The Effects Of Al-10Si-2Fe Master Alloy On Al-Si Alloys Microstructures

Al-Si alloys are widely used in manufacturing industry with a large range of categories. Melts treatment is an effective method to control aluminium alloys microstructures, such as primary aluminium dendrites, primary silicon, eutectic grains and eutectic silicon, so as to improve mechanical properties and processability. At present, the most common way is to add substances that can work as structure matching nucleis into the melts in order to facilitate nucleation of primary or eutectic microstructures. Apart from that, controlling growth process is another effective way. Hence, grains will be refined and multi-properties can be accordingly improved. In the mean time, purification technologies and modifiers are also commonly applied to suppress porosities formation, change porosities distribution and morphology of harmful intermetallic compounds. Recently, researchers reported cluster agglomeration based nucleation mechanism in metallic melts on the basis of classical nucleation framework. According to the mechanism, local ordered (AlFeSi) clusters in Al-Si melts induce pre-nucleation clusters, and then they accelerate silicon nucleation by pre-nucleation clusters accretion. As an inspiring part of the melts solidification theory, it offers a revolutionary way to refine grains in alloys, reducing undesirable effects from defects and thus enhancing properties.In the research, a kind of Al-10Si-2Fe master alloy is added into Al-Si melts to generate (AlFeSi) clusters. The research contains three main parts:1) the effects of local ordering pre-nucleation on silicon nucleation and growth process; 2) mutual impacts between exogenous and endogenous iron and their roles in the formation of iron-rich phases and the changes on eutectic grains during solidification process; 3) the influence of Al-10Si-2Fe master alloys on porosities.We obtained the cooling curves of commercial and high purity Al-15Si alloys inoculated with AlP and Al-10Si-2Fe respectively, compared their optical microstructures and analyzed SEM and TEM images of silicon phases. By doing those, we found that the structures of pre-nucleation clusters induced by (AlFeSi) clusters are totally different from silicon. These pre-nucleation clusters increase nucleation undercooling in high purity melt. While under the help of impurity the local ordering pre-nucleation will decrease undercooling and enhance silicon nucleation effectively. On the other hand, the stacking of pre-nucleation changes silicon growth mechanism and causes the absence of TPRE growth mechanism. All the above contribute to morphology variation and twins vanishing in silicon. As a result, the pre-nucleation clusters are embedded in silicon particles as nanometer sized particle and both primary and eutectic silicon show refined morphology.Aluminum alloys with compositions of Al-10Si-xFe (wt.%, x=0.1,0.4,0.7,1.0, 1.3,1.6) were prepared and characterized by XRD、SEM and stereo-microscopy. The changes in eutectic grains size and types of iron-rich phases after inoculation indicate that Fe element from Al-10Si-2Fe master alloy has an interaction with endogenous Fe existing in melts. Both two participate in the formation of (AlFeSi) clusters whose numbers and structures are related to Fe contents. Working as nucleation templates, these iron-rich clusters obviously enhance iron-rich phases nucleation by reducing nucleation energy barrier with similar local atomic structures and compositions. Therefore, the contents of Fe in melts and Al-10Si-2Fe master alloy play a decisive role in iron-rich phase selection. In addition, eutectic grains are better refined because of increasing number of pre-nucleation clusters at higher Fe contents.We compared several parameters of porosities, such as the average area, maximum diameter and number density, in Al-10Si-0.1Fe and Al-10Si-1.0Fe alloys before and after inoculation. The relationship among eutectic grains size, intermetallic compounds and porosities parameters is also analyzed. The results show that Al-10Si-2Fe master alloy apparently refines porosities. Porosities distribution also becomes more homogeneous on account of Al-10Si-2Fe inoculation. This is mainly due to eutectic grains refinement caused by (AlFeSi) clusters. In addition, the improvement in iron-rich phases morphology and size enhances the tendency.