Microstructural Evolution, Hardness, and Alligatoring in the Mushy State Rolled Cast Al-4.5Cu Alloy and In-Situ Al4.5Cu-
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INTRODUCTION
THERE is a growing demand for light aluminum alloys and composite components for a variety of automotive and aerospace applications.[1] The discontinuously reinforced aluminum alloy (DRA) matrix composites have been found to be attractive due primarily to the properties of high specific strength and stiffness. However, the DRA composites have so far found only limited application, due to their poor ductility and fracture toughness. In addition, the composites are difficult to machine using the conventional cutting tools, which wear out quickly. Thus, the deformation processing of DRA composites into nearnet shapes in a cost-effective manner is highly challenging. While casting provides an economic route for processing Al alloy-matrix composites, the products often have poor mechanical properties, due to the chemical and microstructural heterogeneities accompanying a dendritic matrix microstructure, a nonuniform distribution of particles, and common casting defects, including shrinkage cavities. Thus, hot working or forming of composites in mushy state (partly liquid state) is required for improvement in mechanical properties through microstructural refinement. Metal MERVIN A. HERBERT, Research Scholar, CHANDAN SARKAR, M. Tech. Student, R. MITRA, Associate Professor, and M. CHAKRABORTY, Professor, are with the Department of Metallurgy and Materials Engineering, Indian Institute of Technology, Kharagpur 721302 West Bengal, India. Contact e-mail: [email protected] Manuscript submitted July 4, 2006. Article published online July 21, 2007. 2110—VOLUME 38A, SEPTEMBER 2007
forming in mushy state has gained importance for the manufacturing of Al alloy-based components,[2] because it requires much lower flow stress than does working by conventional methods,[3,4] implying a reduction in the energy requirements for deformation processing. The presence of liquid phase at the grain boundaries weakens the bond between the individual grains and removes the constraint in deformation, allowing the relative movement of grains. The mechanism of plastic deformation in mushy state is sharply different from that in the conventional hot or cold working processes, since rotation and relative sliding between the grains is possible in the former.[5] The externally imposed strain is accommodated by (1) the elastic plastic deformation of the unmelted grains in contact with one another, (2) the deformation or fragmentation of bulk grains, and (3) the rearrangement of grains through sliding.[6] The compression tests with a constant strain rate have shown a reduction in flow stress with increased liquid content.[6,7] A typical compressive stress-strain curve of the semisolid alloy[6] shows three stages, the first involving macroscopically uniform deformation, the second where deformation is localized, and the third consisting of post-localization softening. Mushy state forming is particularly useful for shaping materials, which are otherwise difficult to form through the conventional methods of metal forming. In the past, research a
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