Influence of Melt Superheat, Sr Modifier, and Al-5Ti-1B Grain Refiner on Microstructural Evolution of Secondary Al-Si-Cu
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-SILICON-BASED alloys can be considered as the most important foundry alloys for the production of automotive engine components equipping light duty vehicles, like personal cars and light commercial vehicles, due to their good ratio between weight and mechanical properties, and excellent casting characteristics. The secondary Al process is highly attractive because it requires significantly less energy and emits less CO2 to the atmosphere compared to the primary route. Over 90 pct of the aluminum used in transportation industry comes from secondary sources, i.e., recycled material.[1] However, secondary Al alloys usually contain several types of impurity elements in small amounts, which can exert negative effects on the mechanical properties and the final quality of the cast components.[2] The mechanical properties of Al casting alloys are strongly dependent on the macro- and microstructural features, such as the size, morphology, and the distribution of a-Al dendrites, eutectic Si, and intermetallic compounds, which form during solidification.[3] Some features, such as the grain size, level of eutectic Si modification, dendrite coherency point can be quantified online by using an accurate thermal analysis system.[4]
JOVID RAKHMONOV, Ph.D. Student, GIULIO TIMELLI, Assistant Professor, and FRANCO BONOLLO, Full Professor, are with the Department of Management and Engineering, University of Padova, Stradella S. Nicola, 3 36100 Vicenza, Italy. Contact email: [email protected]. Manuscript submitted February 19, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
The chemical grain refinement and eutectic modification are two critical stages in melt chemical treatments. The grain size and the Si modification level can be significantly improved by introducing a certain amount of Al-Ti-B grain refiner and Sr modifier into the melt. Several research activities have been conducted to understand the effects of chemical grain refinement on the a-Al dendrite features and modification on eutectic Si structure.[5,6] Most studies, however, have focused on primary Al alloys. Investigations on the grain refinement of secondary Al-Si alloys are rather limited.[7] On the other hand, the optimum level of grain refiners in secondary alloys cannot be equally achieved as compared to primary alloys due to the presence of some trace elements that can induce a negative impact on the grain refinement efficiency.[8,9] Zirconium was found to poison the potency of nucleation agents, such as TiB2, particularly at higher melt superheats, ~1073 K (800 C).[8] Furthermore, the grain refinement level can be deteriorated by varying pouring temperatures.[10,11] The effects of Al-Ti-B on the precipitation behavior of Fe-bearing compounds are disputed and still not well understood.[12–14] Iron is considered as one of the most deleterious impurities in secondary Al-Si alloys and due to low solid solubility in the a-Al phase, the Fe-rich compounds precipitate in the microstructure during solidification. The formation of Fe-rich particles, particularly b-Al5FeSi platelets, is
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