Key Engineering Materials Vol. 549

Abstract: The ISO standard 12004-2:2008E for the determination of forming limit curves based on the section method was approved in 2008. About 4 years of measuring experience in different laboratories has shown advantages and weaknesses of the standard and is leading to some minor changes in the specification. In the years from the development of this standard until today a further technical development of the optical measuring devices occurred, so that it is now possible to determine forming limit curves using the time history of the test. This procedure of determination is referred to a time dependent technique and could be the basis of the ISO 12004 part 2 proposal worked out by the work group Erweiterung FLC ISO 12004 of the German group of the IDDRG. This publication recapitulates existing work which was carried out from the IDDRG work group regarding the determination of forming limit curves for sheet metal materials. On one hand known issues with the current section based approach are discussed and on the other hand it deals with a comparison of different algorithms to determine the FLC from the time history of the Nakajima test using strategies to identify the instant of onset of instable necking. The different time dependent algorithms [ utilised are automatically selecting the area where necking is leading to fracture and then analyze the time history of such points using the first or the second time derivative of the true major strain, or of the true thinning strain using methods like: correlation coefficient (modified method based on [2]), gliding correlation coefficient, linear best fit (modified method based on [3]) and gliding difference of mean to median. The resulting experimental FLC points are compared with the results from the section technique described in ISO 12004 part 2 and with the maximum strain values measured in each test. Further a large number of forming limit curves were determined and used for a comparison of these different methods to define the most promising time dependent algorithm, which was selected as a suggestion for the working group defining the new proposed ISO standard 12004 part 2.

Abstract: Different titanium grades are used in aircraft construction because of titaniums unique properties. These materials are mostly joined by different welding methods. Electron beam welding technology is often used in the aircraft industry to join structural elements made of titanium alloys. The goal of the work is a numerical analysis of the electron beam welding process applied to joining thin titanium sheets. The analysis was performed using finite element method, FEM. Temperature distribution, size of heat affected zone (HAZ), depth and width of fusion zone were determined for the assumed heat source model. Thermo-mechanical (TMC) simulation of the electron beam welding process using FEM is presented in the paper. The joining of two sheets, one made of commercially pure titanium Grade 2 and the other made of titanium alloy Grade 5 (Ti6Al4V), is analysed in the work. For the sheet welding process distributions of temperature, effective stress, and sheet deformation were calculated.

Abstract: Numerical modeling of complex sheet stamping operations is well developed and implemented in industry. The weakest link now seems to be appropriate modeling of friction and to some extent also material properties especially when it comes to new lubricants and materials. In modeling of 3-D stamping operations a coefficient of friction μ is often determined by calibration of the simulation results with experimental observations of material flow and/or measured load. In case of modeling of new stamping operations μ is typically selected based on former experience. These procedures are, however, not appropriate when introducing new tribo-systems (lubricant, workpiece material, tool material or tool coating). In order to determine friction under the very varied conditions in sheet stamping simulative testing may be applied, e.g., Plane-Strip-Testing (PST), Draw-Bead-Testing (DBT) and Bending-Under-Tension testing (BUT) but these tests should be analyzed and carefully tuned with the production process in question to ensure useful results. The present paper illustrates how the BUT test combined with classical analytical modeling may lead to very large errors in estimation of the coefficient of friction, whereas detailed numerical simulation of the test can give useful friction values as demonstrated in comparative analysis of an industrial, multistage deep drawing.

Abstract: In the Tube Hydroforming (THF) process, a tube, placed in closed dies, is expanded by a high pressure liquid and two punches push its edges in order to feed the material into the expansion zones. Because of the high pressure and the contact area involved in the process, high friction stresses act on the tube walls restricting the material flow so reducing the amount of fed material, the part formability and affecting the soundness of the final part and its geometry. In fact, previous studies showed that the lower the friction coefficient at the tube-die interface, the more uniform the friction distribution and, therefore, the more uniform the stresses acting on the tube walls. The tube deformation being dependent on the stresses, its final thickness is influenced by friction. Starting from this premise, the authors proposed a reference test which is able to highlight the effect of friction on the final tube thickness. In this test, namely the THF Compression Test, a tube is placed in a cylindrical die having the same diameter as the outside of the tube, it is pressurized and then compressed by the punches. In this way, the tube has no expansion and its final thickness depends on the process parameters (tube material, pressure, punch stroke, tube material and geometry). Using FE simulations, it is possible to express the friction coefficient as a function of the process conditions and to use it in combination with experimental results. In the present paper, the previously validated FE model is used to investigate the influence of the tube material on the compression test results. Therefore, a simulation study was performed using different values of strength and hardening coefficients showing how the method is affected by the tested material thereby giving further indication of the test sensitivity.

Abstract: In the present paper, an EMF numerical model has been developed following an uncoupled approach, being the Lorentz forces acting on the workpiece estimated by solving Maxwells equations and then transferred to solve the mechanical problem. For formability analysis, a fracture indicator based on the linear forming limit diagram was applied through the use of a post-processing tool developed by the authors. To illustrate the applicability of the implemented code in the fracture prediction, an example of electromagnetic tube expansion is presented. The corresponding numerical simulation is performed and its results are compared with experimental obtained from literature for a selected material.

Abstract: Thick plate and sheet materials are often characterised by an inhomogeneous distribution of properties such as yield strength and anisotropy throughout their thickness. Forming of these materials involves further heterogeneous evolution of these properties. A recently developed computational framework [1, now allows these heterogeneities to be modelled via a hierarchical multi-scale material modelling scheme: the evolution of texture and plastic anisotropy can be tracked and individually updated at every integration point in a finite element model, in a computationally efficient manner. In this paper we present the application of this multi-scale model to a benchmark forming simulation, the three point bending test of thick plate steels. A number of hot rolled high strength low alloy steels were considered, two of which are presented here. The results of the simulations are validated against experimental results. Comparison is made between computed and experimental deformed shapes and strain fields, using data acquired by digital image correlation. Predictions of heterogeneously evolved textures are compared with experimental macro-textures, acquired by XRD, at key locations in the final deformed samples. Such models for plate steel forming simulations that are able to provide accurate predictions of deformation textures and derived quantities in the entire volume of the material can be crucial to study further processing steps and properties of the final product.

Abstract: Cylindrical components produced by mecano-welding process are widely used in industries. The mecano-welding process consists of a roll bending sub-process which can produce non-closed cylinders and a welding process which can seam gaps. This paper proposes a numerical model to simulate the process and to get better understanding of the process mechanism. Explicit and implicit solvers are applied to the numerical modeling by using LS-DYNA and ANSYS software. The numerical model can provide a useful tool for design and optimization of the mecano-welding process.

Abstract: Accumulative Roll-Bonding (ARB) process is a severe plastic deformation (SPD) process, capable of developing grains below 1 μm in diameter and improving mechanical properties of the material. In this study, the authors compared two different FE-codes with respect of its applicability for numerical analysis of the ARB process. Modelling this process was achieved using the explicit code for Abaqus/CAE both in 2D and 3D. The proposed model was used to assess the impact of ARB cycles on the final material properties. The numerical results in 2D and 3D were compared and contrasted. The research work presented in this paper is focused on the simulation optimization based on CPU time minimization. The numerical simulations were also validated through a comparison with the experimental results.

Abstract: Laser welding of complex aluminum add-on body parts such as vehicle doors, is a common joining technology in the automotive industry. Besides the many advantages (e.g. high processing speed) laser welding provides, temperature induced distortions are an important task to deal with. In the last twenty years, several simplified FE methods, which predict welding distortion (weld seams, spot welds) of large assemblies, were presented. In order to simulate the distortion of large car body components properly, realistic clamping conditions need to be considered [1, 2, 3]. Furthermore, the calibration process of simplified models has to be examined systematically, to find out their limits and achieve optimal simulation results [4]. In this paper, a new FE model is presented to predict distortion of laser welded structures, based on a shrinkage volume approach. Effective surface based clamping conditions (derived of the real clamping device) and effects of previous forming processes are considered. The simplified model was examined due to an extensive design of experiments. Not only simple, but even complex simulated specimens match with the experimental results very well.

Abstract: As an interstitial atom, nitrogen strengthens the structure of austenitic stainless steel (ASS). It therefore has been used to increase the strength of ASS. On the other hand, work hardening of ASS is a common method to increase the strength of the sheet product. When a work-hardened structure is welded, the strength properties decreases at the melted zone and the heat-affected zone (HAZ) of the weld. The nitrogen content can also be reduced by the effect of the heat input of the weld. Because the width of the soft area of the HAZ depends on the energy input of the weld, the strength of the weld depends on energy input. Therefore, laser welding provides better strength to the welded structure. The role of the shielding gas is also significant. Argon shielding gas is inert, but nitrogen used as a shielding gas can strengthen the weld metal and HAZ microstructure. In this study, the effect of different shielding gases in the laser welding of AISI 201 LN TR type work-hardened ASS are tested and the results are reported. Both non-destructive material and destructive material tests are performed. According to the results of the tensile test, the use of nitrogen as a shielding gas strengthens the laser-welded structure. The results of the low-cycle fatigue test show that fatigue strength improves when nitrogen is used as the shielding gas.