Key Engineering Materials Vols. 611-612

Abstract: Numerical simulation of machining processes represents a promising tool able to reproduce the cutting conditions without the need to perform a large number of experimental tests. In order to obtain reliable results from the finite element method simulation, is then necessary to properly set up the simulation conditions and to implement the most suitable materials behavior according to the real workpiece characteristics. These data are available in commercial softwares libraries but often they have difficulties to properly represent the machined workpiece behavior. Thus, advanced model are implemented in the software to improve the simulations performance and to obtain realistic results. In this work, the more suitable materials flow stress, within those proposed in literature, is sought to simulate the machining process of Ti6Al4V. The results of the simulations have been compared with those obtained experimentally in terms of temperature, chip morphology and cutting force. The results confirm the need to properly select the materials flow stress model according to the physical sample.

Abstract: In order to understand the problems that arising during machining and contribute to find the best way to produce medical prosthesis, an experimental milling analysis was conducted in Ti-6Al-4V and Co-28Cr-6Mo alloys. The thermal gradient and the cutting forces created on the surface during the machining were evaluated for different cutting speeds. Moreover, the roughness and Vickers hardness were evaluated. Based on the results obtained, it is possible to conclude that the difficult to cut of Co-28Cr-6Mo alloy is higher than Ti-6Al-4V alloy.

Abstract: The edge cracking sensitivity of AHSS and UHSS is quite challenging in the cold forming process. Expanding cut holes during flanging operations is rather common in automotive components. During these flanging operations the pierced hole is stretched that its diameter is increased. These flanging operations stretch material that has already been subjected to large amounts of plastic deformation, therefore forming problems may occur. An innovative cutting process decreases micro cracks in the cutting surface and facilitates the subsequent cold forming process. That cutting process consists of two stages, which produces close dimensional tolerance and smooth edges. As a result the hole expanding ratio was increased by nearly 100% when using thick high strength steels for suspension components

Abstract: For many material forming processes steady-state formulations allows reducing numerical simulation time by an order of magnitude with respect to more conventional approaches. In the presented approach, the steady regime is iteratively computed by a free surface algorithm that alternates computations of the metal forming flow over a known geometry and known contact surfaces, with computations of domain corrections to satisfy free and contact surface conditions. Several weak formulations of the second problem equations are investigated to get a robust algorithm suitable for parallel computations with unstructured meshes. Analytical problems show the necessity to introduce an upwind shift within these weak formulations. Contact inequations enforces this necessity by requiring a more dramatic shift. A robust and accurate algorithm is so obtained, which is successfully applied to 3D complex metal forming processes like rolling. In the wire drawing application, computational time is reduced by more than fifteen with respect to the incremental calculation of the steady-state.

Abstract: This paper describes the development of a multiphysics welding simulation model based on the discontinuous Galerkin (DG) finite-element method. Our numerical model implements a classical enthalpy-porosity constitutive law accounting for hydrodynamic and thermal effects occurring during the phase transition from solid to liquid metal. The objective of the study is to present the verification of our numerical framework and explore the applicability of the DG formulation to the simulation of welding processes. Three computational examples of increasing complexity are presented.

Abstract: This paper is concerned with the prediction of part distortion induced by residual stress relief during machining. Aeronautical manufacturing requires nowadays the production of large monolithic parts from workpieces obtained by forging or rolling operations. Residual stresses induced during these forming steps are often consequent and represent a significant obstacle for manufacturers. Re-equilibration of initial residual stresses induced by material removal causes the part to deform.The method presented in this paper provides a tool for predicting changes in the geometry of the part occurring during machining and after unclamping. This prediction is based on an on-line data acquisition. From the predicted shape, the manufacturer is able to anticipate extra conforming steps eventually required afterwards and an updated toolpath accounting for geometrical changes happening during the on-going operation can be derived.The impact of the unknown initial residual stresses on the part deformation during machining is predicted on-line by comparing measured deformations to a reduced basis of mechanically admissible evolutions for the part geometry induced by material removal. This reduced basis is extracted from a larger database constructed off-line thanks to machining simulations and observations of previously machined similar parts.An auxiliary database for post-machining distortion is constructed similarly and makes possible a quick and reliable on-line prediction of the final shape of the part (i.e. after unclamping).

Abstract: This paper presents a fast plastic integration algorithm and compares it with other algorithms for forming process simulations. The iterative Return Mapping Algorithm (RMA) is widely used owing to its accuracy and efficiency, but it is still time consuming and may cause divergence problems. Another algorithm based on the Incremental Deformation Theory (IDT) was proposed, using the deformation theory of plasticity by piecewise; it is very fast but could not well consider the loading history, leading to notable errors. The new Direct Scalar Algorithm (DSA) based on the flow theory of plasticity is proposed in this paper. The basic idea is to transform the constitutive equations in terms of the unknown stress vectors into a scalar equation in terms of the equivalent stresses which can be determined by using the experimental tensile curve; thus, the plastic multiplier λ can be directly calculated without iterative solution. The DSA is a fast and robust plastic integration algorithm. The comparison of the results obtained by using the three algorithms shows the accuracy and efficiency of the DSA.

Abstract: Inertia Friction Welding (IFW) is a solid-state joining process where one rotating (connected to an inertia) and one stationary part are brought together under an axial load, causing frictional heat generation and plastic deformation at the interface; upon cooling a weld is formed between the components. There is evidence in welds between dissimilar materials which show a flow regime that may keep impurities at the weld interface and may have implications for weld strength and fatigue life. Numerical modelling of IFW using Finite Element Analysis (FEA) has allowed the successful prediction of temperature profile, upset (length loss) and flash shape and process parameters such as flywheel slowdown. However, due to the lack of knowledge of the behaviour of the severely plasticised zone (shear zone) and the fluid-like nature of the material near the interface, the use of Computational Fluid Dynamics (CFD) has been considered. This paper presents a method to utilise both FEA and CFD modelling techniques to provide a better modelling strategy for the IFW processes. By using the results of an FEA model as the boundary/initial conditions for the CFD, simple models have allowed comparison between the two numerical approaches and have validated the implementation and consistency of material properties and modelling methodology for both. A model of the interface has been produced with CFD with this method which illustrates the possible material behaviour and material flow in that zone.

Abstract: Modelling the behaviour of metal alloys during their thermo-mechanical processing relies on the physical and mathematical description of numerous phenomena occurring in several space scales and evolving on different characteristic times. Although it is possible to develop complicated multi-scale models, it is often simpler to simulate each phenomenon separately in a single-scale model and link all the models together in a global structure responsible for their good interaction. Such a structure is relatively difficult to design. Both efficiency and flexibility must be well balanced, keeping in mind the character of scientific computing. In that context, the Agile Multiscale Modelling Methodology (AM3) has been developed in order to support the object-oriented designing of complex numerical models [. In this paper, the application of the AM3 for designing a model of the metal alloy behaviour is presented. The basis and some consequences of the application of the Object-Oriented design of a sub-models structure are investigated. The object-oriented (OO) design of a 3 internal variables model of the dislocations evolution is presented and compared to the procedural one. The main advantages and disadvantages of the OO design of numerical models are pointed out.

Abstract: Bending of multilayered sheets like lightweight sandwich sheets or fiber reinforced thermoplastics is dominated by the mechanism of interply-slip. FE-analysis is performed to predict defects depending on this mechanism. The shear and damage behavior of the adhesive layer of sandwich sheets can be modeled by cohesive elements in Abaqus. Forming simulation of fiber reinforced thermoplastics requires coupled thermo-mechanical analysis methods due to temperature dependence. For this, alternative modeling strategies for the inner layer of adhesive or polymer matrices will be tested in this paper that are able to transfer heat. The layer will be presented by solid elements with enriched property definitions or viscoplasticity. Furthermore the thickness of the layer will be neglected and replaced by contact formulations or spring elements.