Effect Of Neodymium On Primary Silicon In Hypereutectic Al-Si Alloys And Modified Mechanism

Hypereutectic Al-Si alloys are important casting alloys and widely used in aviation, aerospace and automotive manufacturing areas due to their series of advantages, such as low thermal expansion coefficient, high ratio of strength to density, good corrosion resistance, excellent wear resistance, and so on. However, with the increase of silicon content, coarse primary silicon phase in hypereutectic Al-Si alloys under conventional casting conditions dissevers the bonding of the matrix and aggravates the alloy’s overall performances. Therefore, it is essential to modify hypereutectic Al-Si alloys to improve shape, size and distribution of primary silicon with an aim of enhancing mechanical properties of the alloys.In this paper hypereutectic Al-Si alloys (Al-17.5%Si, Al-20%Si and Al-25%Si) were prepared by conventional casting process. Pure Nd, as metamorphic agent, was adopted to modify primary silicon of binary hypereutectic Al-Si alloys and metamorphism was investigated. Hypereutectic Al-Si alloys with different content of silicon were modified by controlling process conditions, such as the temperatures of Nd modifier adding in the liquid alloys, holding time and the addition amount of Nd modifier, which have been studied in order to investigate their effect on shape and size of primary silicon in hypereutectic Al-Si alloys and identify the best metamorphism processing conditions. Moreover, mechanical properties and wear resistance of hypereutectic Al-Si alloys were investigated before and after Nd modification. According to the morphology of fracture surface and worn surface of modified alloys, rupture mechanism and wear mechanism of modified alloys were discussed. The distribution and the existing form of pure Nd in hypereutectic Al-Si alloys were investigated and analyzed and modified mechanism of Nd on primary silicon in hypereutectic Al-Si alloys was discussed. The main results and the conclusions are as follows:Primary silicon of binary hypereutectic Al-Si alloys can be refined by Nd and constitutional supercooling theory can be applied to explain modified mechanism of Nd on primary silicon. Adsorption theory and heterogeneous nucleation theory could not explain the experimental results. Metamorphic process research results indicate that metamorphic process conditions have important influence on shape and size of primary silicon of hypereutectic Al-Si alloys. For Al-17.5%Si, Al-20%Si and Al-25%Si alloys, the best metamorphism processing conditions: the temperatures of Nd joined in the liquid alloys are750℃,800℃and850℃respectively, holding time is10min and the best addition amount of Nd alloy is about0.3%.With the addition amount of Nd increasing, the size of primary silicon of hypereutectic Al-Si alloys decreases first and then increases. When the addition amount of Nd is0.3%, the grain size of primary silicon reaches the smallest. The average size of primary silicon of Al-17.5%Si, Al-20%Si and Al-25%Si alloy reduce from30.32μm,63.51μm and90.54μm to13.02μm,20.64μm and37.84μm respectively and decrease by57.1%,68.5%and58.2%. The large lamellar and polyhedron shape primary silicon present in the unmodified Al-17.5%Si alloy doesn’t change after modification. Hypereutectic Al-20%Si alloy and Al-25%Si alloy before and after modification have a similar result. Star and irregular polyhedron shape primary silicon present in the unmodified Al-20%Si alloy and Al-25%Si alloy disappear after modification and the small lamellar and polyhedron shape of primary silicon present in the modified alloys.The test results on mechanical properties of the alloys results show that with the addition amount of Nd increasing, tensile strength of hypereutectic Al-Si alloys increases first and then decreases. When the addition amount of Nd is0.3%, tensile strength of modified alloys reaches the maximum value. Tensile strength of Al-17.5%Si, Al-20%Si and Al-25%Si alloy increase from120MPa,117MPa and113MPa to163MPa,148MPa and132MPa before and after modification respectively and increase by35.8%,26.5%and16.8%. Elongation of Al-17.5%Si, Al-20%Si and Al-25%Si alloy increase from0.8%,0.64%and0.42%to2.2%,1.64%and1.2%respectively and increase by175%,192%and185%. The fracture mode of hypereutectic Al-Si alloys is a mixed fracture mode, including the brittle fracture characteristics of primary silicon and the ductile fracture characteristics of Al matrix.Friction and wear resistance tests show that with the silicon content increasing, weight loss of hypereutectic Al-Si alloys decreases and their wear resistances were significantly improved. The same law presents after Nd adding in the liquid alloys. Weight loss of Al-17.5%Si, Al-20%Si and Al-25%Si alloy reduce from0.0042g,0.0026g and0.0018g before modification to0.0020g,0.0011g and0.0008g after modification respectively and decrease by52.3%,57.7%and55.6%. Wear resistances of hypereutectic Al-Si alloys after modification were significantly improved. The dominant wear mechanism for modified alloy was abrasive wear, adhesive wear and oxidative wear mechanism, but wear mechanism for unmodified alloy was abrasive wear and adhesive wear mechanism.Electron Probe Microanalyzer (EPMA) and Scanning electron microscopy (SEM) analysis show that no Nd element is detected in the center and edge of primary silicon in hypereutectic Al-Si alloys, Nd mainly distributes along the grain boundary. The distribution state may leads to RE distribution along the solid-liquid interface in the melt condensation process, and hinders the growth of interfacial diffusion of silicon to inhibit the growth of silicon phase to refine primary silicon. Therefore constitutional supercooling theory could be appled to explaining the experiments’results.Differential scanning calorimetry (DSC) analysis shows that Nd adding into hypereutectic Al-Si alloys reduces the initial crystallization temperature and the precipitation temperature of primary silicon and shortens the cooling time of primary silicon. Therefore the Si atoms activity was reduced and size of primary silicon was decreased. This shows that modification mechanism of Nd on hypereutectic Al-Si alloys could be explained with constitutional supercooling theory, rather than heterogeneous nucleation theory.Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis show that Nd mainly exists as AlSixNdy ternary phase in the hypereutectic Al-Si alloys. Compared with the unmodified alloys, the new diffraction peaks are formed in the modified alloy and they can be indexed diffraction peaks of AlSixNdy ternary phase. The diffraction intensity of indices of crystal plane of silicon phase before and after modification analysis indicates that Nd can affect the preferred growth orientation of crystal face of silicon phase.