Effect of the Addition of La and Ce on the Solidification Behavior of Al-Cu and Al-Si-Cu Cast Alloys

The present work was undertaken to study the effect of 1.0 and 5.0 wt% additions of rare earths, RE (Ce/La), on the solidification behavior and microstructural characteristics of two as-cast alloys under low solidification rate conditions (0.7 degrees C/s). The alloys studied contain Al-1.2Si-2.4Cu-0.4Mg-0.4Fe-0.6Mn-0.15Ti wt% and Al-8.0Si-2.4Cu-0.4Mg-0.4Fe-0.6Mn-0.15Ti wt%. These two alloys are coded B0 and D0, respectively. Thermal analysis and differential scanning calorimetry (DSC) were used to examine the solidification behavior of the alloys. The thermal analysis results revealed that the main phases observed in the alloys are alpha-Al, alpha-Fe, Al-Si, Al2Cu and Q-Al5Cu2Mg8Si6 phases. The results also showed a drop in the nucleation temperature of alpha-Al and eutectic temperature (Al-Si) caused by the addition of RE (Ce/La); the drop was more readily observed with the 5.0 wt% RE (Ce/La) addition. Differential scanning calorimetry revealed that the addition of La or Ce leads to the appearance of exothermic peaks between the precipitation of the primary alpha-Al phase and the (Al-Al2Cu) eutectic reaction in the solidification and heating curves. Moreover, the peak corresponding to the formation of the Al2Cu phase is clearly noticeable in the base alloys, becomes smaller with 1.0 wt% RE (Ce/La) and then disappears with 5.0 wt% RE (Ce/La) additions. This can be attributed to the interactions between the RE (Ce/La) and Cu in the alloys considered. Since the thickness of precipitated RE intermetallic platelets is about 1 mu m, it is difficult to obtain their stoichiometric compositions using WDS analysis.