Stable Eutectic Formation in Spray-Formed Cast Iron
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Spray forming is an advanced casting process that produces refined and homogenous microstructure directly from the liquid metal regardless of the alloy system. However, the microstructure evolution during spray forming is complex because the process comprises two sequential steps with very different cooling rates, i.e., atomization and deposition. It is well known that the microstructure of cast irons is highly dependent on the chemical composition and the cooling rate imposed to the liquid. In order to better understand the microstructural evolution during solidification by spray forming, this study investigated the solidification of two cast irons with different stable–metastable eutectic temperature interval (ΔTES−M), 30 K and 17 K. The microstructures of both overspray powders presented a dendritic array of austenite with cementite in the inter-dendritic spacing. Despite the high cooling rates imposed to the alloys during the atomization step, the final microstructure was defined by the cooling conditions prevailing during the final step of deposit solidification and stable eutectic was formed. This was ascribed to the dynamic process involving heating and remelting of the low melting temperature phases present in the droplets that arrives completely or partially solid in the deposition zone.
The authors would like to thank the Brazilian research-funding agencies: Fundação de Amparo à Pesquisa do Estado de São Paulo/FAPESP (Grant Nos. 2013/05987-8 and 2016/19326-1), Conselho Nacional de Desenvolvimento Científico e Tecnológico—Brasil/ CNPq, and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001, for the financial support. Additionally, the Laboratory of Structure Characterization at the Department of Materials Engineering at the Federal University of São Carlos (LCE/DEMa/UFSCar) for the microscopy facilities.
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