An Innovative Two-Stage Reheating Process for Wrought Aluminum Alloy During Thixoforming
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INTRODUCTION
THIXOFORMING is a method for the semi-solid processing of aluminum alloys, which typically includes three procedures: metal melting, thermal control within the semi-solid state, and forming.[1] In this case, the prepared material for thixoforming is a partially remelted semi-solid billet with a suitable volume fraction in which the solid phase ideally exhibits a spherical shape.[2] This technique offers practical advantages over traditional casting, including minimal splashing, high production efficiency, and a reduction of porosity and increased formed component density and strength. The thixoforming process can be used for a wide range of applications due to the favorable quality of the resulting products.[3] The key point of thixoforming is the formation of a non-dendritic microstructure during the reheating process at temperatures between the solidus and liquidus, which remarkably affects the subsequent forming process as well as strongly improves the mechanical properties of the finished product. The constitutive behavior of a material during thixoforming is complex, and varies from solid-like character to liquid-like character depending on the shear rate.[4,5] The metal must flow enough to fill a mold, but must also have enough strength to allow the semi-solid slug to JIAOJIAO WANG, Research Assistant, and GUIMIN LU, Professor, are with The Key Laboratory of Pressure Systems and Safety, Ministry of Education, East China University of Science and Technology, Shanghai 200237, P.R. China. Contact e-mail: gmlu@ ecust.edu.cn D. BRABAZON, Professor, is with the School of Mechanical and Manufacturing Engineering, Dublin City University, Dublin 9, Ireland, and also with Advanced Processing Technology Research Centre, Dublin City University, Dublin 9, Ireland. A.B. PHILLION, Assistant Professor, is with the Schools of Engineering, University of British Columbia, Kelowna, BC V1V1V7, Canada. Manuscript submitted October 11, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
maintain its shape under its own weight while it is being transported. The reheating process prior to the forming stage plays a major role in achieving favorable thixotropic behavior during thixoforming. The corresponding grain morphology must be non-dendritic with the solid and liquid phases uniformly distributed throughout the semi-solid state.[6] Although thixoforming is currently used as a commercial process, it has only found real success with Al-Si casting aluminum alloys such as A356 and A357, with strengths between 220 and 260 MPa and 8 to 13 pct elongation.[4] The large eutectic fractions of these alloys aids in the production of the desirable non-dendritic grain morphology in the semi-solid state. At present, there is significant interest in developing the thixoforming process for the production of high-performance wrought aluminum alloys. One of the challenges of casting alloys during thixoforming is grain coarsening. Atkinson and Liu[5] have shown that for wrought alloys, the presence of precipitates and dispersoids at grain boundari
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