**Abstract**

Al-Si alloys are an attractive choice of light alloys due to their low density and high mechanical properties. The application areas include automotive and aerospace parts. With the increased requirements and demands on such applications, the researchers have focused on the enhancement of properties with the addition of novel alloying elements. In the last decade, modification of microstructure by rare-earth element additions has become popular. In this work, systematical tests were carried out by using fluidity test mold that has 8 arms with different thicknesses and tensile test mold that contains 10 bars to statistically analyze the mechanical properties. Different ratios ranging from 0.05 to 1 wt% of Er, Sc, Y, and V were added to Al-7Si alloys. The addition of 0.1 wt% Sc shows the highest fluidity whereas V additions have the lowest. Statistically, Sc addition revealed the highest reproducible results in terms of tensile properties where Y had the highest scatter. Er additions have the highest UTS and elongation at fracture.

**Keywords:** casting, A357, rare earth, tensile, fluidity, microstructure

## **1. Introduction**

Grain refinement is a method used to alter the microstructure from heterogeneous dendrites into smaller and globular morphology. In this way, the casting properties are enhanced significantly. The feedability of the melt increases and thus fluidity increases. Since the interdendritic flow of the liquid resistance is decreased, the shrinkage and porosity decrease. Additionally, obtaining homogeneous and refined grains lead to achieving higher mechanical properties. It is important to note that the reduction of casting defects has the most determining influence on mechanical properties as well as the homogeneous distribution of secondary phases along with the microstructure.

Over the past several years, the modification of aluminum alloys with rare earth elements has gained great attention. The studies are concentrated on the improvement of mechanical properties by grain refinement or modification of Si morphology, particularly in Al-Si alloys. The use of Sc as a grain refiner has almost proven itself to be a key grain refinement agent. However, the expense of using Sc and the high cost have led the application to be limited. Alternative elements such as V, Y,

Er, Eu and Ce have been started to be investigated in detail. Their refinement and modification effects have been found in Al–Si–Mg alloys [1–6].

Mazahery [7] investigated the effect of 600 ppm Eu addition to A360 on mechanical properties. It was found that as SDAS was increased, UTS was decreased by 25% and elongation at fracture was decreased by half. The microstructure and mechanical property change of A356 by Eu additions between 200 and 1000 ppm were investigated by F. Mao [8]. Eu was an effective Si modifier. The eutectic temperature was decreased, and Si morphology was altered from coarse plates into smaller fine spherical fibers which increased the tensile properties by 15% when 1000 ppm Eu was used.

L. Li [9] compared the semisolid die casting method to gravity casting. It was reported that 0.1 wt% RE addition had not affected the grain size but modification of Si to fibrous morphology had resulted in increased elongation at fracture for both methods. Increasing RE content to 0.4 wt% had resulted in the formation of needle-like intermetallic and the material had become brittle. Up to 0.8 wt%, Yb addition had formed globular primary and secondary dendrites with both Si and Fe intermetallic converted from acicular to finer morphology, however, higher amounts of addition of Yb to Al12Si had resulted in decreased mechanical properties [10]. Kabliman [11] added Sc to Al-Mg alloys to produce alternative foil material to Al-Cu alloys where 550 MPa was achieved at 0.5 mm thickness. Elgallad [12] found that in the absence of Sr., the addition of La and Ce to A356 and A413 had not modified Si, instead primary Si phases were formed. In the presence of Sr., much finer Si modification was observed. On the other hand, when Ti was added to these alloys, a high population of intermetallic formation was observed. When Yb was added together with La to A356, spherical Si particles were formed with a depression of 8°C of eutectic temperature [13]. M. Colombo [14] added different amounts of Er to A356 and showed that Er can both grain refine and modify Si at the same time. Best results were obtained at 0.3 wt% Er addition, and increased Er addition had resulted in the formation of intermetallic phases which decreased the ductility of A356 [15–17].

The effect of RE additions on the porosity formation of A356 alloy was investigated by Elgallad [18]. Various amounts of La, Ce, and La + Ce modifications were tried. When the intermetallic was increased (3%), the feedability was decreased which increased the shrinkage and porosity formation. On the other hand, it was reported that Ce was more dominant in pore formation than La. It was also found that Sr. was still the most effective modifier, but it also had a high affinity to RE elements possibly resulting in intermetallic formation [1]. Z. Wang [19] found that the addition of 0.1 wt% La and Ce increased the ductility of the alloy. The formation of fine and distributed secondary phases was observed on the fracture surface as dimples however in the absence of the RE added alloy, the fracture surface consisted of brittle cracks. Asmael [20] investigated the microstructure and mechanical properties of Ce added Al11SiCuMg alloy. Tensile strength and quality index were found to be the highest at 0.1 wt% Ce modified alloy. SDAS was decreased by 36%. When Ce content was changed between 0.5 and 1.0 wt%, it was found that there was no change in the microstructure and tensile properties. Pourbahari [21] studied the properties of thinwalled castings of A357 with La addition. La content was varied between 100 and 1000 ppm. The grain size was reported to be decreased by 50% with an increase of approximately 80% UTS. Sc additions up to 0.4 wt% revealed 50% decrease in SDAS of A356 with obtaining of spherical Si morphology [22]. Thus, the eutectic phase was increased from 40 to 45% with increased tensile property from 180 to 300 MPa. Yii [23] added Ce to Al20Si hypereutetic alloy and reported that wear properties were increased when 1.6 wt% Ce was added. Tzeng [24] added Sc to Al11Si and reported that beta-Fe intermetallic was converted to Chinese script which did not reduce the

*Characterization of Casting Properties of Rare-Earth Modified A356 DOI: http://dx.doi.org/10.5772/intechopen.101722*

ductility of the alloy. The 0.8 Sc refined microstructure eliminated Fe-intermetallic formation increased tensile properties [25].

In this study, different amounts of Er, Sc, Y, and V were added to the A356 alloy. Efforts were made to statistically compare the effect of RE additions on the mechanical properties of A356 alloy. The reproducibility of tensile properties was investigated. The change in the fluidity of alloys was also investigated in the octopus design [26, 27].
