Papers by Author: I. Alfaro

Abstract: The conventional tube extrusion process has been substituted by porthole die extrusion due to relevant advantages in terms of productivity and quality. However, the porthole die has a complex geometry to be effectively designed; consequently, several studies can be found out in the technical literature based on experimental and finite element analyses of the process. From this point of view, while the experimental investigations entail cost and time increasing, due to the die building complexity, finite element techniques present some drawbacks such as the difficulty to simulate material joining and the loss of accuracy due to the heavy mesh distortion and related remeshing. Therefore, the introduction of new numerical techniques for the analyses of this process could have positive effects. In this paper, the Natural Element Method (NEM) together to the alpha shapes and some extra numerical procedures are used in the simulation of tube extrusion, focusing the attention on the simulation of the welding line in a fully 3D analysis. The obtained results are compared with the finite element and experimental ones, measuring the accuracy of the proposed methodology.

Abstract: FEM implicit formulation shows specific limitations in processes such as cutting, where large deformation results in a heavy mesh distortion. Powerful rezoning-remeshing algorithms strongly reduce the effects of such a limitation but the computational times are significantly increased and additional errors are introduced. Nodal Integration is a recently introduced technique that allows finite element method to provide more reliable results when mesh becomes distorted in traditional FEMs. Furthermore, volumetric locking phenomenon seems to be avoided by using this integration technique instead of other methods, such as the coupled formulations. In this paper, a comparison between a “classical” FEM simulation and the Nodal Integration one is carried out taking into account a simple orthogonal cutting process.

Abstract: Porthole die extrusion is a process typology that can give great advantages in the forming processes. Due to the complexity of the die assembly, experimental analyses are often carried out in order to investigate the parameter influence on the quality of the final parts. Finite Element Analyses, however, have been often used for the cost reducing and for a better local investigation of variables like pressure and effective stress inside the welding chamber. In spite of that, up to now, commercial FE codes present a “structural” limit during the welding phase due to the impossibility to simulate element joining when material reaches the required process conditions. From this point of view, the Natural Element Method (NEM) provides significant advantages; in fact, the meshless characteristic of NEM is “natively” able to simulate joining of free surfaces, as it occurs during porthole die extrusion, simulating the welding line formation inside the welding chamber. In this paper, using experimental tests recognizable in literature, the authors tried to validate the effectiveness of this technique; moreover, even a comparison between NEM and FEM results was carried out. More in detail, different geometries of the welding chamber were analyzed; in some cases, the process conditions were suitable to guarantee material welding while, in other cases, the material came out from the porthole die without joint formation. The variable that was used to verify the process goodness is the maximum pressure inside the welding chamber. Furthermore, to evaluate the effectiveness of 2D analyses, even in a complex shape, a significant section was extrapolated for each die, performing a NEM vs. FEM assessment of the results. A good comparison was obtained between the two different methods that, moreover, were in agreement with the experimental tests.