Key Engineering Materials Vols. 622-623

Abstract: Studies of the effects of inhomogeneity of UFG (ultrafine-grained) microstructure evolution have been performed on severely deformed specimens produced by AAD (angular accumulative drawing), followed by wire drawing and wire flattening processes i.e. “top-down” systems of the grain refinement process. In this paper, deformation behavior and size effects are illustrated by means of UFG microalloyed steel with different combinations of microstructure length scale and deformation inhomogeneity. The refined and severely elongated structures were investigated by light microscopy, TEM and EBSD. Of particular importance was the understanding of the effects of strain path, microalloying elements and deformation inhomogeneity on grain refinement and dislocation substructure formation processes.

Abstract: In this study, experimental and numerical analyses of Forming Limit Diagram (FLD) for Advanced High Strength (AHS) steel grade 980 were performed. Forming limit curve was first determined by means of the Nakazima stretch-forming test. Then, analytical calculations of the FLD based on the Marciniak-Kuczynski (M-K) model were carried out. Different yield criteria, namely, Hill’48 (r-value and stress-based), Yld89 (r-value and stress-based) and Barlat2000 (Yld2000-2d) were investigated. The strain hardening law according to Swift was applied. To identify parameters of each model, uniaxial tension, balanced bi-axial bulge test and in-plane biaxial tension test were performed. As a result, predicted plastic flow stresses and plastic anisotropies of the AHS steel by various directions were evaluated. In addition, effects of the anisotropic yield functions, strain rate sensitivities, imperfection values and work hardening coefficient on the predicted FLD were studied and discussed. It was found that the FLD based on the Yld2000-2d yield criterion was in better agreement with the experimental curve. Accuracy of the FLD predictions based on the M-K theory, especially in the biaxial state of stress, significantly depended on the applied yield criteria, for which yield stresses and r-values of different loading directions were required.

Abstract: In the manufacturing process of body in white components made from sheet metal it is state of the art to accompany the process by means of finite element analysis. A main criterion for determining a feasible tool design and production process parameters is the prediction of material failure, which can be categorized in instability and ductile fracture. The ductile fracture failure mode is more likely to occur, as more advanced high strength steels and aluminum alloys are used for body in white components. Therefore different approaches have been presented to model ductile fracture over the past years. This task is more challenging when the material is exposed to arbitrary loading paths that can occur in deep drawing processes. However there is no guideline for sheet metal forming applications to determine which models for predicting ductile fracture are suitable, which experiments are necessary and how calibration of model parameters and validation of model prediction can be performed. Additionally there is no standard established that prescribes the evaluation of limit strain states from experiments. Suitable limit strain states are a basic requirement for prediction of ductile fracture as they are used for calibration of fracture models. In this paper, two methods for evaluation of limit strains are discussed and applied to tensile specimens with circular hole and circular cut outs made from aluminum alloy AlSi0.6Mg0.5. One validation experiment is used to investigate failure prediction that is based on limit strain states from different evaluation methods.

Abstract: The requirement for products to reduce weight while maintaining strength is a major challenge to the development of new advanced materials. Especially in the field of human medicine or aviation and aeronautics new materials are needed to satisfy increasing demands. Therefore the titanium alloy Ti-6Al-4V with its high specific strength and an outstanding corrosion resistance is used for high and reliable performance in sheet metal forming processes as well as in medical applications. Due to a meaningful and accurate numerical process design and to improve the prediction accuracy of the numerical model, advanced material characterization methods are required. To expand the formability and to skillfully use the advantage of Ti-6Al-4V, forming processes are performed at elevated temperatures. Thus the investigation of plastic yielding at different stress states and at an elevated temperature of 400°C is presented in this paper. For this reason biaxial tensile tests with a cruciform shaped specimen are realized at 400°C in addition to uniaxial tensile tests. Moreover the beginning of plastic yielding is analyzed in the first quadrant of the stress space with regard to complex material modeling.

Abstract: A forming process for heated sheet metal, such as hot-stamping, has limited use in deformable shapes. Higher temperature areas which have not yet come into contact with dies are more easily deformed; therefore, local deformation occurs at these areas which leads to breakage. To improve the formability of heated sheet metal, a deformation control technique utilizing the temperature dependence of flow stress is proposed. This technique can suppress local deformation by partial cooling around potential cracking areas to harden them before forming. In order to apply this technique to a variety of product shapes, a procedure to determine a suitable initial temperature distribution for deep drawing and biaxial stretching was developed with a coupled thermal structural simulation. In this procedure, finite elements exceeding forming limit strain are highlighted, and an initial temperature distribution is defined with areas of decreased temperature around the elements to increase their resistance to deformation. Subsequently, the partial cooling technique was applied to a deep drawing test with a heated steel sheet. The results of the experiment showed that the proposed technique improved 71% in the forming limit depth compared with results obtained using a uniform initial temperature distribution.

Abstract: Formability of two magnesium alloys, namely, AZ80 and ZE10, has been investigated. Both alloys were supplied with a thickness of 0.8 mm. The grain structure of the as-received AZ80 alloy showed dislocations, twins and second-phase particles and-/or precipitates distributed uniformly within grains. These were not obvious on the ZE10 alloy. The investigations were carried out at room temperature for both alloys in the as-received and heat treated conditions (410^oC for 1 hour followed by water quench). The heat treatment significantly changed the grain structure of the AZ80 alloy, but did not affect the ZE10 alloy apart from grain enlargement. The formability was studied on the basis of plastic strain ratio (r) and strain hardening coefficient (n) by means of tensile testing. In the as-received condition, the ZE10 alloy had a slightly better formability () than AZ80 alloy. Following heat treatment, however, the formability of the AZ80 alloy was improved significantly (by about 26%), while the ZE10 alloy did not show any significant change.

Abstract: A number of researches have conducted in order to evaluate the ductile fracture occurrence by using forming limit diagram. However, specimen shape and testing machine for obtaining forming limit diagram of sheet metal have some problems. The problem about specimen shape is occurring at the specimen edge. In uniaxial tensile test, the specimen edge may cause a defused neck in width direction and may have influence on fracture occurrence. In biaxial tensile test by using a cruciform specimen, a uniform biaxial deformation is not obtained because uniaxial tensile stress occurs at the specimen edge. Tensile test by using a specimen which does not have such edges should carry out, for example, in bulge test and multi-axial tube expansion test, specimens without edge are used. However, these methods need special machines. Therefore, new biaxial tensile testing method is required. By this method, materials deform depending on biaxial strain state by using popular pressing machines.

Abstract: In recent years, hot stamping of sheet metal parts has emerged to satisfy the contrary demands of the automotive industry for components with increased strength at reduced weight. To analyse the material behaviour during these processes, a hot gas bulge test at high temperatures and high strain rates is promising, since the bulge test at room temperature has already proven itself as a useful test for the material characterization of sheet metals up to high strains. Therefore, a hot gas bulge test at elevated temperatures and high strain rates is being developed at the Institute of Metal Forming (IBF) in cooperation with the Institute for Fluid Power Drives and Controls (IFAS) at the RWTH Aachen. To verify if the concepts of the membrane theory, which are used for the evaluation of bulge tests at room temperature, are adaptable to such a hot gas bulge test, a simulation study using finite element calculations was conducted. The purpose of this simulation study is is to estimate the errors which occur if the equivalent stress at the bulge pole is calculated by using the membrane theory. In addition to this study several approaches were examined to obtain the sheet thickness at the bulge pole by measuring the bulge height. The study showed that a hot gas bulge test can be described very well by the membrane theory if the sheet thickness, the curvature at the bulge pole and the pressure inside the bulge are exactly known. However, substantial errors can occur if the sheet thickness at the bulge pole is determined by measuring the height of the bulge pole.

Abstract: This study investigates the shape of a cruciform specimen that is stretched in the normal direction of the minimum cross section using FEM. In addition, plane strain tensile states exist in the measurement region in order to determine the forming limit diagram not by an arbitrary stress ratio but by the strain ratio. We propose two types of cruciform specimens. One is a flat-type cruciform specimen, which has deep slits in the middle of the arm region in the width direction. The other specimen is a reduced measurement region type, which also has deep slits as well as a shape that is a biaxial combination of two plane strain tensile specimens. We analyze equibiaxial tensile tests of these two proposed cruciform specimen types using FEM.

Abstract: Microalloyed steels have been the subject of theoretical and experimental studies revealing their exceptional mechanical response under nonlinear deformation conditions. In microalloyed steels, especially in multiphase steels, the mechanical properties are adjusted by combination of microstructure components with different levels of theirs mechanical responses, including hardness and ductility. A comprehensive studies have revealed that a transition from the development of usual bulk dislocation microstructures to more architecture ones occurs when the applied strain path allows to cumulate the deformation energy what is also strictly connected with the chemical and structural compositions of analyzed materials. The study presented here aims at understanding the complex strengthening mechanisms as well as microstructure evolution and to provide a link with the mechanical behaviour of investigated steels under nonlinear deformation conditions. The proper choice of the work hardening model for the cyclic plastic deformation is essential for predicting the inhomogeneities occurring during metal forming. Aim of the current work is to discuss the differences between various hardening models with respect to their capabilities in capturing complex deformation models and possibilities of their direct application to finite element modelling of such deformation processes. The results of experimental studies are integrated with computer modelling and dislocation theory to provide insight into the unprecedented combination of properties achieved in certain multiphase steels such as ultra-high flow strengths, good ductility and workability. Finally, based upon results obtained in performed computer simulations, conclusions regarding the possibilities of potential application of the work hardening models in the identification process parameters, trough the inverse analysis, are drawn.