Study On Grain Refinement Of Rare Earth Elements For Al-Si Casting Alloys

Grain refinement can improve the mechanical properties of Al-Si casting alloys, while there are two problems when it comes to the widely used Al-3B grain refiner. On the one hand, it can not cause effective grain refinement of Al-Si casting alloys with the content of Si less than 4%; on the other hand, there is mutual poisoning effects between element B and modification element Sr, so that both the effects of grain refinement and modification will be impaired, leading to the limited improvement of mechanical properties of alloys. Many researches have revealed that rare earth elements can be used as grain refiner and modifier to Al-Si casting alloys, however, the practical effects on both grain refinement and modification are limited when they are added individually. Moreover, the mechanisms of grain refinement are still unclear. Thus, it has the theoretical significance and application value to study the grain refinement of rare earth elements to Al-Si casting alloys.In this paper, the interactions among rare earth element La, grain refinement element B and modification element Sr were studied. The individual addition of La in the Sr-modified Al-11%Si casting alloy not only can not cause efficient grain refinement of a-Al, but also will whittle the effect of modification. The tensile strength and elongation bothdecrease after the addition of La. Compared to the Sr-modified Al-11%Si casting alloy with individual addition of B, combinative addition of La and B can nuetralize the mutual poisoning effect between Sr and B, therefore, the α-Al grains are finer and more equaxed andthe strength and elongation increase at the same time. Fading experiments were carried out in order to study the long-term efficiency of the grain refinement after the combinative addition of La and B. Both the macrostructures and microstructures show that the effect of grain refinement will not fade after 2h. This grain refinement method can also be applied to the Al-Si casting alloys with various Si content.The cooling curves of the alloys were recorded to figure out the effects of La on the solidification process. It is found that the nucleation temperature of a-Al increases, while the undercooling and recalescence temperature of a-Al decrease with the combinative addition of La and B. These results indicate that there are more effective nucleant sites in the melt. While the reduction of eutectic temperature after the combinative addition of La and B reveals the progress in the modification for eutectic Si structure. According to the results of SEM and EDS, the formation of SrB6 consumes Sr in the melt resulting in the mutual poisoning between Sr and B. However, the interaction between Sr and B is eliminated by La addition, so that there is enough Sr in the melt to modify eutectic Si. Meanwhile, LaB6 particle is observed in the middle of a-Al grain. From the point view of E2EM model, there are two possible orientation relationships between LaB6 and a-Al:(100)Al‖(100)LaB6, [100]Al‖[100]LaB6 和(110)A‖(110)LaB6, [100]A1‖[100]LaB6,which declares that LaB6 has the potency to act as heterogeneous nuclei for a-Al grain.Al-La-B master alloys were fabricated employing commercial pure Al, Al-3B master alloy and LaCl3-LiCl salt-melt mixing system. The results of XRD reveal that there are AIB2 particles, LaB6 particles and Al11La3 particles mainly existing in these master alloys. These prepared master alloys can bring about grain refinement in commercial pure Al, while the effect of grain refinement respond to M3, which is consist of 3%La and 1.5%B, is the best among these master alloys. Blocky-like LaB6 particles and AIB2 particles can be seen in M3 alloy, their sizes are from 1μm to 2.5μm, nevertheless, only LaB6 particles are observed in the commercial pure Al refined with M3 alloy.