# Crystallization Kinetics of Sr-Modified Precipitation Hardening Al-Si-Cu Alloys
## 2 Research Material and Methodology  
The sub-eutectic aluminum alloy AlSi6 Cu4 with the chemical composition shown in Table 1 was tested. The alloy chosen for this study was AlSi6 Cu4, which has very good machinability. Due to its properties, it is used for casting machine parts (e.g., vehicles engines) but also for decorative castings. The disadvantages of this alloy are shrinkage porosity and inadequate sealing. The other two alloys are the 0.2% strontium-modified alloy mentioned above and a 1% strontium-modified alloy, which was added to study the effect on the crystallization kinetics of Al-Si-Cu alloys. The test specimens were cylindrical in shape with dimensions \(\omega 40\times 30\) mm. In order to determine the crystallization temperature of the various phases occurring during the solidification process of the AlSi6 Cu4 aluminum casting alloy under study, a thermal-derivative analysis was performed using the UMSA MT5 device (Patent Serial No. PCT/CA02/01903). A K-type thermocouple (HKMQSS-M075U type) from Omega was placed in a thermal junction, the position of which was determined experimentally. The standard measurement error for type K thermocouples is \(\pm 2.2\)\({}^{\circ}\)C. The measurement frequency is 5 Hz. The samples were heated to 750 \({}^{\circ}\)C and held at this temperature for 180 s.  
Standard heat treatment was not applied to these alloys because a thermos-derivative analysis was carried out to melt the entire alloy with all the present phases just to obtain a 100% liquid phase, which was cooled down with a determined cooling rate in the UMSA platform. In industrial casting, aluminum alloys are melted in furnaces to a temperature of approximately 700-750 \({}^{\circ}\)C; the alloy is kept in a liquid state throughout the casting process. The temperature of 750 \({}^{\circ}\)C is the result of technical conditions and long-term studies of the effects that may occur on the liquefaction of all phases of the alloy. The samples were wrapped (3 layers) in 0.002 mm thick stainless steel foil. The steel foil was coated with a BN slurry. The diagram of the heating system is shown in Figure 1. The generator power was set at 2000 W (75% of the generator power was used for heating). Sr was also added to the tested alloy in the form of an AlSr10 mordant to obtain a content of 0.2 wt. %. Sr and 1 wt. %. Sr. The supersaturation was carried out at 480 \({}^{\circ}\)C for 2 h with subsequent cooling in water, while the aging was carried out at 175 \({}^{\circ}\)C for 5 h with further cooling in air. The samples were sanded on sandpaper with grits ranging from 220 to 4000 and subsequent electrolytic polishing in Struers' A2 electrolyte for 4 s at 40 V, using a LectroPol \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \multirow{2}{*}{**Alloy**} & \multicolumn{8}{c}{**Mass Concentration of Elements, \%**} \\ \cline{2-13}  & **Si** & **Fe** & **Cu** & **Mn** & **Mg** & **Cr** & **Ni** & **Zn** & **Pb** & **Sn** & **Ti** & **Sr** \\ \hline AlSi6 Cu4 & 5.82 & 0.43 & 4.30 & 0.48 & 0.28 & 0.04 & 0.02 & 0.64 & 0.02 & 0.01 & 0.15 & - \\ \hline AlSi6 Cu4 Sr0.2 & 5.70 & 0.42 & 4.21 & 0.47 & 0.27 & 0.04 & 0.02 & 0.63 & 0.02 & 0.01 & 0.15 & 0.20 \\ \hline AlSi6 Cu4 Sr1 & 5.24 & 0.39 & 3.87 & 0.43 & 0.25 & 0.04 & 0.02 & 0.58 & 0.02 & 0.01 & 0.13 & 1.00 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the tested aluminum alloys. Figure 1: Heating system: 1—K-type thermocouple, 2—induction coil winding, 3—sample, and 4—ceramic isolation. For surface preparation, the standard metallographic procedure of grinding, polishing and drying was used, and the specimens were mounted in thermosetting resin. The samples were then etched in a 5% HF solution at room temperature for the experimentally chosen time of 25 s for phase etching. For structural etching, the recommended solution is mainly NaOH.