viscosity of the molten alloy (Ns/mm 2 ), P is the pressure (N/mm 2 ), is the density of the molten alloy (Mg/mm 3 ), g is the acceleration of gravity (mm/s 2 ), S is the area (mm 2 ), d is the diameter of the reinforcement(mm), " is the porosity, d ad is the distance between node a and node d, and Z d and Z a are the distances along the vertical coordinate direction (mm). If we ignore gravity in eq. (), we obtain the following expression: ðþ U a,d ¼ K ½P d P a þ gðz d Z a ÞŠ: ð2þ d ad Here, the flow rate from the area of the node is expressed as zero (0), as follows: X n d¼ U ad S ad ¼ 0: The flow of node a can be expressed as a combination of the outside flow and inside flow using eqs. (2) and (3). Accordingly, the pressure distribution can be expressed as follows: 0 ¼ K ( S ad2 d ad2 ½P d2 P a þ gðz d2 Z a ÞŠ þ S ad3 d ad3 ½P d3 P a þ gðz d3 Z a ÞŠ þ S ad d ad ½P d P a þ gðz d Z a ÞŠ þ S ad4 d ad4 ½P d4 P a þ gðz d4 Z a ÞŠ ) : ð3þ ð4þ

3 Analysis of Manufacturing Processes for Metal Fiber Reinforced Aluminum Alloy Composite Fabricated by Low-Pressure Casting 229 Porosity, P (%) (b) Porosity by solidification sec 2sec 5sec Preform (mm) Porosity (%) (c) Porosity by imperfect infitlation sec 2sec 5sec Preform (mm) Fig. 4 Influence of applied pressure (0.4 MPa) and applied pressure acceleration times, sec, 2 sec and 5 sec on porosity (a) position of observed porosity (b) porosity by solidification (c) porosity by imperfect infiltration. 3. Results and Discussion 3. Porosity Figure 4 indicates the porosity at each position of the composites. Figure 4(a) illustrates the observation positions of the porosity inside the composites. The relationship between this porosity and the pressure acceleration times ( s, 2 s, and 5 s) under 0.4 MPa is shown in Fig. 4(b) and (c). We concluded that there are two types of pores, one caused by solidification occurring inside the molten alloy and one caused by imperfect infiltration occurring around the metal fibers. Figure 4(b) shows pores which were caused by the solidification of the molten alloy. The porosity due to solidification was less than 0.2%, and is not related to the pressure acceleration time. Figure 4(c) shows the porosity caused by the imperfect infiltration of the molten alloy. Numerous pores were observed at the side of the mold and around the reinforcing metal fibers. The porosity of the in composites fabricated under the pressure acceleration time of s, 2 s and 5 s was estimated to be 0.36%, 0.48% and 0.78%, respectively, under the applied pressure of 0.4 MPa. The porosity decreased with increasing pressure acceleration time. The porosity due to the imperfect infiltration of the molten alloy depends on the pressure acceleration time. Furthermore, the pores occurred at the side of the mold. An increase in porosity due to a rapid decrease in the temperature of the molten alloy is expected because the mold temperature is low. 3.2 Pressure distribution in the preform Figure 5 shows the results of the pressure distribution measurement inside the preform under the pressure condition Fig. 5 Y Z section of pressure distribution of molten alloy inside preform under steady state (applied pressure, 0.4 MPa. Unit is given by MPa. of 0.4 MPa on molten aluminum and 0. MPa at the air vent. The pressure distribution was calculated and the relationship between the local pressure inside the preform and the porosity was obtained. The result under the constant pressure of 0.4 MPa will be called the steady-state results in this paper. The pressure distribution contour had a linear shape at the center of the air vent. The local pressure at the air vent was low. Hence, the porosity is expected to increase at the air vent parts. 3.3 Relationship between porosity and pressure Figure 6 presents the relationship between the porosity and

4 230 Y. B. Choi, K. Matsugi, G. Sasaki, K. Arita and O. Yanagisawa Porosity (%) y=-3x 5 +92x 4-02x 3 +56x 2-5x +.9 Porosity, P (%) pressure acceleration time (sec) Fig Local pressure (MPa) Relationship between porosity and local pressure inside preform Applied pressure (MPa) Fig. 7 Relationship between applied pressure and porosity in pressure acceleration time, sec. 200 µm 200 µm 200 µm Fig. 8 Optical micrographs of porosity in composites fabricated by low pressure casting with (a) 0.4 MPa, (b) 0.6 MPa, (c) 0.7 MPa and (d) 0.8 MPa. the local pressure inside the composite. The porosity and the local pressure were obtained from experimentally, and the pressure distribution was obtained by calculation. Porosity is inversely proportional to pressure; as expected, the porosity decreased with increasing applied pressure. Figure 7 shows the relationship between the porosity and the applied pressure within the range from 0.4 MPa to 0.8 MPa. No Pores were observed at the applied pressure of 0.8 MPa. This result confirmed that the optimum manufacturing conditions are an applied pressure of 0.8 MPa and a pressure acceleration time of s. Figure 8 shows that the porosity inside the composite varied with different applied pressures. No porosity is seen in this photograph, which shows a composite produced at the applied pressure of 0.8 MPa. Under these conditions, FeCrSi metal-fiber preforms were successfully infiltrated using a low-pressure casting method. 4. Conclusion The novel low-pressure infiltration process presented in

5 Analysis of Manufacturing Processes for Metal Fiber Reinforced Aluminum Alloy Composite Fabricated by Low-Pressure Casting 23 this paper is very effective for FeCrSi-reinforced aluminum alloy composite fabrication. The porosity of the metal-fiberreinforced composite samples examined in this study decreased with increasing pressure acceleration time at the same applied pressures. The porosity decreased as the applied pressure increased for the same pressure acceleration times. Furthermore, we calculated the pressure distribution in the preform by the DFDM based on Darcy s law. We were able to confirm the optimum manufacturing conditions for the fabrication of the metal-fiber-reinforced composite, which is applied pressure 0.8 MPa and a pressure acceleration time of s. Acknowledgements Industrial Science and Technology for financial support of this study. REFERENCES ) H. Westengen and O. Holta: Founder Trade Joumal 63 (989) ) S. Morimoto, N. Ohnishi and S. Okada: AFS Trans. 20 (987) ) N. Onishi, T. Aizawa and S. Okada: JACT News 355 (986) ) T. S. Han and W. S. Hwang: Transactions of Japan Foundary Engineering Society 4 (995) ) N. Oda, M. Fujita, Y. Sugimoto, K. Teshima, K. Arita and T. Kuramoto: Mazuda Tech. Rev. 37 (999) 7. 6) H. Darcy: Les fontaines publiques des la ville de Dijon, Dalmont, Paris (856). We would like to thank Hiroshima Prefectural Institute of