Showing papers by "Gerhard Hirt published in 2020"

Analysis of the bond strength of voids closed by open-die forging

Abstract: Large components, for instance in the energy industry, mining and heavy machinery, are produced from high weight cast as ingots followed by open-die forging. Besides achieving a certain final geometry and microstructure, one of the main objectives during the forging process is the elimination of casting defects, like voids from the solidification shrinkage. This process is divided in the two stages of void closure and void healing. During the healing of the closed void a solid bond is established at high temperature. In literature void closure in open die forging is thoroughly investigated. Concerning the healing by solid bond generation there are only few studies related to voids in open die forging but there is substantial literature related to bond formation in roll hot bonding and diffusion bonding. Most of this work however determines the bond strength after cooling to room temperature. Concerning future appropriate modelling of the closure and healing process in open die forging, it is important to decide, whether a bond, which was established in one forging stroke, would be strong enough to withstand the following strokes. As a first step in this direction, this paper experimentally examines the bond strength directly after bond formation under conditions typical for open die forging strokes. The results quantitatively confirm the expected influence of forming temperature, surface enlargement, holding time and oxide films.

Prediction and analysis of damage evolution during caliber rolling and subsequent cold forward extrusion

Abstract: Damage in the sense of voids influences material as well as product properties and thus is important for the performance of formed components. In this work, the influence of caliber rolling prior to a cold forward extrusion process in terms of damage evolution is investigated. To this end, an uncoupled Lemaitre-type damage model considering the effects of strain-rate and temperature dependent plasticity is employed. The damage-related parameters are identified in an inverse manner based on notched tensile tests. Numerical investigations of the forming processes show that damage increases throughout the process chain. The simulations are validated in terms of density measurements and microscopic investigations. The experimental measurements and numerical simulations are in good qualitative agreement. It is shown that the influence of rolling on subsequent cold forward extrusion is negligible for this experimental setup. For other process parameters, however, the influence could be significant.

Potential and status of damage controlled forming processes

Abstract: In modern process design of metallic components, the influence of the metal forming process on the component properties can be taken into account. However, damage occurring concurrent to forming cannot be accounted for yet. Due to the complex multi-scale, multi-mechanism nature of damage, it is very challenging to predict its evolution through any metal forming process chain. In order to enable such damage controlled forming processes in the future three research questions need to be addressed in detail: How are the mechanisms governing the damage initiation and evolution in metals best characterized? How can the damage mechanisms be described and the damage evolution be predicted using models? How do forming processes influence the damage evolution? Answering these questions and considering damage during process design will in the long term lead to improved lightweight components that do not require conventional safety factors, as their performance is already fully known.

On the Influence of the Tool Path and Intrusion Depth on the Geometrical Accuracy in Incremental Sheet Forming

Abstract: Incremental sheet forming (ISF) is a flexible sheet metal forming process to realize products within short time from design to the first produced part. Although fundamental research on ISF has been carried out around the world, ISF still misses commonly required tolerances for industrial application. In this study, the influences of tool path as well as intrusion depth of the forming tool into the sheet material on the geometrical accuracy were investigated. In the conducted experiments, both flat and stretch-formed sheet metal blanks with different tool paths and intrusion depths were examined. Experimental and numerical investigations showed that changes in the range of a tenth millimeter of the intrusion depth with a consistent tool path lead to different resulting part geometries. A better understanding of the sensitive influence of the tool path and the intrusion depth on the resulting geometry might lead to more accurate parts in the future.

Action Discretization for Robot Arm Teleoperation in Open-Die Forging

Abstract: Action extraction from teleoperated robots can be a crucial step in the direction of full -or shared- autonomy of tasks where human experience is indispensable. This is especially important in tasks that seek dynamic goals, where a human operator needs more control on how the machine behaves to provide assistance or perform a task. Open-die forging is a basic metal-forming process that lacks non-destructive product quality measures. Human experience is therefore imperative. During the process, a robot-arm is operated to place the work-piece between the dies of the forge where it is striked several times to reach a specific geometry. In this paper, we apply a white-box computer vision technique to discretize open-die forging robot-arm teleoperation data into actions as a step in learning the operator’s behavior.

Evaluation of process-induced damage based on dynamic recrystallization during hot caliber rolling

Abstract: The amount of damage induced during hot forming depends not only on the stress state but also on material softening due to dynamic recrystallization (DRX), which can be used to delay or reduce damage initiation. An extension to the Gurson–Tvergaard–Needleman (GTN) model has been proposed recently that includes a new coupled DRX-damage nucleation criterion taking DRX and the stress state into account. In the present paper, a 4-pass caliber rolling process is considered where the bar stock is rotated by 90° after each rolling pass. The extended GTN model is applied to the caliber rolling process to predict the internal damage induced during the rolling process. The simulation results show that the highest void volume fraction (VVF) occurs during the first rolling pass. However, the variations of stress state during each pass assist in reducing the damage in the subsequent passes. The material softening due to increasing DRX in subsequent passes also helps to reduce the void nucleation. The microscopic analysis of the rolled bar confirms the damage distribution predicted by the simulations.

Torsion plastometer trials to investigate the effect of non-proportional loading paths in caliber rolling on damage and performance of metal parts

Abstract: The non-proportional loading path describes a strain-dependent development of stress triaxiality and Lode parameter during metal forming processes. Existing studies suggest a strong dependence of damage evolution on the non-proportional loading path. This work focuses on investigating the influence of non-proportional loading paths observed in hot caliber rolling of the case-hardening steel 16MnCrS5 using laboratory scale experiments. The applied torsion plastometer is highly flexible as it can apply combined loading types (tension, compression and torsion) on notched round specimen and enables deformation at elevated temperature. In this study, two characteristic non-proportional loading paths in caliber rolling and the maximal achievable non-proportional loading path variation were recreated in the torsion plastometer based on both FE simulations and experiments. After deformation, the specimen were further analyzed using Scanning Electron Microscopy (SEM) to quantify the damage. The results indicate an influence of the non-proportional loading path on damage evolution. Furthermore, fatigue tests were employed to characterize the fatigue performance of the deformed specimens. In the torsion plastometer trials carried out no clear correlation of performance and damage was observed. This is most likely due to differences in residual dislocation density after static recrystallization and deviations in the microstructure after hot working. Thus, the superposition of microstructure evolution and damage needs to be considered carefully when testing at elevated temperature.

Influence of process conditions and pore morphology on the closure rate of pores in hot rolling of steel

Abstract: Steel sheets are manufactured from slabs produced in continuous casting, which inevitably results in a porous initial microstructure. These pores are nuclei for ductile damage and need to be closed during rolling in regard of the strict performance requirements of todays advanced high strength steels. Due to the beneficial shape factor of the roll gap, recrystallization and high diffusion rates pore closure and elimination occurs primarily during hot rolling. In this paper, relevant influencing factors on pore closure are investigated based on representative volume elements (RVE). First, the load regime of typical multi-pass hot rolling schedules is calculated via macro scale FEM and then applied to micro scale RVEs. Within the RVE, shape changes of a given pore are tracked and the closure ratio is calculated. This paper elucidates the effect of different roll diameters, absolute reductions per pass, pore shape and pore orientation on the closure rate. The results show that the majority of pores can be closed during the initial hot rolling stages. However, pore closure can be accelerated through suitable process parameters. Furthermore, shape and orientation of pores should be taken into account, if a precise knowledge of the total height reduction, leading to complete pore closure, is desired.

Investigation on the influence of damage on the fatigue strength of hot rolled sheet metal

Abstract: Continuous casting leads to the formation of voids, which can be considered as an initial state of damage. During hot rolling these voids shall be closed, reducing this initial damage and avoiding its negative influence on the mechanical properties and performance of the produced material. However, hot rolling also influences the microstructure, which in turn affects the performance e.g. the fatigue strength. So far, little research has been published on the separation of those influencing factors. This paper is a first attempt to separate the influence of damage from the microstructure in hot rolling. Numerical simulation is utilized to study void closure throughout the multi-pass process while the evolution of microstructure and damage is investigated experimentally using numerous characterization methods. Two process routes with a large and a small pass reduction have been investigated and comparable microstructures have been achieved. A continuous damage reduction throughout the rolling process has been observed by means of void distribution and density measurements. The large pass reduction showed a slightly reduced damage and an increased fatigue strength in all considered thicknesses, however this could not be traced back to the damage reduction exclusively.

Model Predictive Control of an Overactuated Roll Gap with a Moving Manipulated Variable

Abstract: Model predictive control (MPC) has been used in a variety of industrial processes. Due to its large computation time application to fast and complex processes is limited. Therefore, reducing complexity has been focus of intense research. In this paper, a complexity reduction strategy for a linear MPC is developed. It is used for the control of an over-actuated roll gap with two different actuator types in a cold rolling mill. One of the actuators is able to accept new set points only at a much slower sample time than the fast actuator. In order to coordinate the actuators, a moving manipulation point scheme is introduced where a single time-varying optimization variable of the slow actuator moves through the control horizon of the fast actuator. The fast actuator ensures reference tracking regarding the roll gap when the slow one is idle. The proposed scheme reduces the number of optimization variables as well as constraints and thus enables control of faster processes. Simulation results show the proof of concept and an enhancement in computation time. Moreover, a real-time implementation is demonstrated on a cold rolling mill.

Modelling of void healing in hot rolling due to recrystallization

Abstract: Porosities, introduced into the material during continuous casting can impair the mechanical properties and performance of semi-finished material as well as final products. Healing of these porosities through closure and bonding of voids with subsequent dissolution of the interface in further hot working can reduce this detrimental effect. While large deformations under compressive stresses as well as high temperatures are beneficial for void healing, not all regions in the workpiece experience the same conditions. Gradients of these metrics in normal direction and over time create regions of varying probability for void healing. Knowledge on the process conditions and the void shapes in the respective regions is essential to predict void healing successfully and to produce a sound product, especially if the process window is limited. In the present work, a multiscale modelling approach is utilized to predict void closure and recrystallization along the normal direction in the roll gap. A practical void healing criterion is developed by combining these two mechanisms and then used to classify a typical workpiece in normal direction according to the likelihood of void healing. Optimal void healing conditions are found at a relative height of 80% while the center region (50%) of the workpiece exhibits the least favorable conditions for void healing.

Economical production of conically shaped concentrated windings using forming technology for use in wheel hub engines

Abstract: In addition to the electromechanical properties, the design of the stator windings for electric drives depends strongly on the available installation space. In case of the wheel hub engine, the installation space is strongly limited by the rim diameter and the tyre width. Hence, stator windings for use in wheel hub engines should meet two requirements: On the one hand, the stator slot should be optimally filled with electrically conductive material so that a high torque can be guaranteed despite limited installation space. On the other hand, the end windings should be as compact as possible in order to ensure the greatest possible active length of the stator core. Conically shaped concentrated windings meet these requirements and are therefore suitable for use in wheel hub drives. In a cooperation between the Breuckmann company and the Institute of Metal Forming (IBF) of RWTH Aachen University, a multi-stage forming technology has been developed which enables the production of such a winding geometry. In this paper an automated multistage forming tool is described, which allows for an economical mass production of conically shaped concentrated windings. In addition, a 2D FE model is presented, which models the wire cross sectional development during forming. In this way, a suitable number and geometry of the forming stages could be designed prior to tool production and tool costs saved accordingly. Due to the high load on the primary insulation caused by the forming process, the effect of the forming process on the insulation layer thickness is investigated and alternative insulation processes after forming are described.