Abstract
Al–Mg–Si based alloys can provide super ductility to satisfy the demands of thin wall castings in the application of automotive structure. In this work, the effect of iron on the microstructure and mechanical properties of the Al–Mg–Si diecast alloys with different Mn concentrations is investigated. The CALPHAD (acronym of Calculation of Phase Diagrams) modelling with the thermodynamic properties of the multi-component Al–Mg–Si–Mn–Fe and Al–Mg–Si–Fe systems is carried out to understand the role of alloying on the formation of different primary Fe-rich intermetallic compounds. The results showed that the Fe-rich intermetallic phases precipitate in two solidification stages in the high pressure die casting process: one is in the shot sleeve and the other is in the die cavity, resulting in the different morphologies and sizes. In the Al–Mg–Si–Mn alloys, the Fe-rich intermetallic phase formed in the shot sleeve exhibited coarse compact morphology and those formed in the die cavity were fine compact particles. Although with different morphologies, the compact intermetallics were identified as the same α-AlFeMnSi phase with typical composition of Al24(Fe,Mn)6Si2. With increased Fe content, β-AlFe was found in the microstructure with a long needle-shaped morphology, which was identified as Al13(Fe,Mn)4Si0.25. In the Al–Mg–Si alloy, the identified Fe-rich intermetallics included the compact α-AlFeSi phase with typical composition of Al8Fe2Si and the needle-shaped β-AlFe phase with typical composition of Al13Fe4. Generally, the existence of iron in the alloy slightly increases the yield strength, but significantly reduces the elongation. The ultimate tensile strength maintains at similar levels when Fe contents is less than 0.5wt%, but decreases significantly with the further increased Fe concentration in the alloys. CALPHAD modelling shows that the addition of Mn enlarges the Fe tolerance for the formation of α-AlFeMnSi intermetallics and suppresses the formation of β-AlFe phase in the Al–Mg–Si alloys, and thus improves their mechanical properties.
Highlights
In Al–Mg–Si based alloys that can provide super ductility in castings [1], iron is a common impurity element but it is unavoidably picked up during melting and casting, and when the scraped and recycled materials are used
In order to simplifying the explanation, the solidification in the shot sleeve is described as first solidification and the solidification in the die cavity is described as secondary solidification
AlFeMnSi intermetallics can be formed in two solidification stages during high pressure die casting (HPDC), one is in the shot sleeve and the other is in the die cavity, which show the compact morphology with different sizes; (2) manganese increases the Fe content range over which a-AlFeMnSi phase forms; (3) the Mn/Fe ratio at 0.5 is applicable to form b-AlFe intermetallics in the Al–Mg–Si–Mn alloy during HPDC; (4) no b-Al5FeSi and b-AlFeSi intermetallics are observed in the Al–Mg–Si diecast alloys with varied Mn and Fe contents
Summary
In Al–Mg–Si based alloys that can provide super ductility in castings [1], iron is a common impurity element but it is unavoidably picked up during melting and casting, and when the scraped and recycled materials are used. The effect of Fe-rich phases on the mechanical properties of aluminium alloys depends on their type, size and amount in the microstructure. The a-Al8Fe2Si phase has been reported as the compounds with many different types of morphology [10,11]. The morphological changes from plate to Chinese script or compact shapes were reported to enhance mechanical properties [12,13]. In hypoeutectic Al–Si alloys containing Fe, Mn and Mg, three Fe-rich phases of a-Al15(FeMn)3Si2, b-Al5FeSi and p-Al8FeMg3Si6 compounds have been identified [8,15]. In the commonly used Al–Si–Mg cast alloys, with a Mn/Fe ratio of 0.5, the structure of Fe-rich intermetallics is body centred cubic a-Al15(FeMn)3Si2 [16,17], which may appear as hexagonal, star-like, or dendritic crystals at different Mn/Fe ratios [18]. In the various results from previous studies, the effect of Fe on the microstructure is in good
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