Effect of Solidification Rate and Rare Earth Metal Addition on the Microstructural Characteristics and Porosity Formation in A356 Alloy

Abstract

The present study was performed on A356 alloy with the main aim of investigating the effects of La and Ce additions to 356 alloys (with and without 100 ppm Sr) on the microstructure and porosity formation in these alloys. Measured amounts of La, Ce, and Sr were added to the molten alloy. The results showed that, in the absence of Sr, addition of La and Ce leads to an increase in the nucleation temperature of the α-Al dendritic network with a decrease in the temperature of the eutectic Si precipitation, resulting in increasing the freezing range. Addition of 100 ppm Sr results in neutralizing these effects. The presence of La or Ce in the casting has a minor effect on eutectic Si modification, in spite of the observed depression in the eutectic temperature. It should be noted that Ce is more effective than La as an alternate modifying agent. According to the atomic radius ratio, rLa/rSi is 1.604 and rCe/rSi is 1.559, theoretically, which shows that Ce is relatively more effective than La. The present findings confirm that Sr is the most dominating modification agent. Interaction between rare earth (RE) metals and Sr would reduce the effectiveness of Sr. Although modification with Sr causes the formation of shrinkage porosity, it also reacts with RE-rich intermetallics, resulting in their fragmentation.

The Influence of Alloying Elements on the Microstructure and Properties of Al-Si-Based Casting Alloys: A Review

“…This effect can be partially counteracted by higher La additions since it can also modify the eutectic phase, in addition to acting as grain refiner; as a downside, excessive lanthanum additions lead to the precipitation of coarse intermetallics harmful to the alloy's properties [130,145]. In the presence of Sr, the application of adequate solution treatments may cause fragmentation of these intermetallics [130,146]. Also, upon higher cooling rates, La and Ce, just like Mo and Zr, can form fine particles that contribute actively to the alloy's hardening.…”

“…In work on the effect of the addition of these elements to an Al-Si-Mg-Zn alloy, the addition of Mo and Zr led to increased strength and hardness, and the later addition of La and Ce further improved these properties [147]. The addition of La and Ce to the A356 alloy was also acknowledged to increase the solidification interval, hindering the feeding of the melt [146]. Table 5 lists the optimum results of works by several authors whose focus was the addition of rare earth to Al alloys.…”

High Temperature Behavior of Al-7Si-0.4Mg Alloy with Er and Zr Additions

“…The combined addition of Sc, Er and Zr was also proposed in literature to improve the structural properties of Al-Si casting alloys [13,[26][27][28], typically heat treated in a conventional way (SHT + aging). Complementary effects were reported for these minor additions: Zr acts as grain refiner and inoculant [11,29,30] while Er, together with Sc and other rare-earth elements, have modifying effects on eutectic Si, similarly to the widely diffused but more volatile Sr [31][32][33].…”

Applications of Rare Earth Metals in Al-Si Cast Alloys

“…The effects of different addition levels (0.0-1.0 wt.%) of La-based MM and heat treatment on the microstructure and tensile properties of two different sections of Al-Si casting alloy A357 were studied by Mousavi et al [68][69][70]. Optimum recommended levels of MM are 0.1 wt.% and 0.3 wt.% for thin and thick sections of the casting, respectively.…”