Superplastic Deformation Process Research Articles

Early stages of superplasticity and positron lifetime spectroscopy in an Al-Mg-Cu alloy

In the present paper, by using positron lifetime technique, a careful study is carried out to analyze the microstructural changes induced on samples of an Al-based commercial alloy (Al-Mg-Cu-Mn-Cr) by superplastic deformation in the early stages of superplastic behavior of the alloy (strain range from 0.2% to 100%). These results are compared with those obtained on specimens only heat treated at the same temperature and for a time equivalent to the elapsed time during each tensile test, in order to evaluate the thermal contribution to the microstructural changes induced during the superplastic deformation process. Moreover, the positron results were linked with the microstructural evolution of the samples followed by means of optical microscopy and Vickers microhardness technique.

Forming limit during superplastic deformation of sheet metals

In the present paper, a thermo-viscoplastic damage potential and Hill's normal anisotropy (isotropy in plane) yield criterion to be quadratic in the stress components are combined to describe the interaction process of damage and instability during superplastic deformation, based on the kinetic equation for damage during superplastic deformation and the deformation features of superplastic materials. The superplastic deformation process is devided into four stages, namely, stable, quasistable, strain path drift to plane strain and plane strain stages, and a damageinstability model during superplastic deformation of sheet metals is developed through numerical method. On the basis of the above results and taking the occurrence of localized instability (de2=0) or the cavity volume fraction reaching the critical value (fc) as a fracture criterion, the forming limit during superplastic deformation of sheet metals is predicted.

Elevated temperature mechanical properties of magnesium alloys containing Mg2Si

AbstractThe alloys Mg–6Si and Mg–4Si–4Zn (wt-%) were extruded at 573 K and the mechanical properties investigated by tensile testing at strain rates from 2 × 10−5 to 2 × 10−2s−1 and at temperatures in the range 673–773 K. The Mg–Si alloy behaved as M type at any given strain rate. The Mg–Si–Zn alloy behaved as A type at strain rates above 2 × 10−4s−1. The activation energy for the deformation process was higher than those for lattice self diffusion and for the chemical interdiffusion of Zn in Mg. This is probably related to the cross-slip of dislocations. For the Mg–Si–Zn alloy, superplastic behaviour was found at strain rates below 2 × 10−4s−1. The dominant superplastic deformation process is likely to be grain boundary diffusion controlled grain boundary sliding.MST/3192

A kinetic equation for damage during superplastic deformation

In this paper a kinetic equation for the isotropic damage during superplastic deformation is proposed, based on the thermodynamic theory of irreversible processes with internal state variables and the normality criterion. The damage variable D during superplastic deformation is non-linear with the equivalent plastic strain ξ P, and it is shown that D is affected strongly by the triaxiality stress ratio (σ m/σ eq). Furthermore, in order to demonstrate the efficiency to the proposed equation, superplastic deformation experiments were performed on aluminum alloy LY12CZ and brass H62 without any pre-treatment. The comparision with the experimental results is presented on the influence of the triaxiality stress ratio on the equivalent strain to rupture: good correlation is found between the theoretical and experimental results. Therefore, this equation for damage may reflect successfully superplastic damage behaviour during superplastic deformation processes.

Constitutive Equation of a Superplastic Al–Zn–Mg Composite Reinforced with Si<SUB>3</SUB>N<SUB>4</SUB> Whisker

A constitutive equation of a superplastic Al–Zn–Mg alloy composite reinforced with 20 vol.%Si3N4 whisker has been analyzed based on the mechanical properties tested by a constant strain rate tension test mainly at strain rates between 10−4 and 102 s−1 in a temperature range from 798 to 833 K, which includes above and below the partial melting point (828 K) of the composite measured by a differential scanning calorimeter (DSC). After incorporation of the temperature dependence of threshold stress, shear modulus, and grain size, the activation energy for superplastic flow in a temperature range from 798 to 818 K is 148 kJ mol−1, which is similar to or slightly above that for lattice self-diffusion in aluminum (=142 kJ mol−1). This suggests that the dominant process of superplastic deformation in the composite in solid state is gain boundary sliding accommodated by dislocation motion controlled by lattice diffusion. On the other hand, the activation energy for superplastic flow above the partial melting point is much higher than that for lattice self-diffusion in aluminum, indicating that the deformation mechanisms of the composite in liquid and solid state are clearly different from those in solid state only.

Numerical analysis and design of industrial superplastic forming

In the aerospace industry, the production of structural shell components with a prescribed thickness distribution by superplastic forming requires the specification of the initial thickness profile in the undeformed sheet. To this purpose, a finite element simulation methodology developed previously for the analysis of superplastic deformation processes a long with the optimum monitoring of the forming pressure is extended to the determination of the initial thickness profile. The utilization of the procedure is shown for the pre-contouring of the blank for the industrial production of a satellite tank. The examination of the produced part confirms that the requested thickness distribution is achieved fairly well and thus proves the appropriateness of the proposed computational approach as a tool for the process design in superplastic net-shape forming

Deformation behaviour in α/β two-phase super-plastic brass

α/β brass consists of hard α and soft β phase in its superplastic temperature range. Because of the difference between their mechanical properties, the superplastic behaviour of the alloy differs from that of other single-phase and quasi-single-phase alloys. The mechanical properties and microstructure of leaded α/β brass during superplastic deformation were studied and the mechanism is discussed. It is shown that the boundary diffusion controls the superplastic deformation process and boundary sliding is the main deformation mechanism; there is little deformation in the α-phase, and intragranular slip as the accommodating mechanism mainly occurs in β-phase in the superplastic range. It is also shown that the bulk diffusion dominates the deformation process, and intragranular slip is the main mechanism in the non-superplastic range.

Activation Energy of Superplastic Deformation Processes in Aluminium Alloys

Activation Energy of Superplastic Deformation Processes in Aluminium Alloys

A primer on superplasticity in natural formulation

Manufacturing processes rely increasingly on superplastic properties of certain structural materials, such as titanium alloys, and gain significant importance, in particular, in the air- and spacecraft industry. This paper presents a computer-oriented review and analysis of superplastic deformation processes. The modelling of the material response subject to superplastic conditions is attacked first, and relies on recent experimental findings with respect to rate of deformation and grain-growth effects. Multiaxial constitutive relations are based on the homogeneous natural definition of stress and rate of deformation. Computational techniques developed within the framework of finite element discretisations account for the observance of the incompressibility constraint. Nonlinear solution methods for the velocity field are examined as well as the temporal approximation of the unsteady deformation process of superplastic forming. The numerical characteristics of the proposed techniques are demonstrated. A number of examples illustrating superplastic forming processes are investigated and compared, where possible, with experimental data. In this context unsteady contact with the die, frictional sliding, and the forming of an actual structural component are also examined.

Superplasticity in aluminium-palladium eutectic alloy

AbstractThe mechanical behaviour of the hot-rolled AI-Pd eutectic was investigated in uniaxial tension in the temperature range 293–823 K. The fine two-phase microstructure (matrix grain size < 10 μm) was found to exhibit structural superplasticity at high temperatures (723–823 K) manifest by a substantial elongation to failure (maximum value ∼400%) and a high value of the strain-rate sensitivity (m ≃ 0.5). The results are discussed on the basis of the criteria governing the superplastic alloy design. In particular, the important aspect of the grain-size stabilization is explained in terms of the Zener-McLean relation concerning grain-growth inhibition in a two-phase material during elevated-temperature exposure. Moreover, the results qualitatively confirm that the grain-boundary sliding plays a dominant role in the superplastic deformation process.

Transformational Superplasticity of Bi2WO6 and Bi2MoO6

Transformational Superplasticity was studied in the compounds Bi2WO6 and Bi2MoO6. The magnitudes of transitional strain are related to the (Tt/TmP), s of the phase transitions and are proportional to the externally applied stresses. Strain‐rate sensitivities were similar, 0.85 and 0.86; however, the Bi2WO6 exhibited a strain‐axis intercept and the Bi2MoO6 a stress‐axis intercept. The grain‐size effect present in the Bi2WO6 supports an accommodated grain‐boundary sliding mechanism for the superplastic deformation process.

Deformation Mechanisms in Superplasticity

The phenomenon of micro grain superplasticity, in which metals and alloys deform extensively at elevated temperatures under small stresses without risk of rupture, has been well documented over the past decade. Our under­ standing of the rate-controlling mechanisms for superplastic deformation processes in terms of range of stress, temperature, and strain rates is important not only for its intrinsic scientific value but also because of its significance in optimizing the processing parameters for forming opera­ tions (1). The strain rates for stiperplasticity are considerably lower than those for conventional hot-working processes. This factor has so far restricted the extensive adoption of superplastic forming operations. A sound theoretical understanding of the dependence of the superplastic strain rate on temperature, grain size, and flow stress will be of significant use in optimization of these variables, so that the material can be deformed at the highest possible forming rates and still benefit from the exceptionally large, neck-free ductility associated with superplasticity. Therefore, it is not surprising that the considerable attention given to this particular mode of deformation in recent years has resulted in two major advances: (a) several new models have been proposed for the per­ tinent deformation mechanism (2-6), and (b) more reliable new data (71 1, 17)have become available to examine the validity of the proposed models. Many of these advances were recently reviewed in excellent detail (12-15;

Superplasticity in an Al-6 wt.-%Mg alloy

Abstract An Al-6 wt.-%Mg alloy containing Zr, Cr, and Mn was found to deform highly superplastically when produced in a heavily cold-rolled and recrystallized condition. Grain-boundary sliding was detected during superplastic flow of this alloy and dislocations were observed to interact with precipitate particles within grains. The alloy specimens, obtained at 0°, 45°, and 90° to the rolling direction, exhibited anisotropy with respect to their flow stresses and textures. The superplastic deformation process in this alloy can be well explained in terms of grain-boundary sliding accommodated by dislocation motion within grains.

High-temperature plastic flow of low-carbon steel subject to thermal cycling through the superplasticity interval

1. The creep resistance of low-carbon steel is significantly reduced within narrow temperature intervals during thermal cycling under load in accordance with the scheme 750⇌900°C; this is associated with the short-term retention of the alloy under superplasticity conditions. 2. The process of accumulation of irreversible deformation on passage through the super plasticity interval is distinguished by constant “shifts” of points of locally increased deformation throughout structural components of the alloy in contrast to their strict fixity during normal creep. 3. The stress sensitivity of the super plastic-strain rate, which is governed by the structurally unstable state of the material, increases with increasing external stress, remaining, however, substantially lower than that under conditions of normal creep. 4. The temperature interval of the most vigorous development of the process of superplastic deformation during thermal cycling is extremely narrow (e.g., 10–20°C); in this case, this interval widens somewhat with increasing external stress.