A material model for micro/mesoforming simulations

In recent years, finite element simulations have been increasingly used in process design for metal forming applications, and accurate results are obtained. Due to the size effects that occur as a natural result of miniaturization in microforming processes, material models used in conventional metal forming processes are insufficient to use. Microforming is the application of traditional metal forming methods to fabricate products that have critical features within submilimeter dimesions; In this thesis, the surface layer model and the Hall-Petch relationship are combined to model the deformation of materials at the meso/micro-scale. In addition, Sachs model and Taylor model are implemented in Hall-Petch relationship to model the deformation of surface and inner grains, respectiveley. According to the newly developed material model, if the Hall-Petch constants of a material are known, flow curves of the material can be calculated for any specimen size-grain size combination. Alternatively, if a flow curve for a particular grain size of a material is known, the yield curve for that particular grain size for any other sample size can be calculated. In order to validate the newly developed material model, scaled down micro-compression tests were carried out with samples of different grain and specimen sizes which are machined form cold-rolled CuZn30 and flow curves were obtained for 12 different sample size- grain size combinations. The Hall-Petch constants of the test material were determined by using the 3 flow curves of the largest sample size. By implementing these Hall-Petch constants in the material model, 12 experimental yield curves were estimated. A good agreement between the experimental and predicted flow curves was obtained. In addition, the validity of the model has been demonstrated for two different alloys as well. The stress-strain curves for different sample and grain size combinations of these alloys were found in the literature. Again, a good agreement between the experimental and predicted flow stress values are obtained. By this way, the validity of the model is is shown for different materials, as well. Finally, micro-extrusion experiments were carried out to demonstrate the usefulness of the newly developed material model for simulations of micro-forming processes. Billets with two different thicknesses were extruded. By implementining the newly developed model, finite element simulations were performed as well. A reasonable agreement between the experimental and predicted maximum exrusion pressures were obtained. By this way, it has been demonstrated that the developed model can be used in simulations of micro-forming processes such as micro-extrusion.