Thixoforming A356 Aluminum Bipolar Plates at High Solid Fractions
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SEMISOLID metal (SSM) processing is a processing technique used to manufacture near-net-shape metal components in their mushy zone (i.e., semisolid state). The alloys used in SSM processing must exhibit a wide melting range, which enhances the formation of an essential microstructure before the shaping process that ideally consists of solid, nondendritic metal spheroids in a liquid-phase matrix. Such mixtures are known as semisolid slurries, and they behave as thixotropic materials: when they are under shearing, their viscosities fall and flow, but if the shearing is not applied, they withstand their own weight as a solid. The extraordinary thixotropic behavior of metallic materials was first discovered at the Massachusetts Institute of Technology (MIT),[1] and since then, there has been a very strong drive for extensive research among scientists to apply this phenomenon in a manufacturing process.[2–4] SSM processing offers several advantages over casting or solid-state forming, such as die AMIR BOLOURI, Ph.D. Candidate, and CHANG HYUN JANG, Master’s Student, are with Graduate School of Mechanical and Precision Engineering, Pusan National University, Busan 609-735, Republic of Korea. CHUNG GIL KANG, Professor, is with School of Mechanical Engineering, Pusan National University. Contact e-mail: [email protected] Manuscript submitted January 28, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
filling with a slurry at a lower temperature in comparison with casting that flows in a nonturbulent manner[5–7] and forming at lower pressures compared with the conventional forging process.[5–8] There are two basic routes in SSM processing: rheocasting and thixoforming. Rheocasting refers to a process in which the alloy is initially in the liquid state. The alloy is then cooled down into the semisolid state and is simultaneously stirred to produce the preferred semisolid slurry, which is subsequently injected into a die without an intermediate solidification step. In contrast, the alloy feedstock used in thixoforming is initially a solid. The solid feedstock is then rapidly heated to a predetermined temperature at which it is transformed into semisolid slurry that is finally injected into a die.[9] When the semisolid slurry is injected into a relatively cold die, the final microstructure of the SSM-processed components consists of large primary a-Al grains (average size >60 lm) surrounded by the solidified liquid matrix.[9–17] The characteristics of the surrounding solidified liquid matrix vary depending on the processing conditions. For instance, solidified liquid matrix exhibited eutectic solidification with a coarse eutectic silicon phase when A356 aluminum alloy (a bulk component) was rheocasted by the author group.[11] Furthermore, Liu et al.[12,13] have previously thixoformed a number of aluminum alloys under various conditions. They reported that the primary
a-Al equiaxed grains were surrounded by the instantly quenched liquid phase in the final microstructures of the thixoformed components.[12,13] In addition, Fan et al.[14
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