Microstructural Evolution in AlMgSi Alloys during Solidification under Electromagnetic Stirring

Abstract

Equiaxed solidification of AlMgSi alloys with Fe and Mn was studied by electromagnetic stirring to understand the effect of forced flow. The specimens solidified with a low cooling rate, low temperature gradient, and forced convection. Stirring induced by a coil system around the specimens caused a transformation from equiaxed dendritic to rosette morphology with minor dendrites and, occasionally, spheroids. This evolution was quantitatively observed with specific surface S v . The precipitation sequence of the phases was calculated using the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) technique. Melt flow decreased secondary dendrite arm spacing λ 2 in the AlSi5Fe1.0 alloy, while λ 2 increased slightly in Mg-containing alloys. The length of detrimental β-Al 5 FeSi phases decreased only in AlSi5Fe1.0 alloy under stirring, whereas in Mg-containing alloys, changes to the β-Al 5 FeSi phase were negligible; however, in all specimens, the number density increased. The modification of Mn-rich phases, spacing of eutectics, and Mg 2 Si phases was analyzed. It was found that the occurrence of Mg 2 Si phase regions reduced fluid flow in the late stages of solidification and, consequentially, reduced shortening of β-Al 5 FeSi, diminished secondary arm-ripening caused by forced convection, and supported diffusive ripening. However, the Mg 2 Si phase was found to have not disturbed stirring in the early stage of solidification, and transformation from dendrites to rosettes was unaffected.

Mushy Zone Morphology Calculation with Application of CALPHAD Technique

“…According to [26], we may distinguish Mn phases with different morphologies: cube, colony of cubes, tree shaped, platelet shaped, and rib shaped phases. In addition, the formation of other phases caused by the presence of additional alloy components (e.g., Mg) might decrease the fluidity of the solidifying melt [27].…”

Relationship Between Aluminum-Rich/Intermetallic Phases and Microhardness of a Horizontally Solidified AlSiMgFe Alloy

“…This hardening occurs through the formation of barriers that hinder the movement of dislocations. It is highlighted that the Al-(5 to 12)wt%Si casting alloys with addition of magnesium ranging from 0.2 to 0.5 wt% allows the hardening of the matrix by precipitation of second phases, with emphasis to the Mg 2 Si phase, after solution and artificial aging heat treatments [29][30][31][32] .…”

Thermo-Calc Prediction of Mushy Zone in AlSiFeMn Alloys

“…Iron intermetallics form platelets [22] with various dimensions from a few micrometers to 1 mm, while Mn-rich phases [34] may take a shape ranging from compacted cubes (100 µm) to a large complex colony of cubes reaching above 1 mm. The fluidity of the solidifying melt might be decreased by the growth of other phases, e.g., Mg 2 Si in Mg containing alloys [35]. In current study by G = 3 K/mm, v = 0.04 mm/s and cooling rate R = 0.12 K/s only crystalline phases are considered, but flow conditions can change significantly by glassy structure for some alloys like Pd 40 Cu 30 N 10 P 20 [36], where by also lower cooling rate (0.016 K/s, Table 4 in [36]) amorphous phases can precipitate.…”