Materials Science Forum Vol. 735

Abstract: Superplasticity is the ability exhibited by some fine-grained materials to be elongated a great deal with no failure. Such phenomenological definition accounts for the engineering view point of this remarkable property. From a fundamental basis, there is a full consensus to admit that it is essentially linked to the grain boundary motion under invariance of microstructure. Despite the great scientific effort carried out during the two last decades, or probably due to that, there is still a gap on the scientific comprehension of the equation describing superplasticity from a basic point of view, not to say about its potential extension to nanostructured materials. This paper presents the essential basis of a new model proposed to account for the main features of structural ceramics.

Abstract: Ti₃Al intermetallic alloy is a hard formed material with many advantages for aviation and aerospace applications. Superplastic forming (SPF) is an ideal process for Ti₃Al alloy to form sheet metal component. The thickness distribution of U cross-section circular parts, which were superplastically formed in two different female die cavity, were predicted by FEM software. According to female die forming thickness distribution, preforming die cavity was designed and was optimized by FEM software. Superplastic bugling tests with and without preforming were carried out. The thickness of parts were measured and compared with predictions by FEM software. The results showed that preforming can promote thickness uniformity efficiently in female die forming. In this experiment, the thickness difference range of the part formed with preforming reduced 75.8% compared with part formed without preforming.

Abstract: Superplasticity of nanocrystalline materials is a hot spot in the field of scientific research. In this paper, Ni-Co alloy was produced through pulse electrodeposition. Tensile tests were carried out to study the room temperature strength, high temperature plasticity. The superplastic formability under complex stress was evaluated through the superplastic bulging tests. The tests were studied through the methods of resistance heating and furnace temperature heating. The maximum ratios of height and diameter with different heating method were compared. Fracture behavior and microstructure were observed by the method of SEM.

Abstract: An AlCoCrCuFeNi high entropy alloy was multiaxially isothermally forged at 950°C to produce a fine equiaxed structure with the average grain/particle size of ~1.5 µm. The forged alloy exhibited superplastic behavior in the temperature range of 800-1000°C. For example, during deformation at a strain rate of 10-3 s-1, tensile ductility increased from 400% to 860% when the temperature increased from 800°C to 1000°C. An increase in strain rate from 10-4 to 10-2 s-1 at T = 1000°C did not affect ductility: elongation to failure was about 800%. The strain rate sensitivity of the flow stress was rather high, m = 0.6, which is typical to the superplastic behavior. The equiaxed morphology of grains and particles retained after the superplastic deformation, although some grain/particle growth was observed.

Abstract: In the superplastic process, the non-uniformity of the produced part thickness and the possibility of severe thinning are among the major disadvantages. This paper presents a parametric study on the superplastic forming of a Pb-Sn sheet into the shape of a long rectangular pan. A two dimensional plain strain finite element model was used to predict the forming times and thinning profiles of the formed Pb-Sn pan. The effect of varying the sidewall inclination angle was investigated for different friction conditions at the die-sheet interface. Results showed that increasing the side wall inclination angle reduced the forming time and provided a better thickness distribution.

Abstract: The rear part of the APF A380 has a deep drawn shape. In order to develop the forming by SPF process of this part, numerical simulation by finite elements has been performed. Several configurations for 2D and 3D modeling were studied to determine an efficient forming strategy. A double-action solution was chosen. It ensures a satisfactory thickness distribution. This article will deal with the modeling assumptions, the results of individual cases of calculation and comparison with parts obtained at the Airbus plant.

Abstract: In most super-plastic forming (SPF) investigations the focus is usually on the material aspects. In this paper the authors develop a model to improve the heat management of SPF. The model presented improved process possibilities. The improved design involves selective application of heat to the material. Final product shape can easily be controlled by accurate temperature control of the work piece. Numerical simulation has been carried out on various components including a ‘top hat shape‘ and a heat exchanger part. Simulation comparisons are made between selective heating and conventional processing, where all of the formed material is at the same temperature, and greater process efficiency of the selective heating approach is demonstrated.

Abstract: Magnesium alloys are attractive for lightweight structural applications in the transportation industry because of their low density and high specific strength and stiffness [1]. With an ultrafine-grained microstructure, they exhibit superplasticity at relatively low temperatures and high strain rates [2]. Friction stir processing (FSP) was used to obtain a microstructure with ultrafine grains in the magnesium alloy AZ31. Microstructures obtained using different rotational speeds are studied. Free bulge forming of the FS processed AZ31 sheets are carried out to evaluate the superplastic behaviour [3]. The model and the evolution equations are, then, implemented into a commercial FE code and different simulations are conducted to correlate the experimental and numerical results for the model validation [4]. The purpose of this study is to investigate the effect of the microstructure on the superplastic behaviour using free bulge forming and FE simulations.

Abstract: Friction stir process (FSP) is a severe plastic deformation based secondary processing technique that can be utilized to engineer novel microstructures in metallic alloys. It is well known that such techniques are cumbersome and require significant experimental work and material to determine optimum processing conditions. Therefore in this work, we propose a new two step numerical approach, where: (i) CFD simulations coupled with Zener-Holloman relation are used to predict microstructure evolution in stirred, transition and heat affected zones of friction stir processed AZ31 Mg alloy sheets, (ii) Finite element simulations are carried out to evaluate superplastic forming characteristics of different microstructures developed after FSP. Simulation trends including forming pressure profiles, dome height evolution, and thickness distribution of friction stir processed sheets are compared with those of the base material. The proposed combination of numerical approaches to model both processing and forming aspects yields a powerful tool to study and optimize processing and forming technologies with limited experimentation.