Thixoforging of Wrought Aluminum Thin Plates with Microchannels
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INTRODUCTION
SEMISOLID metal (SSM) processing is a near-netshape manufacturing process that is conducted in the mushy zone of alloys. Ideally, feedstock for the SSM processing comprises non-dendritic and globular primary grains, homogenously surrounded by a liquid matrix; such feedstock is generally referred to as a semisolid slurry. The semisolid slurry exhibits a thixotropic behavior, i.e., it flows under shear force; however, it is stable under its own weight when the shear force is removed.[1–3] Semisolid slurries of metallic alloys possess higher viscosities than molten metal. Therefore, when the semisolid slurry is injected into a die cavity, the macroscopic turbulence during die filling is significantly reduced compared to conventional casting processes.[3,4] Consequently, air-entrapment defects are reduced in, or even eliminated from, the resulting castings. This is a major advantage of SSM-processed components over conventional castings. AMIR BOLOURI, Ph. D. Candidate, is with the Graduate School of Mechanical Engineering, Pusan National University, Busan 609735, Republic of Korea. CHUNG-GIL KANG, Professor, is with the School of Mechanical Engineering, Pusan National University, Busan 609-735, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted June 24, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
Furthermore, the presence of a fraction of the solid phase prior to the forming process reduces volume loss during solidification, which, in turn, reduces the solidification shrinkage porosity.[5] These unique characteristics mean that the SSM processing has substantial potential for manufacturing high-quality components with thinner cross sections that are possible using conventional casting techniques.[6–8] The SSM processing proceeds mainly by two routes: the rheo-route and the thixo-route. In the rheo-route, a molten metal is continuously stirred and cooled to obtain the semisolid slurry, which is then shaped inside the die cavity.[9–12] Alternatively, new rheocasting (NRC),[13] swirled enthalpy equilibration device (SEED),[14] and gas induced semisolid (GISS)[15] are based on nucleation enhancing mechanisms. More recently, semisolid rheorolling has been developed. In this method, the semisolid slurry is continuously prepared through a sloping plate and is rolled to fabricate strips.[16,17] In the thixo-route, a solid billet is rapidly heated into the mushy zone of the alloy to obtain the desired semisolid slurry. Subsequently, it is immediately formed inside a die cavity[18–22] or forming is done after some isothermal holdings.[23] For wrought alloys, the initial billet used for the thixo-route is usually an extrusion billet that is warm worked below the recrystallization temperature of the alloy. When the
extrusion billet is heated to achieve the semisolid state in the alloys, recrystallization and partial remelting (RAP) simultaneously occur. Subsequently, the liquid phase penetrates the grain boundaries of the newly recrystallized and equiaxed grains and forms the desired semiso
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