A. Saeed-Akbari

Characterization and Prediction of Flow Behavior in High-Manganese Twinning Induced Plasticity Steels: Part I. Mechanism Maps and Work-Hardening Behavior

Thermodynamic stacking fault energy (SFE) maps were developed using the subregular solution model for the Fe-Mn-Al-C system. These maps were used to explain the variations in the work-hardening behavior of high-manganese steels, both through experiments and by comparison with the published data. The suppression of the transformation induced plasticity (TRIP) mechanism, the similarity between the shape of the work-hardening rate diagrams for the produced iso-SFE materials, and an earlier onset of stage C of work hardening by decreasing SFE were shown to be efficiently predictable by the given mechanism maps. To overcome the limitations arising from studying the deformation response of high-manganese steels by SFE values alone, for example, the different work-hardening rate of iso-SFE materials, an empirical criterion for the occurrence of short-range ordering (SRO) and the consequently enhanced work-hardening, was proposed. The calculated values based on this criterion were superimposed on the thermodynamics-based mechanism maps to establish a more accurate basis for material design in high-manganese iron-based systems. Finally, the given methodology is able to clarify the work-hardening behavior of high-manganese twinning induced plasticity (TWIP) steels across an extensive range of chemical compositions.

Enhanced Mechanical Properties of a Hot-Stamped Advanced High-Strength Steel via Tempering Treatment

The hot stamping process has an extensive range of applications due to its advantages over the traditionally used stamping techniques developed in the past. To enhance the mechanical properties of the indirectly hot-stamped parts, the quenching and partitioning (Q&P) process has been recently applied on boron-alloyed steel. In the current research, it was observed that the tempering treatment on the directly hot-stamped boron steel resulted in better mechanical properties and higher formability index compared with the reported results using the Q&P process. The nano-carbide formation and the dislocation annihilation during the tempering treatment were suggested as the evident reasons for the occurrence of the mentioned robust properties. The ease of the practical implementation of the tempering route together with the markedly enhanced mechanical properties of the tempered parts make the suggested method privileged. Additionally, the variations in the yield strength before and after tempering were quantitatively evaluated.

Investigation on Incremental Sheet Forming Combined with Laser Heating and Stretch Forming for the Production of Lightweight Structures

Asymmetric Incremental Sheet Forming (AISF) is a process for the flexible production of sheet metal parts. In AISF, a part is obtained as the sum of localized plastic deformations produced by a simple forming tool that, in most configurations, moves under CNC control. Flexible processes with low tooling effort like AISF are suitable for sectors with small lot sizes but premium products, e.g. for the aviation and the automotive sector. Four main process limits restrict the range of application of AISF and its take-up in industry. These are: (i) material thinning, (ii) limited geometrical accuracy, (iii) the process duration and (iv) the calculation time and accuracy of process modelling. Moreover, the material spectrum of AISF for structural parts is mostly restricted to cold workable materials like steel and aluminum. This paper presents some new investigations of incremental sheet forming combined with laser heating or stretch forming as possible hybrid approaches to overcome the above mentioned limitations of AISF. These hybrid incremental sheet forming processes can increase the technological and economical potentials of AISF. A possible application is the fabrication of lightweight sheet metal parts as individual parts or small batches, e.g. for the aerospace industry. The present study provides a short overview of the state of the art of AISF, introduces the new hybrid process variations of AISF and compares the capabilities of the hybrid processes and the standard AISF process. Finally, two examples for applications are presented: (i) the production of a part used in an airplane for which the manufacturing steps consist of die manufacture, sheet metal forming by means of stretch forming combined with AISF and a final trimming operation using a single hybrid machine set-up; (ii) laser-assisted AISF for magnesium alloys.

Deformation Mechanisms of Ti6Al4V Sheet Material during the Incremental Sheet Forming with Laser Heating

ncremental sheet metal forming (ISF) is a suitable process for the production of small batch sizes. Due to the minor tooling effort and low forming forces, ISF enables the production of large components with inexpensive and light machine set-ups. Hence, ISF is an interesting manufacturing technique for aeronautical applications. Sheet metal parts in aircrafts are often made of titanium and its alloys like the high strength alloy Ti Grade5 (Ti6Al4V). The characteristic low formability of Ti6Al4V at room temperature requires forming operations on this material to be carried out at the elevated temperatures. The interaction of heating and deformation cycles results in a microstructure evolution, which is believed to have a high impact on formability and product quality. In the present work, the temperature-dependent microstructural evolution of the as-deformed parts was investigated. Longitudinal pockets with different depths were formed using a laser-assisted ISF process. The microstructural evolution and hardening of the material were analyzed with respect to the local strain in different forming depths and pocket zones. The formability of the material together with the deformation depth and the sheet thickness-reduction were found to be strongly dependent on the applied process temperatures and the activated deformation mechanisms like dislocation glide and dynamic recrystallization.

Hybrid Production Systems

While virtual product development allows great freedom in terms of design, actual development processes are rather restricted. Those boundary conditions are at best hardly possible to exert influence on. Therefore, future research has to focus both on the realisation of the concept of one-piece-flow while simultaneously increasing flexibility and productivity and on the technological advancement. Hence, hybridisation of manufacturing processes is a promising approach, which often allows tapping potentials in all the aforementioned dimensions.

Charakterisierung der Verformungsmechanismen bei hoch Mn-legierten Stählen unter einachsiger Zugbeanspruchung∗

Kurzfassung In hoch Mn-legierten Stählen treten in Abhängigkeit von der Stapelfehlerenergie (SFE) Verformungsmechanismen wie verformungsinduzierte Martensitbildung (TRIP), mechanische Zwillingsbildung (TWIP) und homogenes Versetzungsgleiten (SLIP) während der plastischen Umformung auf. Die SFE kann thermodynamisch berechnet und in sogenannten Mechanismenkarten grafisch dargestellt werden. Zur Beschreibung des Zusammenhangs zwischen Verfestigungsverhalten und Verformungsmechanismus im System Fe-Mn-C wurde zunächst die standardisierte Messtechnik im einachsigen Zugversuch mit zusätzlichen orts- und zeitauflösenden Verfahren kombiniert. Mittels Infrarot-Thermografie lässt sich das Auftreten von Dehnungsbändern infolge dynamischer Reckalterung detektieren. Der Einsatz lokaler Dehnungsanalyse auf der Probenoberfläche ermöglicht die quantitative Analyse der lokal erreichbaren Dehnung. In den sog. TORNADO-Diagrammen wird das Auftreten charakteristischer Muster in der Entwicklung der momentanen Dehnrate über der wahren Spannung zur Bestimmung der dominierenden Verformungsmechanismen genutzt. Die Oberflächentopografieänderung infolge Martensit- und Zwillingsbildung sowie prozessbedingter Einflüsse wie der Textur wurde mittels Konfokalmikroskopie untersucht.