Materials Science Forum Vol. 735

Abstract: A time-dependent material constitutive model is developed for the deformation of wrought Mg AZ31 sheet material at 450°C. This material model is used to simulate gas-pressure bulge forming of AZ31 sheet into hemispherical domes. Finite-element-method (FEM) simulations using this material model are compared against experimental data obtained for dome height as a function of forming time under forming conditions identical to those assumed in the simulations. The time-dependent material model predicts experimental dome heights during forming with a quite useful accuracy. The most significant advantage of the time-dependent material model is that it can address the effect of preheating time on forming. Preheating times shorter than ~120 s produce an increase in forming rate. This material model provides a quantitative means of accounting for that effect.

Abstract: This paper describes a new approach for identification of the optimum pressure history for SPF processes, based on mechanisms-based hyperbolic constitutive equations. This equation set has been modified to incorporate the effect of the damage behaviour the material suffers due to the cavitational evolution of Al-5083 superplastic alloy. A large deformation, multiaxial formulation of the constitutive equation set is implemented and applied to finite element modelling of a bulge test forming process to characterise the cavitation evolution behaviour in the bulge test, using conventional (constant strain rate) and the newly proposed (variable strain rate) strategy.

Abstract: In this paper, we proposed a new superplastic forming and diffusion bonding (SPF/DB) structure and processed validation between traditional SPF/DB four sheets structure and the new structure at the coincidence boundary conditions by FEM method. The calculation results show that the new structure can decreases the maximum stress at the joining part by 82% and decreases the maximum displacement at the aerofoil tip by 51% at the cost of the mass increases less than 20%. It induces that the new structure presents better stiffness and greater load carrying capacity than traditional ones, therefore, it could be used in titanium main load-carrying structure especially the structure which has the assembling and lose weight requirement very effectively.

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: Superplasticity is characterized by high elongations under a high strain rate sensibility, and it’s variation with strain rate, temperature and grain size. This parameter is often obtained from uniaxial tensile test. However, superplastic deformation is a biaxial process; hence there is a need to develop a way to get this parameter in a biaxial test. This work aims to set up the instrumentation to record and control a biaxial superplastic forming in a Pb-Sn alloy. The control system project has been divided into tracking variables: strain and pressure. The instrumentation is able to predict the breaking point at the beginning of the superplastic forming process from biaxial testing.

Abstract: Transient regimes of deforming are always present in any technological process and can be taken into account and used more widely if properly studied. The behavior of the materials under such regimes is becoming even more interesting if initial microstructure is coarse grained and undergoes transformation in the process of deforming. One of the transient processes which happen in any Superplastic deformation is the initial stage of loading, before steady superplastic flow starts. Initial parts of stress-strain curves during superplastic deformation are not frequently studied experimentally but provide very important information about mechanical properties of material. They are also necessary for development and verification of the constitutive equations. The results of experimental analysis of the behaviour of titanium alloys under superplastic conditions at the initial stages of loading and also under unloading are presented here. Another type of transient regimes of deforming is represented by the strain rate jumps. In such kind of experiments if the amplitudes of the jumps are big enough, the shifts of the corresponding parts of the stress-strain curves about the basic ones (hardening or softening) can be observed depending on the amplitude of the jump and microstructure of the material. Some experimental results related to this effect are discussed in this paper. The applicability of some constitutive equations for description of the observed results is discussed. The necessity of involving visco-elastic properties of material for proper description of its behavior in some regimes of deforming is also mentioned.

Abstract: In the current study, the finite element simulation for superplastic blow forming of a toroidal Ti-6Al-4V fuel tank is discussed. 3 types of preforms are investigated in order to obtain defect free final shape with desirable thickness distribution. From the simulation result, forming tool is designed so that the hydraulic pressure is not used. The forming test is carried out using forming pressure profile obtained from the simulation, and the validity of the selected perform is investigated in terms of thickness distribution and deformed shape.

Abstract: Structural superplasticity is observed in materials of different classes with μm–, sub–μm– or nm– grain size. In all cases mesoscopic grain/interphase boundary sliding (~ grain diameter or more) is suggested to be the rate controlling mechanism. Sub–μm grained metallic and ceramic systems are analyzed here and good agreement with experimental results is established. Compared with earlier works, the numerical procedure is more robust and the free energy of activation for the rate controlling process is matched with the value for the same obtained using Eshelby’s equation.

Abstract: Multidirectional forging has been developed to produce an ultrafine-grain (UFG) microstructure in the two-phase titanium alloy Ti-6Al-4V. A microstructure with a grain size of 135 nm was attained, enabling low-temperature superplasticity (LTSP) at 550°C. A total elongation of 1000% and strain-rate-sensitivity coefficient m=0.47 were obtained at the optimal strain rate of 2×10^-4 s^-1. Important features of the microstructure and superplastic behavior of the alloy are summarized in the present work. It is shown that microstructure evolution during low-temperature deformation plays a key role in superplastic flow behavior.

Abstract: Mg96Zn2Y2 (at.%) extruded alloy was fabricated by hot-extrusion of the Mg96Zn2Y2 machined chip. The Mg96Zn2Y2 extruded alloy consisted of a long period stacking ordered (LPSO)-, Mg3Zn3Y2- and Mg- phases. The Mg phase with mean grain size of 450 nm was confirmed by TEM. However, the LPSO- and Mg3Zn3Y2- phases had relatively large grain size compared with Mg phase. The Mg96Zn2Y2 extruded alloy also showed superplasticity at temperatures of 623 K and 723 K with initial strain rates from 2×10−1 s−1 to 2×10−3 s−1. The maximum elongation of 450 % was achieved at 723 K with an initial strain rate of 2×10−3 s−1. From TEM observation, it is considered that grain boundary sliding of Mg grains was dominant deformation mechanism of the Mg96Zn2Y2 extruded alloy at high temperature range.