Mushy Zone Morphology Calculation with Application of CALPHAD Technique

Abstract

Mushy zone morphology in AlSiMn alloys was studied using directional solidification, and the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) technique was applied for thermodynamic calculations. The specimens solidified with forced convection presented segregation across the sample diameter, and the measured compositions were located on the Al-Si-Mn phase diagram. Scheil-Gulliver calculations for measured compositions were used to determine various solidification paths that may occur in specimens. Property diagrams and solidification paths presented the segregation effect on the characteristic temperatures, mushy zone length and the sequence of occurring phases whilst 2D maps enabled visualization of the mushy zone during directional solidification. Melt stirring was found to change solidification range, as well as mushy zone length and shape, and the dendrite tips formed a rough profile across the specimens. The study revealed mushy zones with dense dendritic structure and liquid channels empty of Mn phases, where intermetallics had no possibility to flow in the liquid, whilst in other samples with channels filled with Al 15 Si 2 Mn 4 , Mn-precipitates also flowed above the α-Al. The melt flow may lead to a mainly dendritic mushy zone or to a mushy zone with dendrites reaching only lower half of mushy length with intermetallics forming and freely flowing above dendrites in the liquid upper half.

Thermo-Calc Prediction of Mushy Zone in AlSiFeMn Alloys

“…The main cause for the liquid channel is segregation of Si and results are similar to [23,31,33]. The segregation of Mn and Fe is not so uniform as in Fe-containing AlSi alloys [31].…”

“…In previous papers predictions of the mushy zone have been presented for AlSiFe [31] and AlSiMn [33] The mushy zone length varies according to the composition. The dendritic area is about 20 mm (575-635 • C) long for all specimens, but the whole predicted mushy zone differs according to Fe and Mn content.…”

“…In [31] maps similar to those calculated in [23] were presented with 12-15 mm deep liquid channel in an AlSi7Fe1.0 alloy and for AlSi7Fe0.5 the depth reached 9-13 mm. Study [33] (specimen AlSi7Mn1.0) showed 13 mm long α-Al area and liquid channel (575-615 • C) and 28 mm long area (575-660 • C) for Al 15 Si 2 Mn 4 with Mn-phases growing above dendrites and in the channel. Current results conducted for 5% Si are similar to [31,33] The possibility of Fe-or Mn-phases moving through the liquid depends strongly on the morphology and dimension of intermetallic Fe-and Mn-phases, and the differences in the shape are well visible on Figure 13.…”

“…Study [33] (specimen AlSi7Mn1.0) showed 13 mm long α-Al area and liquid channel (575-615 • C) and 28 mm long area (575-660 • C) for Al 15 Si 2 Mn 4 with Mn-phases growing above dendrites and in the channel. Current results conducted for 5% Si are similar to [31,33] The possibility of Fe-or Mn-phases moving through the liquid depends strongly on the morphology and dimension of intermetallic Fe-and Mn-phases, and the differences in the shape are well visible on Figure 13. 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.…”

Morphology and Distribution of α-Al and Mn-rich Phases in Al-Si-Mn Alloys under an Electromagnetic Stirring

“…These phases in aluminum alloys are known to author from earlier studies, projects and papers (e.g. in [1][2][3][14][15][16][17][18][19]). The composition of the proposed alloys was first determined based on available in the literature phase diagrams, but the exact composition and resulting precipitation sequence needed detailed calculations, in order to achieve the proposed, e.g.…”