Modeling of microstructural evolutions in machining of dual phase alloys

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Tabei, Seyed Ali
Liang, Steven Y.
Garmestani, Hamid
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Depending on the material system and machining conditions, the localized strain, strain rate and temperature fields induced to the material during the machining process can be intense. Therefore, a wide variety of microstructural evolutions are likely to occur below the machined surface. These microstructural changes take place at various scales. First of all, due to the severe plastic deformation below the machined surface, the crystallographic orientation of grains can change dramatically. In addition, if the levels of the induced temperature and strain are high enough, recrystallization may occur, new grains may form and subsequently grow. Additionally, contingent upon the duration of the machining process, partial grain growth might also happen. Last but not least, if the material is consisted of more than one phase, the microstructural characteristics of secondary phases will also evolve. The ultimate result of all the aforementioned evolutions produces remarkable changes in the mechanical and thermal (and almost all other) properties of the material, which consequently affect the response of the material during service. A comprehensive modeling framework that reliably captures all the aspects of the above microstructural evolutions in machining is absent in the open literature. This work coalesces concrete and all-inclusive modeling toolsets into a unified scheme to follow the mentioned phenomena in machining of aluminum alloy 7075. The modeling outcomes are verified by experimental results to assure reliability. Finite element analyses were applied to obtain the stress, temperature, strain and strain rate fields developed in the material during machining at different parameters. Kinetic-based models were exploited to determine the possible recrystallization or grain growth. A viscoplastic self-consistent crystal plasticity model was utilized to investigate texture evolution below the machined surface. Also for multi-phase materials, the first steps in developing a totally new constitutive model to yield the extent of the possible refinement in the second phase precipitates, were taken. The main goal of the work was to link the above-mentioned microstructural evolutions to process parameters of machining by mathematical derivation of process path functions. Therefore, prediction of microstructural changes as a result of changing the process parameters became possible; which has significant industrial potential and importance. Additionally, such a direct and complete linkage between machining and microstructure is completely new to the scientific community in manufacturing and design fields.
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