"Continued weight reduction which involves replacing steel components with lightweight materials is one of the major strategies to achieve the average fuel economy standards in the automotive industry. Based on the significant success of aluminum alloys in non-structural components, aluminum alloys in structural applications attract particular interest. To this end, the ultimate objective is to achieve aluminum alloys with high ductility and strength. However, conventional aluminum-silicon (Al-Si) based casting alloys possess low ductility as compensation for the required high strength, since the presence of irregular eutectic silicon crystals in the as-cast condition promotes crack initiation and propagation. Strontium (Sr) is widely used to modify the Si eutectic in commercially available "high ductility" cast aluminum alloys. However, fading issues arise due to Sr oxidation at the casting temperature, and become major concerns for aluminum foundries. Therefore, it is important to develop a universal modifier that can be used in alloys with a larger range of Si content. In this thesis, trisilanol polyhedral silsesquioxane (TSP) was first incorporated in Sn-based binary and ternary alloys, leading to microstructure refinement and high creep resistance. Based on these preliminary results, TSP was further applied to Al based alloys. First, TSP was coated on Al-12Si powders using dip-coating approach. After melting, the microstructure of both primary Al and eutectic microconstituents of Al-12Si ingots was refined. Ductility increased from 5% with no TSP treatment to 18% when treated with TSP. More interestingly, the modified structure was maintained with 150 ppm of TSP addition. The incorporation of TSP reduced the undercooling and arrest temperatures of Al-Si eutectic during solidification. These results suggest TSP bonds with Al to slow down the Al segregation from Al-Si melt during eutectic reaction, leading to the microstructural refinement of Al-Si eutectic microconstituents. Based on the laboratory-scale results, a new TSP master was produced to facilitate the incorporation of TSP into traditional foundry process. The optimized master composition contained 6%TSP in an Al-12Si alloy, and 10% Al-12Si-6TSP master was applied to modify Al-Si binary and commercial alloys at a 100 lbs. scale. In Al-Si binary alloys, TSP addition results 50% improvement in ductility over the Al-7.5Si base alloy without sacrificing the strength. The microstructure of both primary Al and eutectic microconstituents of the Al-7.5Si alloy with TSP addition was refined. Minimal fade was observed up to a 192-hour hold at 720°C, which sheds light on the Sr fading issue. In Aural(TM) 2 and W319 -type commercial alloys, TSP additions reduced the solidification shrinkage of castings. Metallography showed refined Si in the Al-Si eutectic microconstituent and reduced Al secondary dendrite arm spacing (SDAS), resulting in an 100% improvement in ductility without sacrificing strength. Besides microstructural refinement, the mass fraction of theta (Al2Cu) increased while Q (Al4Cu2Mg8Si7) decreased with TSP addition, suggesting TSP changes the Cu intermetallic compounds (IMCs) to favor the formation of theta, which demonstrated its potential as a strengthening precipitate. This study addresses fundamental questions on how the addition of TSP influences microstructure and solidification behavior of Al-Si based casting alloys, thus providing solutions to reinforce aluminum alloys in body structures for the environmental and fuel economy benefits."--Pages ii-iii.
Read
