A Microstructure-Based Constitutive Model for Superplastic Forming
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HOT forming processes, including superplastic forming (SPF) and quick-plastic forming (QPF), are used to manufacture components with complex shapes that usually cannot be produced by cold forming. Especially superplastically formed parts find many applications, particularly in aerospace and transportation industries, where weight reduction is critical to meet product performance requirements.[1,2] Finite-element simulations of hot forming are of great interest as a tool for process design. In hot forming, many parameters affect the flow stress such as strain rate, temperature, and microstructure. Therefore, the accuracy of constitutive models is currently the most significant issue in forming simulations. There is also great interest in developing a better understanding of the dominant mechanisms responsible for hot forming to extend desirable forming conditions. Constitutive models of hot forming have been historically constructed from uniaxial tension tests.[1–5] In these works, several tests are performed on materials with various microstructures, temperatures, and strain rates. Then, a function is fitted to the resulting data. This function in conjunction with a potential surface REZA JAFARI NEDOUSHAN, PhD Student, MAHMOUD FARZIN, Professor, and MOHAMMAD MASHAYEKHI, Assistant Professor, are with the Department of Mechanical Engineering, Isfahan University of Technology, Isfahan, 84156 Iran. DOREL BANABIC, Professor, is with the Mechanical Technology Department, Technical University of Cluj-Napoca, 3400 Cluj-Napoca, Romania. Contact e-mail: [email protected] Manuscript submitted July 30, 2011. Article published online May 23, 2012 4266—VOLUME 43A, NOVEMBER 2012
that relates different stress states to an equivalent stress is used to predict material behavior in various situations.[6–13] Researchers have tried to improve their understanding about dominant mechanisms responsible for hot forming by developing microstructural constitutive models. In practice, it has been noticed that microstructural forming mechanisms affect the stress–strain rate relation. Chandra[14] proposed a constitutive model that considered microstructural forming mechanisms and compared it with other available constitutive models. Some researchers developed a microstructural-based model and investigated superplastic forming mechanisms.[15–19] More investigations showed that mechanisms of deformation in hot forming and especially superplastic forming differ substantially from cold forming.[14] Besides deformation within the grains, which is the dominant mechanism in cold forming, other mechanisms may play a role in hot forming, including grain boundary sliding, grain boundary diffusion, and grain boundary migration. Many researchers considered the contribution of intergranular deformation as well as grain boundary sliding in total deformation and obtained a better fit to tensile test data.[20–23] Some others also considered contribution of grain boundary diffusion.[14,16] The aim of the current work is to propose a constitutive model based on m
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