Fast simulation of a flat cross wedge rolling process

In this paper, a new approach for a Fast Simulation of flat cross wedge rolling processes is presented. The approach is based on a preliminary research project of the Institute of Forming Technology and Machines (IFUM) of the Leibniz Universität Hannover, in which a software prototype for a Fast Simulation of a radial-axial ring rolling process was developed. Both simulations are based on geometric-kinematic models that allow a faster calculation of the material flow compared to the Finite Element-simulation (FE-simulation). The goal of the Fast Simulation for the flat cross wedge process is to support the designer in the challenging design phase of the flat cross wedge tool as well as in the planning phase of the process parameters. In this phase it shall be easier in future to determine the best geometric parameters for the design of flat cross wedge tools, to attain the necessary material flow and geometry, before starting with the first FE-simulation. With this preliminary information from the Fast Simulation it will be possible to reduce the number of iteration loops for the time-consuming FE-simulations of incremental forming processes

Investigation on temperature control in the SPS process with titanium aluminides

Spark Plasma Sintering (SPS) or Field-Assisted Sintering Technique (FAST) is a powder metallurgical (PM) heat-assisted process, where loose metal powder is pressed to a compact. Powders, which are difficult to consolidate, can be processed much more efficiently than within the conventional PM process route, i.e. die pressing followed by sintering. However, temperature measurement in the SPS process is challenging since it can only be done on the die surface or within the die wall without influencing the process. In case of γ-based Titanium aluminide (TiAl) powders, a suitable sintering temperature allows for a modification of the three grain morphologies lamellar, equiaxed and duplex, and thus for the mechanical properties to be selectively adjusted. In this study, temperature control during the SPS process is investigated using the commercially available pre-alloyed TiAl48-2Cr-2Nb (GE48) powder. For this purpose, temperature measurement with thermocouples and a pyrometer is considered. Furthermore, the influences of graphite and boron nitride lubricants or rather release agents and insulators on the temperature profile are investigated. The evaluation is carried out based on microstructure analysis and hardness measurements. The results show that a suitable temperature control of the process is possible. In addition, it seems that the choice of release agent has a significant influence on the temperature and thus on microstructure and mechanical properties

Production, Bonding and Application of Metal Matrix Composite Hot Forging Tool Components

Metal matrix composite materials are of high interest for their increased stiffness, strength or wear resistance. Wear resistant composites contain hard ceramic particles to reduce microcutting and grooving of the metal matrix surface. In this paper, a gas atomised hot work tool steel X40CrMoV5-1 (1.2344/AISI H13) was combined with fused tungsten carbide (FTC) particles in order to create forging tools with increased abrasive wear resistance. For that purpose, tool components were manufactured by sinter-forging of stacked powder layers to build up a graded hard phase concentration of up to 10 vol.-%. Subsequently, sinter-forged specimens were combined with basic hot work tool steel components and joined by diffusion bonding to assemble the complete tool. In order to evaluate their performance, the tools were examined in a hot backward can extrusion process of low-alloyed steel. Optical geometry measurements, light microscopy and scanning electron microscopy of the worn tool radii indicated a significant decrease in abrasive wear when using FTC-reinforced tools rather than conventional hardened tool steel

On modelling of shear fracture in deep drawing of a high-strength dual-phase sheet steel

The paper presents application of fracture behaviour characterisation results of a dual-phase sheet steel DP600 to an FEA of its deep-drawing for shear fracture prediction. The characterisation results were obtained with the help of a characterisation method based on a tensile test on a novel butterfly specimen and published previously by the authors. The aim of the present paper is to evaluate that characterisation method on a deep-drawing process. Based on the previous results of the authors, the fracture behaviour is modelled here with the help of the modified Mohr-Coloumb fracture model. The obtained FEA results reveal that shear fracture of the studied material is predicted too early by the used MMC fracture model. A novel adjustment of the model is proposed yielding infinitely high fracture strains at strongly pressure-superimposed stress states. As it is often the case in the state-of-the-art fracture characterisation of high-strenght sheet steels, such stress states were not tested during the previously performed fracture characterisation but occur during the studied deep drawing process. With the help of the adjusted MMC fracture model, it is possible to predict the crack initiation moment very accurately and the crack initiation location sufficiently accurately. © Published under licence by IOP Publishing Ltd

Forming sheets of metal and fibre-reinforced plastics to hybrid parts in one deep drawing process

Each material has its own advantages and disadvantages in terms of its mechanical, chemical and physical properties. Metallic materials are comparatively ductile and easy to process. Fibre reinforced plastics are very stiff and endure high tensile stresses based on their weight. By intelligent combination of these materials into one overall-part light but strong components may be established. However, the conventional production of a separate fibre reinforced plastic (FRP)-component and a metal component and a subsequent joining is time- And labour-intensive and therefore not economical in mass-production. Thus in this paper a new fabrication technology is presented.German Federation of Industrial Research Associations (AiF)European Research Association for Sheet Metal Working (EFB)Federal Ministry of Economics and Technology (BMWi

Experimental and numerical characterization method for forming behavior of thermoplastics reinforced with woven fabrics

The automotive and aviation industry has to achieve significant weight reduction in order to fulfil legal obligations. This leads to an increasing use of new materials or new material combinations like fibre-reinforced plastics (FRP) as they provide a high lightweight potential due to the combination of low density and high tensile strength. Meanwhile pre-impregnated sheets with a thermoplastic matrix reinforced with woven carbon fibres are commercially available. This has led in a significant cost reduction and hence, the FRP have become affordable for large scale production. The material properties, in particular the forming and failure behaviour of the FRP, differ strongly from that of conventional metal materials like steel or aluminium. Therefore, new material characterisation techniques, investigation methods as well as numerical models are required. The main focus of this paper lies on the development of a non-orthogonal material model for the FRP, its implementation in a commercial FE-software as well as on the use of a combined experimental-numerical procedure for material characterisation. Since the properties of these materials are strongly temperature dependent, the forming process of reinforced thermoplastics is typically carried out at elevated temperatures. Thus, temperature sensitivity has to be taken into account during experimental testing as well as in the model approach. The model parameterisation is carried out based on an iterative numerical optimization procedure. For this purpose, the experimentally obtained results are investigated by means of digital image correlation and linked with the numerical model in combination with an automated optimization process

Investigation of surface topography evolution of sheet aluminum under pressure and tension

Besides tool parameters and work piece properties, the accuracy of the finite element analysis depends much on contact conditions between the sheet and the tool. To a large extent, the contact conditions are determined by the material properties of the parts in contact as well as the relative motion and the contact pressure between them. A further factor influencing friction is the sheet surface topography which is changing due to tension, contact and sheet deformation. In this paper, the surface topography evolution under pressure and under tension is presented and discussed. Therefore, basic experiments were done. In the experiments, the influence of different parameters like tension and contact pressure is determined. To analyze the surface topography evolution, pre-defined spots on the samples were traced during the experiments. The changes in the surface topography before and after forming were analyzed by means of roughness measurement. The intention is to get a correlation between flattening or roughness evolution and the parameters of the deformation. This correlation can be used to improve existing friction models used in the FE analysi

Failure Modelling of CP800 Using Acoustic Emission Analysis

Advanced high-strength steels (AHHS) are widely used in many production lines of car components. For efficient design of the forming processes, numerical methods are frequently applied in the automotive industry. To model the forming processes realistically, exact material data and analytical models are required. With respect to failure modelling, the accurate determination of failure onset continues to be a challenge. In this article, the complex phase (CP) steel CP800 is characterised for its failure characteristics using tensile tests with butterfly specimens. The material failure was determined by three evaluation methods: mechanically by a sudden drop in the forming force, optically by a crack appearing on the specimen surface, and acoustically by burst signals. As to be expected, the mechanical evaluation method determined material failure the latest, while the optical and acoustical methods showed similar values. Numerical models of the butterfly tests were created using boundary conditions determined by each evaluation method. A comparison of the experiments, regarding the forming force and the distribution of the equivalent plastic strain, showed sufficient agreement. Based on the numerical models, the characteristic stress states of each test were evaluated, which showed similar values for the mechanical and optical evaluation method. The characteristic stress states derived from the acoustical evaluation method were shifted to higher triaxialities, compared to the other methods. Matching the point in time of material failure, the equivalent plastic strain at failure was highest for the mechanical evaluation method, with lower values for the other two methods. Furter, three Johnson–Cook (JC) failure models were parametrised and subsequently compared. The major difference was in the slope of the failure models, of which the optical evaluation method showed the lowest slope. The reasons for the differences are the different stress states and the different equivalent plastic strains due to different evaluation areas

Fracture Characterisation and Modelling of AHSS Using Acoustic Emission Analysis for Deep Drawing †

Driven by high energy prices, AHSS are still gaining importance in the automotive industry regarding electric vehicles and their battery range. Simulation-based design of forming processes can contribute to exploiting their potential for lightweight design. Fracture models are frequently used to predict the material’s failure and are often parametrised using different tensile tests with optical measurements. Hereby, the fracture is determined by a surface crack. However, for many steels, the fracture initiation already occurs inside the specimen prior to a crack on the surface. This leads to inaccuracies and more imprecise fracture models. Using a method that detects the fracture initiation within the specimen, such as acoustic emission analysis, has a high potential to improve the modelling accuracy. In the presented paper, tests for fracture characterisation with two AHSS were performed for a wide range of stress states and measured with a conventional optical as well as a new acoustical measurement system. The tests were analysed regarding the fracture initiation using both measurement systems. Numerical models of the tests were created, and the EMC fracture model was parametrised based on the two evaluation areas: a surface crack as usual and a fracture from the inside as a novelty. The two fracture models were used in a deep drawing simulation for analysis, comparison and validation with deep drawing experiments. It was shown that the evaluation area for the fracture initiation had a significant impact on the fracture model. Hence, the failure prediction of the EMC fracture model from the acoustic evaluation method showed a higher agreement in the numerical simulations with the experiments than the model from the optical evaluation

Stress-state dependent fracture characterisation and modelling of an AZ31 magnesium sheet alloy at elevated temperatures

Due to a high specific strength, magnesium alloys have a high potential to be considered for lightweight solutions in automotive industry. For the numerical design of forming processes, it is important to describe the yielding as well as the fracture behaviour of a material as precisely as possible. In order to fully characterise the fracture behaviour of an AZ31 magnesium sheet alloy at elevated temperatures, a heated test setup for uniaxial tensile machines was developed. The setup allows an adjustment of the load application angle whereby a stress variation is achieved in the centre of the specimen. In order to determine the fracture strain for different temperatures and for varying stress states, a shear stress specimen (also known as butterfly specimen) was considered to perform mechanical experiments by means of this setup. Using numerical simulations, the specific stress development and strain value in the fracture zone, which is needed to calibrate stress state fracture models, was determined for each loading angle and temperature. For this purpose, an orthotropic yield criterion CPB06, which is suitable for depiction of the particular flow behaviour of magnesium alloys (e. g. compression-tension asymmetry), was used. By this means, sufficient data for the calibration of common stress state based fracture models could be provided and the MMC- (Modified Mohr-Coulomb) fracture model was parameterised