Materials Science Forum Vols. 551-552

Abstract: It is established that some superplastic materials undergo significant cavitation during deformation. Cavitation not only limits the superplastic ductility of the material, but also reduces the service properties and the fatigue performance of the formed parts. Experimental results have shown that an effective method to eliminate cavitation is the application of hydrostatic pressure during deformation. In this work, finite element simulations are carried out to study the effects of hydrostatic pressure on damage evolution during SPF. The analysis is conducted for the superplastic copper based alloy Coronze-638 at 550 °C. The results clearly demonstrate the effectiveness of the superimposition of hydrostatic pressure in reducing the amount of cavities generated during SPF and improving the integrity of the formed part.

Abstract: A numerical simulation of superplastic backward extrusion of a magnesium alloy part is presented in this paper. In fact, the simulated superplastic forming is not a pure superplastic forming because of the billet with coarse cylindrical grains. The forming may become a pure superplastic forming only after dynamic recrystallization and grain refinement appear and the grain boundary sliding has been the main deformation mechanism. In order to simulate the special forming process, a constitutive relation considering dynamic recrystallization and the multiform deform mechanism and the parameter identification of the constitutive relattion are studied. The program for simulation is able to predict the grain refinement and the transform between different deformation mechanisms. Finally the calculated results on the grain size and dynamic rerystallization are presented. A comparison between the calculated and the experimental results shows there is a good agreement between calculated results and experimental results.

Abstract: In this paper, the flow stress model has been established based on the isothermal compression data at deformation temperature of 800~1050oC, strain rate of 0.001~0.1 s-1 with the help of the Zener-Hollomon parameter and the Arrehnius’ equation. For the forging of Ti-6.0Al-2.0Zr-1.0Mo-1.0V alloy with 3.0 mm in thickness and 66.0 mm in height, the equivalent stress, strain, strain rate distribution and temperature rise in the superplastic extrusion process have been simulated through FEM. The simulated results show that the punch velocity has significant effect on the equivalent stress, temperature rise and extrusion load.

Abstract: A comparative study of different element formulations in simulating superplastic forming with the MARC finite element code is performed in the paper. Simulations were accomplished with solid, shell, membrane elements to predict forming characteristics and pressure-time curves. Finite element analysis (FEA) predictions of SPF pressure-time curves were found to be greatly affected by the element type and the strain rate control algorithms. Two strain rate control algorithms were applied in the present study: an algorithm based on limiting the rate of deformation with the average strain rate of all the elements, i.e. the build-in method in MARC, and a second algorithm which limits the rate of deformation based on the average strain rate of the elements with the 20 highest strain rates. The resulting pressure-time curves for each of these formulations were compared with respect to each type of element. Under the guide of the analysis, the die was fabricated and the AA5083 bracket was successfully manufactured. Good agreement was obtained between predicted and measured thickness in the part.

Abstract: FEM analysis has proved to be a powerful investigative tool capable of encompassing all the aspects that characterise an SPF process. In this paper, with the aid of a commercial finite element software is used to simulate high strain rate superplastic bulging process of Al-6Mg-0.2Sc alloy sheet box part under different technological parameters such as with and without lubrication. Base on obtaining optimum the loading curve, the strain rate distributions and thickness distribution in forming process by FE simulation, the relative validation experiments be carried out. It was found that the thickness gradient of the formed parts will be effectively improved reducing the interfacial friction by use of a lubrication at a higher strain rate forming. The results of the experiments are in good accordance with the FE numerical predictions both in terms of thickness distribution and forming times.

Abstract: The aim of the study on the deformation defect is to prevent defects and to improve the quality and to control the deformation. It is important that the preventive measure could be established to advance the scientificity of the product and processing design. In this paper, the reasons of the outer defects such as cracks are analyzed based on the superplasticity of the copper alloy and the process of solid cages in the superplastic extrusion. The prediction and numerical simulation of these typical defects are also carried out using the Deform-3D software grounded on the model of the rigid-visco plastic FEM. The position, time of crack appearance and relation between the strain rate and the defects are also studied. The simulated results are in equivalent agreement with the experiment.

Abstract: The Chinese nickel-base powder metallurgy (PM) superalloy FGH96, which was processed through hot isostatic pressing, is very difficult to deform. FGH96 superalloy has better superplasticity in special deformation conditions and superplastic isothermal forging is the best formation method at present. The accurate constitutive equations of the FGH96 alloy was established depended on the isothermal compression experiments. A two dimensional and thermomechanical coupled axisymmetric finite element(FE) model in which both part and die were taken in consideration was established to fully simulate the FGH96 superalloy turbine disk superplastic isothermal forging process. Some physical parameters about the turbine disk forging process, such as load, stress field and strain field were calculated at different temperature within the forging range of FGH96. The regularity of peak equivalent stress acted on die cavity surface, yield limit and ultimate strength of die material during the forging process was found. Based on the regulation, peak equivalent stress acted on cavity surface must be extremely less than yield limit of die material, the optimized processing parameter 1050°C that is the best deformation temperature for the alloy was determined. That was proved better in practice and high quality disk was forged.

Abstract: Superplastic forming has emerged as an important manufacturing process, large deformation always occurs during superplastic forming, time-consuming remeshing is necessary while the finite element method (FEM) is used to analyze metal forming process. Meshless methods with no meshes can avoid this problem and overcome those problems in FEM. In this paper a meshless method based on the reproducing kernel particle method (RKPM) is applied to analyze Magnesium Alloy (MB15) thin sheet superplastic tension forming. A superplastic meshless method modeling program is set up, and background cells are used to compute the integrations in weak form equations and the mixed transformation method (MTM) is used to impose the essential boundary condition exactly. Numerical example demonstrates the effectiveness of the method in superplastic forming.

Abstract: Superplastic large deformation problems always meet handicaps associated with severe mesh distortion and iterative remeshing when traditional mesh-based numerical approaches are used. To eliminate these difficulties, meshless simulation of element free Galerkin method (EFGM) for rigid-viscoplastic(RVP) model and incompressible materials is built upon the Moving Least Squares (MLS) approximation and modified Markov variational principles. Also, a numerical quadrature that aligns integration cells with the local supports of shape functions is presented to compute integrations involved in the objective functions. Satisfying results of a superplastic upsetting example with very large compression ratio demonstrate the feasibility and accuracy for the meshfree method to simulate superplastic large deformation process.