Abstract
Iron (Fe) is the most common and the most detrimental impurity element in Al alloys due to the formation of Fe-containing intermetallic compounds (IMCs), which are harmful to mechanical performance of the Al-alloy components. In this paper we investigate the formation of Fe-containing IMCs during solidification of an Al-5Mg-2Si-0.7Mn-1.1Fe alloy under varied solidification conditions. We found that the primary Fe-containing intermetallic compound (P-IMC) in the alloy is the BCC α-Al15(Fe,Mn)3Si2 phase and has a polyhedral morphology with {1 1 0} surface termination. The formation of the P-IMCs can be easily suppressed by increasing the melt superheat and/or cooling rate, suggesting that the nucleation of the α-Al15(Fe,Mn)3Si2 phase is difficult. In addition, we found that the IMCs with a Chinese script morphology is initiated on the {1 0 0} surfaces of the P-IMCs during the binary eutectic reaction with the α-Al phase. Both the binary and ternary eutectic IMCs are also identified as the BCC α-Al15(Fe,Mn)3Si2 phase. Furthermore, we found that the Fe content increases and the Mn content decreases in the Fe-containing intermetallic compounds with the decrease of the formation temperature, although the sum of the Fe and Mn contents in all of the IMCs is constant.
Highlights
IRON (Fe) is the most common and undesirable impurity element in Al alloys
We have developed a holistic approach to mitigate the harmful effect of Fe impurity in Al alloys; one is to develop an effective Fe-removal technique based on intensive shearing to reduce the excessive Fe contents to a minimum,[24] the other is to refine the Fe-bearing intermetallic compounds (IMCs) through enhanced heterogeneous nucleation of the IMCs so that the resultant IMCs have a refined size, compact morphology and uniform distribution in the alloy matrix.[25]
Both the primary Fe-containing intermetallic compound (P-IMC) and BE- IMCs were suppressed when the alloy was solidified at a cooling rate of ~ 10 K/s using the twin-roll caster (TRC) process; and only ternary eutectic IMC (TE-IMC) were found in the ternary eutectic regions located in the inter-dendritic regions of the a-Al phase (Figure 10)
Summary
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. 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.[4,5,6,7,8,9] Among these IMCs, b-AlFeSi and a-AlFeSi are the most common ones present in Al-Si based alloys widely used for automotive components.[10,11,12] Research effort has been focused on these two types of IMCs to control their formation during solidification 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. We investigate the formation of Fe-rich intermetallic compounds in a newly developed Al-5Mg-2Si-0.7Mn-1.1Fe alloy for automotive applications
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