18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS THERMAL CONDUCTIVITY AND EXPANSION OF MAGNESIUM ALLOY MATRIX COMPOSITES BY LIQUID PRESSING PROCESS S.J. Hong, S.-B. Lee*, H.B. Kim, J.-B. Kim and S.-K. Lee Composite Materials Research group, Korea Institute of Materials Science, 797 Changwon- daero, Changwon, Gyeongnam, 642-831, Korea *Corresponding author (leesb@kims.re.kr) Keywords : Metal Matrix Composite, Magnesium, Coefficient of thermal expansion, Thermal conductivity, Liquid pressing process Unidirectional carbon fiber (UD C f , Mitsubishi 1 Introduction Rayon TR-50S) and approximately 10 m m sized Magnesium alloy matrix composites are offering silicon carbide particle (SiC p , Saint-Gobain SIKA F- several advantages over other MMCs, including 600) were used as reinforcements of the composites. lightweight, better specific strength and modulus. [1- Figure 1 shows the schematic illustration of liquid 5] Despite the practical significance, relatively little pressing process. A reinforcement preform and research has been conducted on thermal property of AZ91 magnesium alloy were inserted into the mold, magnesium alloy matrix composites. [6-8] degassed, and vacuumed. The mold was heated to Coefficient of thermal expansion (CTE) and thermal 695 o C, held for 5 minutes, and then pressed under a conductivity (TC) of liquid pressed AZ91 pressure of about 10 MPa. The fabricated magnesium alloy matrix composites could be composites were sectioned three different directions affected by reinforcement shape. (x, y and z) to observe microstructure using scanning The squeeze casting process, one of the conventional electron microscope (SEM). For the quantitative fabrication methods for the metal matrix composites, CTE and TC measurements, dilatometer and laser has merits such as high productivity and easiness for flash method were used. The effects of near-net-shape fabrication, but has shortcomings of reinforcement shape on thermal properties were poor reliability, the requirement of high-pressure analyzed by several prediction models. loading of 50 MPa or more in order to enhance the wettability between reinforcements and matrix, and Table 1. Chemical compositions of AZ91 easy collapse of preform patterns. The liquid magnesium alloy. pressing process is new-concept process using low pressure near to the theoretically required minimum Ele. Mg Al Zn Si Cu Mn Ni loading pressure to infiltrate the metallic melt in Wt.% Bal. 9.0 1.0 0.30 0.10 0.13 0.010 reinforcement preform. The objective of this study is to examine the thermal properties of liquid pressed AZ91 magnesium alloy matrix composites. The change in thermal properties with two different shapes of reinforcement is discussed based on thermal property prediction models and micrographic observations. 2 Experimental The AZ91 magnesium alloy was used as a matrix of composites. Table 1 shows the chemical Fig. 1. Schematic illustration of liquid pressing composition of AZ91 magnesium alloy. process.
3 Results and Discussion continuous carbon fiber. On the other hand, the SiC p /AZ91 composite (Figure 2(b)) has isotropic 3.1 Microstructures microstructures due to discontinuous SiC p . 3.2 Thermal Properties (a) Fig. 3. Experimental and predicted CTE of liquid pressed SiC p /AZ91 composite as a function of temperature. Figure 3 shows the experimental and predicted CTE of liquid pressed SiC p /AZ91 composite with the raw materials of SiC p and AZ91. The CTE of SiC p /AZ91 increased from 11.55 to 13.08 ppm/K with increment of temperature. In order to understand the effects of reinforcement shape on CTE, thermal property predictions were conducted. There are several models to predict the CTE of the metal matrix composites. In this study, the thermal expansion of SiC p reinforced AZ91 composite was calculated using two different models as follows. (b) Rule of mixture Fig. 2. SEM micrographs of liquid pressed (a) UD C f /AZ91 and (b) SiC p /AZ91 composites, ( ) (1) a = 1 - a + a V V respectively. c f m f f Kerner’s model [9] Figure 2 shows the SEM micrographs of AZ91 magnesium alloy matrix composites fabricated with ( )( )( ) - a - a - V 1 V K K ( ) different reinforcement shapes of UD C f and SiC p , a = - a + a + f f f m f m 1 V V ( ) c f m f f - + + respectively. The reinforcements of UD C f and SiC p 1 V K V K 3 K K / 4 G f m f f f m m are uniformly distributed through the composites (2) and there is no evidence of pores or separated with interface. Figure 2(a) showing that the UD C f /AZ91 E (3) = K ( ) composite has anisotropic microstructures due to - 3 3 E / G
THERMAL CONDUCTIVITY AND EXPANSION OF MAGNESIUM ALLOY MATRIX COMPOSITES BY LIQUID PRESSING PROCESS where: a = CTE, V = volume fraction, K = bulk Chamis’s model [10] modulus, and subscript c = composite, f = ( ) ( ) (5) reinforcement, m = matrix a = a f + - + n m f a m V 1 V 1 V E / E 2 2 f f f 1 1 As shown in Figure 3, the predicted Kerner’s model, where: a = CTE, V = volume fraction, F = packing completely matches the experimental result of the factor, E = young’s modulus, and subscript 1 = 1- SiC p /AZ91 composite. direction, 2 = 2-direction, f = reinforcement, m = matrix Fig. 4. Experimental and predicted CTE of liquid Fig. 5. TC of liquid pressed SiC p /AZ91 as a function pressed UD C f /AZ91 composite as a function of of temperature. temperature. Figure 4 shows the experimental and predicted CTE of the UD C f /AZ91 composites. The predicted models do not match the experimental result. The discrepancy between the experimental result and the calculated results of UD C f /AZ91 composite was considered due to increase in constrain effect of the rigid continuous UD C f as a softening of magnesium alloy matrix. The stiffness of magnesium alloy matrix decreases with increasing temperature, but there is little change in that of continuous UD C f at the experimental temperature range. Therefore, the effect of continuous UD C f on the resistance of thermal expansion increases and the CTE of UD C f /AZ91 composite decreases. Fig. 6. TC of liquid pressed UD C f /AZ91 as a The thermal expansion of UD C f reinforced AZ91 function of temperature. composite was also calculated using two different models as follows. Figure 5 and Figure 6 show the TC of SiC p /AZ91 and UD C f /AZ91 composites with the raw materials Chamberlain’s model [10] of SiC p , UD C f , and AZ91. The TC of SiC p /AZ91 ( ) composite decreased from 84.80 at room a f - a m 2 V a = a + 2 f m ( ) ( ) ( )( ) ( ) temperature to 77.05 W/mK at 200 ℃ . On the 2 n m - + + + m f - n f - F V F V E / E 1 F V f f 1 12 f contrary, the TC of UD C f /AZ91 composite (4) 3
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