Effects of metallurgical parameters on the decomposition of pi-AlFeMgSi phase in Al-Si-Mg alloys and its influence on the mechanical properties

The formation of the pi-AlFeMgSi iron intermetallic phase in Al-Si-Mg alloys is known for its detrimental effect on ductility and strength, in that it is controlled by the Fe and Mg content of the alloy, as well as by the cooling rate. The current study was carried out with a view to investigating all the metallurgical parameters affecting the formation of the pi-phase iron intermetallic and, in turn, the role of the pi-phase as it relates to the tensile and impact properties of Al-Si-Mg alloys.;The effects of different solution treatment times on the decomposition of the pi-phase were investigated in order to examine how this type of decomposition affected the chemistry of the matrix itself. After 8 hours of solution heat treatment and at Mg content of 0.4wt%, the pi-phase showed complete decomposition into fine beta-phase needles. The a-phase, however, showed only partial decomposition into beta-AlFeSi phase needles at Mg levels of over 0.4%wt. This type of decomposition was examined for the purposes of this study over extended periods of solution heat treatment time in Al-7Si-0.55Mg-0.1Fe alloy samples obtained at different cooling rates in order to evaluate the mechanism of pi to beta-phase decomposition. The results obtained show that the volume fraction of pi-AlFeMgSi phase decreases significantly at prolonged solution treatment times. The highest amount of the newly-formed beta-phase was observed in the solution treatment time range of 60 to 80 hours. An analysis of the chemical composition of the matrix using wavelength-dispersive spectroscopy (WDS) at different stages of solution heat treatment revealed that the pi to beta-phase decomposition during solution heat treatment results in a distinct increase in the Mg content of the matrix. Furthermore, no changes were observed in the calculated stoichiometries of the pi-phase or the beta-phase intermetallics during solution treatment in all the alloy samples studied.;The study also investigated the decomposition of pi-AlFeMgSi into beta-phase needles during extended periods of solution heat treatment and its effects on the mechanical properties of Al-7Si-0.55Mg-0.1Fe alloys. The results obtained from the calculated quality index values show that the optimum solution treatment time for Sr-modified alloys is of the order of 12 hours. Using prolonged solution treatment time leads to the decomposition of a large amount of pi-phase into beta-phase needles, approximately 85%, thereby providing a slight improvement in the tensile properties at 80 hrs compared to standard heat treatment times; this improvement may be attributed to the increased amount of Mg in the matrix resulting from the decomposition of the pi-phase, and which is then available for precipitation as Mg2Si upon subsequent aging.;An analysis of the results obtained from the Charpy impact test using unnotched samples shows that the greatest improvement in the initiation and propagation energies is obtained for the as-cast and heat-treated alloys when these alloys are solidified at a low cooling rate and modified with strontium. An increase in the solution treatment time improves the impact properties of the alloys compared to the as-cast condition. In accordance with this finding, the recommended solution treatment time at which the maximum initiation and propagation energy values can be obtained is 20 hours for all alloys studied. The results also show that the impact properties are more sensitive to the changes occurring in the microstructure which result from solution heat treatment and Sr modification, namely, the eutectic Si and pi-phase morphologies, rather than those related to the tensile properties i.e., to the Mg content in the matrix. (Abstract shortened by UMI.);Microstructural assessment was carried out by means of quantitative metallography using electron probe microanalysis (EPMA) and scanning electron microscopy (SEM). The results indicate that increasing the Mg and Fe content increases the amount of the pi-AlMgFeSi phase formed. All the alloys containing low levels of iron regardless of the amount of Mg-content show low amounts of pi-phase iron intermetallic. The addition of trace amounts of Be has an observable effect in reducing the amount of the pi-phase formed in all the alloys studied. The pi-phase iron intermetallic particles appear to be segregated away from the modified Si in the Sr-modified alloys, particularly those solidified at a low cooling rate.