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.     

Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

Abstract

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10^-6 to 2.8 × 10^-2 s^-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ? = 2.8 × 10^-4 s^-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.     

Microstructural evolution and plastic stability during superplastic flow in a 7475 aluminium alloy

Abstract

Superplastic behaviour and microstructural evolution were examined at 788 K for strain rates in the range 2 × 10^-4–2 × 10^-3 s^-1 in a 7475 aluminium alloy of nominal composition Al–(1·2–1·9)Cu–(5·2–6·2)Zn–(1· 9–2·6)Mg (wt-%). In addition, the variation of the strain hardening and plastic stability parameters with strain was investigated based on experimental grain growth and cavitation data. The strain hardening parameter at 2 × 10^-4 s^-1 was high over a wide range of strain because of the high grain growth rate. Decrease in the strain hardening parameter due to cavitation was negligible. The highest plastic stability parameter was attained at 2 × 10^-4 s^-1, although the strain rate sensitivity was the lowest for the strain rate range investigated. This demonstrates the influence of grain growth on high plastic stability during superplastic deformation.     

Fabrication of fibre composites using an aluminium superplastic alloy as matrix

An aluminium superplastic alloy has been used as the matrix for a variety of fibre reinforcements. It is shown that, by hot pressing in the superplastic regime of the alloy, a number of different reinforcements can be incorporated into the matrix. Tensile tests on composites with up to 25 vol % of reinforcement showed good agreement with the rule of mixtures.     

Effects of intermetallic particles on cavitation during superplastic forming of aluminium alloy

An Al-4.6%Mg-1.5%Mn-0.27%Fe alloy was specially processed with friction stir processing followed by cold rolling. Half of the sheet thickness contains a large number of blocky or irregular-shaped Al₆(Mn,Fe) coarse intermetallic particles, while the other half, smaller and more spherical ones. The particle-induced cavitation upon uni-axial tension at 475°C with 2?×?10^?4–2?×?10^?2?s^?1 strain rates was investigated. The density of nuclei for cavities was estimated based on quantitative image analysis of the particles, and the strain controlled growth rate, calculated assuming the cavities are nucleated before or in the early stage of straining. The number, size and morphology of intermetallic particles are found to control the cavitation by determining the nucleation rate, but the strain controlled growth rate appears unaffected.     

Influence of electric current on superplastic deformation mechanism of 5083 aluminium alloy

5083 aluminium alloy superplastic forming adopted resistance heating can not only improve efficiency and cut energy but also generate electroplastic/electrosuperplastic effect to make the material deformation possess lower flow stress and higher plasticity. By analysing the influence of current on dislocation slipping, grain boundary migration and dynamic recrystallisation, it is found that the electron wind force can enhance the mobility of dislocations; meanwhile, the current also can reduce the activation heat of dislocation motion by joule heating effect. What is more, the grain size of resistance heating forming sample is significantly smaller than furnace heating, and the cavities in the sample become small and dispersive, so the resistance heating forming specimen possesses better performance.     

Microstructure and thermal stabilityof superplastic aluminium—lithium alloy after severe plastic deformation

Structure and phase composition of 1420 (Al-Mg-Li-Zr) alloy obtained by equal channel angular pressing during mechanical tensile testing and annealing within the temperature range 423–723 K were studied by means of TEM and XRDA. It was revealed that structural changes of the alloy after the mechanical tests corresponded to two stages of the deformation. The alloy structure was unstable due to its non-equilibrium nature and came to the equilibrium state during the annealing. Changes in the grain size and phase composition during the alloy annealing resulted in a significant decrease of its hardness.     

The tensile properties of a superplastic aluminium bronze

The high temperature tensile properties of a micrograin Cu-9.5% Al-4% Fe alloy, which is superplastic at 800° C, have been determined. Elongations at fracture of greater than 700% are achieved when the nominal strain-rate is in the range 3.9×10^–2 min^–1 to 7.9×10^–2 min^–1. The nature of plastic instability in superplastic materials is considered and it is shown that the amount of strain at the onset of plastic instability is inversely related to the applied strain-rate and is relatively independent of the strain-rate sensitivity exponent, m. The onset of plastic instability during a tensile test results in an increase of local strain-rate at the point of minimum cross-section and this, together with the existence of a triaxial stress state in the necked region, may produce errors in the m versus strain-rate plot if m is determined by the change-rate method. The initial strain-rate for maximum elongation is lower than the strain-rate for maximum m. This may be ascribed either to the influence of plastic instability or the formation of cavities at the higher strain-rates.     

Internal variable approach to microstructural change in 7475 aluminium alloy during superplastic deformation

Abstract

Flow behaviour and microstructural changes in fine grained 7475 Al during superplastic deformation have been investigated. A series of mechanical tests and transmission electron micrography has been conducted at various temperatures ranging from 430 to 516°C. Quantitative constitutive parameters have been determined from load relaxation tests by applying the internal variable theory of structural superplasticity proposed recently. The validity of the mechanical analysis is proved by the evolution of the microstructure (precipitate free zones PFZs) during the accommodation process. Additional information about the mechanism of PFZ formation was determined by microchemical analysis.     

Effect of bonding variables on bonding mechanisms in press bonding superplastic 8090 aluminium alloy

Abstract

Press bonding was carried out to study the bonding ability of superplastic 8090 aluminium alloy sheet in air. Surface roughness, deformation, time, and microstructure were the bonding variables chosen, to investigate the effects of these parameters on the bonding mechanisms. The mechanisms involved in press bonding to eliminate voids along the bond interface were instantaneous deformation on loading, plastic flow during heating, and a diffusion dominant mechanism. The contributions of the bonding mechanisms to the bonding effect were highly dependent on the original status of the specimens and the bonding variables. Plastic flow during heating was the major effect during bonding. The bonding effect response to time was related to the surface roughness and evolution of the microstructure during bonding. The effect of surface roughness on bonding depended on the bonding mechanisms.     

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     

Analytical prediction of pressure‐time curve for free inflation of superplastic sheet into hemispherical dome

Abstract

Blow forming is a commonly used production method to deform superplastic sheets. Several papers have appeared to analytically predict the pressurization sequence needed to control the inflation process at a desired strain rate. Some of them, however, are based on an over‐simplified assumption of uniform thickness reduction during the inflation, which often leads to non‐conservative prediction of pressure‐time path. Others have been more rigorous in the formulation, which on the other hand, results in more extensive equations requiring iterative method with a computer program. This paper presents a modified approach. Ignoring the assumption of uniform thinning, a set of physically sound yet easy‐to‐use equations for obtaining pressure‐time curves, heights of the deforming dome, and thickness variations both in space and time, are derived. The analytical results compare favorably with the experimental data and more elaborate finite element solutions.     

Constant stress creep and constant true strain-rate tensile tests of the superplastic alloy PbSn

Strain-time curves and stress-strain curves have been obtained for the superplastically deformed PbSn eutectic tested in creep under constant stress and in tension under true strain-rate conditions at temperatures ranging between –44 and 30° C. It is shown that the flow stress does not depend on strain and time and is only a function of the true strain rate, of the temperature and of the initial grain size. The result is that this superplastic alloy does not strain-harden and that the grain size is constant. The apparent activation energy does not depend on stress and temperature and is equal to 11.5±0.5 kcal/mole.     

Effect of inverted pressurisation profile on deformation characteristics of 5083 aluminium alloy during superplastic forming

Abstract

Decreasing the cycle time for superplastic forming of a commercially available superplastic 5083 aluminium alloy has been studied in the present work by use of an inverted pressurisation profile. A right cylindrical cup with a depth/ diameter ratio of 0·5 could be superplastically gas pressure formed in less than 100 s. The deformation behaviour was similar to that of constant strain forming during the free bulging stage. In this stage, a stress state gradient from the pole to the edge of the formed dome was observed. Plasticity controlled growth of cavities was thought to be the mechanism for the increase of cavity volume fraction during forming. After the centre point of the deformed sheet touched the die surface, the metal flow pattern was found to be different from that of the traditional approach. The minimum thickness was not located at around the bottom corner of the cylindrical cup rather it was located ~ 7.5 mm away from the bottom centre of the cup with radius 20 mm. Significant cavity nucleation and coalescence caused higher cavity growth rates at large strains, owing to the continuous increase in strain rate resulting from the imposed pressurisation profile.     

Miniature thermal cycling tests on aluminium alloy metal matrix composites

Abstract

A new miniaturised electrothermomechanical test system has been used to study the thermal cycling response of a number of aluminium alloy metal matrix composites reinforced with either Al₂0₃ or SiC particles. Tests were also performed on a monolithic 2618 aluminium alloy for comparison. The system showed good test discrimination between the different materials for both constant load-constant temperature (creep) tests and constant load-temperature cycling (50–200°C) tests. The system was also used to compare the yield behaviour at 200°C, and the thermal expansion and thermal diffusivity of several of the materials.     

Tensile flow and fracture behaviour of a superplastic Al-Ca-Zn alloy

The tensile flow behaviour in the range 275 to 550 ° C of an ultra-fine-grained superplastic Al-Ca-Zn alloy is reported. Under certain conditions of temperature and strain rate, superplastic ductility could be established. Fracture surfaces of tensile specimens tested in the above temperature range were examined by scanning electron microscopy and a correlation could be obtained between the ductility, as revealed by the tension tests, and the fracture behaviour. The fractographic studies also suggested a transition in the deformation process from grain deformation (mainly slip) at the lower temperatures to grain-boundary deformation (predominantly grain-boundary sliding) in the vicinity of 425 ° C.