Solidification Behavior of Intensively Sheared Hypoeutectic Al-Si Alloy Liquid
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AL-SI casting alloys are being used increasingly in the automotive and aerospace industries for critical structural applications because of their low density, excellent castability, weldability, corrosion resistance and, in particular, good tensile and fatigue properties.[1] The presence of silicon in these cast alloys provides a high fluidity imparted by the presence of a relatively large volume of Al-Si eutectic, which reduces shrinkage during solidification as well as the coefficient of thermal expansion of the cast products.[2] The mechanical properties of Al-Si alloys can be greatly altered by changing the microstructure of the primary a-Al and the eutectic phase, which can be controlled by the solidification process. Al-Si alloys are commonly fabricated by a highpressure die cast (HPDC) process because of their high fluidity and short solidification temperature range. The HPDC process has been explored extensively to meet the demands of modern automobile industries for efficient production and low cost.[3] The microstructure of HPDC produced samples is complex, and influenced by many factors associated with liquid and semisolid melt processing, flow as well as heat-transfer phenomena.[4–6] The general microstructure of HPDC hypoeutectic Al-Si alloy consists of a dendritic primary a-Al solid solution surrounded by eutectic regions. The complex geometry of these microstructural features, their locations, and their arrangements are often H.R. KOTADIA, formerly Ph.D. Student, BCAST, Brunel University, Uxbridge, Middlesex UB8 3PH, UK, is now Post-Doctoral Researcher, Department of Mechanical Engineering, King’s College London, Strand, London WC2R 2LS, UK. Contact e-mail: hiren. [email protected] N. HARI BABU, Lecturer, is with BCAST, Brunel University. H. ZHANG, formerly Post-Doctoral Researcher, BCAST, Brunel University, is now Assistant Professor, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P.R. China. S. ARUMUGANATHAR, formerly Post-Doctoral Researcher, BCAST, Brunel University, is now Post-Doctoral Researcher, School of Engineering and Computing, Coventry University, Priory Street, Coventry CV1 5FB, UK, and Z. FAN, Professor, is with BCAST, Brunel University. Manuscript submitted December 8, 2009. Article published online November 5, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
nonuniform, and usually a strong spatial correlation exists between microstructural features and mechanical properties. A fine and uniform microstructure with minimum casting defects are important to obtain better mechanical properties. Continuous research has been undertaken over the past few years to enhance the mechanical properties by changing the solidification conditions and the mechanisms responsible for the formation of casting defects such as defect bands. A defect band is a common feature observed in HPDC aluminum and magnesium alloys, particularly for thinwalled castings. A defect band is characterized by a region consisting of a higher concentration of the eutectic phase, interdendritic porosity,
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