Key Engineering Materials Vols. 554-557

Abstract: To provide a high quality of forged products, a homogeneous distribution of material properties has to be achieved inside the ingot. As the properties are not visible from the outside, an online monitoring during the forging process is required. By using modern measuring equipment and fast calculation models, the equivalent strain, temperature and average grain size in the core fibre of a forging ingot can be calculated parallel to the process. Software implementing the fast calculation models has been established and connected to the measuring system of two different open die forging presses. Two experimental forging processes with ten passes have been performed (20 ton steel ingot, 750 kg Ni-base alloy ingot). Parallel to the process, the current strain, temperature and average grain size in the centre of the ingot are visualised in the graphic user interface and recorded by the process monitor. It was shown that the calculation speed is high enough to allow online capability. After finishing the process, the developed software can further be used to analyse in detail the impact of every single stroke or pass on the whole process. Additionally, information like minimum or maximum grain size or recrystallized fraction is calculated and can be used to get insight into the process and optimize its design. Comparing the metallographically measured average grain size from experiment with the grain size estimated by the process monitor, the average deviation of three measured points is less than 13 %.

Abstract: Due to the constant development in the automotive industry, where high performance shared with the maximal comfort and safety at low car body weight are the primary goals, gains the lightweight construction in importance. Materials with light weight, high strength and toughness are being engaged. With this background the material aluminum and its alloys become highly attractive to manufacturers. There are mainly two ways of forming the metal materials: casting or forming. Apart from substitution of one method by another there are also many examples of combining of casting and forging processes in practice. Such approach allows using the advantages of both methods, shortening the process chains and saving energy and resources at the same time. Furthermore the form flexibility can be increased and the product quality can be improved. For higher process efficiency a direct transition from casting to forging operation should be applied, so that the heat loss decreases and no additional heat treatment between these operations is necessary. There are processes known, which allow producing the final parts by casting and forging from one a single heat. The application of such processes requires materials, which have simultaneously good casting and forging properties. The Institute of Metal forming TU Freiberg works intensively on development of combined casting-forging technologies for lightweight aluminum parts. A technological chain for this coupled process followed by precipitation hardening heat treatment was developed (Figure 1). Heat treatable aluminum cast and wrought alloys with 1 – 7 % silicon were applied. By the variation of silicon content the optimal cast, forging and hardening properties were achieved. This technology with high energy efficiency allows producing durable light weight parts from aluminum alloys while the mechanical properties of the final parts are equal to or even higher than those in the conventional processes.

Abstract: The microstructure and the resulting mechanical properties of Twin Roll-Cast (TRC) AZ31 strips have been investigated after rolled from thickness of 5.2 mm to 1.00 mm and 1.25 mm, respectively. Twin-Roll-Casting (TRC) was used to produce AZ31 strips with a near-net final thickness directly from the liquefied material. The two-stage rolling experiments were carried out on the four high reversing mill at the Institute of Metal Forming in Freiberg. The first stage was two roughing passes followed by intermediate annealing. The second stage was subsequent finish rolling with 1 to 3 rolling passes. The influence of the finish-rolling on the properties of the final strip was investigated, including the variations of rolling pass and pass reduction. The TRC strip exhibits a heterogeneous microstructure with random texture due to a deformed structure combined with partial cast columnar and equiaxed grains. Significant grain refinement was achieved using high deformation degree per pass (> 30 %). Increasing rolling passes to 3 during finish-rolling reduces the strain per pass and also leads to a temperature drop so that incomplete dynamic recrystallization after the final rolling pass occured, leading to a more coarse and heterogeneous microstructure. It was found, that a 2-pass finish rolling provides the optimum strategy for the material properties as well as the process stability. Due to homogeneous fine grained microstructure, the 2-pass rolled strip showes high mechanical properties with low anisotropy. This includes yield point of 234 MPa, tensile strength of 285 MPa and total elongations of 25 %.

Abstract: With cold forging processes it is possible to produce parts characterized by high strength, high dimensional accuracy and high surface quality. In order to optimize the forming process and to be able to use the advantages of cold forging specifically and combined, it is necessary to find correlations between the manufacturing parameters, the strength and the properties, like hardness distribution and surface quality, of the component. The research work covered in this paper focuses on the influence of the components properties influenced by its manufacturing history on their fatigue strength. The used component is a flange hub, produced by a four-stage cold forging process. To obtain different component properties the production process of the hub was varied for example by changing the heat treatment or by producing the parts by turning process. First of all, the components’ properties, for example hardness distribution, remaining residual stresses, orientation of fibers and surface quality, were determined. Moreover, a special experimental setup for the fatigue tests has been designed. The components’ fatigue behavior was determined using a high frequency pulsator and evaluated in terms of finite life fatigue strength and fatigue endurance limit. These examinations were used to produce Woehler curves for the differently manufactured components with a certain statistical data analysis method.

Abstract: The presented work is dedicated to studying the forgeability of bimaterial cladded workpiece. Hot upsetting tests of cylindrical low carbon steel (C15) billets weld cladded (MIG) by stainless steel (SS316L) are experimentally and numerically studied. Upsetting tests with different upsetting ratios are performed in different tribology conditions at 1050°C which is within the better forgeability temperature range of both substrate and cladding materials[ ]. Slab model and finite-element simulation are conducted to parametrically study the potential forgeability of the bimaterial cladded workpiece. The viscoplastic law is adopted to model the friction at the die/billet interface. The friction condition at the die/billet interface has a great impact on the final material distribution, forging effort and cracking occurrence. With Latham and Cockcroft Criterion, the possibility and potential position of cracks could be predicted.

Abstract: Gear rolling is a manufacturing technique for gears with many advantages like reduced material consumption, reduced scrap generation, fast cycle times, good surface quality and improved final properties of the gear wheels compared to conventional production technology based on machining. In order to make use of all these advantages it is desired to reach the final shape of the gear wheel already after rolling. This means that post treatments like grinding should be avoided. This puts high requirements on the shape accuracy after gear rolling. In this paper it was studied if finite element simulation could be used to evaluate the shape accuracy after gear rolling. The measurement of shape accuracy of gear wheels is specified in standards like ISO1328-1. The allowed deviations from nominal shape are often of the order of 10-30 μm for very good qualities. So if such evaluation shall be possible from a finite element simulation the accuracy must be of the same order. In order to have sufficient accuracy of the finite element simulation 2D simulations were performed on a spur gear. The FE code DEFORM was utilized. The shape accuracy was evaluated for gear rolling of two cases. One case had gears with the module of 1 mm. The other case involved gears with a significantly larger module of 4 mm. This was an interesting case since it is known that it is more difficult to roll the gear with good accuracy in large modules.

Abstract: Gearing components are an example for widely used machining parts in engines. Nowadays the development and optimization of materials and process chains are driven towards a concurrent improvement of final product properties and production efficiency. Excellent mechanical properties needed for gearing components e.g. high load capacity and high fatigue resistance depend on a fine homogeneous microstructure in the final product. Efficiency in gear manufacturing can be optimized by increasing the temperature during processing, which allows for lower forging loads and lower die stresses, thus improving die life in terms of mechanical fatigue. Additionally, increasing the temperature during case hardening reduces the process duration significantly. Hence process efficiency also increases. To meet the need of a fine homogenous microstructure, dynamic recrystallization has to be initiated during hot forging and grain growth has to be avoided during dwell times and case hardening. This grain size control can be achieved by applying micro-alloying concepts. Recently, an Nb-Ti-based alloying concept for case hardening steels was introduced, which increases fine grain stability and therefore potentially allows for higher forging and case hardening temperatures, leading to improved process efficiency [1]. In this paper a 25MoCr4-Nb-Ti steel grade is characterized in terms of flow resistance and microstructure evolution by hot compression tests and annealing experiments. The processing limits of this material in terms of abnormal grain growth are determined and a JMAK-based microstructure model considering these limits is presented and implemented in the FE-Software DEFORM 3dTM. The model is used in a case study to design a laboratory scale forging process for lowest possible die stresses and finest possible grain sizes. Experimentally measured grain sizes and forging loads from forgings at the laboratory scale are used to evaluate the process design. It is shown that considering microstructure evolution in process design is absolutely necessary to jointly optimize for process efficiency and final properties. The application of the Nb-Ti-based micro-alloying concepts allows for lower die stresses and thus seems to reduce mechanical fatigue of the dies compared to conventional case-hardening steels. [1] S. Konovalov et. al.: Testcase gearing component. In: G. J. Schmitz, U. Prahl (Ed.): Integrative Computational Materials Engineering, Wiley-VCH Verlag GmbH & Co. KGaA, 2012, ISBN 978-3-527-33081-2

Abstract: Developing green processes establishes new possibilities for cold forging industry. Current technological developments require automotive parts with less mass, but higher material-efficiency. To achieve these goals, high-strength steels and complex geometries are used. The rising process forces lead to increased tool loads and subsequently elastic tool deformation resulting in early tool failure or dimensional deviations. A numerical determination of tool loads during process enables their reduction by a load-dependent design of the tool geometry. Aim of this work is a time-efficient and precise determination of tool loads considering the complete tool system using the example of a lateral extrusion process. By domain decomposition into Finite Element Method (FEM) and Boundary Element Method (BEM) domains and subsequently an integrated FEM/BEM simulation, a significant computation time reduction towards a conventional FEM model is achieved. Experiments of the examined lateral extrusion process provide data for the verification of the investigated process simulation models. In order to be able to validate the simulated elastic tool deformations, strain gauges are installed on the die insert and allow an experimental measurement of the elastic radial die strains. Additionally the simulated process force development and the final workpiece geometry of the simulation models are compared with experimental results.

Abstract: A workpiece is considered as “long” when one dimension is larger than the two others, for example, railways or guides of manufacturing machines. The customers’ requirements’ evolutions asks the question of workpieces end parts straightness. This paper deals with the means to measure and correct the straightness, a way to prevent flaws in adapting the process and it is especially focused on improving quality for the end parts of the long workpiece. At first, to get the right measure of the straightness, we take into account the fact that the workpiece is deformable; consequently, we assumed the hypothesis that the measured straightness is composed of the intrinsic straightness and the elastic one. Then an approach was developed in order to measure any straightness. Secondly, after getting the proof that the measures are reliable; the correction of the straightness has been studied. The method of semi-automated straightening has been developed in order to improve the productivity. Some experiments have been realized and the results have been compared to the theoretical ones. The conclusions will help to find ways to modify the process in order to prevent the flaws of straightness of long workpieces.

Abstract: Ring Rolling is a complex hot forming process used for the production of shaped rings, seamless and axis symmetrical workpieces. The main advantage of workpieces produced by ring rolling, compared to other technological processes, is given by the size and orientation of grains, especially on the worked surface which give to the final product excellent mechanical properties. In this process different rolls (Idle, Axial, Guide and Driver) are involved in generating the desired ring shape. Because each roll is characterized by a speed law that could be set independently by the speed law imposed to the other rolls an optimization is more critical compared with other deformation processes. Usually in industrial environment a milling curve is introduced in order to correlate the Idle and Axial roll displacement, however it must be underlined that different milling curves lead to different loads and energy for ring realization.In this work an industrial case study was modeled by a numerical approach: different milling curves characterized by different Idle and Axial roll speeds laws (constant, linear and quadratic) were designed and simulated. The results were compared in order to identify the best set of Idle and Axial roll speed laws that guarantee a good quality produced ring (lower fishtail) with lower manufacturing loads and energy.