Formation of the Fe-Containing Intermetallic Compounds during Solidification of Al-5Mg-2Si-0.7Mn-1.1Fe Alloy
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TRODUCTION
IRON (Fe) is the most common and undesirable impurity element in Al alloys. Fe is inevitably picked up during the fabrication and recycling processes of Al alloys. Fe-rich intermetallic compounds (IMCs) are easily formed during solidification processing due to the low solid solubility of Fe in Al. The presence of Fe-rich IMCs is one of the main reasons for deterioration of the mechanical properties of cast aluminum alloys.[1,2] Significant effort has been made to remove Fe or reduce its detrimental effect on mechanical properties. Previous work[3] has revealed that the concentration of Fe can be reduced relatively easily down to 0.7 wt pct in recycled Al alloys. However, it is considerably more difficult to reduce the Fe concentration further. Therefore, to minimize the detrimental effect of the remaining Fe in recycled Al alloys, it is desirable to understand the nature of the Fe-containing IMCs and their formation
ZHONGPING QUE, YUN WANG, and ZHONGYUN FAN are with the BCAST, Brunel University London, Uxbridge, Middlesex, UB8 3PH, UK. Contact e-mail: [email protected] Manuscript submitted November 23, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanisms during solidification so that their morphology, size and distribution can be effectively controlled. During the past few decades, more than 20 different Fe-containing IMCs have been reported to exist in various Al alloys, including b-Al5FeSi (b-AlFeSi for short), a-Al15(Fe,Mn)3Si2 (a-AlFeSi for short), Al13Fe4 and Al6Fe, and so on.[4–9] Among these IMCs, b-AlFeSi and a-AlFeSi are the most common ones present in Al-Si based alloys widely used for automotive components.[10–12] Research effort has been focused on these two types of IMCs to control their formation during solidification. Technologically, physical processes, such as plastic deformation,[13] ultrasonic vibration[14] and electromagnetic stirring,[15] were employed to break up and/or refine such Fe-containing IMCs with some degree of success. Plastic deformation during thermomechanical processing resulted in aligned IMCs with a reduced particle size, from a few tens of microns to a few microns.[13] Ultrasonic vibration[14] and electromagnetic stirring[15] provide intensive forced convection during solidification processing and have been reported to be beneficial to the phase transition from the platelet-like b-AlFeSi to the more compact a-AlFeSi or Chinese script a-AlFeSi. However, the majority of the prior research has been concentrated on elemental additions (e.g., Mn, Cr, Co, Sr, Li, and K) to modify the morphology of the Fe-containing IMCs, from platelet to polyhedral, in order to reduce their detrimental effect
on mechanical properties.[16–21]. For instance, Mn addition with the Fe/Mn ratio lower than 2:1 can promote the transition from platelet b-AlFeSi to polyhedral a-AlFeSi. To further minimize the detrimental effect of the IMCs, the size of the a-AlFeSi phase must be refined and its morphology modified to be more compact. Few studies have been carried out to significantly refine the a-AlF
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