Abstract: In this work we propose a new model (SOFTMAR) to describe the transient softening observed during severe plastic deformation of Al-30wt%Zn alloys. The model is divided in two main parts. The first one describes the softening process based on the evolution law of the mean free path of dislocations (L) with plastic strain . In the second part of the model emphazis is given to the relationship between the grain size and strain rate that in turn depends on diffusion-driven grain-boundary sliding.
Abstract: The aim of the study was to investigate correlation between bendability and tensile properties of high-strength steels. Strength and elongation in tensile test have traditionally been basic values in evaluating bending properties. Advanced high-strength (AHS) steels often have tendency for strain localization, causing risk of fractures and impairing the shape of the bend. Practice has shown that tensile test results, indicating bendability well, are not sufficiently accurate when using AHS steels. Since tensile test is a fast, simple and cheap testing method, it would be beneficial to rework it to suit better for predicting of bendability. In this study, the usability of tensile test results to predict failures in bending AHS steels has been investigated. The most common failures and failure mechanisms in bending are also presented. Test materials used were 6 mm thick AHS wear-resistant, protection and structural steels with good and poor bending properties. Minimum bending radii were determined and then compared with ten-sile test results to estimate the correlation. Conventional tensile test results, fracture surfaces and necking through width and thickness were analyzed. Correlation coefficient for measured tension properties and minimum bending radius was calculated. Results showed that in tensile test, have the best correlation with minimum bending radius with necking through the thickness and actual strain in necking area.
Abstract: In the SFB 692 HALS (High-strength aluminum based lightweight materials for safety components), subproject B-3, the production of an aluminum magnesium compound by a hydrostatic co-extrusion process was investigated. The quality of these semi-finished products, especially the stability and robustness of the interface between the aluminum (AlMgSi1) sleeve and magnesium (AZ31) core, was of particular interest. Previous papers have described the first process optimization steps as the improvement of the die design as well as the numerical methods for identification of important process parameters and the development of a quality model for the interface. This paper describes the formability of such semi-finished products with subsequent forging processes, especially die forging. Therefore, two different die forging strategies were investigated. In the first approach the strand-shaped work piece, with a circular cross-section, was formed along its longitudinal axis with die forging. In the second approach the same geometry was radially formed with die forging. Thereby, the compound was formed in longitudinal direction up to an analytical equivalent strain value of 1.61 and in radial direction up to 1.38. First results showed that the interface of the aluminum magnesium compound is very stable and ductile enough to be forged. Dye penetration tests were performed to prove the stability of the interface in a first step. Then, micro sections were made to investigate the interface metallographically. No cracks or damages were detected with both test methods in the interface of the forged aluminum magnesium compound. Furthermore, numerical simulations were performed to analyze the forging processes in detail. Therefore, a full 3D simulation model was set-up with Forge2011 and the calibration was performed with the press force as well as the geometry aspects. The correlations between experiments and simulations are very well. By means of the calibrated simulation detailed analyses of interface section are performed and the stability of the interface was investigated. This shows that the compound quality reached by the hydrostatic co-extrusion process is very suitable for subsequent forming steps as die forging. The investigations show the potential of such hybrid compounds and clarify their application, especially in the automotive sector.
Abstract: Titanium alloys, such as Ti-6Al-4V, offer favorable characteristics as significant strength, biocompatibility and metallurgical stability at elevated temperatures. These advantages afford the application of parts out of Ti-6Al-4V in a wide field within aerospace, astronautic and medical technologies. Most applied shaping operations for parts out of titanium alloys are forging, casting, forming and machining. In order to develop and improve forming operations numerical simulations are applied during preprocessing. For that purpose mechanical properties of the material such as yield stress and Lankford parameter have to be determined. Due to the two-phase (α + β) microstructure of Ti-6Al-4V, forming operations have to be carried out at elevated temperatures to reduce the required forming force and extend forming limits. Taking the temperature and stress state dependency of the material into consideration, uniaxial tensile and compression tests are accomplished at elevated temperatures, ranging from 400 to 600 °C. Furthermore, the experimentally determined yield stress and Lankford parameter are approximated with the yield loci model proposed by Barlat 2000. The model predicts the flow response of the material, thus provides input data for the finite element analysis of forming processes at different temperature levels.
Abstract: Optical measuring systems provide much more detail on the deformation of the blank in the bulge test than conventional contact height measuring systems. A significant increase in accuracy of the stress-strain curve can be achieved by fitting the surface to more complicated equations than the traditional spherical surface and by considering the local strain data to approximate the curvature for the midplane. In particular an ellipsoid shape is shown to be very accurate in describing the surface of the blank. Contact height measuring systems provide insufficient data to fit a surface to an ellipsoid shape and to establish local strain data. Pragmatic equations are proposed using the work hardening coefficient from the tensile test to approximate the same accuracy in stress-strain curves as obtained by optical measuring systems using the before mentioned evaluation method.
Abstract: A modular ductile failure model is presented and applied to the forming of an AA5182 aluminium alloy sheet. A detailed description of the failure model and its calibration is provided. The final application of the calibrated failure model to the deep drawing of a cruciform cup reveals a good correlation with the experimental findings. Finally, a study on the influence of the r-value on formability is conducted.
Abstract: Since the last two decades, the automotive industry has dedicated an increasing attention to the manufacturing of sheet components made of high-resistant aluminium alloys; the superplastic AA5083 grade is currently utilized in both the conventional superplastic forming and the recently patented quick plastic forming, which assures higher productivity compared to that of superplastic forming, while the commercial AA5083 grade is rarely employed. The objective of the paper is to compare the hot tensile behaviour of commercial and fine-grained AA5083 sheets when processed at high temperature and strain rate, which are typical of hot stamping processes. The results are presented and commented in terms of flow stress, anisotropy, strain at failure, microstructural and hardness features as a function of temperature and strain rate. On the basis of the obtained results, the set of optimal forming conditions for the two grades is identified.
Abstract: Magnesium (Mg) alloys are the lightest metals that can be used for structural components, and the press forming of Mg alloy sheets has recently attracted attention in automobile and electrical industries. To increase the number of applications of the press forming, it is crucial to understand mechanical properties of Mg alloy sheets. Because Mg alloys are hexagonal close-packed (hcp) materials, mechanical properties of Mg alloy sheets are significantly different from those of conventional structural sheet metals that have cubic structures. Crystal plasticity models can analyze numerically the interaction between mesoscopic crystalline and macroscopic deformation in metals; thus the models are powerful tools to further understand the mechanical properties of Mg alloy sheets. In the present study, the nonlinear response that arose during unloading under in-plane compression of a rolled magnesium alloy sheet was investigated using a crystal-plasticity finite-element method, focusing on the effects of twinning and detwinning. The mechanism that the nonlinear response was more pronounced under in-plane compression than that under in-plane tension was also discussed. In the simulation, a twinning and detwinning model that has originally been proposed by Van Houtte (1978) and recently extended to the detwinning process by the authors [Hama and Takuda, 2012] was employed. From numerical experiments, it was confirmed that, as already pointed out in literature, the activity of detwinning played an important role on the nonlinear response during unloading. On the other hand, it was also found that the basal slip systems could be very active during unloading because of the dispersion of crystal orientations owing to the activity of twinning during loading, which increased the nonlinear response. It was concluded that the nonlinear response during unloading was more pronounced under in-plane compression than that under in-plane tension owing to these two factors that did not present under in-plane tension.