Deformation Prediction of a Heavy Hydro Turbine Blade During the Casting Process with Consideration of Martensitic Trans
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TRODUCTION
DEFORMATION is a common defect in casting production, which mainly results from uneven cooling. Modeling and numerical simulation can be used to predict stress and deformation in which thermomechanical modeling, thermal and mechanical boundary conditions between casting and mold, and the analysis of deformation are the main concerns. Thermo-elastic and thermo-elasto–plastic models are commonly used in stress analysis. Because both the liquid and solid phases and their mixture are involved in the casting process, thermo-elasto-visco–plastic or rheological models are also applied, especially for the prediction of hot tearing.[1–3] The effect of the mold on stress and strain results is very complicated, and it used to be considered as rigid or elastic. In a contact problem between the casting and mold, sometimes a gap forms and sometimes forced contact occurs. Liu et al.[4] considered the effect of sand mold on casting using the contact element method. These results showed that the mold had an obvious effect on the stress and deformation of heavy runner band castings. On the other hand, the casting/mold interface heat transfer is affected by the formation of the air gap resulting from deformation of both the casting and mold. Xu et al.[5] coupled the JINWU KANG, Associate Professor, TIANJIAO WANG, Research Scientist, TIANYOU HUANG and BAICHENG LIU, Professors, are with the School of Materials Science and Engineering, Key Laboratory for Advanced Materials Processing Technology (Ministry of Education), Tsinghua University, Beijing 100084, P.R. China. Contact e-mail: [email protected] Manuscript submitted July 6, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
thermal and mechanical boundaries between the casting and mold, using the contact element method for the mechanical action between the casting and mold, and the air gap resulting from deformation is fed back to the thermal contact condition by modifying the interfacial heat transfer coefficient. Kron et al.[6] systemically investigated the effect of the thermo-mechanical boundary on the thermal histories and displacements of the casting and mold. These results showed that heat transfer between the casting and mold is affected by air gap formation. Shake-out of casting alters the mechanical boundary condition, which was studied by Lee and Lee[7] with the effect of shake-out on the deformation of a large marine propeller casting during solidification and the subsequent cooling. It is found that the later shake-out of the casting, the smaller its deformation. In order to control the deformation of the propeller casting, slow shake-out was much better than sudden shake-out. The aim of deformation prediction is to control deformation finally. So, Chen et al.[8] carried out the stress analysis of an impeller casting, and the displacement was annulled by modifying the casting geometry in the subsequent simulations. Iterations were performed until uniform machining allowances were achieved. Usually, displacement, a kind of direct result of stress analysis, is used to
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