Key Engineering Materials Vols. 554-557

Abstract: Real-time control of manufacturing processes is a challenging issue for nowadays industry. The need for ever more efficient production requires new strategies in order to make correct decisions in an acceptable time. In a large number of cases, operators working on a CNC machine tool have a reduced number of possibilities for interacting in real-time with the machine. Numerical simulation based control is in that sense an appealing alternative to the conventional approach since it provides the operator with an additional source of information, confirming his choices or in reverse suggesting a more adapted strategy. The main goal of this work is to propose a method to move from a bilateral approach (operator and CNC controller) to a trilateral one where the simulation is an active component of the manufacturing process. This paper focuses on a simple issue sometimes encountered in milling processes: how to remove a constant thickness of material at the surface of a part whose exact geometry is unknown The difficulty lies in the choice of an appropriate trajectory for the tool. So far the method which is employed consists in acquiring the geometry of the part thanks to a palpation step made prior to milling. However, this step has to be repeated for each part and can become rather fastidious as the size of the part increases. The approach presented here gets rid of the palpation step and makes use of online measurements for identifying the real geometry and correcting the trajectory of the tool in accordance. By monitoring the forces applying on the tool (directly on the NC), we have access to the milling depth and therefore to the geometry of the part at several locations along the trajectory of the tool. This information is used as an input data for our numerical model running on an external device, which finally derives an approximation for the geometry. An optimized trajectory is then obtained and is updated on the machine. This procedure is repeated as the tool moves forward and it allows for a fast and robust on-line correction of the toolpath.

Abstract: Within the general context of solid-state sintering process, this work presents a numericalmodelling approach, at the grain scale, of ceramic grain packing consolidation. Typically, the sinteringprocess triggers several matter diffusion routes that are thermally activated: surface, grain boundaryand volume diffusions. Including this physics into a high-performance computing framework wouldpermit to investigate and to track the changes occurring into a granular packing during sintering. Inperforming this kind of simulations, one will face several challenges: the strong topological changesappear during sintering simulation at the grains scale, the evolution of the structure is mainly drivenby the surface tension phenomena through the Laplace's law, and the mechanical properties of thegrains could, possibly, be different. The proposed numerical simulations are carried out within anEulerian Finite Element framework and the Level-Set method is used to cope with changes in themicrostructure. The results obtained with this numerical strategy are compared with success to theusual geometrical models.

Abstract: This paper presents a level set framework for the modelling of doping effect during surfacediffusion phenomena in a granular packing. The molecular flux of the doped compound is related tothe chemical potentials of all the diffusion species. The evolution of the grain compact is simulatedin three dimensions, based on the resulting kinetic law relating the surface diffusion velocity to thethermodynamic driving force. An anisotropic adaptive mesh, based on the level set function propertiesis used to refine the mesh in the surroundings of the grain surface. The simulations have been perfomedby using parallel computing strategy.

Abstract: It is difficult to predict springback, particularly in torsion, with high accuracy by FE simulation. Generally, more accurate springback prediction havs been achieved mainly by the improvement of material modeling such as Baushinger effect and plastic anisotropy models. It is also proved that tool deformations can greatly influence on the accuracy of torsion springback prediction as shown in the authors’ study [Esaform 2012]. The study shows that FE simulation using elastic tool model has 30% more accuracy in predicting torsional spring back in a curved hat shape than that by rigid tool model. But full elastic tool modeling is tedious work and FE calculation with the elastic tool model needs enormous time.There are two kinds of tool deformation during a press forming: tool deflection as a whole, and surface deformation where the tool is in contact with the steel sheet. Three forming experiments were carried out with an insert block of different stiffness, which touch steel sheets directly, in this study. The results revealed that surface deformation of a tool has great influence on torsion springback of a curved hat shape. Based on the results, a new tool modeling is proposed in this study. In the model, the part of a tool in direct contact with a blank sheet is elastic and the other part is rigid. That means the model deals with only surface deformations of tools in FE simulation. By the new model, the accuracy of torsion springback prediction of a curved hat shape was improved with less calculation time.

Abstract: Electromagnetic forming is a non-conventional forming process and is classified as a high-speed forming process. It provides certain advantages as compared to conventional forming processes: improved formability, high repeatability and productivity, reduction in tooling cost and reduction of springback and of wrinkling. However, various process parameters affect the performance of the electromagnetic forming system. Finite element simulations are very useful to optimize a process because they can reduce time and costs. With the aim of investigating the effects of the process parameters on the deformed blank geometry, finite element simulations of an electromagnetic sheet bulging test have been performed in this work. Furthermore the role of first impulse of discharged current is also investigated.

Abstract: Heavy steel plates are among the most essential construction elements for plant and heavy machinery. Their production involves hot rolling, followed by accelerated cooling and leveling. In this work, the focus is put on modelling the accelerated cooling step. It is the goal to build an algorithm which can be used for a virtual design of the accelerated cooling process in order to minimize distortion. Simulation of accelerated cooling requires a complex material model since various physical effects are involved, when plates are cooled down from 850°C to room temperature. Above all, the material undergoes a phase transformation from the austenitic parent phase to the bainitic product phase. The phase transformation is accompanied by metallurgical strains as well as transformation induced plasticity [1]. The highly nonlinear material behavior calls for implicit local integration. To this end, an implicit procedure is formulated within the plane stress theory accounting for plasticity and TRIP. Moreover, it provides the consistent elasto-plastic material stiffness. The global level normally requires an excessive number of DOFs for reliable predictions. It is a challenging task to make accurate predictions about the plate behavior (especially curvature) at the same time avoiding an excessive number of DOFs. A 3D calculation (Fig. 1) is still needed to verify the assumptions that are permitted to reduce the number of DOFs without losing accuracy. Eventually, the fast computation algorithm describes the approximate deformation of the plate by 3 DOFs only, enabling a reduction of computation time by several orders of magnitude. Due to its speed the algorithm can be used as an efficient tool for a virtual process design to obtain minimized distortion at the end of the cooling step (Fig. 2).

Abstract: Due to their low density and high specific strengths Mg-alloys provide an excellent potential to be used for light weight constructions. However, their equilibrium potential is very low resulting in relatively low corrosion resistance, especially in contact with other, more noble metals. In order to separate Mg from a corrosive environment hybrid billets with Al-alloy coating and AZ31-core were coextruded. Thus, the extrusion and coating of Mg-profiles can be done in a single production step, resulting in aluminum coated Mg-profiles. The influence of the extrusion ratio as well as of different die angles on the formation of diffusion layers at the interface was investigated. Furthermore the phases formed in the diffusion zones were analyzed using EDS and synchrotron XRD. Additionally, FEM-simulations were conducted in order to reveal the material flow of core and shell material during the forming process and to identify differences in using different die angles. The FEM-results were verified by comparison with the real extrusion experiments. Finally, the shear strengths of the produced compounds were evaluated in push-out tests.

Abstract: This study investigates the velocity fields that are descriptive for the forward, backward and friction assisted extrusion of axisymmetric rods. The Avitzur theory was used to calculate the velocity field and strain rate in extrusion of Al alloys. Several simulations have also been performed by using finite element analysis (FEA) with DEFORM 2D, in order to find the admissible velocity field for different conditions of friction including high and low friction. The results from FEA and theory of axisymmetric extrusion are compared to see if there is good agreement. The correlation between the data obtained by theory and FEA is discussed.

Abstract: The material flow in porthole dies is of crucial importance with regard to the seam weld quality in aluminum extrusion. Thus, experimental as well as numerical investigations on the effect of die geometry on the material flow were conducted. The experimental tests were performed on a 10 MN laboratory extrusion press. During the experimental trials, the extrusion ratio was varied by means of exchangeable die plates. Since the modular die allows removal of the aluminum in the welding chamber as well as in the feeders after the process, the material flow could be inspected in detail. The experimental results were used to improve the accuracy of FEA simulations, which were also conducted by commercial software. An attempt was made to improve the result quality of Eulerian FEA model regarding the simulation of an extrusion process with a gas pocket in the welding chamber. The influence of the modeling approach on the predicted material flow and on the contact pressure was analyzed and finally linked to the seam weld quality.