Thixoforming of an ECAPed Aluminum A356 Alloy: Microstructure Evolution, Rheological Behavior, and Mechanical Properties
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SEMISOLID metal (SSM) processing is an alternative route to conventional metal shaping processes such as casting and forging. It has been widely exploited since the discovery of the thixotropic behavior of semisolid alloys by Flemings in the early 1970s,[1–3] and to date, a number of components are produced by this technique, primarily in the automotive industry. The advantages of SSM processing include high energy and production efficiency, low forming loads, near-net-shape capability, and reduced porosity. In a practical example, Sim et al.[4] demonstrated its process advantages comparing the traditional die casting of a diesel engine block with a new Advanced Rheocasting Technology system, which allowed die filling without turbulence and consequent pore-free structure. Thixoforming is a common approach for SSM processing. It involves the partial melting of a solid billet, isothermal heat treatment within the semisolid state, and component forming. Most of the research and commercial applications of thixoforming processes have been KAIO NIITSU CAMPO, Ph.D. Student, and EUGEˆNIO JOSE´ ZOQUI, Professor, are with the Materials and Manufacturing Department, School of Mechanical Engineering, University of Campinas UNICAMP, Campinas, SP, 13083-860 Brazil. Contact e-mail: [email protected] Manuscript submitted July 22, 2015. Article published online January 29, 2016 1792—VOLUME 47A, APRIL 2016
performed on aluminum (Al) alloys, mainly the conventional casting A356 and A357 compositions.[3] The amount of silicon (Si) in these alloys (6.5 to 7.5 wt pct) provides a microstructure formed by approximately 50 pct of the primary a-Al phase and 50 pct of the Al–Si eutectic prior to melting. As their chemical composition guarantees this arrangement of phases, which has been considered ideal for good thixoformability,[5] they must be prepared focusing on microstructure refinement. This is because a non-dendritic (globular) microstructure is essential to thixoforming.[1–3] Many globularization-processing routes have been proposed and tested. In a previous paper,[6] the efficiency of four different routes for the production of Al A356 alloy for thixoforming was evaluated. Among all the tested routes, the stress-induced melt-activated (SIMA) one, which combines plastic deformation and recrystallization, yielded the most suitable thixotropic material. In this case, the deformation process utilized was the equal channel angular pressing (ECAP). Unlike the rolling and forging processes, ECAP is a very interesting approach to thixoforming because it is capable of imposing a large homogeneous deformation without modifying the cross section of the material.[7] This process feature enables the simultaneous achievement of both severely deformed and bulk materials. Promising results using ECAP and thixoforming have also been reported by other authors. Zhao et al.[8] determined the tensile mechanical properties of a thixoforged magnesium alloy pretreated with two ECAP METALLURGICAL AND MATERIALS TRANSACTIONS A
passes. The thixoforged s
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