A mechanism of porosity formation in metal composite casts with saturated reinforcement
Jacek Jackowski Politechnika Poznańska, Wydział Budowy Maszyn i Zarządzania, Instytut Technologii Materiałów, Zakład Odlewnictwa, ul. Piotrowo 3, 60-965 Poznań
Quarterly No. 3, 2005 pages 102-105
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abstract The paper presents the results of analyses and studies related to formation of porosity in the structure of composite casts manufactured by saturation of a porous reinforcement with liquid metal matrix. The analysis includes the phenomena undergoing in the time interval between the beginning of the reinforcement saturation and the end of solidification of the composite matrix. The analysis has shown that the main factors leading to porosity during the casting process are as follows: generation of gas occlusions in the volume of a saturated reinforcement profile and shrinkage phenomena accompanying the processes of cooling in liquid state and solidification of composite matrix. In the reinforcement profile under saturation the gas occlusions appear that are initially immobile and, with growing pressure, are subject to compression (Fig. 1). Cooling in the liquid state and solidification of the composite matrix are accompanied by the decrease of its volume that, if not compensated, leads to decompression of the occlusions and growing porosity of the composite material (Fig. 2). Distribution of porosity in trial composite casts has been gravimetrically determined in the samples obtained from cut casts (Fig. 3). The distribution found this way and presented in Figure 4 authenticates the considered mechanism of porosity formation and provides an obvious prove of existence of insulated regions in the trail casts, in spite of using different values of the matrix reinforcement pressure. Hence, in order to obtain composite casts (with saturated reinforcement) of low-porous structure a pre-determined saturation pressure should be maintained in the whole area of the manufactured cast until the time of solidification of the reinforcing matrix. Key words: composite casts, structure, porosity