Prevention of macrosegregation in squeeze casting of an Al-4.5 wt pct Cu alloy
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I.
INTRODUCTION
THE squeeze casting process, combining the advantages of the casting and forging processes, has been widely used to produce quality castings. Because of the high pressure applied during solidification, porosities caused by both gas and shrinkage can be prevented or eliminated. The cooling rate of the casting can be increased by applying high pressure during solidification, since thermal contact between the casting and the die is improved by pressurization, which results in the formation of fine-grained structures. However, macrosegregation has been known to be easily formed in most squeeze castings, which leads to nonuniform macrostructures and mechanical properties. It is generally considered that pressurization during solidification prevents the formation of shrinkage defects. However, it enhances the formation of macrosegregates in squeeze castings of aluminum alloys.[1,2,3] Formation of macrosegregates in castings or ingots has been reported to be caused by interdendritic fluid flow, which is driven by solidification contraction, differences in density, etc.[4,5,6] Extensive efforts have been made to investigate how heat transfer characteristics affect the solidification behavior of aluminum alloys in squeeze casting.[3,7,8] Some qualitative explanations on the formation of channel macrosegregates have been made in the squeeze casting of an Al-4.5 wt pct Cu alloy.[1,2,8] Attention has also been given to the exploration of practical methods of eliminating casting defects in squeeze castings.[8–12] However, it is generally considered difficult to prevent the formation of macrosegregates in squeeze casting, since the condition for producing sound castings without the formation of macrosegregates and shrinkage defects is very limited. No quantitative work has reported on the control of solidification characteristics to C.P. HONG, Professor, H.F. SHEN, Research Fellow, and I.S. CHO, Graduate Student, are with the Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Korea. Manuscript submitted May 12, 1997.
METALLURGICAL AND MATERIALS TRANSACTIONS A
prevent the formation of macrosegregates in the squeeze casting process. The purpose of the present study is to investigate the conditions for the formation of the macrodefects, such as macrosegregates and shrinkage defects in squeeze castings, and also to explore optimum process conditions for obtaining sound castings. The squeeze casting of an Al-4.5 wt pct Cu alloy was performed to evaluate the correlation of macrosegregation with the formation of shrinkage defects. The effects of the process parameters, such as applied pressure, pouring temperature, die temperature, delay time for pressurization, and humidity, on the formation of macrodefects were investigated in order to provide practical guidance for optimizing the squeeze casting process. II.
EXPERIMENTAL
An Al-4.5 wt pct Cu alloy was used as the casting material. After melting in a high frequency induction furnace, liquid metal with a certain superheat was poured into th
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