Limit Diagrams for Selecting Process Parameters to Prevent Defects Formation during Forward Bar Extrusion Using FEM

Finite Element Method (FEM) has becoming more influences in analyzing and solving metal forming problems from the beginning of punch and die designed up to setting the appropriated surrounding constrains in the deformation processes. In this research, the study of forward bar extrusion process of titanium alloy; TiAl6V4, was considered by using a commercial FE program. The parameters that were investigated in extrusion process were die angle, 2α, and area reduction ratio, ε_A, while Coulomb friction coefficient, μ, between workpiece and die were assumed according to the result from compression test. The billet had initial diameter of 24.8 mm. and length of 35 mm. The area reduction ratios in the forming process were varied from 30% to 90% and the die angles were varied from 30° to 90°. The punch and die in the simulation were assumed to be rigid, which neglected small effect of elastic deformation in order to simplify the process. The material property was assumed to be isotropic, behaved according to constitutive equation of power law, and deformed rigid-plastic, which followed Von Misses yield criterion. In case of heat that had generated and dissipated between the workpiece and die in the extrusion process, they have little affected to the change of material property. The process was assumed to be done isothermally at room temperature of 20° Celsius to simplify the process of heat transfer. From the simulation results, the approximated prediction for the process condition with defect-free products in the central axis was obtained by using hydrostatic tensile stress and maximum principle stress. The predicting results, which were compared to the analytical results of Avitzur, had shown a similar trend and good correspondence. The safe condition for extrusion process was obtained, when the die angle was small and the area reduction ratio was large. Thus, by increasing in the area reduction ratio had caused an increased in hydrostatic compression stress, which retarded the occurrence of damage in the central part of the workpiece. In contrary, the increasing in die angle had caused an increase in hydrostatic tensile stress in the central part of the workpiece, which hasten the occurrence of damage. Thus at large die angle and small area reduction ratio, the extruded products were obtained with defects in the central part, which caused by high hydrostatic tensile stress. The forming limit diagram was obtained, which provided information and preference for extrusion process with defect free products. Furthermore, these data were considered as additional information in this field of research and development.