Applications of Rare Earth Metals in Al-Si Cast Alloys

Abstract

The present article reviews a large number of research publications on the effect of mischmetal (MM), rare earth metals (RE), La or Ce, and combinations of La + Ce on the performance of Al-Si cast alloys mainly 319, 356, 380, 413, and 390 alloys. Most of these articles focused on the use of rare earth metals as a substitute for strontium (Sr) as a eutectic silicon (Si) modifier if added in low percentage (< 1 wt.%) to avoid precipitation of a significant amount of insoluble intermetallics and hence poor mechanical properties. Other points that were considered were the affinity of RE to react with Sr., reducing its effectiveness as modifier, as well as the grain refining efficiency of the added RE in any form. None of these articles mentioned the exact composition of the RE used and percentage of tramp elements inherited from the parent ore. Using high purity La or Ce proved to have no effect on the Si shape, size or distribution, in particular at low solidification rates (thick sections). However, regardless the source of the RE, its addition to Sr-modified alloys reduced the modification effect. As for grain refining, apparently a high percentage of RE (> 1 wt.%) is required to achieve a noticeable reduction in grain size, however at the cost of alloy brittleness.

Effect of Casting Processes, Rare Earth Metals, and Sr Addition on Porosity Formation in Al-Si Cast Alloys

“…According to Mahmoud et al [37][38][39][40], Ce reacts with La and other alloying elements mainly Al, Fe, Sr, and Cu. Although the added amount of RE is double that of a single element addition, the resulting volume fraction of intermetallics is higher than that obtained from individual elements but not necessarily double the volume fraction shown in Figure 10.…”

Effect of Cooling Rate on Nano-Eutectic Formation in Laser Surface Remelted and Rare Earth Modified Hypereutectic Al-20Si Alloys

“…Chemical modification often leads to thermodynamically favorable phases other than primary Silicon, thereby reducing the degree of Si segregation. However, the excess addition of rare earth elements leads to the formation of intermetallic phases that can be detrimental to the ductility and toughness of the alloy [25]. Melt spinning [26], thin film deposition [27], and gas atomization [28] and friction stir spot processing [29], have also been used for micro to nanoscale research and applications for refined Si phases.…”