Solidification characteristics of the Al-8.3Fe-0.8V-0.9Si alloy
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I. INTRODUCTION
THE properties of cast Al-Fe-V-Si alloys are determined by the fineness of their microstructures and the distribution of their phases. Two major factors that affect these are the solidification rate and alloy composition.[1] Observable changes in microstructure may occasionally be brought about by chemical treatment of an alloy melt, the notable example being the modification of Al-Si alloys.[2–5] Thermal analysis is a powerful tool for understanding the transformation that occurs during solidification and solid-state cooling. We use this tool, in the present investigation, to attempt to understand the changes in the microstructures and microanalysis of phases in the Al-Fe-V-Si alloys that result due to variations in cooling rates between 1 8C/s and 20 8C/s. These cooling rates approximate the usual sand- and chill-casting situations in the foundries. In addition, the feasibility of modifying the complex intermetallic phases in Al-Fe-V-Si alloys by a magnesium treatment has been examined. Normally, the Al-Fe-V-Si alloys are processed through a rapid solidification–powder compaction route.[6,7] Development of a suitable modification technique may enable the processing of these alloys through the casting route. II. EXPERIMENTAL PROCEDURE The compositions of different alloys investigated in the present study are shown in Table I. The experimental alloys were prepared in an electric resistance heating furnace in a clay-bonded graphite crucible under the cover of a sodiumfree flux (COVERAL 33FF*), which made up 2 pct of the *COVERAL is a trademark of FOSECO (F.S.) Limited, Staffordshire, United Kingdom.
melt. For alloy preparation, Al-28.6 pct Fe, Al-9.85 pct Fe-5.3 pct V, and Al-19.8 pct Si master alloys were used. (Compositions are given in weight percent unless otherwise mentioned.) Weighed quantities of the master alloys were
K.L. SAHOO, Scientist, and C.S. SIVARAMAKRISHNAN, Senior Deputy Director, are with the National Metallurgical Laboratory, Jamshedpur831007, India. A.K. CHAKRABARTI, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur-721302, India. Manuscript submitted January 12, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
charge in the crucible coated with alumina paint and heated to about 600 8C. At this stage, a 99.9 pct pure aluminium ingot (ALCAN, Ontario, Canada) was charged into the crucible. The major impurities present in the aluminium ingot used in the present study were iron (0.05 pct maximum) and silicon (0.04 pct maximum). Traces of copper (0.01 pct maximum), manganese (0.01 pct maximum), and zinc (0.004 pct maximum) were also detected. After melting, sufficient time was given for complete homogenization of the melt. The melt was intermittently agitated with a graphite rod for complete mixing. The melt was then degassed with dry argon (impurities were 2 ppm oxygen, 3 ppm nitrogen, and 0.2 ppm hydrocarbons) introduced through lancing tubes, after the cover flux was skimmed, and subsequently cast into differ
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