The solidification characteristics of Fe-rich intermetallics in Al-11.5Si-0.4Mg cast alloys
- PDF / 4,136,150 Bytes
- 11 Pages / 606.24 x 786 pts Page_size
- 89 Downloads / 179 Views
25/3/04
11:28
Page 1425
The Solidification Characteristics of Fe-Rich Intermetallics in Al-11.5Si-0.4Mg Cast Alloys X. CAO and J. CAMPBELL After the nucleation and sedimentation of primary Fe-rich phases, the microstructures of Al-11.5Si-0.4Mg cast alloys with 0.35-1.03Fe and 0.18-0.59Mn have been studied to investigate the solidification characteristics of Fe-rich phases. Depending on the iron and manganese contents as well as cooling rates, Fe-rich phases may solidify as predendritic (primary), pre-eutectic, coeutectic, and posteutectic intermetallics at the different stages of solidification through three types of reactions: (1) predendritic (primary), (2) eutectic, and (3) peritectic reactions. It seems that Fe-rich phases may nucleate on the wetted sides of double oxide films, while the gap of the dry sides of oxide films constitutes the cracks commonly observed in the Fe-rich phases and aluminum matrix. Conventional metallurgical observations also suggest that the Fe-rich phases nucleated early during the solidification might act as nuclei for those formed subsequently, although it has not been ruled out that these phases may share the same oxide substrates. It is probable that these nucleation events may all work as suggested in the possible nucleation hierarchy for Al-11.5Si0.4Mg cast alloys.
I. INTRODUCTION
CASTING quality is primarily influenced by the casting processes and subsequent treatments such as hot isostatic pressing, heat treatment, etc. Casting techniques developed to improve the quality of cast aluminum alloys can be categorized as (1) control of the liquid metal quality prior to casting, (2) control of the pouring of liquid metal into a mold, and (3) control of casting microstructures and defects during solidification. Because of the commercial and technological importance of cast aluminum alloys, these processes have been the subject of extensive research for several decades aiming at improving the quality of cast alloys. Liquid aluminum is usually laden with oxide films. The films are frozen into castings as cracks. The entrained oxide cracks are a source of concern because they are often not detectable by normal nondestructive testing techniques but are highly damaging to casting quality.[1] For many years, the separation and removal of oxide films from aluminum melts has been a key research field. Basically, there are four techniques currently used to clean molten aluminum alloys: sedimentation, fluxing, degassing (flotation), and filtration.[2,3] Success, however, using these conventional methods has been mixed.[4,5] A new technique, termed the heat treatment of liquid metal (sedimentation of primary intermetallic compounds from melts), has been explored to remove both oxide films and primary intermetallic compounds from the aluminum melt.[4–10] The concept was to remove oxide crack defects from liquid metal by the nucleation of primary intermetallic compounds on the oxide films, causing them to sink under
X. CAO, formerly PhD Student, Department of Metallurgy and Materials, Univer
Data Loading...
