# Solid Fraction Determination at the Rigidity Point by Advanced Thermal Analysis
## 4 Discussion
The Fe-rich phases with Cr and Mn and specially Al\({}_{3}\)Ni phases are created at temperatures higher than the liquidus temperature of the alloy and are embedded in the aluminum matrix. As the Cu content in an alloy increases, the solidification time increases its value and the liquidus temperature decreases, mainly changing the alloy towards a more eutectic alloy. Cu precipitates mainly as Al\({}_{2}\)Cu in the final stage of the solidification process, at temperatures very close to the solidus temperature, with an enrichment of Cu in the remaining eutectic liquid. Thus, the Cu precipitates do not decrease the ability of the solid  
Figure 8: SEM+EDS analysis of alloy 16. Figure 7: Microstructure of alloy 16.  
particles to move in the remaining liquid (fluidity) [45], nor thermal conductivity, because as the particles precipitates at the final stage, there are few solid particles in the remaining liquid until the end of the solidification process. Cu also has another preferred way of precipitating, which it is in combination with Fe, modifying the structure of \(\beta\)-Al\({}_{5}\)FeSi phase from acicular or plate-like to skeleton structures, such as in the Q-Al\({}_{5}\)MgSCu\({}_{2}\)Si\({}_{6}\) phase [46; 47; 48; 49], increasing the fluidity. In Figure 9 we can observe the microstructure of alloy 6, where Cu is present as Al\({}_{2}\)Cu in the interdendritic space. The high percentage of Cr and Mn modify the \(\beta\)-Al\({}_{5}\)FeSi to a skeleton-like structure. Many of them can be detected and also some remaining \(\beta\)-Al\({}_{5}\)FeSi. Ni is presented as Al\({}_{3}\)Ni acicular phase. The FRP value of this alloy is 48.23%, a value close to the average of all the samples studied. In this case, the increase in fluidity linked to the presence of Cu is partially limited by the presence of acicular phases. Mn changes the morphology of the iron-rich \(\beta\)-Al\({}_{5}\)FeSi phase from its typical acicular shape to a more cubic and compact Al\({}_{15}\)(MnFe)\({}_{3}\)Si\({}_{2}\) shape [49; 50]. The shape change allows easier movement of the compact phases over the remaining liquid in the interdendritic space, increasing the solidification time and reducing the stiffness of the melt. The \(\alpha\)-Al\({}_{15}\) (Fe,Mn)\({}_{3}\)Si\({}_{2}\) phase precipitates at high temperatures, reducing the coarsening ability of the possible remaining \(\beta\)-Al\({}_{5}\)FeSi phase and thus increasing the fluidity of the alloy. It also promotes a very fine eutectic structure, as shown in Figure 10.  
Ti is a refining element that promotes grain size reduction. Grain size reduction increases the fluidity of the alloy and the solidification time. It also increases the heat  
Figure 10: Microstructure of alloy 10. Figure 9: Microstructure of alloy 6.  
transfer from the solidifying alloy to the surroundings. We can observe in Figure 10 a well-refined structure, with small aluminum dendrites and the presence of shrinkage porosity. Zn and Sn at the concentrations studied in this work tend to be dissolved in the alloy, solidifying within the aluminum dendrites in the aluminum matrix. An increase in the percentage of Zn and Sn promotes a more eutectic alloy, with a reduction in FRP values. Pb appears as isolated dots at the grain border, being the element that is one of the last to precipitate in the final interdendritic space. It promotes a more eutectic alloy, increasing the solidification time and decreasing FRP values. In the case of the rest of the alloying elements studied in this work, we can observe how Mg, Fe, Ni, Cr and Sr promote the increase of FRP values, pushing the stiffened lattice to form faster. Mg precipitates as Mg\({}_{2}\)Si in the inter-grain space, mainly from the remaining eutectic liquid within the interdendritic space. It promotes faster rigidity, because it reduces the solidification time and because it hinders the movement of the remaining eutectic liquid.