Papers by Keyword: Super Plastic Forming (SPF)

Abstract: The paper describes a finite element method in 2D and 3D to simulate the super plastic forming of a demonstrator jet engine fan blade made from Titanium alloy sheet. The fan blade is an assembly of three sheets in which a single inner (core) sheet is diffusion bonded to the two outer (skin) sheets at prescribed zones, which is then super-plastically formed to a desired fan profile. In the model, the diffusion bonded zones between the core and skin sheets are simulated using tied interfaces. The thickness of each skin sheet is not uniform and significant change in thickness can occur over a short distance as well as gradually over the entire skin sheet. The thickness of the core sheet which is smaller than the thickness of each skin sheet remains uniform. The paper describes the design for a scaled-down demonstrator fan blade and model build process. Selected results and evaluations of finite element simulations are presented and discussed.

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: Constitutive models based on dominant mechanisms in hot forming are proposed. These models consider inter-granular deformation, grain boundary sliding, grain boundary diffusion and grain growth. New stress-strain rate relationships are proposed to predict deformation due to grain boundary sliding and grain boundary diffusion. Beside a Taylor type polycrystalline constitutive model, a visco-plastic relation in conjunction with a yield function is used to predict inter-granular deformation with much less computational costs. The proposed models are calibrated with tensile test data of AA5083 at . The calibrated models closely fit simple tension experimental data for various strain rates and strains. Then as an example the models are used to simulate a tray forming experiment. Dome heights and tray thicknesses at various positions during forming time can well predict experimental observations.

Abstract: A constitutive model is proposed for simulations of hot forming processes. Dominant mechanisms in hot forming including inter-granular deformation, grain boundary sliding and grain boundary diffusion are considered in the constitutive model. A Taylor type polycrystalline model is used to predict inter-granular deformation. Previous works on grain boundary sliding and grain boundary diffusion are extended to drive three dimensional macro stress-strain rate relationships for each mechanism. In these relationships, the effect of grain size is also taken into account. It is shown that for grain boundary diffusion, stress-strain rate relationship obeys the Prandtl-Reuss flow rule. The proposed model is used to simulate step strain rate tests and the results are compared with experimental data. It is concluded that the model can be used to predict flow stress for various grain sizes and strain rates. The proposed model can be directly used in simulation of hot forming processes and as an example the bulge forming process is simulated and the results are compared with experimental data.

Abstract: High strain rate superplastic forming is the key issue of the industrial application of superplastic ceramic and is also an effective approach to achieve “near-net shaping” of ceramic parts with complicated shape. The sol-gel method was adapted to synthesize the nano-sized composite powders. A three-phase nanocomposite ceramic Al2O3-30mol%YSZ-30mol%MgAl2O4 (AZ30S30) was prepared by using hot-pressing sintering in vacuum. Then the superplastic forming of a ceramic part with relatively complicated shape was accomplished and some new characters in superplastic forming were studied. The results demonstrate that as-sintered ceramic is a typical inter/intra granular nano-structured composite. Because the dispersed second phase inhibited the growth of the alumina matrix grains in deformation as well as in sintering, the material behaves excellent superplastic formability. The extrusion forming at a high strain rate was achieved and the part in cup shape was obtained. The yttrium element segregated obviously at the grain boundary after deformation, which demonstrated that the yttrium element directionally diffused during the deformation. The mechanical properties of the ceramic did not change much after extrusion. The maximum hardness of as-deformed composite ceramic is 32GPa, a litter lower than that of 35GPa before deformation.

Abstract: A critical analysis of approaches to find constitutive relations from the tests on uniaxial and two axial loading is presented. Then we report on the methodology of fitting parameters of constitutive relations for superplastic forming based on the results of cone cup and elongated wedge cup tests. Optimal cone and wedge angles are estimated analytically and also from the results of finite element simulations. Our methodology is based on comparison of the experimental results and the results of numerical simulations of the corresponding tests in frame of the commercial software such as ANSYS10.0 (ED). We carry out numerical simulations of the cone cup and the wedge cup tests for different sets of parameters of the chosen constitutive relation and different values of friction coefficient in order to reproduce the experimental results. The parameters corresponding to the best fit of the experimental data are then recommended for the use in computer simulations of the superplastic forming of commercial products.

Abstract: Superplastic behavior was investigated using an extruded Mg-Zn-Y alloy with the dispersion of the quasicrystal phase particle in fine-grained matrix. Tensile tests showed that the low temperature superplasticity was behaved at a temperature of 473 K and maximum elongation was 462 % at 573 K in 1  10-5 s-1. The deformed microstructure observation showed that the dominant deformation process was grain boundary sliding. The present alloy also demonstrated a high possibility for secondary forming, such as superplastic forge forming. Furthermore, the forged alloy had a homogeneous microstructures, no mechanical anisotropy and uniform micro-hardness properties in any portion of a forged product.

Abstract: While magnesium alloys are routinely used in engineering applications in the form of net shape castings, applications for sheet product have been limited due to the poor cold formability of magnesium combined with the perceived expense of sheet. The issues associated with poor cold formability could largely be overcome if magnesium alloys were to be superplastically formed. Superplasticity in magnesium is well established with research papers on the subject dating back to the late 1960s. In recent years, interest in this area has grown to the point where a number of companies have successfully superplastically formed prototype automotive panels from magnesium alloy sheet. Concurrent to this the scientific community have demonstrated superplasticity in a wide range of magnesium alloys using processing techniques ranging from the exotic (severe plastic deformation) to the mundane (traditional warm rolling). Work by the current authors has shown, rather surprisingly, that superplasticity can be achieved in magnesium alloys in the as-cast condition. This has led to some initial exploratory work involving twin roll casting. The concept being that affordable superplastic magnesium sheet could be produced via twin roll casting with only limited rolling reduction to final gauge. This paper describes the superplastic behaviour (in uniaxial tension) and microstructure of sheet processed from strip cast AZ31 and AZ91. The experimental material has included strip cast AZ91 subjected to large shear strains immediately prior to casting. The material was tested in the as-cast condition and after warm rolling to a number of gauges. Industrially useful superplastic capability was demonstrated in the strip cast alloys. Furthermore, good superplastic capability was also demonstrated in sheet subsequently rolled from the cast metal and rolling strain did not significantly influence the ductilities obtained. The mechanism for achieving superplasticity in as-cast magnesium alloys will be considered and the contrasting deformation characteristics of AZ31 and AZ91 will be discussed in terms of m value analysis and microstructural characterisation.

Abstract: This paper describes high temperature cyclic and creep relaxation testing and modelling of a high nickel-chromium material (XN40F) for application to life prediction of superplastic forming (SPF) tools. An experimental test programme to (i) characterise the high temperature cyclic elastic-plastic-creep behaviour of the material over a range of temperatures between 20oC and 900oC, including cyclic controlled strain-range tests at different strain-rates and creep relaxation tests, and (ii) identify the material constants relevant to thermo-mechanical fatigue (TMF) life prediction, is described. The objective of the material testing is the development of high temperature material and failure models for cyclic analyses and life prediction of SPF and diffusion bonding (DB) dies for titanium aerospace components.

Abstract: Numerical simulation took root in the last few decades in the superplastic forming field as one of the most dominant tools for process analysis and optimization. The big role of the simulation can be found in many areas concerning the study and the implementation of the forming process. The purpose of this paper is to outline some of the main applications of the numerical simulation in superplastic forming that can be found in the material characterization phase, in the simulation of forming tests and in the optimization of the process. A brief overview of results that can be found in literature is given with special regard to Finite Element numerical simulation of metal sheet Superplastic Forming.