The Solidification Microstructure of Ai-Cu-Si Alloys Metal Matrix Composites
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**LEMIT-CIC, Calle 52 entre 121 y 122, (1900) La Plata, Argentina, [email protected] ABSTRACT The relationship between solidification microstructure and fluidity in MMC was studied. The composites were fabricated by infiltration of liquid metal into a alumina SAFFIL fibers preform under a gas pressure, using alloys of the AlCuSi system as matrices. The fluidity was measured in terms of classic foundry practice (i.e., the distance of flow liquid metal into the preform, while solidifying). The characterization of solidification microstructure in the cast composite was analysed and correlated with the results of fluidity. The attention was particularly focused on such effects as the presence or absence of selective nucleation, the refinement of certain solidifying phases in the presence of fibers and their influence on microstructure formation and segregation of certain elements present in the liquid at the fiber matrix interface. By comparing reinforced and non reinforced zones, it was shown that the presence of fibers resulted in a refinement of the dendritic arm spacing of the a-Al phase, with nucleation of Si on the fibers and without nucleation of primary Al dendrites. The results were discussed and compared with the microstructures and fluidity test of the unreinforced Al-Cu-Si alloys. INTRODUCTION An extensive study of the fluidity and microstructure in the Al-rich comer of the Al-Cu-Si system was carried out by us in a previous work [1], with a vacuum linear device, using information from the phase fields indicated by the equilibrium phase diagram. It is considered that a proper knowledge of these relationships can be used in order to predict the behavior of the
molten metal when infiltration experiments inmetal matrix composites (MMCs) are performed. In contrast to the extensive data available on fluidity of Al-Cu-Si foundry alloys, relatively little is documented about their composites. Among the various techniques for fabricating MMCs, Pressure Infiltration is being currently used to evaluate the fluidity of liquid metals through preforms of short fibers [2]. The fluidity during the liquid metal matrix infiltration of fibers preforms can be discussed in terms of a classic foundry practice (i.e., the length the liquid metal flows in a channel packed with a SAFFIL alumina chopped fiber preform while solidifying). Fundamental principles governing the kinetics of infiltration, the observation of the microstructure of MMCs, micro and macrosegregation and the formation of various kinds of processing defects, etc., have been considered in previous publications[2-8]. The authors of this paper have recently reported [9] the influence of the parameters process such as, applied pressure (P.), fiber preform temperature (Tf), fiber fraction volume (Vf) and melt superheat (AT) on infiltration length of Al-5%Cu, Al-5%Si and Al- 1.7%Si matrix alloys in SAFFIL chopped fiber preform. The interpretation of the deformation of fiber preform during infiltration also have been considered. The results of this study indicated that the
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