# Tailoring Microstructure and Mechanical Properties of the Al-7Si-0.35Mg-0.35Fe Alloy by Cr Addition: A Study on Fe-Rich Phase Modification

## TL;DR

Adding chromium to an aluminum alloy changes harmful iron-rich phases into less damaging forms, improving the alloy's strength and flexibility.

## Contribution

This study demonstrates that chromium addition modifies Fe-rich phases in Al-7Si-0.35Mg-0.35Fe alloy, significantly enhancing mechanical properties.

## Key findings

- Chromium addition refines the secondary dendrite arm spacing in the alloy.
- Fe-rich phases transition from needle-like β-Al5FeSi to short rod-like or blocky α-Al15(Fe,Cr)3Si2 with increasing Cr content.
- The 0.20Cr alloy shows a 19% increase in tensile strength and 107% increase in elongation compared to the 0Cr alloy.

## Abstract

Fe-rich phases are unavoidable intermetallic compounds in aluminum alloys, particularly in recycled aluminum. Their needle-like morphology not only impairs the mechanical performance of the alloy by disrupting the continuity of the matrix but also significantly reduces the allowable addition of recycled aluminum materials. Based on this, this study focuses on the Al-7Si-0.35Mg-0.35Fe alloy with a high Fe content. The Cr was introduced to modify the characteristics of the Fe-rich phase, and the microstructural evolution and mechanical properties of the aluminum alloy with different Cr content (0–0.25 wt.%) were investigated. Experimental results show that the secondary dendrite arm spacing of the alloy is significantly refined after Cr addition. Meanwhile, the Fe-rich phase gradually transitions from β-Al5FeSi with needle-like morphology to α-Al15(Fe,Cr)3Si2 with short rod-like or blocky morphology as the Cr content increases. Notably, the Fe-rich phase in the 0.20Cr alloy exhibits an approximately 65% increase in sphericity and an 84% reduction in equivalent diameter compared to those in the 0Cr alloy. The morphological blunting and dispersed distribution of Fe-rich phases lead to a broad effective Cr addition range of 0.05–0.20 wt% in the alloy. Among them, the 0.20Cr alloy exhibited the best comprehensive mechanical properties, with its ultimate tensile strength and elongation approximately 19% and 107% higher than those of the 0Cr alloy, respectively. Furthermore, the fracture morphology and the relationship between the Fe-rich phase and microcracks in Al-7Si-0.35Mg-0.35Fe alloys with different Cr contents were also analyzed.