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Hot stamping is a widely used technology to produce ultra-high strength steel parts in the automotive industry. This method combines traditional heat treatment and cold stamping technologies. To avoid surface oxidation at high temperatures, Aluminum-based coated steel sheets are used for hot stamping of boron steels. These Al-based coatings prevent surface oxidation, decarburization and enhance corrosion resistance of the hot stamped parts. Apart from the advantages of the Al-based (Al-Si) coatings, introducing them to the hot stamping of the boron steels complicates the heat treatment and sheet metal forming processes. Therefore, a profound knowledge and control of the heat treatment procedure, bimetal system interface properties, and subsequently their mechanical behavior during forming stage is required. Localized cracking and delamination of the Al-Si coatings are the major problems which can affect the determining process factors such as coating frictional behavior and tool wear. This may consequently lead to a less efficient forming process, oxidation of the substrate and reduction in its corrosion resistance. Currently, there is not any comprehensive knowledge about the initiation and propagation of the micro-cracks during the heat treatment and subsequently cracking and interfacial debonding of the coating during the sheet metal forming. The aim of this project is first characterizing the initiation of the micro-cracks at the surface and the coating-substrate interface, second multi-scale modeling of the fracture behavior of the multi-layered Al-Si coating based on different layers of the intermetallic compounds in the coating (obtained from different heat treatment variables: time, temperature and cooling/heating rate), and third optimizing hot stamping process parameters (both thermal and mechanical) to avoid cracks in the coating based on fracture behavior of the coating layer. We anticipate that the optimization of the process will consequently lead to enhanced frictional behavior and tool wear in hot stamping of the Al-Si coated boron steels.
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