چكيده به لاتين
Producing high strength tubes via conventional severe plastic deformation methods in industrial scale is a challenging issue. For this purpose, friction assisted tube straining (FATS), friction assisted tube forming (FATF) and friction assisted tube extrusion (FATE) were introduced to produce high strength tubes in desirable dimensions. Applicability of the presented methods were experimentally examined on commercially pure copper and yellow brass. The considerable grain refinement after 7 passes of the FATS was obtained where the initial microstructure with average grain size 30µm was refined to 1.4µm. Mechanical properties of the FATSed tubes showed significant changes where yield stress was increased to 374MPa from an initial value of 40MPa. A same trend was observed for hardness and ultimate tensile strength with 3.5 and 1.4 times enhancements. Finite element simulations of the FATS process were executed adding automatic re-meshing capability to ABAQUS/Standard via Python scripts, providing high quality mesh during element distortion. The comparison of the FE results with the experimental tests for determination of temperature, extrusion force histories and hardness revealed the good agreements between them. DOE and Taguchi method were utilized to arrange the parametric study tests to investigate effect of the process parameters on microstructure and mechanical properties of the FATSed tubes. The results revealed that, increasing feed rate and reducing rotary speed decrease grain size and enhance material yield stress while die angle showe insignificant effect. A fully couple thermomechanical analytical model of the FATS process was presented, calculating the admissible velocity field, the processing power and the heat transfer equations. The derived equations were coupled and were solved in many time increments. The results of the computational modeling revealed that the slope of the deformation area increased when friction coefficient was enhanced. In addition, the presented model was more accurate in the friction coefficient limit between 0.3-0.5. The comparison of the uncoupled and the coupled solutions resulted diminishing the temperature of the deformation area and the extrusion force for the coupled solution. In addition, the analytical solution results were in good agreement with the FE and the experimental tests on the temperature and the extrusion force histories and the torsion angle. The numerical and the experimental investigations of the FATF and the FATE processes on the copper tubes showed 4, 2, 2.4 and 6 times enhancemeent in yield stress, hardness, imposed plastic strain and the required force respectively. Also, grain size is decreased to 8µm from the initial value 55µm.
