Microsegregation during Solidification of Graphitic Fiber-Reinforced Aluminum Alloys under External Heat Sinks

Abstract

Squeeze casting and melt infiltration were employed in processing continuous graphitic fiberreinforced aluminum matrix composites. The fiber reinforcements were (1) uncoated carbon fiber (UNC-CF), (2) Ni-coated carbon fiber (NiC-CF), and (3) bare graphite fibers (GRFs), and they were externally cooled to enhance the local solidification of the matrix alloy. The solidified microstructures and their composition profiles were examined using optical microscopy, scanning electron microscopy-energy-dispersive X-ray, and electron probe microanalysis-wavelengh-dispersive X-ray. The resultant microstructures in the UNC-CF and NiC-CF-reinforced composites exhibited significant differences from those found in the GRF-reinforced composite, in terms of solidified morphologies and compositions. It was found that coarse columnar dendrites developed in the fiber-free matrix, fine equiaxed dendrites in the chilled matrix, and columnar-like arms in the fiber-reinforced matrices. In contrast, in bare GRF-reinforced composites, two distinct regions were clearly distinguished: (1) a region consisting of coarse equiaxed dendrites in the fiber-free matrix and (2) a featureless morphology within the fiber reinforcement regions. These distinct microstructures were attributed to preferential heat extraction through the GRFs, which possess a relatively high thermal conductivity. Apparently, heat extraction through the GRFs led to the formation of single a-Al envelopes on the fiber surfaces. In addition, the extent of solute segregation found in the GRF-reinforced alloy composite was relatively small when compared with the CF-reinforced alloy composites.

Interface structure in carbon and graphite fiber reinforced 2014 aluminum alloy processed with active fiber cooling

“…Fiber bundles of 40 cm length were longitudinally placed along the parting line of a BN-coated rectangular steel mold and preheated in a resistance furnace as shown in Fig. 1 [13]. Both ends of the reinforcing fibers were extended outside the mold for direct heat extraction from the solidifying melt during infiltration and solidification; the ends of the fiber were in contact with ambient air leading to a high temperature gradient and heat transfer along their lengths [9], possibly enhancing the cooling rate and leading to the formation of the primary phase on the surfaces of cooled fibers.…”

Modeling the Effect of Active Fiber Cooling on the Microstructure of Fiber-Reinforced Metal Matrix Composites

“…Similar segregation effects were also seen in other composite systems. [8] If the inflection point is reached in a short time, the diffusion direction above the inflection point will then provide more favorable conditions in which the nucleation of a-Al can begin close to the fiber surface. The direction of the heat flow is shown clearly in Figure 9 for one of the analyzed conditions.…”

“…[6][7][8] These reinforcing fibers are used for the additional function of extracting the heat from the composite melt in order to control the cooling rate. This modified process cannot eliminate the inherent limitations in the infiltration processes, but it can provide better control over the solidification microstructure during the solidification step and can improve the quality of the resulting composites.…”

Effect of holding pressure on densification and mechanical properties of Cf/Mg composites

“…The contact angle θ between carbon fibres and molten magnesium alloy was more than 90°, so the solidification interface would be bulged, and the tip of the dendrite growth tendency was located in the space between adjacent carbon fibres [30]. When the holding pressure was too high, the damage on the carbon fibres occurred, which directly reduced the mechanical properties of Cf/Mg composite materials [31]. In Fig.…”