Deformation behaviour in superplastic 5083 Al alloy during biaxial gas pressure forming with lubrication

Abstract

Effect of lubrication on deformation behaviour of a superplastic material has been given little attention, although it is important for industrial application. In this paper, a superplastic 5083 Al alloy under biaxial deformation was investigated by deforming the sheet into a cylindrical die cavity with and without lubrication. Several interrupted tests were performed to bulge the sheets to various depths for two different strain rates, the formed parts were then utilised to evaluate the effect of lubrication on metal flow, thickness distribution and cavitation. It was found that reducing the interfacial friction by use of a lubricant improved the metal flow after the deformed sheet had made contact with the bottom surface of die. Changes of the metal flow during forming not only developed a better thickness distribution of the formed part, but also reduced cavitation levels.     

Rapid forming of superplastic aluminium alloy 5083

Abstract

Decreasing the cycle time significantly for forming the commercially available superplastic aluminium alloy 5083 has been achieved. Forming results and conditions are compared with previous relevant works which are actually scarce. A circular cup having a depthdiameter ratio of 1:2 can be formed in 70 s. This ratio requires flat sheet to be stretched in area by up to three times, which should be large enough when dealing with actual industrial sheet forming. On average, the thickness is decreased by two-thirds; in fact, the thickness distribution is not uniform and the gradient is concentrated at the wall of the cup. The location of minimum thickness in rapid forming is different from that in conventional forming. Disregarding the traditional approach, the pressure-time profile employed in this work was not restricted to yield the so called optimum strain rate, which is usually low. Following the same processing profile, but proceeding in stages of partial forming, a series of progressive forming configurations was obtained in order to analyse the strain rate path leading to the successful rapid forming. For a specimen processed at 500C, the maximum volume fraction of cavities is 4 existing at the location of minimum thickness.     

Influence of Deformation Variables on Cavitation of a Superplastic 5083 Aluminum Alloy

A series of experiments were performed under various strain rates and strain states during superplastic deformation of a fine-grained 5083 aluminum alloy. Uniaxial tensile tests, rectangular pans, hemispherical and cylindrical capped parts were superplastically deformed to study the effects of forming rates and strain states on the nucleation and growth characteristics of cavities. The results showed that cavity fraction increased with increasing temperature due to greater grain growth. Strain states did have an effect on the cavity fraction in formed parts. A two-step strain rate deformation process could reduce cavitation.     

Superplastic deformation and postformed mechanical properties of high temperature titanium alloy IMI 834

Abstract

Superplastic forming is particularly attractive for high temperature Ti alloys because of the much lower forming stresses compared with those encountered during forging. The superplastic deformation parameters of IMI 834 sheet were obtained at 900, 940, and 990°C. At 990°C, IMI 834 shows low flow stresses, high values of strain rate sensitivity, and minimum strain anisotropy, however, 300% superplastic elongation was readily obtained at the lower forming temperature of 940°C but with a higher flow stress. A reduction in the room temperature and 600°C tensile properties with superplastic strain resulted from strain enhanced grain growth during superplastic deformation; this effect was greatest at 990°C. Aging of post 990°C superplastically formed material was studied. The creep performance of IMI 834 was found to be slightly reduced by superplastic forming. These properties and the changes in the microstructure and texture are compared with other Ti alloys under superplastic conditions.

MST/1822     

Deformation characteristics of fine grained magnesium alloy AZ31B thin sheet during rapid gas blow forming

Abstract

Decreasing the forming time in gas blow forming using fine grained Mg alloy AZ31B thin sheet with a thickness of 0·6 mm was studied in this work. Tensile tests and gas blow forming using stepwise pressurisation profiles were performed to explore the deformation behaviour of a fine grained AZ31B Mg alloy sheet. The alloy sheets were successfully deformed into hemispherical domes using two proposed stepwise pressurisation profiles during gas blow forming. As a result, significant reduction in forming time was achieved. Maximum effective deformation rates of 1·02 × 10^–2 and 1·98 × 10^–2 s^–1 were obtained at 300 and 370°C respectively. It was feasible to form a hemispherical dome with a height of 20 mm in less than 80 s at 370°C. The results confirmed that the thickness distribution along the centreline of the formed dome was sensitive to the pressurisation profiles. A higher thinning effect was observed at 370°C due to the higher deformation rate imposed during forming. Grain growth was not a serious problem for forming even at 370°C, and static grain growth should be the major factor resulting in grain growth during forming.     

Superplastic forming behaviour of complex shapes and post-forming mechanical properties of aluminium based SiCp reinforced metal matrix composites

Abstract

From a commercial viewpoint superplastic forming of complex shapes using only a single operation and one surface tool is appealing, especially for metal matrix composites (MMCs) that are hard to form even at elevated temperatures due to low ductility and toughness. Furthermore, secondary machining operations are difficult due to the presence of extremely hard ceramic reinforcements such as SiC. A range of aluminium alloy based MMCs have indeed been shown to exhibit superplastic properties although most of these studies have been concerned with microstructural characterisation using small uniaxial tensile specimens. This paper therefore concentrates on high strain rate biaxial superplastic forming of complex shapes (critical feature) in MMCs where a forming envelope has been defined and post-forming mechanical properties investigated. Particulate reinforced MMCs based on aluminium alloys 7475 and 7178 were superplastically formed in a die with a 45° step at a range of temperatures and pressures. Formed specimens were sectioned to investigate cavitation and cross-sectional thinning. Tensile tests were performed on parent and formed material to investigate the effect of superplastic forming on mechanical properties. The MMCs were successfully formed over the temperature range 450–550°C achieving step angles α of 22–42°. This study has shown that high strain rate superplasticity (～10^-1 s^-1)can be achieved giving a strain of 70% in only 3.5 s without SiC fracture, reinforcement–matrix decohesion or matrix cavitation making this technique economic and very attractive for commercial exploitation. Cross-sectional thinning was found to be uniform and in the order of ～25% which could be accounted for at the design stage. The high strain rate superplasticity was found to be grain size dependent (<3 µm) but greater profile definitions were achieved when forming took place just above the matrix solidus. Superplastic forming above the matrix solidus temperature resulted in the achievement of the highest step angles in the complex shapes but had a detrimental effect on mechanical properties. This is thought to be due to the liquid phase present that aids grain boundary and interfacial sliding but has a similar effect to overheating during solution treatment and brittle phases are formed at the grain boundaries.     

Effect of stress state on cavitation and hole growth in superplastic AA 7475 aluminium alloy

Abstract

A study has been made of the growth of cavities and of artificial holes in AA 7475 alloy sheet material during both uniaxial and equibiaxial tensile straining, with the object of clarifying the effect of stress state on cavitation during superplastic flow. The growth rate of cavities with strain was observed to be lower for uniaxial tension than for equibiaxial tension. An analysis of artificial hole growth data supports these observations, and is consistent with the view that continuous cavity nucleation and cavity coalescence lead to an increase in the apparent cavity growth rate during superplastic flow.

MST/1149     

Cavitation characteristics of a superplastic 5083 Al alloy during gas blow forming

Cavitation behavior of a superplastic 5083 Al alloy during gas blow forming has been investigated by deforming the sheet into a die with a rectangular cavity. Cavitation characteristics could be separated into two stages. In stage I, the sheet deformed freely as part of a hemi-cylindrical shape, cavity volume increased exponentially with deformation. The evolution of cavity volume was due to both nucleation and growth of cavities. In the second stage the surface friction would restrict thinning of the sheet, and the cavity volume first increased and then decreased with forming time for all test-forming rates. Decrease in cavity volume in the later stage could be related to the cavity shrinkage rising from sintering effect. A higher strain rate utilizing a higher imposed pressure during blow forming led to a greater average cavity shrinkage rate.     

Effect of strain rate sensitivity and cavity growth rate on failure of superplastic material

Abstract

Necking development and fracture strain of superplastic material under tensile load are analysed by introducing a model of cavity growth into the long wavelength approximation analysis which can describe the external neck development of specimens during deformation. The results show that both strain rate sensitivity m and cavity growth rate η have an important influence on the fracture strain of superplastic material. According to these results, a fracture diagram is presented in m–η coordinates, which is divided into three: a region in which material fails by macroscopic external necking, a region where cavity growth is predominant leading to fracture without pronounced external necking, and an intermediate region where both fracture modes occur. The prediction of fracture strain for various superplastic alloys exhibiting cavity growth during deformation is in good agreement with experimental results. The present analysis thus enables quantitative prediction of the effects of both strain rate sensitivity and cavity growth on superplastic fracture under uniaxial tension.

MST/491     

The development of internal cavitation in a superplastic zinc–aluminum alloy processed by ECAP

A Zn-22% Al eutectoid alloy was processed by Equal-Channel Angular Pressing (ECAP) to produce an ultrafine grain size and then pulled in tension at elevated temperatures to evaluate the role of internal cavitation under superplastic conditions. Tensile testing yielded a highest elongation of 2,230% at a strain rate of 1.0 × 10^?2 s^?1 at 473 K representing high strain rate superplasticity. Quantitative cavity measurements were taken to investigate the significance of the internal cavities formed during superplastic deformation. The results demonstrate that cavity nucleation occurs continuously throughout superplastic flow, and there is a transition in the cavity growth mechanism from superplastic diffusion growth at the smaller cavity sizes to plasticity-controlled growth at the larger sizes.     

Effects of Loading Paths on Hydrodynamic Deep Drawing with Independent Radial Hydraulic Pressure of Aluminum Alloy Based on Numerical Simulation

In order to meet the forming demands for low plasticity materials and large height-diameter ratio parts, a new process of hydrodynamic deep drawing （HDD） with independent radial hydraulic pressure is proposed. To investigate the effects of loading paths on the HDD with independent radial hydraulic pressure, the forming process of 5A06 aluminum alloy cylindrical cup with a hemispherical bottom was studied by numerical simulation. By employing the dynamic explicit analytical software ETA/Dynaform based on LS-DYNA3D, the effects of loading paths on the sheet-thickness distribution and surface quality were analyzed. The corresponding relations of the radial hydraulic pressure loading paths and the part＇s strain status on the forming limit diagram （FLD） were also discussed. The results indicated that a sound match between liquid chamber pressure and independent radial hydraulic pressure could restrain the serious thinning at the hemisphere bottom and that through adjusting radial hydraulic pressure could reduce the radial tensile strain and change the strain paths. Therefore, the drawing limit of the aluminum cylindrical cup with a hemispherical bottom could be increased significantly.     

Numerical analysis of effect of material and processing parameters on thickness distribution during superplastic die bulging of cavity sensitive materials

Abstract

The superplastic bulging of circular sheets clamped against axisymmetrical cylindrical dies has been analysed numerically by means of a rigid–viscoplastic finite element method, in which four node quadrilateral isoparametric elements are used with a Newton–Raphson non­linear solution scheme. Both effects of normal anisotropy and strain hardening in the material are considered and a modified Coulomb friction law is adopted. At the same time, the yield criterion suited for the superplastic forming process and the cavity damage evolution model deduced from continuum damage mechanics are applied to a finite element formulation. The influences of material parameters (the strain rate sensitivity exponent m, the strain hardening exponent n, the coefficient of normal anisotropy R) and processing parameters (pressure cycle, lubrication condition, die geometry) on the inhomogeneity of the thickness distribution are studied and discussed. A selection of the simulated results is compared with the experimental results, with good agreement.     

Superplastic Forming of an Al-Li-Cu-Mg-Zr Alloy Using Various Forming Paths

A series of experiments were performed by use of various forming paths during superplastic forming of an Al-Li based alloy. A fully recrystallized Al-Li-Cu-Mg-Zr alloy could be formed superplastically using paths of either a constant or a variable strain rate. The distribution of thickness along the formed sheet was confirmed by the results as being not much sensitive to the forming paths. The uniformity of the formed part affected the microstructural characteristics; the grain size was larger in regions at which the plastic deformation was larger, an effect was believed due to strain-induced grain growth. Static growth of grain showed little effect on an Al-Li based alloy during superplastic forming. The strain-induced grain growth influenced the distribution of grain size, resulting in cavitation occurring in the regions of large grain size. A two-step variable forming path could reduce cavitation in the formed part.     

Detailed experimental and numerical analysis of a cylindrical cup deep drawing: Pros and cons of using solid-shell elements

The Swift test was originally proposed as a formability test to reproduce the conditions observed in deep drawing operations. This test consists on forming a cylindrical cup from a circular blank, using a flat bottom cylindrical punch and has been extensively studied using both analytical and numerical methods. This test can also be combined with the Demeri test, which consists in cutting a ring from the wall of a cylindrical cup, in order to open it afterwards to measure the springback. This combination allows their use as benchmark test, in order to improve the knowledge concerning the numerical simulation models, through the comparison between experimental and numerical results. The focus of this study is the experimental and numerical analyses of the Swift cup test, followed by the Demeri test, performed with an AA5754-O alloy at room temperature. In this context, a detailed analysis of the punch force evolution, the thickness evolution along the cup wall, the earing profile, the strain paths and their evolution and the ring opening is performed. The numerical simulation is performed using the finite element code ABAQUS, with solid and solid-shell elements, in order to compare the computational efficiency of these type of elements. The results show that the solid-shell element is more cost-effective than the solid, presenting global accurate predictions, excepted for the thinning zones. Both the von Mises and the Hill48 yield criteria predict the strain distributions in the final cup quite accurately. However, improved knowledge concerning the stress states is still required, because the Hill48 criterion showed difficulties in the correct prediction of the springback, whatever the type of finite element adopted.     

Effect of coalescence on cavity growth during superplastic deformation

Abstract

The accumulation of cavitation damage with increasing strain during the biaxial deformation of two superplastic aluminium-base alloys has been studied using densitometry and quantitative metallography. A model based on constitutive relationships for the diffusive and plastic growth of voids has been extended to account for cavity coalescence, a phenomenon commonly observed during superplastic deformation. Good agreement between the measured and calculated cavity growth rates was obtained for one alloy (AI 7475), while discrepancies observed for the second alloy (Supral 220) are explained in terms of the effects of continuous cavity nucleation.

MST/189     

Numerical analysis of superplastic blow forming of Ti–6Al–4V alloys

The superplastic blow forming of a Ti–6Al–4V sheet into a cylindrical cup has been numerically analysed based on the actual forming process using ABAQUS. A detailed element type study has been performed to eliminate the element dependency in the finite element analysis. The accuracy and reliability of the proposed finite element model has been validated in comparison with experimental data. The validation proves that, there is a good agreement between the simulation and the experiment. In addition, the best prediction of the thickness distribution can be obtained using the continuum element. Furthermore, the effects of major factors such as friction coefficient and the strain rate sensitivity index upon the optimum forming pressure-time and thickness distribution of the component have been studied systematically using the proposed finite element model.     

MICROSTRUCTURAL MODELING AND NUMERICAL ANALYSIS FOR THE SUPERPLASTIC FORMING PROCESS

Superplastic forming is a manufacturing process during which a sheet is blow formed into a die to produce lightweight and strong components. In this paper, the microstructural mechanism of grain growth during superplastic deformation is studied. A new model, which considers grain growth, is proposed and applied to conventional superplastic materials. The relationships among the strain, strain rate, test temperature, initial grain size, and grain growth in superplastic materials are discussed. According to the proposed model, theoretical predictions for superplastic forming processes are presented, and comparison with experimental data is given. The new constitutive equation of superplasticity is introduced into a finite element method program to study superplastic blow forming. The effects of the geometric shape parameters of the die on the superplastic blow forming process are investigated, and the inhomogeneity in the thickness distribution of the specimen is analyzed.     

Numerical study of superplastic deformation with superimposed pressure for cavity sensitive materials

Abstract

The non-uniform deformation (necking and thinning) development and fracture of superplastic materials under both uniaxial tension and circular sheet bulging are numerically analysed by considering the effects of strain rate sensitivity and cavity growth with superimposed pressure. It is found that the fracture mode, which is controlled by both strain rate sensitivity and cavity growth rate, can be changed by superimposed pressure from fracture without external necking for cavity sensitive alloys at zero pressure to fracture with necking development or extensive thinning at pressure large enough to completely suppress cavity growth. Fracture mechanism diagrams are presented which enable prediction of the fracture mode to be made as a function of material parameters and pressure conditions for uniaxial tension and bulging.

MST/724     

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.     

Effect of strain on cavity development during Al–Zn–Mg–Cu alloy superplastic flow

Cavitation behaviour has been investigated in an Al–Zn–Mg–Cu alloy with an average grain size of 10?µm during superplastic deformation. The superplastic tensile tests were interrupted at different true strains at 530°C and 3?×?10^?4?s^?1. The results showed that cavity nucleation occurred above a critical strain in the optimum loading condition. It was easy for cavities to form at the triple junction due to the stress concentration caused by cooperative grain boundary sliding. Since the tensile stress was higher in the middle of the sample, the cavities were arranged in a straight line parallel to the tensile axis in the centre of the sample. A more appropriate cavity growth equation considering the critical strain was proposed to describe the cavitation behaviour.