Study on the Microstructure and Liquid Phase Formation in a Semisolid Gray Cast Iron
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SEMISOLID processes use metal alloys in the semisolid state (i.e., between the solidus and liquidus temperatures) to exploit the thixotropic behavior of these materials, which is characterized by a time-varying dependency on shear rate. For the material to exhibit thixotropic behavior, the microstructure must consist of globular particles of solid phase immersed in a liquid matrix (non-dendritic microstructure).[1,2] As described by Atkinson,[2] ‘‘semisolid processing covers a family of processes and generally is divided into two groups: rheocasting and thixoforming. Rheocasting
DAVI MUNHOZ BENATI and EUGENIO JOSE´ ZOQUI are with the Department of Materials and Manufacturing Engineering, School of Mechanical Engineering, University of Campinas, Campinas, Sa˜o Paulo 13083860, Brazil. Contact e-mail: [email protected] KAZUHIRO ITO, KAZUYUKI KOHAMA and HAJIME YAMAMOTO are with the Department of Welding Mechanism, Research Division of Materials Joining Mechanism, Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 5670047, Japan. Manuscript submitted December 20, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
refers to the processes where the alloy is cooled into the semisolid state and injected into a die without an intermediate solidification step—the non-dendritic microstructure can be obtained by a variety of means during cooling. Thixoforming refers to the processes where an intermediate solidification step does occur. The alloy is solid and has been treated in such a way (e.g., grain refinement) that when it is heated into the semisolid state it will have a non-dendritic microstructure.’’ It has been reported in several studies that solid particles can agglomerate even for moderate solid fractions, resulting in a more or less connected skeleton while the liquid phase may be entrapped in the solid phase or spatially continuous and free to flow.[2,3] Factors such as liquid fraction, solid-phase morphology (how globular the particles are), solid-phase distribution (the structure of the solid skeleton) and liquid-phase distribution around the solid particles play an essential role during semisolid processing as they determine the thixotropic properties of the metal alloy in the semisolid state, i.e., its flow behavior as it is formed.[1–3] Thus, understanding the phase transformations that lead to the microstructure observed in the
semisolid state and how this microstructure is linked to the performance of the material during semisolid processing is key to the development of high-quality semisolid raw materials. The solidification behavior of nearly all industrial alloys is now well defined as CALPHAD methods can reliably predict phase transformations during solidification. However, semisolid processing also requires an understanding of the microstructural evolution during heating to the semisolid state, i.e., the melting behavior of the material. In recent years, several in situ techniques have been successfully used to study the microstructural evolution of semisolid alloys.[4–7] High-
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