The current study is to discuss that the degree of lubrication might be significantly different at different positions of a SPFed product. Numerous comparisons between experimental and predicted thickness profiles for open-die free bulging or close-die forming were made, using simulations based on systematic combinations of m, n and μ values. Current results suggest that the experimental thickness profiles fit best with the simulated distribution when using m=0.5 together with μ=∞ for the die entry portion and μ=0.01 for the bottom region. It we use the same friction coefficient for all positions, the predicted thickness profiles can never be close to the experimental ones. It was latter found that the lubricant powders would slide down along with the deformed sheet, from the upper die entry position down to the bottom center. The lubrication effect after the onset of SPF becomes different at different positions. There is almost no lubrication at the die entry (hence μ∼∞) and near complete lubrication and free sliding at the central bottom region (hence μ∼0). The discrepancy in experimental and predicted thickness profiles might not simply due to the wrong estimation of m-values, but could also be owing to different lubrication effects at different positions.

The surface topography of an 8090 aluminum alloy was studied after uniaxial or biaxial superplastic deformation, with particular reference to grain-boundary sliding (GBS) offsets, grain rotation angles, formation of striated bands (SBs) or fibers, cavity distribution, and cavity formation mechanisms. Additionally, the contribution of GBS or grain separation to the overall strain was evaluated. Striated bands were observed and are thought to be the newly exposed faces of the grains inclined to the specimen’s surface. They were formed by sliding of grains upward and downward relative to the specimen surface. Grooves and crests inside SBs were formed from the relative motion of grainboundary defects. Fibers were observed and are thought to be the further development of the SBs resulting from the formation of elongated cavities and grain separation. More cavitation was found in equibiaxially strained regions than in other regions subjected to approximately equivalent levels of strain. About 50 pct of the total strain was contributed by GBS in the uniaxial tensile-loaded specimens, as compared to about 30 pct in the biaxial-strained specimens. The effects of grain separation, grain rotation, and secondary GBS may be the reasons for the reduction of the observed strain contribution from GBS in biaxially strained specimens.

A mathematical model for analyzing the plastic deformation behavior of a sheet during a conical closed-die blow-forming is proposed. In the formulation of this mathematical model, uniform thinning in the free bulged region is considered. Using this model, the effects of various forming parameters, such as the die entry radius, friction coefficient, and inclined angle of the die, on the optimized pressurization profile, forming time and the thickness distribution of products can be discussed systematically. Furthermore, experiments using 8090 Al-Li sheets are carried out in conical closed-die superplastic blow-forming. The predicted thickness distributions of the SPFed product obtained with different friction coefficients along the die enty, sidewall and bottom are found to be closer to the experimental data than those obtained with a constant friction coefficient.

A mathematical model using the finite-difference method has been proposed in this work to examine the plastic deformation behavior of the sheet during blow-forming in a conical closed die. In the formulation of this mathematical model, nonuniform thinning in the free bulged region and the contact condition including the sticking and sliding friction modes between the sheet and die are considered. Effects of various forming parameters such as the die entry radius, friction coefficient, inclined angle of the die, etc., upon the optimized pressurization profile, forming time and the thickness distribution of products were discussed systematically. Furthermore, experiments on superplastic blow-forming in a conical closed die were carried out using 8090 Al-Li sheets. It is found that the theoretical predictions agree with experimental results. The thickness distributions of the SPF-ed product obtained with different friction coefficients along the die entry, sidewall and bottom are closer to the experimental data than those obtained with a constant friction coefficient.

A mathematical model using the finite-difference method and considering non-uniform thinning in the free bulged region has been proposed in this work to examine the plastic deformation behavior of the sheet during blow-forming in a circular closed-die. Optimized pressurization profiles during the blow-forming process are obtained by this model, the use of which the strain rate at the pole of the free bulged region corresponding to the maximum m-value is maintained during the blow-forming process. The effects of various forming parameters, such as the die entry radius, the friction coefficient, the aspect ratio of the die, etc., upon the optimized pressurization profile, the forming time and the thickness distribution of the products, are discussed systematically. Furthermore, experiments on superplastic blow-forming in a circular closed-die were carried out using 8090 AlLi sheets. It is found that the thickness distribution of the formed product employing the optimized pressurization profile predicted by this model is more uniform than that by the previous model considering uniform thinning in the free bulged region.

Hemisphere free bulging of a superplastic 8090 Al-Li sheet was carried out, with particular emphasis given to the superplastic behaviour over the low strain regime epsilon = 0-0.7. Various pressuring cycles, including constant pressure, constant strain rate, and multiple strain rate bulging, were performed to characterise the superplastic behaviour in terms of strain rate variation, thickness distribution, and evaluation of the strain rate sensitivity (m) of the sheet during biaxial bulging. For constant strain rate bulging, a modified Ghosh and Hamilton (GH) model and a model developed by the present alcohols (HL) have been used to simulate the necessary pressure profiles. Two different constitutive equations, extracted from uniaxial tensile tests, were used in the study. The modified GH model applied sigma = K epsilon over dot (m) as the constitutive equation, i.e. considering only the m value in the simulation, while the HL model incorporated the strain hardening exponent (n) additionally into the constitutive equation for the simulation: i.e. the equation sigma = K'epsilon(n) epsilon over dot (m) was used. The HL model cart also be applied to simulate the multiple stage strain late forming route. Based on the analyses, the material superplasticity characteristics obtained from uniaxial tensile tests were seen to be reliably applicable to equibiaxial sheet bulging. The equibiaxial straining condition was not obeyed exactly except at the pole. With the consideration of initial work hardening contribution towards the constitutive equation, the simulated results seemed to agree better with the experimental data. With further straining, the rate sensitivity increased and work hardening decreased, thereby a multiple strain rate bulging path, including a high initial rate followed by several lower strain rates, would be able to fully utilise the higher values of m and n during each straining stage. The thickness uniformity as well as the total forming time could thus be improved and shortened. (C) 1996 The Institute of Materials.

Quench sensitivity of the superplastic 8090 Al-Li thin sheets was investigated from the strengthening point of view of the various precipitates formed in the alloy. Specimens subjected to different cooling rates (water quenching, silicon oil cooling or air cooling) from solution treatment (or superplastic forming) temperature were examined by tensile testing, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Experimental results show that delta' is the predominating phase in the alloy under all cooling conditions. Since delta' could be formed in a similar amount and size after the T6 treatment and since the amount of S' precipitates and T-2 particles were extremely low, the superplastic 8090 Al-Li alloy would thus always exhibit low quench sensitivity.