Key Engineering Materials Vols. 504-506

Abstract: In this note, techniques for goal oriented error control of finite element discretizations are proposed for frictional contact problems. The finite element discretization is based on a mixed method, where Lagrange multipliers are introduced to capture the geometrical and frictional contact conditions. A posteriori error estimates for user-defined, probably non-linear quantities of interest are derived using the dual weighted residual method (DWR). Numerical results substantiate the applicability of the presented techniques to the simulation of metal forming processes.

Abstract: Sheet-bulk-metal forming processes require an accurate material model which is derived in this contribution. The microscopic model is based on a simulation of a real microstructure. A validation on the macroscopical scale is performed through the reproduction of the experimentally calculated yield surface based on the homogenised structural response of a corresponding deformed representative volume element (RVE). The microstructural material model is also compared with a macroscopical phenomenological model based on logarithmic strains. The homogenised microscopic model and the phenomenological macroscopic model are in good agreement with the evolution of the stresses and strains obtained during the experiments.

Abstract: The present contribution deals with the process-machine interaction and its impacts on the workpiece quality by forming parts with sheet-bulk metal forming. It focuses on the manufacturing of complex asymmetrical parts with functional elements. The functional applications of these elements such as fixation, motion and load transmission require compliance with high geometrical accuracy as well as high precision regarding the final mechanical properties of the part. The high process forces in horizontal as well as in vertical direction to form these elements cause displacements of the tool and press components, which lead to workpiece defects retroactively. Although the interaction between the forming machine and process affects the quality of the finished part significantly, the machine influence is usually ignored in the analysis of the forming process with the Finite-Element-Method. The challenges mentioned are demonstrated by forming of a complex asymmetrical part with gear teeths and carrier elements. As typical for sheet-bulk metal forming, the dimensions of the finished gear teeths (4mm) are much higher as the initial thickness of the sheet metal (3mm). The displacements of the press components during the process have been determined by means of optical measuring systems. The results are presented in all three spatial directions. The conclusions about the acting process forces in horizontal and vertical direction have been made by analyzing the press displacements by means of a new developed three-axial load device. Finally, this paper introduces a suitable approach to represent the machine characteristics in order to improve the computational accuracy of sheet-bulk metal forming simulations and gives an overview about the possibilities to improve the process stability by improving the mechanical components of forming machines.

Abstract: Sheet bulk metal forming is a new forming technology, currently developed by several companies and research institutes. It creates high demands on the inspection of parts and tools, especially in the field of in-situ abrasion detection of the forming tool and its impacts on the work piece. This manuscript introduces two optical testing methods for fulfilling these inspection tasks: On the one hand the endoscopic fringe projection as a flexible small scale optical measurement principal with high depth of focus and accuracy for the acquisition of filigree form elements for a continuous abrasion determination and one the other hand the multi-scaled fringe projection for a holistic one shot measurement of the work piece for an adapted, multiscale deviation analysis. The development and advantages of both systems for the sheet bulk metal forming process are shown as well as potentials of the combination of the both systems close to the proposed application next to the production line.

Abstract: Forming processes are characterized by high cost-effectiveness due to high material utilization and short forming process time. Therefore the development of new forming processes aims for overcoming the current process limitations to produce function extended components with a wider range of application and lower manufacturing costs. The comprehensive and complex development investigations of cause-effect relationships and interdependencies lead to an intransparent development status, whose assessment is often based on undefined and not reproducible criteria. This can result in wrong decisions with vain modifications that require subsequent changes associated with an increased effort. A valid characterization of the development status of new forming processes by maturity determination in early development phases can detect improvement potentials in order to apply effective measures with reduced efforts. Maturity models provide essential indicators and assign their values to maturity levels, whereby a uniform assessment base is created and comparability of the determined maturity values is ensured. On the basis of a combined reference model maturity-related indicators for assessing the development status of new forming processes are defined. The designed maturity model has been applied in the development of the Sheet-Bulk Metal Forming.

Abstract: Nitride layers on the high-speed steel 1.3343 were produced by means of transmitted plasma arc. The surface treatment with plasma arc leads to high thermal stress reliefs caused by the tracing point of the arc. To use the plasma arc to build up a nitrided layer without a deep heat penetration of the base metal requires a good knowledge of the temperature profile on and under the surface. The investigations show that the steel needs a stable temperature on the surface to reach maximum nitrogen input. At the same time the surface temperature must not reach the melting point of the material. To satisfy these two conflicting conditions an accurate temperature control is necessary. Surface temperatures, temperatures in the bulk and under the surface were measured by conductive and thermographic methods and were correlated with investigations of the resulting metallographic structure. It was shown that the temperature distribution in and under the surface zone during the atmospheric plasma-nitriding shows a large gradient and the material temperature at a depth of 100 µm is not more than 200 °C.

Abstract: Manufacturing of functional sheet metal products by forming can be realised with the application of conventional bulk forming operations on sheet metals. The challenges of those sheet bulk metal forming processes are high resulting forming forces and the demand on a specific control of material flow. To meet these challenges well-directed thinning of blanks as well as accumulations of material to form functional elements is employed. Due to local loads, simultaneous 2D and 3D stress and strain states occur. Process adapted semi-finished products, containing a defined sheet thickness characteristic, are formed in the presented work by the technologies upsetting and orbital forming. Orbital forming is an incremental bulk forming operation to decrease the forming zone extension and consequently the required process force. Afterwards a process combination of deep drawing and upsetting in order to manufacture a cup-shaped workpiece with external gearing is presented. The results of this integrated single-stage forming process are discussed and subsequently the potential to enhance the process limits is shown by using process adapted semi-finished products.

Abstract: Sheet-bulk metal forming is a novel manufacturing technology, which unites the advantages and design solutions of sheet metal and bulk metal forming. To challenge the high forming force the process is superimposed with an oscillation in the main flow of the process. The paper focuses on the characterization of the material behavior under cyclic load and the effects for the sheet bulk metal forming process.

Abstract: Innovative trends like increasing component functionality, the demand for automotive lightweight constructions and the economic issue to optimize existing process chains, require new ways in manufacturing. Today, the traditional sheet metal and bulk metal forming processes are often reaching their limits if closely-tolerated complex functional components with variants have to be produced. A promising approach is the direct forming of high-precision shapes starting from blanks. Thus, classic sheet metal forming operations, such as deep drawing, are combined with bulk metal forming operations like extrusion of complex variants as for example teeth. This combination of sheet and bulk metal forming operations leads to a side by side situation of different tribological conditions according to the locally varying load situations within the same forming process. This new class of forming processes is defined as sheet-bulk metal forming (SBMF). The tribological conditions in sheet-bulk metal forming processes are of major importance for the process realization, its stability and for the quality of the produced part. The objective of this paper is the investigation of material flow in SBMF in general and the attempt to improve the material flow by local adapted tribological conditions. First the material flow was analyzed by FE-simulation of a model geometry that is typical for SBMF. The investigations with FE-simulation have shown, locally adapted tribological conditions are leading to an improvement in material flow and thus to an increased mould filling. As frictional conditions are directly connected to the topography of workpiece and tool, the modification of the workpiece topography is leading to an alteration in friction values. For the modification of workpiece topography grit blasting was used. The increase in friction of grit blasted surface towards untreated surface was investigates by using the laboratory friction tests. To manufacture specimens with locally adapted topographies for forming tests a masking technique has been developed. The masks are designed after the preliminary findings determined by FE-simulation.

Abstract: This paper presents an analysis of roll bite heat transfers during hot steel strip rolling. Two types of temperature sensors (drilled sensor /slot sensor) implemented near roll surface and heat transfer models are used to identify in the roll bite interfacial heat flux, temperature and Heat Transfer Coefficient HTCroll-bite during pilot rolling tests. It is shown that: - the slot type sensor is much more efficient than the drilled type sensor to capture correctly fast roll temperature changes in the bite during hot rolling but life’s duration of the slot sensor is shorter. - average HTCroll-bite, identified with roll sensors temperature signals is within the range 15-26 kW/m2/K: the higher the strip reduction is, the higher the HTCroll-bite is. - scale thickness at strip surface tends to decrease heat transfers from strip to roll in the roll bite. - HTCroll-bite appears not uniform along the roll-strip contact, in contrast to usual assumptions made in existing models - Heat dissipated by friction at roll-strip interface and its partitioning through roll and strip respectively seems over-estimated in the existing thermal roll gap model [1]. Modeling of interfacial friction heat dissipation should be reviewed and verified. The above results show the interest of roll temperature sensors to determine accurately roll bite heat transfers and evaluate more precisely the corresponding roll thermal fatigue degradation.