МОДЕЛИРОВАНИЕ ПРОЦЕССА СВОБОДНОЙ ГАЗОВОЙ ФОРМОВКИ ЛИСТОВОЙ ЗАГОТОВКИ
This study proposes a method for determining the material constitutive equations and optimal forming conditions on the basis of free bulging tests. The blow-forming tests were carried out at the temperature of 420 °C using aluminum alloy (AMg-6) sheets of a 1 mm thickness. Each test was performed at constant pressure. For each fixed value of the pressure, a series of experiments was carried out with different forming times to obtain evolutions of dome height H and thickness s. These data were processed by the proposed method to obtain the flow stress dependence on the effective strain rate. The constitutive equations were obtained by least squares minimization of deviations between the experimental variations of H and s and ones predicted by a simplified engineering model. On the basis of the obtained data, the optimum strain rate for AMg-6 processing was determined as one corresponding to the maximum strain rate sensitivity.
Different test methods are described to determine the mechanical properties of materials in the superplastic state. The flow stress depends on strain and strain rate, the structural parameter of the materials, and temperature. The rheology and mechanics of superplastic deformation are discussed. The methods have been checked for reliability and produced good results in testing titanium alloys and constructing mathematical models as part of an order submitted by the company EADS (Airbus). The information given on the test methods and the subsequent approximation of materials’ mechanical properties is of considerable interest for making reliable predictions of the deformation of materials during shaping operations. © 2015 Springer Science+Business Media New York
Computer simulation of mechanical testing is used primarily for the correct interpretation of their results and is particularly relevant in cases, when the properties of the material during deformation are essentially nonlinear. For example: when we study mechanical properties of materials with high rate sensitivity. First of all it is superplastic titanium alloys. Superplastic materials exhibit the ability to severe plastic deformation without discontinuities if forming occurs in a narrow range of strain rates, specific to each alloy and temperature-dependent. In the study of superplastic materials, it’s necessary to maintain a constant rate of deformation of the sample. This is achieved by conducting an experiment with a special program loading, crosshead speed at which change during the experiment.
The purpose of this study is to find out the characteristics of hot forming of Ti-6Al-4V titanium alloy in order to determine the conditions of its superplastic behavior. The experiments were performed in two stages: the stepped tensile-tests series (temperature range 700 – 925 °С) and the constant strain rate tensile-test series (temperature range 775 – 925 °С). By the results of stepped tensile tests the constitutive equations which describe relationship between stress and strain rate for each temperature were constructed. On the base of obtained data, the temperature and strain-rate ranges which ensure the realization of superplasticity at forming of Ti-6Al-4V alloy as well as optimal strain rates which corresponds to the maximum value of strain rate sensitivity exponent were determined. In was shown that at low temperatures (700 – 775C) the Ti-6Al-4V alloy shows all signs of superplasticity, however at these temperatures the optimal strain rates are too slow for industrial technological procedures. The dependence between optimum strain rate and reciprocal temperature appears to be well fitted by exponential low. At the second stage of the experimental research, the tensile-tests with a constant, optimum for each temperature strain-rate were carried in order, to estimate the real initial flow stress and the character of strain hardening of the material during the deformation with optimum strain rate. In was found that flow stress values obtained by stepped tensile tests matches the values form constant-strain-rate tests with effective strain value equal to 0,2 and the strain hardening during the deformation with optimal strain rates is significant.
This aim of this paper is the interpretation of the results of mechanical testing of materials to determine their properties under hot deformation. As an example, a simulation of rod stretching in superplasticity mode was considered. Comparing obtained data with the analytical solution was conducted.
This paper presents the research of the flow characteristics of the Ti-6V-4Al alloy in wide ranges of temperature (725 ‑ 950 °C) and strain rate (10-5 ‑ 10-2 s-1). The material processing maps were constructed based on the basis of dynamic materials model (DMM) developed by Prassad and modified by Narayana Murty. For the construction of such maps the data of the material’s flow stress at different temperatures and strain rates is necessary. To obtain such data the stepped tensile tests which allow get the stress - strain rate dependence at a given temperature are ideal. The experiments conducted consist of the tensile-testing of samples’ series at various temperatures with stepped change of the deformation speed. By the results of these tests the constitutive equations, which describe relationship between stress and strain rate for each temperature were obtained. The data was analyzed in terms of the two different approaches proposed by Prassad and Narayana Murty to assess the impact of deformation conditions on the formability and flow stability of the material. Based on these approaches, the processing maps which allow identifying the conditions of the Ti-6V-4Al alloy superplasticity were constructed.