Superplastic Aluminum Research Articles

Microstructural evolution and deformation mechanisms of superplastic aluminium alloys: A review

Aluminium alloy is one of the earliest and most widely used superplastic materials. The objective of this work is to review the scientific advances in superplastic Al alloys. Particularly, the emphasis is placed on the microstructural evolution and deformation mechanisms of Al alloys during superplastic deformation. The evolution of grain structure, texture, secondary phase, and cavities during superplastic flow in typical superplastic Al alloys is discussed in detail. The quantitative evaluation of different deformation mechanisms based on the focus ion beam (FIB)-assisted surface study provides new insights into the superplasticity of Al alloys. The main features, such as grain boundary sliding, intragranular dislocation slip, and diffusion creep can be observed intuitively and analyzed quantitatively. This study provides some reference for the research of superplastic deformation mechanism and the development of superplastic Al alloys.

Determination of biaxial stress–strain curves for superplastic materials by means of bulge forming tests at constant stress

As the characterization of superplastic materials requires elevated temperatures and strain rate control, standard bulge testing procedure with optical measuring systems is not feasible for the determination of biaxial stress–strain curves. Standard superplastic bulge forming tests performed at constant pressure can be used for the identification of material constants or formability evaluation, but they are not applicable for accurate characterization of deformation behavior providing stress–strain curves at constant strain rates. The present paper is aimed to provide a technique for the direct characterization of stress–strain behavior of superplastic materials in conditions of biaxial tension. This technique is based on bulge forming testing with a closed-loop pressure control procedure allowing one to maintain the value of the effective stress at the dome pole at the predefined constant level. Thus, the variation of strain rate is reduced compared to the constant pressure testing. The results are evaluated using a double-step numerical procedure which provides the way to calculate the stress–strain curves, corresponding to constant referenced strain rates. The developed technique was used to characterize superplastic aluminum alloy Alnovi-U in conditions of biaxial tension at 500°C.

Investigation of stress-strain behavior of a sheet material using free bulging test

Abstract In this work, a new experimental-numerical technique is developed in order to investigate the constitutive behaviour of a sheet material in conditions of superplastic forming. The principal feature of this technique is that unlike classical tensile testing it allows one to obtain stress-strain curves for a material formed in biaxial tension conditions produced by free bulging process. These conditions are much closer to the ones that the material undergoes during the superplastic forming process. Consequently, they give more accurate information about the material behaviour than the ones coming from tensile tests data. The drawback is that the strain (and similarly its time derivative) cannot be directly measured and controlled during free bulging test but its value has to be derived from other macroscopic measurement. Towards this end, a blow forming machine was equipped with a position transducer for the measurement of the dome height during the test. In order to control the stress in the dome apex at a predetermined level the applied pressure was continuously adjusted to current dome height using a special algorithm. After the test, the dome height data were processed to obtain the evolution of stress, strain and strain rate at the dome apex as well as the stress strain curves for constant referenced strain rates. The tests were performed on superplastic aluminium alloy (ALNOVI-U) sheets of 1.35 mm initial thickness at 500 °C. Using the data from two tests with different strain rate paths the stress-strain curves and the strain rate sensitivity index evolution were calculated for two constant referenced strain rates. The obtained constitutive data were verified by finite element simulation of a blow forming.

Strain behaviour of a friction stir processed superplastic aluminium alloy sheet during free inflation tests

In this work, the strain behaviour of friction stir processed aluminium superplastic sheet was investigated. The aim was to study the feasibility of the combination of the friction stir processing technique with the superplastic forming process. Main friction stir process parameters (tool rotational speed and transverse speed) were varied in order to study the effect of these changes on the strain behaviour and on the formability of the processed sheet. For this purpose, free inflation tests were carried out on the processed sheet at a temperature at which the base material exhibits an optimal superplastic behaviour. A strong influence of the stirring parameters on the strain behaviour of the processed sheet was found denoting only a specified set of friction stir parameters made the processed sheet behave like the base material. The microstructure evolution of the stirred zone played an important role in this direction. Higher tool rotational speeds led to abnormal grain growth and, consequently, to a slower deformation in the processed material. Thickness measurements made after free inflation tests also confirmed this behaviour: the material processed with a higher tool rotational speed had a thickness that is twice the unprocessed one.

Chronicling the Development of a High Strength 5xxx-Based Superplastic Aluminium Alloy

In recent years there has been a largely unspoken demand for a high strength, non-heat treatable aluminium alloy for superplastic forming applications. This is particularly true for the automotive industry since the high strength, superplastic aluminium alloys, such as AA7475, are both too time consuming (in forming and heat treatment) and too expensive. Compound this with the expense of corrosion protection and almost all aluminium alloys except for AA5083 fall by the wayside for the automobile industry.However, the need for a higher strength alloy has remained. To achieve this Hydro has systematically investigated the basis behind the superplastic forming of AA5083. On this basis a new high strength 5xxx alloy was extrapolated. The resulting alloy was then characterised and benchmarked against the existing SPF alloy, AA5083. The new alloy, an AA5456-type alloy demonstrated a higher strength than AA5083 while improving the formability and rate of forming. This paper will discuss some of the lessons learned during the development of this alloy.

超々ジュラルミン開発の伝統を受け継いで—超塑性材料の組織制御から学んだこと—

超々ジュラルミン開発の伝統を受け継いで—超塑性材料の組織制御から学んだこと—

Analysis of the strain behaviour of a friction stir processed superplastic aluminium alloy

Superplastic forming is a well-established process by which very large, very complex shaped and also multi-sheets components can be manufactured in a single step. Combining this process with a suitable joining technique is of great industrial interest. In this work the strain behaviour of a friction stir processed aluminium alloy was investigated through free inflation tests. Principal parameters of the friction stir process were changed and free inflation tests were performed to assess the formability of the processed sheet. A strong influence of the friction stir process parameters was recorded on the formability of the processed material. Only a specified set of parameters assured a strain behaviour close to the one of the base material.

Preliminary Study on the Formability of a Laser-Welded Superplastic Aluminum Alloy

In this work, the effect of the laser-material interaction on the formability of a superplastic aluminum alloy was investigated. In applications such as Tailor-Welded Blanks and in the manufacturing of very large components with a complex shape, laser welding combined with superplastic forming may be a very fitting industrial tool. Bead on plate tests were carried out in order to simulate the laser-welding process and then, free inflation tests were performed to evaluate the compatibility of these two processing techniques. The Al-Mg alloy used in this work has a very small grain size which ensures the superplastic behavior. With the aim of preserving this peculiarity, the following aspects on the formability were investigated: (i) the surface condition of the bead before the forming test (with and without the removal of the excess of metal); (ii) the effect of the travel speed of the laser source on the mean grain size; (iii) the introduction of a refiner, commonly used in aluminum casts, in the molten pool in order to further reduce the mean grain size.

Two-Stage Superplastic Forming of a V-Shaped Aluminum Sheet into a Trough with Deep and Irregular Contour

A sheet metal trough of aluminum alloy is manufactured by a two-stage gas forming process at 500 °C. The product with sloped side walls is of ~1.2 m length and ~260 mm opening width, comprising two near-conical-shaped sinks at two ends. The depth of one sink apex is ~350 mm, which results in the depth/width ratio reaching 1.4. To form such a deep and irregular trough, a superplastic aluminum alloy 5083 initially bent into a V-shaped groove was prepared prior to the gas forming work. Within a single die, gas pressure is used to form the V-shaped blank into a preform die cavity prior to the pressure being reversed to form the sheet into the final component cavity. The preforming of the V blank creates a uniform length of line in order to improve the thickness profile of the final part. In this work, a preform has been designed to improve the forming of a complex component by providing a superior thickness profile as compared to a conventional single-stage forming cycle. However, a serious wrinkling situation was encountered, the cause of which has been analyzed using basic mechanics as well as numerical simulation.

Pressure Profile Optimization on a Superplastic Aluminium Alloy

In this work authors present a study of the influence of the pressure profile on forming time and on post-forming characteristics of superplastically formed parts. Material parameters of an aluminium superplastic alloy (ALNOVI-U) were estimated on the basis of experimental tests and of a numerical approach. A numerical model of the forming process was then created and used for evaluating the pressure profile able to keep the maximum strain rate value close to a target value. Pressure profiles were calculated using a strain rate control algorithm, firstly following a conventional approach and monitoring the whole sheet, and then considering only the elements most deformed at the end of the forming process. Experimental results from different numerical pressure profiles are then compared, in terms post-forming characteristics, to test the effectiveness of the approach.

Characterization of a superplastic aluminium alloy ALNOVI-U through free inflation tests and inverse analysis

Numerical methods are widespread in forming applications since a deeper understanding and a finer calibration of the process can be reached without most of the assumptions used in analytical approaches. In this calibration procedure the characterization of the material behaviour is an important preliminary step that cannot be avoided. Experimental tests can be numerically modelled and material constants can be found by inverse methods making numerical results as close as possible to experimental ones. In this work material parameters of a superplastic aluminium alloy have been found by experimental forming tests and an inverse analysis. Constant pressure free inflation tests were firstly performed to find the optimal range for temperature and strain rate values. Material constants were then calculated, on the basis of these tests, minimizing errors between experimental and numerical data through a gradient based optimization iterative procedure. Constant strain rate experimental tests were finally used to refine material parameters and to gain a better agreement between experiments and numerical simulations.

Comparison study of two constitutive equations for Al‐5083 superplastic aluminium alloy

AbstractThis paper compares a double power law constitutive equation and a hyperbolic constitutive equation for the prediction of superplastic behaviour of a commercial Al‐5083 superplastic aluminium alloy. These constitutive equations are firstly fitted to tensile test data at different strain rates and then they are compared taking into account their capacity to describe the tensile test curves and the mechanical behaviour of the material in general.

Gas Forming a V-Shape Aluminum Sheet into a Trough with Contour Like the Back of a Twin-Hump-Camel

A ~1.2 meter long trough comprising of variable cross-sections, with the highest ratio of depth to opening width reaching 1.4, is gas-formed at 500°C with superplastic aluminum alloy 5083. It is a challenge to successfully make such an industrial scale part as demonstrated in this paper. Starting as an expanded V-shaped groove, pressurized gas is manipulated to flow into the forming die, and the pressure vs. time profile needs to be calculated. Thus, the flow stress in the deforming material is optimal to maintain desired strain rate correspondingly. Thickness distribution over the formed product is far from uniform. The other even undesired result is that the thinnest region suffers severe cavitation

AZ31 합금의 부풀림 성형시 공공의 거동

Based on the facts that AZ31 magnesium alloy can be blow formed just like superplastic aluminum alloys and that most superplastic alloys fail by cavitation, the present study was undertaken to investigate the cavitation behavior of a fine-grained AZ31 sheet during blow forming at the elevated temperature. Other points of interest included the much lower strain rate and temperature dependencies of the magnesium alloy compared with conventional superplastic alloys. It was also aimed to find if cavitation in the AZ31 alloy can be suppressed by hydrostatic pressure, as is the case in most superplastic alloys. Interestingly, the application of hydrostatic pressure did not increase the blow formability of AZ31 sheet, even though it reduced the degree of cavitation. A possible reason for this behavior is discussed.

Effects of Interfacial Friction Distribution on the Superplastic Forming of AA5083

This paper studies the effects of interfacial friction distribution on the integrity of superplastic formed parts. For that purpose, the deformation of AA5083 superplastic aluminum alloy into a long rectangular box is investigated. The die surface is divided into five regions for local application of friction coefficients. The commercial finite element code, ABAQUSTM, is used to carry out the forming simulations and calculate the thickness distribution, forming time, and forming pressure profile for different combinations of friction coefficients. It is found that friction distribution at the die-sheet interface strongly affects the metal flow during the forming process, which has a direct impact on deformation stability and strain localization. With the proposed optimal variable friction distribution, the quality of the formed parts has been enhanced, while reducing the required forming time.

Cavitation behavior of fine grained AZ31 alloy sheet during hot blow forming

Cavitation behaviors of fine grained AZ31 alloy sheet during high temperature blow forming have been investigated in terms of deformation and microstructure evolution. Especially, effect of hydrostatic stress by applying backward pressure on blow formability has been investigated by examining cavitation behavior. A commercial grade AZ31 magnesium alloy sheet with the grain size of 15μm could have an elongation over 300% over 400°C. However, elongation did not show a strong strain rate and temperature dependencies which could be easily found in most superplastic aluminum alloys. It might result from that AZ31 would be vulnerable to cavitation even at the early stage of deformation. It was also found that hydrostatic stress was not effective to prevent evolution of cavities during blow forming, which was much different with the case in most superplastic alloys. Both low strain rate dependency of elongation and ineffectiveness of hydrostatic stress might be originated from poor accommodation slip for grain boundary sliding of AZ31 even at high temperature where non basal slip systems were activated.

Grain dynamics and plastic properties of highly refined materials

It has been shown that a grain boundary may undergo two competing classes of elastic instability when the in-plane shear stress exceeds the proper critical values. It may buckle acquiring a sinusoidal shape or may develop a periodic series of fissures, separating bands with a sigmoidal profile. The two instabilities lead to grain sliding, but the corresponding expressions relating the relative velocity between adjacent grains with stress do differ. The plastic properties for small strains were calculated for the two force laws, which we called force models A and B. A comparison of the theoretical results with published experimental data shows that model A, while giving predictions within the experimental uncertainties for a series of superplastic aluminium and titanium alloys, fails for Avesta 2304 steel. However, excellent results are obtained when model B is applied for this steel.

Mechanical experiments on the superplastic material ALNOVI-1, including leak information

In subatomic particle physics, unstable particles can be detected with a so-called vertex detector, placed inside a particle accelerator. A detecting unit close to the accelerator bunch of charged particles must be separated from the accelerator vacuum. A thin sheet with a complex 3D shape prevents the detector vacuum from polluting the accelerator vacuum. Therefore, this sheet has to be completely leak tight. However, this can conflict with restrictions concerning maximum sheet thickness of the product. To produce such a complex thin sheet, superplastic forming can be very attractive in cases where a small number of products is needed. In order to predict gas permeability of these formed sheets, many mechanical experiments are necessary, where the gas leak has to be measured. To obtain insight in the mechanical behaviour of the used material, ALNOVI-1, tensile experiments were performed to describe the uniaxial stress–strain behaviour. From these experiments, a high strain rate sensitivity was measured. The flow stress of this material under superplastic conditions was low and the material behaved in an isotropic manner upon large plastic strains. The results of these experiments were used to predict the forming pressure as a function of time in a free bulge experiment, such that a predefined target strain rate will not be exceeded in the material. An extra parameter within these bulging experiments is the application of a hydrostatic pressure during the forming process. Such a pressure postpones the nucleation and growth of internal cavities, which means that higher plastic strains can be reached before failure. Results from these experiments showed that at higher hydrostatic pressures, higher bulges were made. All these bulges were leak tested, showing also that higher hydrostatic pressures lead to a lower void volume fraction at higher hydrostatic pressures, since these bulges were more leak tight at the same bulge height than bulges made without the application of this pressure. This article describes the setup and results of the uniaxial (tensile) and biaxial (bulging) experiments on the superplastic aluminium ALNOVI-1.

Forty Years since the Invention of SUPRAL Alloys and Look where we Are now

Prior to 1969 the pioneering work carried out by Backofen and Fields in the USA and Johnson and Hundy in the UK demonstrated the 'promise' of Superplastic Forming. Using fine grained dual phase alloys, typically of eutectic or eutectoid compositions, they produced some of the very first superplastically formed prototype components. Although not always 'practical', these dual phase alloys were stable when heated and if appropriately processed often proved to be very superplastic. At that time 'dilute' alloys, including the majority of commercial aluminum alloys, having only a small volume fraction of alloying additions, were thought not to be capable of superplastic behavior due to their propensity to grain coarsen when heated. Breakthrough came in 1969 when at the research labs of Tube Investments, Hinxton Hall Nr Cambridge UK; the first 'SUPRAL' type dilute superplastic aluminum alloys were created. This paper describes the events and 'science' that led up to this development and the remarkable technology that has emerged since the authors began their superplasticity careers more than forty years ago. The future direction that this intriguing technology is likely to take is also explored.

Forty Years since the Invention of SUPRAL Alloys and Look where we Are now

Prior to 1969 the pioneering work carried out by Backofen and Fields in the USA and Johnson and Hundy in the UK demonstrated the 'promise' of Superplastic Forming. Using fine grained dual phase alloys, typically of eutectic or eutectoid compositions, they produced some of the very first superplastically formed prototype components. Although not always 'practical', these dual phase alloys were stable when heated and if appropriately processed often proved to be very superplastic. At that time 'dilute' alloys, including the majority of commercial aluminum alloys, having only a small volume fraction of alloying additions, were thought not to be capable of superplastic behavior due to their propensity to grain coarsen when heated. Breakthrough came in 1969 when at the research labs of Tube Investments, Hinxton Hall Nr Cambridge UK; the first 'SUPRAL' type dilute superplastic aluminum alloys were created. This paper describes the events and 'science' that led up to this development and the remarkable technology that has emerged since the authors began their superplasticity careers more than forty years ago. The future direction that this intriguing technology is likely to take is also explored.