Characterization of superplastic materials by results of free bulging tests
Determination of material constants describing its behavior during superplastic gas
forming is the main subject of this study. The main feature of free bulging tests is the stress-strain
conditions which are very similar to ones occurring in the most of gas forming processes. On the
other hand, the interpretation of the results of such tests is a complicated procedure. The paper
presents a simple technique for the characterization of materials superplasticity by free bulging tests,
which is based on inverse analysis. The main idea of this technique is a semianalytical solution of
the direct problem instead of finite element simulation which allows one to reduce the calculation
time significantly. At the same time the results this simplified solution are accurate enough to obtain
realistic material constants.
The paper presents a simple technique for the characterization of materials superplasticity by free bulging tests, which is based on inverse analysis. The main idea of this technique is a semianalytical solution of the direct problem instead of finite element simulation which allows one to reduce the calculation time significantly. Presented method use experimental time-thickness and time-dome height of the workpiece dependancies as initial experimental data. Presented method has been applied for AZ31 magnesium alloy at 520. Received properties have been veracity via simulation by finite element method. Obtained time-height relations were comparison with the data presented in the literature.
Superplastic behaviour and the microstructural evolution were investigated.
Alloy with Al3(Sc,Zr) particles and alloy containing Al3Zr and Al3Ni particles.
Alloy with Ni and Zr exhibits a 1.5–2 times finer grains during deformation.
Al3Ni particles provide a larger number of nuclei for recrystallization.
The superplastic properties of two high strength aluminium alloys are investigated. The first alloy contains fine, coherent Al3(Zr,Sc) particles, and the second alloy contains both fine, coherent Al3Zr particles and coarse, eutectic Al3Ni particles. Due to its fine, stable grain structure, the alloy containing both Al3Zr and Al3Ni particles exhibits significantly better superplastic properties. The superplastic behaviour in tension under both constant and initial strain rates was compared, and the effective activation energy of superplastic deformation was calculated. The alloy containing both fine and coarse particles exhibit 1100%–1200% elongation at strain rates of 1 × 10−2 s−1–1 × 10−1 s−1 and temperature of 440 °C.
Mechanical performances of titanium biomedical implants manufactured by superplastic forming are strongly related to the process parameters: the thickness distribution along the formed sheet has a key role in the evaluation of post-forming characteristics of the prosthesis. In this work, a finite element model able to reliably predict the thickness distribution after the superplastic forming operation was developed and validated in a case study. The material model was built for the investigated titanium alloy (Ti6Al4V-ELI) upon results achieved through free inflation tests in different pressure regimes. Thus, a strain and strain rate dependent material behaviour was implemented in the numerical model. It was found that, especially for relatively low strain rates, the strain rate sensitivity index of the investigated titanium alloy significantly decreases during the deformation process. Results on the case study highlighted that the strain rate has a strong influence on the thickness profile, both on its minimum value and on the position in which such a minimum is found.
This study proposes a method for determination of material characteristics by inverse analysis of free bulging tests results. The blow-forming tests were carried out at the temperature of 415 °C using aluminum alloy (AMg-6) sheets of a 0.92 mm thickness. Each test was performed at constant pressure. For each fixed value of pressure, a series of experiments was carried out with different forming times to obtain evolutions of dome height H and thickness s. Two different constitutive equations were used to describe the dependence of flow stress on the effective strain rate: the Backofen power equation and the Smirnov one taking into account an s-shape of stress-strain rate curve in the logarithmic scale. The constants of these 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 formulated for this purpose. Using the Smirnov constitutive model to describe the dependence of flow stress on strain rate, unlike the classical power law, makes it possible to analyze the variation of strain rate sensitivity index m with strain rate. On the basis of the obtained data, the optimum strain rate for AMg-6 processing was estimated as one corresponding to the maximum of strain rate sensitivity index. The validity of the proposed method was examined by finite element simulation of free bulging process.
A method based on the spectral analysis of thermowave oscillations formed under the effect of radiation of lasers operated in a periodic pulsed mode is developed for investigating the state of the interface of multilayered systems. The method is based on high sensitivity of the shape of the oscillating component of the pyrometric signal to adhesion characteristics of the phase interface. The shape of the signal is quantitatively estimated using the correlation coefficient (for a film–interface system) and the transfer function (for multilayered specimens).