Papers by Author: Gow Yi Tzou

Abstract: The friction caused power losses and bulging deformation are always inevitable. The rotating compression forming can reduce the compression force and bulging deformation in metal forming. Furthermore, the bounded double-layer metal material is able to have advantage in proper material cost saving and meeting the required strength for customer. Accordingly, this present study proposes the analysis based on Slab Method (SM) and Finite Element Method (FEM) under constant shear friction to predict the compression force and the rotating moment in the rotating compression forming. Moreover the bulging deformation profiles, the effective stress, the effective strain, the velocity field, the compression force, the rotating moment, and the twist angle can be obtained from FEM simulations. It is also shown that the predicted results have the same trend to verify the acceptance of analysis models.

Abstract: Bulging deformation during cylinder compression forming results in geometrical imprecision and flow stress prediction error. Consequently, the present study proposes a hybrid technique comprising the Upper Bound Method (UBM) and geometrical fitting method. It is noted that a volume conservation constraint and an elliptical curve-fitting method with UBM for predicting both the compression force and the bulge profile. It is shown that the predicted results for the compression force based on UBM under certain reduction ratios of 30%, 40%, 50% and 60% are in good agreement with those obtained by experiment, Finite Element Method (FEM), and Slab Method (SM). In addition, it is revealed that the use of an elliptical curve-fitting technique provides a reasonable approximation of the bulge profile obtained by experiment and FEM simulations. Generally, the proposed method effectively provides a low-cost, computationally straightforward means of estimating the compression force and bulge profile in cylinder compression forming. Therefore it can be utilized as the widespread applications in the metal forming field.

Abstract: This study proposes a new piercing technology with rotating punch; it carries out an FEM simulation on rotating piercing process using DEFORM-3D commercial software. Frictions among the punch, the blank holder, the dies and the work-piece material are assumed as Coulomb friction, but can be different. The surface of the inner diameter, the effective stress, the effective strain, velocity field, damage, burr and the shearing force can be determined form the FEM simulation. In this study, effects of various piercing conditions such as the clearance, the punch nose angle, the frictional coefficient, the rotating angular velocity, the shearing force, and burr on shearing characteristics are explored effectively to realize the feasibility of FEM model.

Abstract: The paper concerns with an effect of plastic anisotropy on the load required to deform hollow cylinders between two parallel, rough dies. It is assumed that the material obeys Hill’s quadratic yield criterion and its associated flow rule. The friction stress is supposed to be proportional to the corresponding shear yield stress, including the maximum friction law as a special case. The kinematically admissible velocity field is chosen such that the stress field following from the associated flow rule satisfies the boundary condition at the plane of symmetry. Moreover, this velocity field is singular in the vicinity of the friction surface. Therefore, in the case of the maximum friction law the friction law is satisfied, again if the associated flow rule is combined with the velocity field. A significant effect of plastic anisotropy on the limit load is illustrated.

Abstract: This study investigates a novel changing channel angular (CCA) extrusion process, in which high strains are induced within the billet by passing it through a series of channels of unequal cross-sections arranged such that they form specified internal angles. Using commercial DEFORMTM 2D rigid-plastic finite element code, the plastic deformation behavior of CuZn37 brass alloy is examined during one-turn and two-turn CCA extrusion processing in dies with internal angles of φ =90o, 120o, 135o or 150o, respectively. The simulations focus specifically on the effects of the processing conditions on the effective strain, the rotation angle and the effective stress induced within the extruded billet. The numerical results provide valuable insights into the shear plastic deformation behavior of CuZn37 brass alloy during the CCA extrusion process.

Abstract: An investigation into the compression forming of cylinder using the commercial code SUPERFORM is developed. The cylinder billet compressed between the upper and lower dies is meshed by a quadrangle elastic-plastic element. The numerical simulation based on the FEM also compares with the slab method established by us. In the slab method analysis, the stress distributions are estimated by considering the coulomb friction between the dies and the cylinder. Throughout this study, the effects of frictional coefficient, rotating angular speed, reduction and aspect ratio etc upon the compression force, the effective stress and the effective strain, and velocity field are discussed systematically. For verifying the validity of two models, comparisons of compression forces based both modes are carried out to prove the feasibility of both models.

Abstract: The shear plastic deformation behavior of a material during equal channel angular (ECA) extrusion is governed primarily by the die geometry, the material properties, and the processing conditions. Using commercial DEFORMTM 2D rigid-plastic finite element code, this study investigates the plastic deformation behavior of Ti-6Al-4V titanium alloy during 1- and 2-turn ECA extrusion processing in dies containing right-angle turns. The simulations investigate the distributions of the billet mesh, effective stress and effective strain under various processing conditions. The respective influences of the channel curvatures in the inner and outer regions of the channel corner are systematically examined. The numerical results provide valuable insights into the shear plastic deformation behavior of Ti-6Al-4V titanium alloy during ECA extrusion.

Abstract: The choice of a kinematically admissible velocity field has a great effect on the predictive capacity of upper bound solutions. It is always advantageous, in addition to the formal requirements of the upper bound theorem, to select a class of velocity fields satisfying some additional conditions that follow from the exact formulation of the problem. In the case of maximum friction law, such an additional condition is that the real velocity field is singular in the vicinity of the friction surface. In the present paper this additional condition is incorporated in the class of kinematically admissible velocity fields chosen for a theoretical analysis of two - layer cylinders subject to compression and twist. An effect of the angular velocity of the die on process parameters is emphasized and discussed.