Key Engineering Materials Vols. 504-506

Abstract: In this paper, a numerical problem with contacting solid metal flows is presented and solved with an arbitrary Lagrangian-Eulerian (ALE) finite element method. The problem consists of two domains which mechanically interact with each other. For this simulation a new free surface boundary condition is implemented for remeshing of the boundary elements. It uses explicitly that the integral of the convective velocity along a boundary element remains zero. Steady state solutions are obtained only if the integral of the convective velocities along each free surface boundary element remains zero. The new remeshing option for the free surface is tested on a cladding problem employing friction stir welding (FSW). The problem describes two elasto-viscoplastic aluminum material flows which mechanically interact.

Abstract: It is difficult to predict springback, torsion in particular, with great accuracy by FE simulation. In this study, the effects of anisotropic yield functions of the blank and elastic deformation of the tools on springback are investigated in order to improve the accuracy of torsional springback prediction in high strength steel part forming. The effects of anisotropic yield functions on the torsional springback in curved hat forming were compared, using parameter sets derived from various experimental methods: uniaxial tensile tests only; hydrostatic bulge tests and uniaxial tension tests; uniaxial and biaxial tensile tests. FE analysis of press forming taking account of tool deformations was carried out and the results were compared with the ones of FE simulation with rigid tools and experimental results. It is demonstrated that FE simulated torsional springback with deformable tool model has better correlation with experimental one than with rigid tool model.

Abstract: In this work a material model for hardening development in sheet metals during forming processes involving loading path changes is formulated. In particular, such hardening development is due to the formation and interaction of dislocation microstructures in the material, resulting in an evolution in the size, center and shape of the yield surface. Such yield surface evolution is accounted for in the current model with the help of an evolving structure tensor. The model is intended for an air hardening steel and takes therefore thermomechanics into account in particular phase transformations from ferrite to austenite and from austenite to martensite. As numerical examples a tension shear test and a heating-cooling sequence are simulated.

Abstract: The trends in automobile industry always include the use of new materials such as those needed for the passive safety of vehicles and they are one of the most important strategies to reduce injury of passengers during traffic accidents. Associated with the development of security systems, there is the possibility of improving efficiency by the introduction of materials that lead to weight reduction, having a direct impact on fuel consumption and lower carbon emissions. The present work aims to study the behaviour of sandwich structures, composed by a foam core with two outer layers of metal sheet (all structure being aluminium). The study of the composite structure behaviour, its mechanical characterization and numerical modelling is essential to analyse the mechanical performance of structures based on this type of materials. This step is fundamental in preliminary design, since the different materials of the composite structure show different mechanical responses. The differences in mechanical behaviour are demonstrated by the axisymmetric compressive stress states tests and also by the influence of hydrostatic pressure in the yield of the aluminium foam porous material, while the yield of the homogeneous solid material (aluminium sheet) can be considered as pressure insensitive. In order to correctly characterize separately these two materials of the composite (outer layers and core), a set of tests were performed. The characterization of the aluminium sheet was performed in a series of tensile tests, using three different rolling directions. For the metal foam core characterization a series of uniaxial compression tests were performed. The experimentally obtained results were applied in the development of numerical models for this kind of sandwich structure. The models include elastoplastic constitutive relation, where a distinct plastic domain for different materials is accounted for, as well as, the influence of hydrostatic pressure in the yield of the porous material. Also, the validation of the elastoplastic models is performed by comparing results obtained by numerical simulations with those obtained experimentally.

Abstract: In metal forming simulations, the transfer of data from one mesh to another can be very often required such as, in ALE formulation at each time step or in Lagrangian formulation at each remeshing step. Its accuracy is one of the main concerns for researchers, since the accumulation of generated diffusion can lead to larger numerical error and create convergence problems. Since 1992, the Super convergent Patch Recovery method (SPR) introduced by Zienkiewicz & Zhu [1], was a major breakthrough for the methods to estimate errors in FE solution. Later, it has also been used to recover nodal fields from integration points, in order to transfer data between two meshes [10]. The original method raises some difficulties to treat the domain boundaries. They have not quite properly been handled, in particular in the frame of parallel implementation, despite the fact that, surface phenomena, such as contact and friction, play such an important role in metal forming applications. In the present paper, it is presented a modified iterative SPR method which deals with boundary points with the same order of accuracy as the interior points. It does not require increasing the patch size and it is easier to implement in parallel environment. Also, when interpolating the field on new mesh, a new and consistent technique (P0+ transport) of enriched field has been used. It involves building a P1+ field by mixing the recovered SPR nodal field with the known field at integration points. Results are presented in form of convergence rate and L2 error norm, for several analytical functions for a SPR based transfer operator against a simple volumetric based transfer operator, before being applied to an actual metal forming problem. All computations presented here have been done by using four processors in parallel environment.

Abstract: Many models encountered in computer science remain intractable because of their tremendouscomplexity. Among them, the numerical modeling of manufacturing processes involving severalcharacteristic times is a challenging issue. Classical incremental methods often fail for solving efficientlysuch transient models. In that sense model reduction based simulation appears to be a verypromising alternative. Multidimensional parametric models can be solved within the context of theProper Generalized Decomposition (PGD). It opens new horizons regarding time parallelization. Indeed,by no more considering the initial condition of a transient problem as a static input data butas an extra- coordinate similarly to space and time, we demonstrate that it is possible to parallelizeefficiently the computation and even reach real-time in some cases.

Abstract: We analyse here how Dynamic Data Driven Application Systems (DDDAS) can constitute a valuable tool in the field of forming processes. Simulation tools in the field of DDDAS are required to provide a response in real-time, a requisite that is often too severe for complex problems. Here, we consider that of hyperelasticity, commonly used in different fields such as rubber manufacturing or surgery, for instance. We analyse here how model reduction techniques, and particularly Proper Generalized Decompositions (PGD) methods can provide a suitable response to the strong requirements posed by DDDAS. We will consider two different approaches to the problem. The first one is an explicit approach that nevertheless provides with good results. The second one is based on the use of Asymptotic Numerical Method.

Abstract: The material flow in a screw extruder of aluminium was investigated. Screw extrusion of aluminium is a new solid state process where granulate aluminium is consolidated and extruded in a single continuous process using a rotating screw as the pressure generating device. Understanding the material flow is vital for predicting the consolidation and welding process thereby making it possible to optimize screw and container design to increase the capacity, ekstrudate quality and process stability. Using a prototype screw extruder, aluminium alloy AA6060 was extruded together with contrast material using different feeding schemes to visualize the material flow. Material left in the extruder was inspected visually, sectioned and etched in NaOH to reveal the interaction between the contrast material and the AA6060. It was found that newly fed contrast material primarily displaced material in the centre of the screw channel and the extrusion chamber hereby revealing the main paths of the material flow. Areas of sticking friction, material dead zones and zones of slow material displacement rate were effectively identified using this method.

Abstract: Aluminum matrix composite extrusions reinforced with wires made of high strength stainless steel represent an innovative material concept for lightweight structures. The use of reinforcing elements should improve the mechanical properties and the performance of lightweight structures. This study deals with the process chain of extrusion and die forging to manufacture steel-reinforced products. The production of discontinuously-reinforced, semi-finished aluminum profiles by co-extrusion is in focus on the extrusion part. The material flow is analysed in order to understand, and further to influence, where the steel-reinforcements are appear in the strand. For the forging part the extruded profiles are continuous-reinforced by means of steel wires as well as partially by means of steels elements. For the process design the geometry of the forging die cavity and the material flow are of vital importance. A Finite Element Analysis is carried out in order to predict the position of the elements in the forging parts depending on the position in the extrusions.

Abstract: The increasing attention to magnesium alloys in extruded profiles, especially in the transportation industry, is related to their low density associated with good mechanical properties and complete recyclability. This allows to push towards both increasing efficiency and pollution restrictions. However, these advantages are negatively balanced by the production rates drop in relation to dangerous profile temperatures increasing that force to keep low velocities. In this context, a novel porthole die has been purposely designed for magnesium alloys allowing an increasing of the process velocity up to four times with respect to past solutions. The mandrel consisted of three ports made by 120° bridges that created an equal number of seam welds. The extruded tubes, made in ZM21, were 50 mm in diameter and 2 mm in thickness and were tested under different process conditions. In the present work, the quality of the seam welds has been investigated in relation to each process condition by means of the rubber plug testing method that allowed to applied an hydrostatic tensile state.