Superplastic Metal Research Articles

Application of Finite Element Method to the Design of Superplastic Forming Processes

Finite element method is used as a design tool in the prediction of process parameters for superplastic metal forming processes. The method is used in the design of various plane strain and axisymmetric components. The effect of varying die radii and die friction are studied in the forming characteristics of a simple pan. The cone test used for the mechanical characterization of superplastic materials is simulated. A complex component typically used in the aerospace industry is analyzed and the output pressure-time loading and the resulting thickness distributions are determined.

Effect of grain size on superplastic behavior of Y-TZP

This paper reports that grain size strongly influences the properties of fine-grained superplastic metal alloys. In principle, fine grain sizes enhance grain boundary sliding which is the dominant superplastic deformation mechanism. Recently, some fine-grained ceramics have been shown to be superplastic in tension. A maximum tensile elongation of about 800% has been recorded in a yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). During superplastic deformation, concurrent grain growth, and in particular dynamic grain growth, was observed in this material. It has been demonstrated that this concurrent grain growth strongly affects the superplastic flow stress in the material, and also confuses the accurate determination of the stress dependency of the superplastic strain rate. It is important, therefore, to understand the precise grain size dependence of superplastic flow in ceramics. It has been shown that the superplastic strain rate of Y-TZP is inversely proportional to the grain size raised to a 1.8 power. The effect of concurrent grain growth, however, was not considered in their analyses.

Superplastic Deformation of YBa2Cu3O7−x in a Superplastic Metal Matrix

ABSTRACTThe new class of high-transition-temperature ceramic superconductors (e.g. Y-Ba-Cu-O) show a fine grain size polycrystalline structure, similar to that shown by the superplastic metals. The material behaves in a brittle manner with a strain to fracture below 0.5 % at room temperature. One of the reasons for this mechanical behavior is that the grain boundaries are easily separated when a stress is applied. It was found in this work that a deformation of the superconductor ceramic (scc) in superplastic metal (spin) matrix reduces the separation of the grain boundaries in such materials so that they can be deformed at room temperature. The spin matrixes used in this work were Zn-Cd, Bi-Sn and Cd-Sn. It was found that the highest the yield point of the matrix the highest the deformation induced in the scc. The Cd-Zn alloy was the most effective in avoiding the separation of the grain boundaries during the deformation of a composite formed by a cylinder of scc embedded in a spin matrix. Meissner effect was observed in the scc, after more than 160 % of plastic deformation. SEM observations show that deformation takes place by grain boundary sliding and some grain refinement was observed.

Analysis of superplastic metal forming by a finite element method

AbstractA finite element velocity method for analysing the superplastic sheet metal forming process is presented. This method is developed from the principle of virtual work and is based on the use of isoparametric continuum elements. The large inelastic deformation of the superplastic material is modelled as the behaviour of an incompressible non‐linear viscous flow material. The contact and friction problem is solved by using the compatibility load step method, which is an extension of an earlier work. The finite element method is applied to selected problems to illustrate the applicability of the solution procedure.

Flow localization and neck formation in a superplastic metal

Experiments were conducted on the superplastic Zn22% Al eutectoid alloy to investigate the extent of flow localization and neck formation in the three regions of plastic flow. The results show that the behavior is qualitatively similar in the low stress region I and the high stress region III, but there are very significant differences in the superplastic region II. Deformation is essentially uniform in region II up to at least ~800% and quasi-uniform thereafter, whereas there are marked deviations from uniform flow in regions I and III at ~200%. Failure occurs by the formation and catastrophic growth of a sharp neck in regions I and III, whereas necking is diffuse at high elongations in region II. The results are consistent with a genuine decrease in the value of the strain rate sensitivity at low strain rates in region I.

A quantitative model of the blow-forming of spherical surfaces in superplastic sheet metal

The blow-forming of a flat superplastic metal sheet to a spherical surface is analysed with a mathematical model based on the classical theory of liquid bubbles. The model enables a quantitative account to be given of the effect of the strain-rate sensitivity, flow stress and temperature of the metal on the rate of blow-forming and the replication of detail. Formulae are derived which enable the superplastic flow stress and strain-rate sensitivity to be calculated from simple blow-forming tests.