Key Engineering Materials Vols. 651-653

Abstract: The state of deformation in deep drawing operations is characterized by superimposed stretching and bending (i.e. stretch-bending). Bending effects, especially for Advanced High Strength Steels (AHSS) are known to influence the material formability. Traditional formability measures such as the Forming Limit Curve (FLC) fail to reliably predict stretch-bending formability. Consequently, to ensure an efficient and economical use of AHSS in the industrial application, current research work is focusing on the reliable numerical prediction of stretch-bending formability of AHSS sheets.Within this work, a phenomenological concept to predict the forming limit (e.g. the onset of necking) in deep drawing processes taking bending effects into account is presented. The proposed concept is based on curvature-dependent (i.e. regarding the principle curvatures κ₁ and κ₂ of the stretch-bend (convex) sheet surface) forming limit surfaces representing the probability of failure and is calibrated with experimental results from stretch-bending tests and conventional forming test such as a Nakazima test. The results of the phenomenological forming limit criterion are promising and show a more accurate prediction of the drawing depth at failure than the conventional FLC approach. The method contributes also to a probabilistic view on the forming limit of deep drawing parts.

Abstract: This work concerns with two successive modifications of the Lemaitre's damage model tomeet the requirements of formability prediction for today's modern steels. The first one is the quasiunilateraldamage evolution which modifies the damage driving force by scaling the elastic energyrelease rate due to compressive principal stress components. The second one is the shear modificationby which the damage rate is multiplied by a normalized maximum shear stress dependent factor.With the assumption of non-rotating principal axes of deformation, proportional strain paths and rigidplasticity, closed form expressions for the isochronous fracture surfaces are derived for each modelvariant and resulting surface plots at various spaces are compared. The findings show that the formermodification not only remedies the pathological reflective symmetry of the fracture surface acrossthe plane with vanishing stress triaxiality ratio, but also allows hindering fracture under uniaxialcompression. The latter modification by adding a direct Lode parameter dependence to the damageevolution function allows prediction of premature fracture at generalized shear stress states, a conditionobserved for certain advanced high strength metallic sheets. Parameter calibration is realized foreach model variant using the experimental data from the literature. It is shown that the fracture modelwith both the shear modification and the quasi-unilateral enhancement shows the best fitting quality.Finally, the models are implemented as user subroutines for ABAQUS/EXPLICIT and used in predictionof initiation and propagation of cracks for a series of deep-drawing punch tests. A good agreementwith the outputs reported in the literature is observed in terms of the shear damage occurrence zonesas well as corresponding punch force-displacement diagrams.

Abstract: A numerical model to predict forming limit diagrams (FLD) for polycrystalline metal sheets is presented. In it, the Marciniak-Kuczynski (MK) approach is incorporated into the framework of the viscoplastic self-consistent (VPSC) crystal plasticity model. The current model, dubbed the VPSC-FLD, can run simulations along individual loading paths in parallel, which can make use of a CPU-cluster to enhance the computational speed. The main objective of the current work is to provide a detailed sensitivity report based on the VPSC-FLD. First of all, the influence of the initial inhomogeneity, f , as defined in the MK approach, is illustrated. Secondly, FLDs resulting from various sizes of the statistical population for the crystallographic texture are examined. Lastly, the computation time spent for various sizes of the statistical population is given.

Abstract: As important light-weight structure material, aluminum alloys have been widely used in automotive and aerospace industries. In the last years, the manufacturing of parts with high strength and good dimensional accuracy has become the main objective in industrial applications. Within the available aluminum alloys, the 7xxx series has attract the interest of the industrial designers due to the high yield strength and ultimate tensile strength they present. However, the formability of these alloys in as-received industrial condition is very poor at room temperature and various studies are being carried out to develop efficient warm and hot forming processes to form them industrially using heated tools. In the present paper, the W-Temper forming is studied as an alternative to the warm and hot forming processes. Heat treatment temperatures and critical times are presented and an industrial B-Pillar is formed to validate the new process. In the last chapter, the final mechanical properties of the part are reported, before and after a virtual e-coat process where the W-Temper forming is compared with a hot stamping process.

Abstract: This paper introduces a concept for the simulation of temperature distribution in reversing hot rolling of magnesium strip. The technological chain consists of reheating the casted rough strip, carrying the coil form furnace to coiler, setting down on mandrel, and hot reversing rolling. The paper represents the current state of work. The numerical calculation is based on an object-oriented FEM toolkit written in MATLAB™ and is carried out in three spatial dimensions and time.

Abstract: Metal forming simulations based on the finite element method are a frequently used tool for the prediction of the deformed shape, material state and reaction forces. The most critical prerequisite for any reliable result is a reasonable description of the constitutive behavior of the underlying material. The presented work focusses on the latter for the case of molybdenum via advanced formulations for the temperature and strain rate dependence. The quality of the results is compared for several approaches. Sheet metal rolling serves as an example application. Verification is based on comparison with data from industrial processes.

Abstract: Cold roll forming is a process for plastic deformation, which allows realizing profiles, with a defined section and established length, from the plastic deformation of a metal sheet. The sheet is induced to cross several stands of rolls, arranged along the same axis of advancing. The rolls induce plastic deformation in the sheet and then lead it to the desired geometric configuration. In order to control the geometric parameters of the plate during the profiling, it was created a FEM model to simulate the final stage of the technological process, developed by an industrial production line of a company located in Naples (Italy), that sells tubes with several cross sections. In this phase, the semi-finished product, having a circular cross section, is forced to cross through four stands of rolls. In this way, it changes the geometric condition of the cross section from circular to square. The model was carried out using a non-linear calculation code, which allows analyzing the parameters of interest in the different process steps. The results, obtained numerically, were compared with the experimental ones through the measurement of five specimens, obtained directly from technological process. The values of percentage deviation, regarding the external dimension and the thickness, for each step of advancement, do not exceed the 3% of error. Then, the analysis results denote the capability to simulate the cold roll forming process using finite element method.

Abstract: Titanium forging has been encountering a growing interest in the scientific and industrial communities because of the distinct advantages it provides with respect to machining, in terms of both mechanical properties of the product and material waste, thus significantly reducing the Buy to Fly ratio. In the paper, a numerical FE model, based on a tri-coupled approach and able to predict the microstructural evolutions of the workpiece during the process, is developed and set up. Calculated results are compared to experiments for a few industrial case studies. The final phases distribution in the forged parts is experimentally measured and compared to the FE model output finding satisfying overlapping.

Abstract: Abstract. Residual stress gradients across the wall of seamless steel tubes influence decisively the mechanical stability and reliability of automotive and industrial constructions. Irreversible bending moments imposed on the tubes induce gradual and asymmetric elasto-plastic deformation across the tube cross-sections which result in very complex residual stress distributions. The aim of this contribution is to present a novel methodology as well as complementary modeling approach to assess the three-dimensional distribution of triaxial residual stresses in bent steel tubes. The stress characterization was performed using high energy X-ray diffraction at the HEMS beamline of PETRAIII synchrotron source in Hamburg as well as using laboratory Drill-hole method. For the complementary modeling of the stress distribution, a FEM software package DEFORM ^HT was used. The results reveal that the stress gradients across the tube wall are primarily influenced by the martensite profile predetermined by the parameters for thermo-mechanical treatment of the tubes. The tube bending causes the formation of continually varying compressive and tensile stresses across the tube circumference whereas the stress magnitude across the wall thickness scales again with the martensite appearance. Finally the results document the importance of the cooling process control and the influence of the applied bending radius on the resulting stress distributions as well as related mechanical parameters like fracture toughness and fatigue behavior.

Abstract: The ingot forging process is numerically simulated applying both the Shima-Oyane porous plasticity model as a coupled damage model and the uncoupled normalized Cockcroft & Latham criterion. Four different cases including two different lower die angles (120^o and 180^o) and two different sizes of feed (400mm and 800mm) are analysed. Comparison of the simulation results with recommendations in literature on ingot forging, indicates the normalized Cockcroft & Latham damage criterion to be the most realistic of the two.