Formation of intermetallic phases during solidification in Al-Mg-Mn 5xxx alloys with various Mg levels

In the present study, four Al-Mg-Mn 5xxx alloys with different Mg levels (2-5 wt.%) were investigated for better understanding the evolution of intermetallic phases formed during solidification. Optical and scanning electron microscopes, electron backscattered diffraction and differential scanning calorimetry analyses in combination with thermodynamic calculation were used to identify various intermetallic phases. Results showed that the most dominant intermetallic phases are Al6(Mn,Fe), α-Al(Fe,Mn)Si, Al3Fe, Alm(Mn,Fe) and Mg2Si in experimental Al-Mg-Mn alloys, which is greatly dependant on the Mg levels. It is found that Chinese script α-Al(Fe,Mn)Si is the dominant iron-rich intermetallic phase for the alloys containing 2-3 wt.% Mg, while blocky Al6(Mn,Fe) and needle-like Al3(Mn,Fe) become the major phases for the alloy containing 4 wt.% Mg. Further increasing Mg content to 5 wt. %, the dominant phase transfers to blocky Al6(Mn,Fe) intermetallic. Meanwhile, the morphology of primary Mg2Si is changed from well-branched to plate-like with increasing Mg contents. In addition, β-Al3Mg2 and τ-Al6CuMg4 eutectic phases have been observed in the alloys with 3-5 wt. % Mg. A comparison on various intermetallic phases from the Scheil simulation and the actual as-cast microstructure is provided.

Prediction of Yield Strength at Room Temperature for Squeeze Cast A226

Predicting yield strength of the cast is difficult, mainly due to inherent chemical inhomogeneity of the microstructure and metal matrix composite nature of the cast. In our approach to predict the yield strength of as cast material AlSi9Cu3(Fe), Scheil-Gulliver model has been used to calculate the phase fraction and chemical composition of each phase during solidification and at each temperature step. Inhomogeneity of the microstructure has been taken into account by considering the evolution of pre-eutectic and eutectic fractions separately. The solidification time-temperature data and cooling to room temperature are recorded using thermocouples and serve as input for the thermo-kinetic software “MatCalc”, that has been used for Scheil simulation and takes into account the evolution of microstructure after solidification and during any arbitrary cooling rate. The strengthening model takes into account the contributions of the intrinsic yield strength of the aluminum matrix, solid solution strengthening, precipitation hardening, effect of eutectic silicon portion and dendrite arm spacing size effect. The phases taken in to consideration include α-Al, Intermetallics, Si and Cu-rich precipitates. The predicted yield strength values are validated by comparing with the experimental values. This approach is extendable to calculate yield strength of the as-cast and heat-treated aluminum alloys.

Thermodynamic Aspects of Tin Segregation during Solidification of Aluminium Alloys

When trying to calculate the approximate constitution of as-cast tin containing aluminium alloys one has to cope with a combination of intricacies: (i) Scheil solidification simulation may reflect strong enrichment of alloying components, especially in multicomponent alloys, thus leaving the safe ground of the underlying thermodynamic database. (ii) Liquid demixing often intensifies by addition of many components to Al-Sn alloys, thus forming monotectic reactions, boosting the segregation and aggravating the first effect. (iii) Scheil simulation in multicomponent Al-x-y-z-Sn alloys not only combines the first two problems, moreover, the current versions of major thermodynamic software packages are not able to perform the Scheil simulation if liquid demixing and monotectic reactions occur. These intricacies are worked out and the development of a dedicated Al-Si-Cu-Mg-Sn thermodynamic database for large composition ranges is presented. Calculations are compared to experimental data of an Al-7.5Si-3.5Cu-0.3Mg-0.1Sn alloy and the need for specific follow-up work is identified.