Search

Undergoing the Tailored Forming process chain, coaxial aluminium-steel profiles joined by co-extrusion are formed into hybrid bearing bushings by die forging. During the joining of aluminium and steel, intermetallic phases may develop. As these phases are very hard and brittle, it is important to be able to predict the width of the resulting intermetallic layer because it is likely to reduce the strength of the compound for the subsequent forging step. In the scope of this paper, a possibility for numerical calculation of the resulting phase thickness during the co-extrusion of aluminium and steel, by means of Lateral Angular Co-Extrusion (LACE), is presented. In the first step, an analogy test on a forming dilatometer was developed for the experimental investigation of the intermetallic phase formation. The width of the intermetallic phase seam was determined by means of scanning electron microscopy using an image processing tool. Based on the experimental results, a calculation instruction was defined to describe the intermetallic phase thickness as a function of temperature and contact time. The function was implemented in a commercial finite element (FE) software by means of a user-defined subroutine and validated on the basis of experimental data. [ABSTRACT FROM AUTHOR]

In the Tailored Forming process chain, serially arranged aluminium-steel semi-finished products, joined by friction welding, are formed into hybrid shafts by a forward extrusion process. In the extrusion of serially arranged hybrid semi-finished products, it is crucial that the yield stress differences between the two materials are as small as possible. If the yield stress difference between the material components is too high, the local deformation is not sufficient and the different materials flow successively into the conical taper area with only a parallel displacement of the interface. In the preliminary work of the Collaborative Research Centre (CRC) 1153, the yield stress difference between the steel and aluminium alloy could not be compensated despite a developed inductive heating strategy, whereby the previously numerically determined optimised joint zone shape could not be achieved. In addition to the adapted heating strategy, the geometry of the joining zone can also be influenced by geometrically adapted joining surfaces in the friction welding process. In the context of this technical contribution, an individual adaptation of the joining zone geometry of the semi-finished products before friction welding is presented, whereby the numerically determined joining zone geometry can be achieved. With the conical welding surface geometries, an increase in bond strength of approx. 18 % was reached in contrast to plane surfaces. In addition to an extension of the joining zone surface, the relative velocity in the sample centre could also be increased, which has shown a positive effect on the bond strength. [ABSTRACT FROM AUTHOR]

Today’s competitive manufacturing world needs machine components which are high quality, cost-efficient and light weight. The welding of different materials is a common technique to produce machine components capable of matching these goals, although some welding processes may negatively impact the quality of the components produced. “Tailored Forming” is a novel processing concept in which hybrid workpieces are fabricated by welding different metals together and subjecting them to deformation processing at elevated temperatures. The combination of plastic straining and high temperatures during forming leads to the activation of the recovery and recrystallization processes. These processes act to improve the mechanical properties of the component, by replacing the directional and coarse microstructure of the weld metal, with an equiaxed, fine microstructure after forging. The hybrid forging billets used in this study combine alloy steel (41Cr4) with plain carbon steel (C22.8), and are manufactured by laser beam welding. An experimental test matrix is employed, which utilises the variations of process parameters, to influence the geometry and microstructure of the materials’ joining zone. The effects of the deformation processing of laser welded hybrid workpieces are characterised by destructive testing and metallography. [ABSTRACT FROM AUTHOR]

Thixoforming as a forming technology takes advantage of the semi-solid material state in order to produce geometrically complex parts. In the automotive industry, several aluminium components are partly manufactured by thixoforming processes. However, the production of steel parts is still challenging and has not yet been established in the industry. One of the main reasons is a short tool life due to high process temperatures. In this paper, the Sehitoglu’s model is introduced as a life prediction approach for forging tools. For this purpose, results of low-cycle fatigue tests are presented. The findings are used for the determination of the model’s material parameters. Numerical calculation of the thermo-mechanical tool loads during thixoforging was carried out. Fatigue tool life can be calculated on the basis of these data by implementing the Sehitoglu’s model in the commercial FE software Simufact.forming by means of user-defined subroutines. [ABSTRACT FROM AUTHOR]

Forming of already joined semi-finished products is an innovative approach to manufacture components which are well-adapted to external loads. This approach results in an economically and ecologically improved production by the targeted use of high-quality materials in component areas, which undergo high stresses. One possible production method for hybrid semi-finished products is friction welding. This welding method allows for the production of hybrid semi-finished products made of aluminium and steel as well as steel and steel. In this paper, the thermomechanical tensile and shear stresses causing a failure of the joined zone are experimentally determined through tension tests. These tests are performed with specimens whose joint zones are aligned with different angles to the load direction. [ABSTRACT FROM AUTHOR]

Hot forging is an effective production method producing safety relevant parts with excellent mechanical properties. The economic efficiency directly depends on the occurring wear of the tools, which limits service lifetime. Several approaches of the presenting research group aim at minimizing the wear caused by interacting mechanical and thermal loads by using enhanced nitriding technology. Thus, by modifying the surface zone layer it is possible to create a resistance against thermal softening provoking plastic deformation and pronounced abrasive wear. As a disadvantage, intensely nitrided surfaces may possibly include the risk of increased crack sensitivity and therefore feature the chipping of material at the treated surface. Recent projects (evaluated in several industrial applications) show the high technological potential of adapted treatments: A first approach evaluated localized treatments by preventing areas from nitrogen diffusion with applied pastes or other coverages. Now, further ideas are to use this principle to structure the surface with differently designed patterns generating smaller ductile zones beneath nitrided ones. The selection of suitable designs is subject to certain geo-metrical requirements though. The intention of this approach is to prevent the formation and propagation of cracks under thermal shock conditions. Analytical characterization methods for crack sensitivity of surface zone layers and an accurate system of testing rigs for thermal shock conditions verified the treatment concepts. Additionally, serial forging tests using adapted testing geometries and finally, tests in the industrial production field were performed. Besides stabilizing the service lifetime and decreasing specific wear mechanisms caused by thermal influences, the crack behavior was influenced positively. This leads to a higher efficiency of the industrial production process and enables higher output in forging campaigns of industrial partners. [ABSTRACT FROM AUTHOR]

In order to save weight and costs, different materials can be combined within one component. In the novel process chain being developed within the Collaborative Research Centre (CRC) 1153, joined semi-finished workpieces are used to produce hybrid solid components with locally adapted properties. Different materials are joined in an initial step before the forming process takes place. Hereby, the quality of the joining zone is improved by means of the thermo-mechanical treatment during the forming and machining processes. The lateral angular co-extrusion (LACE) approach is used to produce semi-finished workpieces because it allows for the production of coaxial semi-finished products consisting of aluminium and steel. In the further process chain, these semi-finished products are processed into hybrid bearing bushings with locally adapted properties by die forging. In the scope of this work, numerical investigations of the co-extrusion of aluminium-steel compounds were carried out using finite element (FE) simulation in order to examine the influence of the process parameters on the co-extrusion process. For this purpose, the relevant material properties of the aluminium alloy EN AW-6082 were determined experimentally and subsequently implemented in the numerical model. The obtained numerical model was used to study the impact of different ram speeds, press ratios and billet temperatures on the resulting extrusion forces and the material flow. The numerical results have been validated using force-time curves obtained from experimental extrusion tests carried out on a 2.5 MN laboratory extrusion press. [ABSTRACT FROM AUTHOR]

A sectionwise defined material model has been developed for the numerical description of thixoforming processes. It consists of two sections. The first one describes the material behaviour below the solidus temperature and comprises an approach from structure mechanics, whereas the second section model describes the thixotropic behaviour above the solidus temperature based on the Ostwald-de Waele power law. The material model has been implemented in a commercial FE software Simufact Forming by means of user-defined subroutines. Numerical and experimental investigations of special upsetting tests have been designed and carried out with Armco iron-coated specimens. Finally, the model parameters were fitted by reverse engineering. [ABSTRACT FROM AUTHOR]

Solid-forming components are often used in areas where they are subjected to very high loads. For most solid components locally divergent and sometimes contradictory requirements exist. Despite these contradictory requirements, almost exclusively monomaterials are nowadays used for the production of solid components. These components often reach their material-specific limits because of increasing demands on the products. Thus a significant increase in product quality and profitability would result from combining different materials in order to create tailored properties. In the Collaborative Research Center (CRC) 1153 "Tailored Forming" at the Leibniz Universität Hannover, this topic is investigated. The primary objective of the CRC 1153 is to develop and investigate new tailored manufacturing processes to produce reliable hybrid solid semi-finished components. In contrast to existing production processes of hybrid solid components, semi-finished workpieces in the CRC 1153 are joined before the forming phase. Thus, it will be possible to produce complex and highly stressable solid components made of different metals, which cannot be produced yet with the current used technologies. In this work the material and friction characteristics are investigated and the forming tool for the production of hybrid bevel gears made of different steel alloys (C22 and 41Cr4) is designed by numerical simulations. For this purpose, flow curves of both materials are determined by means of upsetting tests at process-related forming temperatures and strain rates. The temperature range for the forming process of the semi-finished product is determined by comparing the respective flow curves regarding similar flow stresses. Furthermore, the friction between the tool and the joining materials is investigated by means of ring upsetting tests at a process-relevant temperature. Finally, a stress analysis of the forming tools is carried out. [ABSTRACT FROM AUTHOR]

Nowadays the design of single process steps and even of whole process chains is realized by the use of numerical simulation, in particular finite element (FE) based methods. A detailed numerical simulation of hot forging processes requires realistic models, which consider the relevant material-specific parameters to characterize the material behavior, the surface phenomena, the dies as well as models for the machine kinematic. This data exists partial for several materials, but general information on steel groups depending on alloying elements are not available. In order to generate the scientific input data regarding to material modelling, it is necessary to take into account the mathematical functions for deformation behavior as well as recrystallization kinetic, which depends alloying elements, initial microstructure and reheating mode. Besides the material flow characterization, a detailed description of surface changes caused by oxide scale is gaining in importance, as these phenomena affect the material flow and the component quality. Experiments to investigate the influence of only one chemical element on the oxide scale kinetic and the inner structure at high temperatures are still not available. Most data concerning these characteristics is provided for the steel grade C45, so this steel will be used as basis for the tests. In order to identify the effect of the carbon content on the material and oxidation behavior, the steel grades C15 and C60 will be investigated. This paper gives first approaches with regard to the influence of the carbon content on the oxide scale kinetic and the flow stresses combined with the initial microstructure. [ABSTRACT FROM AUTHOR]

In industrial hot forging processes, the forging dies underlie cyclic thermo-mechanical loads, which can lead to failure of the tools. Besides die wear (abrasive and adhesive) on the surface, fatigue crack initiation with a following fracture of the tool is one of the most frequent failures occurring in hot forging processes. In this study an industrial hot forging process is considered for fatigue analysis. Tool geometry is designed for a defined life time until crack initiated failure of the tool. Material characterization tests in form of tensile tests as well as cylindrical compression tests and low cycle fatigue tests were carried out. The data will be implemented in a commercial software-system to realize a qualitative good prediction of die life regarding crack wiation. [ABSTRACT FROM AUTHOR]

The combination of light metals and steels allows for new lightweight components with wear-resistant functional surfaces. Within the Collaborative Research Centre 1153 novel process chains are developed for the manufacture of such hybrid components. Here, the production process of a hybrid bearing bushing made of the aluminium alloy EN AW-6082 and the case-hardened steel 20MnCr5 is developed. Hybrid semi-finished products are an attractive alternative to conventional ones resulting from massive forming processes where the individual components are joined after the forming process. The actual hybrid semi-finished products were manufactured using a lateral angular co-extrusion (LACE) process. The bearing bushings are subsequently produced by die forging. In the present study, a tool concept for the LACE process is described, which renders the continuous joining of a steel rod with an aluminium tube possible. During the LACE process, the rod is fed into the extrusion die at an angle of approx. 90°. Metallographic analysis of the hybrid profile showed that the mechanical bonding between the different materials begins about 75 mm after the edge of the aluminium sheath. In order to improve the bonding strength, the steel rod is to be preheated during extrusion. Systematic investigations using a dilatometer, considering the maximum possible co-extrusion process parameters, were carried out. The variable parameters for the dilatometer experiments were determined by numerical simulation. In order to form a bond between the materials, the oxide layer needs to be disrupted during the co-extrusion process. In an attempt to better understand this effect, a modified sample geometry with chamfered steel was developed for the dilatometer experiments. The influence of the process parameters on the formation of the intermetallic phase at the interface was analysed by scanning electron microscopy and X-ray diffraction. [ABSTRACT FROM AUTHOR]

In order to meet requirements by automotive industry like decreasing the CO2 emissions, which reflects in reducing vehicles mass in the car body, the chassis and the powertrain, the continuous innovation and further development of existing production processes are required. In sheet metal forming processes the process limits and components characteristics are defined through the process specific loads. While exceeding the load limits, a failure in the material occurs, which can be avoided by additional force transmission activated in the deep drawing process before the process limit is achieved. This contribution deals with experimental investigations of a forming process with additional force transmission regarding the extension of the process limits. Based on FEA a tool system is designed and developed by IFUM. For this purpose, the steel material HCT600 is analyzed numerically. Within the experimental investigations, the deep drawing processes, with and without the additional force transmission are carried out. Here, a comparison of the produced rectangle cups is done. Subsequently, the identical deep drawing processes are investigated numerically. Thereby, the values of the punch reaction force and displacement are estimated and compared with experimental results. Thus, the validation of material model is successfully carried out on process scale. For further quantitative verification of the FEA results the experimental determined geometry of the rectangular cup is measured optically with ATOS system of the company GOM mbH and digitally compared with external software Geomagic® QualifyTM. The goal of this paper is the verification of the transferability of the FEA model for a conventional deep drawing process to a deep drawing process with additional force transmission with a counter punch. [ABSTRACT FROM AUTHOR]

As a part of the newly established Collaborative Research Center 1153 (SFB 1153) „Process chain for the manufacturing of hybrid high-performance components by tailored forming" at the Leibniz Universität Hannover, the Institute of Forming Technology and Machines (IFUM) examines the influence of thermo-mechanical stresses on the reduced Young's modulus as well as the hardness of hybrid (steel-steel compound) joined semi-finished products. Currently the expertise in the production of bulk metal formed parts is limited to mono-materials. For manufacturing parts of hybrid materials and also for the methods of the new process routes, practical experience has to be gained. The subproject C1 within the collaborative research center 1153 with the short title "Failure Prediction" deals with the question, if the hybrid semi-finished products fulfill the thermo-mechanical demands or if they fail at the joining zone (JZ) during forging. For this purpose, stresses similar to those in the process were imposed on hybrid semi-finished products by torsion tests by using the thermo-mechanical test system Gleeble 3800. Afterwards, the specimens were examined metallographically and by nanoindentations with the help of a TriboIndenter TI950. Thus, first knowledge on the behaviour of thermo-mechanical stresses on the reduced Young's modulus and the hardness of hybrid joined semifinished parts was gained. [ABSTRACT FROM AUTHOR]

A structural concept in multi-material design is used in the automotive industry with the aim of achieving significant weight reductions of conventional car bodies. In this respect, the use of aluminum and short fiber reinforced plastics represents an interesting material combination. A wide acceptance of such a material combination requires a suitable joining technique. Among different joining techniques, clinching represents one of the most appealing alternative for automotive applications. This contribution deals with the FE simulation of the clinching process of two representative materials PA6GF30 and EN AW 5754 using the FE software LS-DYNA. With regard to the material modelling of the aluminum sheet, an isotropic material model based on the von Mises plasticity implemented in LS-DYNA was chosen. Analogous to aluminum, the same material model is used for modelling the short fiber reinforced thermoplastic. Additionally, a semi-analytical model for polymers (SAMP-1) also available in LS-DYNA was taken. Finally, the FEA of clinching process is carried out and the comparison of the simulation results is presented above. [ABSTRACT FROM AUTHOR]

Powder metallurgy is known for its high potential for producing near net-shape products. A material utilization of up to 95 %, paired with comparably low energy costs, allows powder metallurgy to fulfil the requirements of modern manufacturing processes. By combining different powders, a wide range of products can be manufactured. An innovative powder metallurgical method currently being investigated at the Institute of Forming Technology and Machines (IFUM) within the subproject E3 of the Collaborative Research Centre 653 is the generation of controlled cavities inside a sintered part. For this purpose, a foreign element with a lower melting point than the base powder is embedded inside the green body. Depending on the sintering temperature, the foreign element can be firmly bonded with or melted out of the base powder, creating a defined cavity. Being attached to an external cooling system, the cavity can be applied as a closed circuit for circulating a cooling medium within the tool. The approach in this work is the development of a sintered forging die, equipped with an active temperature regulation which can react autonomously to process variations. The cooling temperature is controlled by measuring the operating temperature within the forging die. For measuring purposes, the cavities can also be used for integrating temperature sensors. The main aspect of these studies is the characterization of the compaction and melting behavior of the foreign material. Since the location of the foreign element within the base powder can differ due to the pressing force, the prediction of its final position based on the initial position and the process conditions is of high importance. For this aim, numerical simulations are employed to develop an optimized cooling layout. A numerical model is used to describe the compaction behaviour of the powder as an elastoplastic compressible continuum and its interdependency with the integrated elements. The studies also cover the influence of surface contours of the foreign elements (corrugated, plain) on their melting behavior as well as the resulting inner surface of the cooling channel. [ABSTRACT FROM AUTHOR]

This paper compares methods used for integration of cooling system in the press hardening simulation. For this purpose an experimental tool was developed and constructed. This tool makes it possible to compare and validate different simulation methods applicable for cooling systems and resolves the question of their importance. The simplified shape of the tool has been chosen to break down the problem to less numerous parameters limited to thermodynamics and fluid dynamics. Such a simplified problem can better address the parameters like heat transfer coefficient in channels depending on channel geometry and flow speed of cooling water, and can easily relate them to the reality. A temperature distribution on the tool surface for different geometries of channels is presented under chosen flow speeds of cooling medium and heating power of integrated heating elements and compared to the results from both FEM and CFD simulation methods. Resultant heat transfer coefficients for different flow speeds of water and cooling channels geometries will be calculated through the inverse calculation, compared to the analytical calculation. The conclusion should reveal whether it is better, for a given geometry, to use a constant analytically calculated heat transfer coefficient, constant coefficient from the inverse calculation, or variable heat transfer coefficient from CFD simulation. [ABSTRACT FROM AUTHOR]

Limited natural resources increase the demand on highly efficient machinery and transportation means. New energy-saving mobility concepts call for design optimisation through downsizing of components and choice of corrosion resistant materials possessing high strength to density ratios. Component downsizing can be performed either by constructive structural optimisation or by substituting heavy materials with lighter high-strength ones. In this context, forging plays an important role in manufacturing load-optimised structural components. At the Institute of Metal Forming and Metal-Forming Machines (IFUM) various innovative forging technologies have been developed. With regard to structural optimisation, different strategies for localised reinforcement of components were investigated. Locally induced strain hardening by means of cold forging under a superimposed hydrostatic pressure could be realised. In addition, controlled martensitic zones could be created through forming induced phase conversion in metastable austenitic steels. Other research focused on the replacement of heavy steel parts with high-strength nonferrous alloys or hybrid material compounds. Several forging processes of magnesium, aluminium and titanium alloys for different aeronautical and automotive applications were developed. The whole process chain from material characterisation via simulation-based process design to the production of the parts has been considered. The feasibility of forging complex shaped geometries using these alloys was confirmed. In spite of the difficulties encountered due to machine noise and high temperature, acoustic emission (AE) technique has been successfully applied for online monitoring of forging defects. New AE analysis algorithm has been developed, so that different signal patterns due to various events such as product/die cracking or die wear could be detected and classified. Further, the feasibility of the mentioned forging technologies was proven by means of the finite element analysis (FEA). For example, the integrity of forging dies with respect to crack initiation due to thermo-mechanical fatigue as well as the ductile damage of forgings was investigated with the help of cumulative damage models. In this paper some of the mentioned approaches are described. [ABSTRACT FROM AUTHOR]

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 analysis (FEA) may serve as a means for process design and process optimisation. So far the FEA 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. Two industrial forging processes, where clear fatigue crack initiation has been observed are considered for a fatigue analysis. For this purpose the relevant tool components are modelled with elasto-plastic material behaviour. The predicted sites, where crack initiation occurs, agree with the ones observed on the real die component. [ABSTRACT FROM AUTHOR]