Key Engineering Materials Vols. 504-506

Abstract: The paper deals with a mathematical model able to describe the presence of lattice defects of the crystalline materials, such as dislocation and disclination. Within the constitutive framework of second order plasticity developed by the author, the evolution equations to describe the disclinations that are compatible with the screw dislocations are derived.

Abstract: Resistance spot welding (RSW) is commonly employed in the automotive industry to join sheet metal parts, which often involves several thousands of welds for one vehicle. Therefore, the proper evaluation of their formability/failure behavior is so vital to accurately predict overall crash performance of vehicles. In this work, the failure performance of spot welds has been experimentally and numerically analyzed for welds in which the base material is the TRIP (Transformation induced plasticity steel) 980 sheet. The mechanical properties of the base and welds have been characterized utilizing the inverse method based on the standard and miniature simple tension tests along with the lap-shear tests.

Abstract: The conventional manufacturing of solar absorbers has the disadvantage that only simple pipe branch channel geometries are possible. For this reason, a new approach of manufacturing solar absorbers is presented with which it is possible to design the channel geometry as a quasi-fractal structure (FracTherm®, developed by Fraunhofer Institute for Solar Energy Systems) and which reduces the pressure drop and the required energy for the pump. To manufacture these new absorbers, a fast, nearly continuous production process was developed which consists of partial cold roll bonding and subsequent hydroforming, similar to symmetric hydroforming of sheet metal pairs. Various channel geometries, which are developed by Fraunhofer ISE, within the hybrid sheet metal, can be applied. Therefore, three technologies are combined to manufacture a new type of solar absorber. A special look is taken at cold roll bonding. By this process, the material for the new solar absorbers (copper-steel-copper/copper-steel-copper hybrid sheet material) is produced using a suitable release agent to avoid the bonding process along the subsequent water channels. To establish manufacturing guidelines, the characterization of the material is necessary. A peel test like ISO 11339 is used to determine the strength of the layers of the hybrid material. At least the forming characteristics of the hybrid material are dominated by the basic material, in this case steel. A further look is given to the hydroforming of the material. Different cross-section geometries have been tested by a simplified experiment to find out the best geometry for the absorber channels, and have been compared with simulation results, too. Hence, for a crack-free forming operation the choice of the die radius and corner radius is decisive as they are within the same range. To show the feasibility of the hydroforming process and validate the process chain, a small demonstrator was manufactured. Finally it is possible to manufacture a steel based solar absorber with a quasi-fractal structure by combining partial cold roll bonding and hydroforming. All these results are used as guidelines for the production of a new absorber with quasi-fractal structure (FracTherm®).

Abstract: The effect of short-term electrical pulses on metallic single crystals, with and without the application of external mechanical stresses, was investigated with the aid of deformation reliefs. The nickel-cobalt single crystals were subjected to electrical pulses and subsequently microscopically measured. In doing this, it was established that an electrical pulse without a simultaneously applied mechanical stress, has no influence on the deformation relief. It was possible to show that on loading the single crystal with a mechanical stress, the deformation relief significantly changes even when the stress was markedly below the flow stress.

Abstract: Classical forging process chains for the production of high performance parts consist of heating, hot forming, machining and heat treatment stages. Especially the last stage comprises numerous energy consumptive heating and cooling procedures. Consolidation of these process chains has a large energy-saving potential and thus can lead to ecological and economic advantages. In addition, the optimization may result in well enhanced local material properties in the final part. Therefore, the collaborative research project “EcoForge: Resource-efficient process chains for high performance parts” has been initiated. Five renowned research institutes aim at optimising the industrially relevant process chain in order to facilitate the resource-efficient production of forgings with increased load bearing capacity. The six subprojects of EcoForge include both, numerical investigations on the microstructure evolution and cooling behaviour of the forged parts as well as numerous experiments which will be carried out in collaboration with the project’s industrial consulting committee, summarising up to 50 relevant participating companies. The joint project is funded by the AiF and BMWi aiming at major areas of technical developments in Germany.

Abstract: Computer aided design of the manufacturing technology for the fasteners is presented the paper. The particular objectives of the work were twofold. The first objective is evaluation of applicability of various materials for fasteners. Analysis of different technological variants is the second objective of the research. In the material part, bainitic steels are considered as an alternative for the commonly used carbon-manganese steels. This is a continuation of [1,2]. Possibility of elimination of the heat treatment was evaluated. Main features of the new generation of bainitic steels are discussed briefly in the paper. Rheological models for all steels investigated in the project were developed. The models were implemented into the finite element code for simulations of drawing and multi stage forging. Simulations of various variants of manufacturing chain were performed and the best alternative was selected. Criteria for the selection composed dimensional accuracy and tool life. Industrial trials were performed for the selected cycle and the efficiency of this cycle was evaluated. Finally, the optimization task was formulated. However, solution of the optimization problem is costly at this stage and improvement of the efficiency of the formulation will be the objective of further research. References 1. Kuziak R., Skóra M., Węglarczyk S., Paćko M., Pietrzyk M., Computer aided design of the manufacturing chain for fasteners, Computer Methods in Materials Science, 11, 2011, 243-250. 2. Kuziak R., Pidvysots’kyy V., Węglarczyk S., Pietrzyk M., Bainitic steels as alternative for conventional carbon-manganese steels in manufacturing of fasteners - simulation of production chain, Computer Methods in Materials Science, 11, 2011, 443 – 462.

Abstract: Hot forging dies are exposed during service to a combination of cyclic thermo-mechanical, tribological and chemical loads. Besides abrasive and adhesive wear on the die surface, fatigue crack initiation with subsequent fracture is one of the most frequent causes of failure. In order to extend the tool life, the finite element method (FEM) may serve as a means for process design and process optimisation. So far the FEM based estimation of the production cycles until initial cracking is limited as tool material behaviour due to repeated loading is not captured with the required accuracy. Material models which are able to account for cyclic effects are not verified for the fatigue life predictions of forging dies. Furthermore fatigue properties from strain controlled fatigue tests of relevant hot work steels are to date not available to allow for a close-to-reality fatigue life prediction. An industrial forging process, where clear fatigue crack initiation has been observed is considered for a fatigue analysis. For this purpose the relevant tool component is modelled with elasto-plastic material behaviour. The predi

Abstract: In the work developed in this paper, the model of the microstructural behavior of Inconel 718 for the rotary forging process is studied. This process is presented as an alternative to the conventional forging. The window process of Inconel 718 is very narrow, so the requirements for manufacturing Inconel are very restrictive. Optimization of the rotary forging process was carried out in order to manufacture Inconel pieces with good microstructural distribution. Microstructural specification was given by aeronautic sector. Numerical work was done in order to simulate the microstructural behavior during the forming process. This method was used: a) to understand the recrystallization mechanism that take place in rotary forging processes, b) to compare the microstructure between a piece done in conventional forging and another done in rotary forging and c) to study the influence of the initial grain size in the final piece.

Abstract: Compound forging is a technology to successfully manufacture hybrid parts by applying resource-saving process steps. During compound forging of steel-aluminum parts the formation of intermetallic phases is benefited. The thickness of these intermetallic phases influences the bonding and thus the global part quality. According to literature, specific coating elements reduce the phase seam thickness. In powder-metallurgically manufactured parts it is possible to selectively insert specific elements in the surface area. Therefore, a time intensive coating process can be avoided. The applicability of combining the technologies of powder-metallurgy and compound forging is discussed in this paper. Powder-metallurgically manufactured and solid parts made of steel and aluminum are compound forged and the influences on deformation behavior and the joining zone are investigated.

Abstract: Lightweight design of cars is one way to reduce fuel consumption and increase the range of cars. This is an important factor to attain the EU limit values for CO2 emissions for vehicles and so to escape penalties for exceeding these limits as of the year 2012. The increasing uses of high-strength steels or of lightweight structures are adequate means to reducing weight. At IPH – Institut für Integrierte Produktion Hannover gGmbH a method to produce hollow profiles made of aluminum was developed. The method combines flashless forging and tube hydroforming. This is called hydroforging. With this method it is possible to produce thick-walled hollow aluminum profiles with undercuts without complex tool kinematics. The forging is supported by an active medium. A tool concept has been developed that uses the drives of a hydraulic press with die cushion. With this tool various component geometries were produced and analysed. To produce the tools interior pressure and the forming pressure liquid tin as incompressible active medium was used. The forming is initiated by an upsetting of the aluminum profiles and supported by the active medium, so that the profile is pushed against the engraving of the dies. The process was designed based on simulations and verified by practical experiments. This paper describes the development of the forging process with active medium and the discussion of the forging experiment results.