Decomposition of Fe-intermetallics in Sr-modified cast 6XXX type aluminum alloys for automotive skin

Abstract

The present study was carried out on two 6XXX type aluminum alloys: an experimental alloy and commercial 6111 alloy, containing Fe, Si, Mn, and Mg as the principal elements, and with Fe/Si ratios of 0.28 and 0.49, respectively. Measured amounts of Sr (up to 1500 ppm) were added to these alloys. The alloy melts were cast in a graphite mold heated at 600 °C (cooling rate ∼0.7 °C s−1). Some of the castings were homogenized for 8 hours at 540 °C. The microstructural constituents were evaluated using thermal analysis and metallographic techniques. The various phases observed were identified using electron probe microanalysis (EPMA) and energy dispersive X-ray (EDX) analysis. Results show that, when the Fe/free Si ratio is <1 (free Si being the Si remaining above that required for Mg2Si formation), addition of Sr (300 to 500 ppm) destabilizes the β-Al5FeSi phase platelets, leading to rejection of Si ahead of the β-platelet/Al matrix interface. Increasing the concentration of Sr (∼800 ppm) leads to the precipitation of a new intermetallic, possibly Al2Si2Sr (in the form of short needles or rods). The iron can react with the Al to form Al6Fe, or with the remaining Si to precipitate as α-Al8Fe2Si in Chinese script form. A new mechanism for the formation of Al2Si2Sr phase has been proposed, based on the possibility that the rejected Si ahead of the destablized β-Al5FeSi platelets reacts with the surrounding Sr and Al to form polygonal (rounded or squarelike) particles of Al2Si2Sr. These particles are normally observed attached to the decomposed β phase (or Al6Fe). During homogenization treatment, the β-Al5FeSi decomposes into Al6Fe and Si. The decomposition takes place much more easily in the Sr-containing alloys. In contrast, both Al2Si2Sr and Al8Fe2Si phases are stable at temperatures as high as 540 °C. The introduction of Mn and Mg to the Al-Fe-Si system (viz., 6111 alloy, with a Fe/free Si ratio of 1.55) results in the precipitation of several Chinese script phases, namely, Al15(Fe,Mn)3Si2, Al8(Fe,Mn)2Si, Al8Mg3FeSi6, and Mg2Si. Compared to the β-Al5FeSi phase, the compositions of these script phases are more stable in the presence of Sr. At higher Sr levels (∼800 ppm or more), partial decomposition of α-Al15(Fe,Mn)3Si2 into Al6.9(Fe,Mn)Si and Si can occur. Fragmentation of the α-Al15(Fe,Mn)3Si2 phase takes place only at Sr concentrations on the order of ∼1500 ppm.