Effect of Solid Fraction and Pressure on Microstructure and Mechanical Properties and Reduction in Liquid Segregation in
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TRODUCTION
DUE to its excellent die filling characteristics, the thixo-die-casting process has attracted wide industrial interest and is considered to be a promising near netshape metal-forming process for future use.[1,2] The thixo-die-casting process has several distinguishing advantages compared to the conventional die-casting process. The relatively high viscosities of thixo slurries reduce turbulent flow during die filling. Consequently, products made by the thixo-die-casting process have fewer defects and higher qualities.[3,4] The partially remaining latent heat of the fusion of thixo slurries also reduces cycling time and decreases the thermal impact imposed on the surface of the die cavity.[1,2,5] In addition, thixo metallic slurries behave differently with superheated molten metal when the temperature is altered. When temperature decreases from a liquidus to solidus line, solid fraction increases from 0 to 1. Accordingly, the viscosity of thixo slurries changes dramatically. Viscosity is also influenced by many other factors, such as the morphology of the primary phases, the shearing rate, and the agitation history of the slurry. Therefore, it is of great importance to find the optimal conditions for the thixo-die-casting process. The thixo-die-casting process has been investigated extensively in recent years. Cho et al.[6] have investigated the effect of die temperature on the thixo-die-casting process. It was found that a high die temperature not only makes it difficult to lock the die, but also decreases C.G. KANG, Professor, School of Mechanical Engineering, National Research Labratory of Thixo/Rheo Forming, and S.M. LEE, Assistant Professor, Engineering Research Center for Net Shape and Die Manufacturing, are with the Pusan National University, Pusan 609-735, South Korea. Contact e-mail: [email protected] Manuscript submitted June 18, 2007. Article published online March 7, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
surface hardness of the cast products. Midson et al.[7] examined the effect of gate size and location with which semisolid A357 alloy filled a plate-shaped cavity. Sauermann et al.[8] have reported the microstructure characteristics of thixo die casting with an aluminum-lithium alloy and thus demonstrated the great feasibility of semisolid die casting. Pola et al.[9] conducted an experimental study of alloy processing by means of an instrumental crucible in order to verify thermal simulation. Forn et al.[10] studied the influence of heat treatments on the anodic oxidation layer formed on A357 aluminum alloy in the die-casting process. To optimize the design of the filling system, Yitao et al.[11] performed a computer simulation and introduced a new method to prevent defects arising from production of automotive parts by using semisolid die casting. It was proven that it is important to control the width, thickness, and location of the gate during the thixo-die-casting process. Kang et al.[12] illustrated that a larger gate results in smoother die filling. In addition, Kang et al.[13] investigated
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