## 1 Introduction  
The Al-Si alloy, which accounts for 80% of the aluminum alloy market, has many advantages including excellent castability ratings, high fluidity, and good corrosion resistance, etc. However, it is difficult to branch into additional application fields, due to the coarse microstructures. The Al-Si alloys were strengthened by the additional metal elements, such as Tungsten [1]. [PERSON] et al. [2] considered the effects that Sr additions had on the microstructures and the mechanical properties of Al3 Ti/ ADC12 (Aluminum-Alloy Die Castings) composites. They found that when the Sr content increase to 0.25 wt.%, the coarse dendritic of the primary \(\alpha\)-Al phases completely refines, and the acicular or short rod shape of the eutectic silicon changes into a granular shape. The mechanical properties are improved by the addition of Sr, where good UTS and breaking elongation were obtained. Studies [3; 4; 5; 6; 7] show that some rare earth (RE) elements play a role in modifying the Al-Si cast aluminum alloys. [PERSON] et al. [8] investigated the impact that La had on the microstructure and hot crack resistance of an ADC12 alloy. The morphology of the \(\alpha\)-Al grain changes from a dendritic crystal into fine equiaxed or spheroidal crystals when the quantity of La reaches 0.6 wt.%, and the size of the eutectic silicon varies from the needle-like to a tabular shape into fine rod-like shape without edges and corners. During the modification process of rare earths, the performance of aluminum alloys change by the different rare earth elements or processing techniques. Studies show that the addition of rare earth minerals modify the Fe phase to less harmful forms for Al-Si alloys [9]. [PERSON] et al. [10] investigated the effects that Sm additions had on the eutectic Si, \(\beta\)-Al5 FeSi phases of the ADC12 as-cast alloy. When the Sm addition increased to 1.0 wt.%, the needle-like \(\beta\)-Fe modifies into the Chinese script or the spherical \(\alpha\)-Fe phase.  
Previous studies [2; 3; 4; 5; 6; 7; 8; 9; 10] regarding the modification of the Al-Si alloy by mono-rare earth elements are informative. The similarity effect that the mixed rare earths had on the Al alloys is known, which alleviates the restrictions with solubility for the mono-rare earth elements, and the formation of the coarser intermetallic compounds is avoided, which improves heterogeneous nucleation. [PERSON] et al. [11] found that the sizes of the primary \(\alpha\)-Al phase and the eutectic Si phase were reduced as the (La+Ce) mass fraction increased in the range between 0.1 wt.% \(\sim\) 0.3 wt.%. [PERSON] et al. [12] investigated the effects that the (La+Yb) addition had on the microstructure and the tensile properties of the AlSi10 Cu3 alloy. They detected that, the primary \(\alpha\)-Al phase and the acicular \(\beta\)-Al5 FeSi phases were obviously refined because of the (La+Yb) addition, where the coarse acicular eutectic Si phases were modified into fine particles or a short rod morphology.  
There are two hypotheses regarding the modification mechanism of the eutectic Si that rare commonly recognized. One view is that modifiers are forced to 'poison' Si and trigger the multiply twinning (IIT theory) [13]. There is also a TPRE (Twin Plane Reentrant Edge) mechanism [14] view that states the modifier can poison the re-entrant of the twin grooves and changes the morphology of the eutectic Si. The optimal ratio of the atomic radius of the modifier with regard to that of Si (Rmodifier/Rsi) to induce IIT is about 1.646 [13]. Pr and Ce have, atomic radius ratios of Pr/Si and Ce/Si that were 1.573 and 1.556. These values are close to the optimal atomic radius ratio. The purpose of this study is to investigate the (Pr+Ce) addition effect on the AlSi5 Cu1 Mg alloy's microstructures and the mechanical properties. The mechanism of the (Pr+Ce) modification is discussed.