Key Engineering Materials Vol. 716

Abstract: Short Cycle Stretch Forming (SCS) is an innovative stretch forming technology developed at the Institute for Metal Forming Technology (IFU) at the University of Stuttgart. The SCS technology combines plane pre-stretching of blank and subsequent deep drawing operations within the same stroke of press ram. The sheet metal thickness is reduced while denting resistance and yield stress increases due to hardening effects.Current research work focuses on applying SCS-technology to rotational-symmetrical bodies. A process simulation for SCS-Cupping process was performed for food cans. Based on these results a tool was manufactured and commissioned. The results showed that the thickness of cup bottoms of two-piece drawn and ironed (D&I) steel cans can be reduced. Therefore, it is possible to save material costs in serial production based on a reduced blank diameter.In this paper the different effects leading to the thinning of steel can bottom and failure types, such as, wrinkling and cracking are observed in a number of experimental series. Based on these results, the tool geometry was optimized and an advanced tool was manufactured. The results of this paper show that SCS-Cupping offers promising potential to save material, as well as outlining the main effects for this technology.

Abstract: The following paper deals with the production of Interpenetrating Phase Composites (IPC) using semi-solid forming technology. Therefore, adequate ceramic foams were selected and infiltrated by processing the aluminium alloy A356 in the semi-solid state. In the studies presented in this paper, the infiltrations of two ceramic materials (Al₂O₃ and SiC) with three different pore sizes (10, 20 and 30 ppi) were investigated. During the forming process the liquid phase fraction of the aluminium was varied to analyze infiltration effects in relation to the raw material´s liquid phase fraction. Afterwards, microsections of the produced specimens were analyzed in order to characterize their microstructure and the quality of infiltration. The results showed that completely filled composite components with good mechanical properties can be produced by infiltrating ceramic preforms with a semi-solid aluminium alloy.

Abstract: Blanking noise and blanking vibration are causes of pollution. At the same time, blanking vibration disturbs the high precision of a press work product, and shortens the life expectancy of the press machine and the press die. Recently, the use of a servo press has been developed to improve these problems, using low speed blanking and pulse vibration banking. However, these methods lengthen the blanking time, so it is necessary to control the servo press properly in the case of blanking. In this study, we describe a principle for the reduction of blanking vibration when using a servo press, and we propose an active vibration control method for the reduction of blanking vibration. We used a crank servo press of 450kN in an experiment, which has a pulse motion of 10Hz. We conducted a simulation and an experiment of blanking using an active vibration control method, which we named “counter pulse blanking.” We confirmed that counter pulse blanking is effective in reducing vibration as a result of the simulation and the experiment.

Abstract: Residual stresses are often introduced into aluminum alloys through quenching processes performed to generate the required microstructure. Such residual stresses are known to be deleterious to the integrity of the component. Methods to mitigate residual stresses in quenched components are therefore of great importance. Cold rolling has been proposed as an effective technique to remove residual stresses in large components. In this work, the effectiveness of cold rolling in reducing the residual stresses in quenched blocks AA7050 has been quantified using the neutron diffraction technique. Neutron diffraction measurements have been performed on two blocks one quenched and the other quenched & cold rolled block. Comparing the residual stress distributions pre and post rolling it has been found that cold rolling almost eliminates the tensile residual stresses in the core of the block, however it generates large tensile residual stresses d in a shallow region near the surface of the block.

Abstract: The servo press has high potential for producing high precision mechanical parts. However, small gaps between dies and workpieces tend to exist even in servo press stamping, and the potential of the servo press has not yet been fully utilized. The reason for this is conventional presses do not have feedback control systems, and the lack of a suitable method of sensing contact information in real time causes deterioration in the accuracy of products. If slide motion could be controlled by contact information, the small gaps could be removed. To solve this problem, the authors have developed a method of monitoring the contact states between dies and workpieces during the stamping process. The method uses ultrasonic wave reflection and transmission at the contact surfaces and was proved to be able to monitor contact pressure by using a simple geometry experimental die apparatus. Finite-difference time-domain (FDTD) numerical simulation was conducted in this study to obtain better understanding of wave propagation through dies and workpieces. The results obtained from this FDTD simulation visualized wave propagation that could not be experimentally measured. Some of the major results obtained are as follows. 1) When a thin metal sheet is pressed between dies that have inclined stamping surfaces, ultrasonic elastic waves are reflected and transmitted multiple times. 2) Modal conversion occurs at the die-workpiece boundary in such a way that normal waves with an inclined incident angle are transformed into normal and shear waves. 3) Elastic waves sent out from an ultrasonic transducer are mixtures of normal waves with flat wave fronts along the propagation path axis, normal waves with circular or spherical wave fronts expanding from both sides of the transducer, and shear waves. These results brought about much useful information for setting ultrasonic transducers and analyzing collected signals.

Abstract: To comply with increasing product requirements, the use of function-optimized materialsis claimed. Joining technology thereby becomes increasingly important to use high strength materialonly in postulated sections. Staking is a joining by forming technology that is highly reliable andcost efficient. High process forces and sufficient formability of the material limit the suitability inclaimed miniaturization for use in industrial applications. A promising approach to break these processlimitations is the use of superposed high frequency oscillation, whereby joining forces could bedecreased. The present study indicates first trials of an ultrasonic (US) assisted staking process of highstrength martensitic steel. Based on high temporal instrumentation, such as laser vibrometer, contactdetection and high-resolution force measurement, the process sequence is characterized and studiedin detail. The researches confirm high potential in force reduction of mean values due to superimposedhigh frequency oscillation with a high dependency on amplitudes. In process, two differentforce-characteristics within three regimes can be identified. Since US assisted forming processes arewell known in literature with harmonic oscillating force signals during process, hammering and soirregular force peaks with changes in contact signal within process, are identified for first time anddemonstrate a highly promising field of application.

Abstract: Common limitations in bulk metal forming processes are the maximum available press force and forming characteristics of metallic materials. Conventional measures to overcome those limitations, such as forming at elevated temperature, are not always applicable. An alternative approach is the use of a superimposed axial tool vibration with ultrasonic frequency. This enables a considerable reduction of required forming forces. The underlying mechanisms of this phenomenon have so far mainly been investigated for frequencies above 20 kHz and easily deformable materials, like copper. Due to limitations concerning the system technology, materials with higher strength have been considered only to a very limited extent. In order to allow investigations on the deformation behavior of materials with higher strength during ultrasonic-assisted upsetting, a tool setup containing a 15 kHz oscillating system has been developed. It offers a larger loading capacity in comparison to industrially available systems with higher frequency. Ultrasonic-assisted upsetting experiments with varying amplitude and press velocity are carried out to examine vibration-induced changes in the flow behavior of steel S235JR. Changes in the material characteristics are analyzed by evaluating the mean upsetting force as well as the microstructure of the upsetting specimen before and after forming. The resulting forces show a strong process dependence regarding the oscillation amplitude. The static press velocity and contact status between tool and specimen also influence the process forces, but to a much lower extent than the amplitude. Concerning the microstructure a rising oscillation amplitude leads to an increased radial elongation and axial compression of ferrite grains at the strongly deformed specimen center.

Abstract: The present work explores the importance of model parameters and input variables when simulating the quenching of thick sectioned nuclear forgings. The modelling approach adopted uses values of specific heat capacity, containing latent heat release, to simulate cooling curves; rather than calculating transformation kinetics based upon a mathematical model. Termed the effective specific heat (Cp_eff), two different methods were used to establish values: differential scanning calorimetry (DSC) and thermos dynamic predictive software. Values were then included in finite element (FE) models to simulate the characteristic cooling at the mid-wall position in a thick section forging and were validated against production thermocouple data. The investigation found that the formation of ferrite, bainite and martensite or lower bainite were all represented by the data established using DSC and critical formation temperatures were comparable with others in the literature. Conversely, values calculated using the thermodynamic software failed to represent ferrite formation and predicted different critical transformation temperatures for bainite. The simulated cooling curve that used the software predicted Cp_eff data was comparable to the thermocouple data either side of the bainite transformation, however during the transformation the effects of latent heat on cooling rate were over predicting leading to disparities. The equivalent DSC cooling curves produced a near exact match.

Abstract: Rolling Models have come a long way from the first empirical relations about forward slip and bite conditions to their current state, which allows local quantities to be calculated in two and three dimensions. In this paper, state-of-the-art of analytical modelling of the rolling process is shown with a fully three-dimensional rolling model for hot and cold strip rolling with stress distributions in the longitudinal, vertical and lateral directions. For this purpose, von Karman’s strip approach is extended to account for the stress gradient in lateral direction, as was already shown in different papers. The stress gradient in the vertical (through-thickness) direction is introduced by a modern implementation of Orowan’s inhomogeneous deformation theory. The local stress distributions are compared to results from Finite-Element Calculations obtained with modern FEM codes. It will be shown, under which circumstances expensive FEM calculations can be replaced by simpler models like the one proposed here, which are more time and cost-effective without a significant loss in result precision. The rolling model is extended with a Finite Element Beam Model for work and backup roll deformation, as well as local work roll flattening and thermal crown for hot rolling. The Effects of those features on stress distribution and exit strip profile are shown for hot and cold rolling.

Abstract: Large size forged ingots, made of high strength steel, are widely used in aerospace, transport and energy applications. The presence of internal voids in the as-cast ingot may significantly affect the mechanical properties of final products. Thus, such internal defects must be eliminated during first steps of the open die forging process. In this paper, the effect of in-billet void positioning on void closure throughout the ingot breakdown process and specifically the upsetting step in a large ingot size steel is quantitatively investigated. The developed Hansel-Spittel material model for new high strength steel is used in this study. The ingot forging process (3D simulation) was simulated with Forge NxT 1.0^® according to existing industrial data. A degree of closure of ten virtual existing voids was evaluated using a semi-analytical void closure model. It is found that the upsetting process is most effective for void closure in core regions and central upper billet including certain areas within the dead metal zone (DMZ). The volumetric strain rate is determined and two types of inertial effects are observed. The dependence of void closure on accumulated equivalent deformation is calculated and discussed in relation to void in-billet locations. The original combination of information from both relative void closure and the volumetric strain rate provides a way to optimize the forging process in terms of void elimination.