The Influence of Ultrasonic Cavitation on the Formation of Fe-Rich Intermetallics in A383 Alloy
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UCTION
AL-SI-CU alloys have been widely used in the automotive industry because of their good castability and high strength-to-weight ratio.[1] Iron is a common impurity and is regarded as the most detrimental element that lessens the mechanical properties of Al-Si-Cu alloys.[2,3] Iron does have the positive effect of preventing die soldering during casting processes.[4,5] Depending on the cooling rate and amount of Fe and Mn, Fe-rich intermetallics may form as pre-dendritic, pre-eutectic, co-eutectic and post-eutectic phases during the solidification process.[6] The Fe-rich intermetallics that precipitate before the formation of the Al dendritic phase are called pre-dendritic phases. These Fe-rich intermetallics have polyhedral morphology. According to Fabrizi,[7] the 3D morphology of pre-dendritic Fe-rich intermetallics corresponds to a regular rhombic dodecahedron. The pre-dendritic Fe-rich intermetallics can cause severe stress concentration at the matrix/ intermetallics interface because of their faceted
YANG XUAN, TAO LIU, LAURENTIU NASTAC, and LUKE BREWER are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487. Contact e-mail: [email protected] ILYA LEVIN and VISH ARVIKAR are with Nemak Alabama, Sylacauga, AL 35150. Manuscript submitted January 26, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
geometry. The Fe-rich intermetallics that precipitate after the formation of the primary Al dendrites are called post-dendritic phases, and the most common phases are the a-Fe (Al15(Fe, Mn)3Si2) phases with Chinese-script morphologies[8] and b-Fe (Al5FeSi) intermetallics, which have long, needle-like shapes.[9] These post-dendritic Fe-rich phases can block the feeding channels of the interdendritic network, resulting in the formation of a large number of porosities and shrinkage in the cast Al alloy, which in turn will significantly decrease its ductility. The platelet b-Fe phases, which are hard and brittle and have a relatively low bonding strength with the matrix Al, are regarded as the most detrimental of the Fe-rich intermetallics, and they can act as stress concentrators, which result in the brittleness of these cast Al alloys.[10] In this study, we will follow the literature conventions and use the term ‘‘sludge’’ for the pre-dendritic Fe-rich intermetallics phase,[11,12] and the term ‘‘a-Fe’’ and ‘‘b-Fe’’ will be used for the post-dendritic a-Fe phase and b-Fe phase, respectively. It is important to control the number and morphology of the Fe-rich intermetallics in Al-Si-Cu alloys to improve their mechanical performance. It has been indicated that a rapid solidification rate or the addition of Sr will promote the formation of the more compact, less harmful a-Fe phase rather than b-Fe intermetallics.[13] Basak et al.[14] introduced a gravitational segregation method to eliminate b-Fe intermetallics in Al-Si alloys. Some other physical segregation
techniques, such as magnetic segregation and eddy current segregation, are also used in industry to mini
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