Materials Science Forum Vol. 949

Abstract: During the hot rolling of bainitic steels, time and temperature must be controlled within narrow limits to avoid undesirable ferritic or martensitic phase fractions. In order to design a reliable process window for the production of bainitic steels, the effects of the different process parameters on the phase transformation and the final properties of a microalloyed and a non-microalloyed steel were investigated. Thermomechanical tests with the possibility of producing secondary samples were conducted to analyze the influence on the mechanical properties strength and toughness. Transmission electron microscopy (TEM) and electron probe micro analysis (EPMA) were used to investigate the origin of the differing properties. In particular, it has been found that thermomechanical rolling of the microalloyed steel leads to an improvement in strength. This is partly due to the transformation kinetics and partly to strain-induced precipitations. Further, the hardening behavior is affected by the secondary phase within the bainitic matrix configured through the cooling strategy. Coarse Martensite/Austenite (MA) structures reduce toughness, whereas finely dispersed MA islands increase the hardening potential. Furthermore, the results from the material experiments were used to develop a rate model in combination with a nucleation model to predict the kinetics of the phase transformation and the shape of the bainitic microstructure.

Abstract: One of the strategies employed to lower weight and to decrease material consumption is reducing part thickness itself. Thus, functionally graded materials in which structural reinforcement is adjusted locally, are of great interest. With regard to conventional industrial processes, such as extrusion or flexible cold rolling, thickness variations can only be achieved either longitudinally or through the cross-section of the semi-finished products. Hence, a combined thickness variation (along both axes) is difficult to generate solely by extrusion or rolling. A simultaneous thickness variation in both directions, however, would enable further weight savings in structural components such as car body parts. In this study, a promising approach with extruded shapes, serving as a billet for a flexible hot rolling process, is elaborated upon. By employing the described process modification, shapes with simultaneous thickness variations in longitudinal as well as in transverse direction are feasible. Initial numerical analysis reveals the weight-saving potential of using these semi-finished products for structural parts in a car body. A demonstration of the production process for the semi-finished parts and the occurring challenges are discussed. To verify and adjust the new technology, a numerical model of the flexible hot rolling process has been created based on the finite element software QForm VX. This model is also employed for tool design optimization to produce semi-finished components with the required geometrical quality. Finally, the results of hot rolling experiments conducted using the adjusted roll design are presented.

Abstract: The understanding of the softening behaviour during the hot rolling process is required to optimize the hot rolling schedule. Therefore, the microstructural evolution in the hot rolling of austenitic stainless steel was simulated. In this work, kinetics of grain growth was investigated by means of compression tests using the Gleeble HDS V40 and described by appropriate kinetic equations based on the obtained experimental results. Moreover, numerical simulation was performed using the Simufact.forming software. The results of the numerical simulation were further validated by experimental data, which were obtained from the labour continuous hot rolling of the austenitic stainless steel.

Abstract: Transport industry faces challenges steadily due to rising fuel costs and stricter regulations for the emission of air pollutants. Technological developments that reduce fuel consumption are necessary for sustainable and resource-efficient transport. Innovative production technologies utilising multi-material designs come to the fore in an attempt to fabricate lightweight products with extended functionality. Departing from this motivation, novel process chain concepts for the manufacturing of bi-material forged products are being researched at the Leibniz Universität Hannover in the context of the Collaborative Research Centre (CRC) 1153. The developed technology is referred as Tailored Forming and deals with the deformation and subsequent processing of joined hybrid workpieces to produce application-oriented products. Deformation processes are carried out at elevated temperatures for thermomechanical treatment of the joining zone properties. Researchers make use of numerical simulation in each step in the process chains. This paper explains the challenges associated with induction heating and impact extrusion of bi-material forging billets and presents our solution approaches with the aid of numerical modelling. Experimental validation results and analysis of deformed workpieces are also shown.

Abstract: In this work, the influences of deviations of material properties (used material is aluminium for both metal sheets), hole geometry (diameter, chamfer at the bottom and rounding at the top) and offset between punch and hole on the quality of a clinched connection are analysed. The analyses were done with numerical forming simulations, which were validated by experimental tests. For each process parameter, models were built up to simulate the forming process. After simulation of the forming process, it was possible to measure the resulting undercut and to identify the dependency between process parameters and width of undercut. This shows the influence of each investigated parameter on clinch quality and enables to set tolerances as high as possible but small enough to get the required undercut in the clinched connection.

Abstract: Ship transport continues to be the most economical intercontinental means of transportation, which is becoming more and more under observation due to increasing international environmental protection regulations. In order to protect the marine biotope, the government is introducing stricter exhaust emission regulations, which mean the further development of low-emission large engines and thus make the development of new combustion chamber components indispensable. This paper presents FE simulations of a friction welding process for two metallic materials, which are being developed as part of a BMWi joint project for the production of innovative combustion chamber components. In addition, the first results of the simulations and their comparison with real tests are presented.

Abstract: The incremental forming process Radial Rotation Profile-Forming (RRPF) has been developed to enable the production of profiled hollow parts with low sheet thinning and high geometrical accuracy. As a result of low thinning, a smaller initial sheet thickness can be used, while material and weight can be saved.The two principal forming steps are the production of the preform by Rotational Swing-Folding (RSB) and the subsequent radial profile forming of the hollow part in one clamping position. The special feature are the purposed wrinkles in the first process step, which formed in the indentation of the profiled mandrel. This is an advantage, because of the additional thickening. Due to the radial profiled forming in the second process step, the axial profile can be formed with less thinning.The focus of the article is on the development of the forming tools for the second process step of RRPF. Based on the general law of gearing, the forming tools for the production of a component are developed. First, a forming simulation of an example component validates the approach. For this purpose, some simplifications have been made in order to consider the profile forming process separately. Afterwards, the experimental results of the incremental sheet metal forming process for the production of the profiled hollow parts are presented.