Superplastic Materials Research Articles

A New Approach to Characterize Superplastic Materials from Free-Forming Test and Inverse Analysis

For about 60 years, the aerospace industry has been strongly interested in superplastic forming processes to produce extremely light and complex-shaped components. Superplastic characteristics are found in lightweight metallic materials such as titanium-based, aluminum-based, and, more recently, magnesium-based alloys. Since the high ductility exhibited by superplastic materials is two orders of magnitude higher than that of conventional materials, complex-shaped components can be obtained. If made with conventional materials, they require expensive assembly operations. The behaviour of superplastic materials is summarized by a constitutive equation commonly obtained via tensile testing that subjects the tested material to a one-dimensional stress state. On the contrary, free-forming tests allows us to test the material by subjecting it to a stress state similar to that determined during a real superplastic-forming process. The aim of this work is to define the characteristic parameters of superplastic materials by free-forming tests. The behaviour of superplastic materials is commonly modelled using a power law which puts the material into a stress-to-strain-rate relationship. This law needs to identify two parameters characterizing superplastic materials: the strain rate sensitivity index and the strength coefficient. In this work, a new procedure is presented that implies the two material parameters vary with strain. It allows for a reduction in the number of constants needed to determine the material constitutive equation, thus requiring low simulation time compared to models that adopt the multiple-objective optimization based on genetic algorithms (GAs). It is more suitable to be used in the industrial field. Furthermore, the proposed procedure is compared with a conventional procedure which is also based on the inverse analysis carried out through the use of a finite element analysis. The results of the conventional procedure, based on the inverse analysis, which is conducted through the use of a finite element analysis, are used to calculate the material constants, and are compared with those coming from the procedure proposed in this work. The proposed procedure appears equally simple and gives more accurate results compared to the conventional procedure. In fact, the maximum percentage error, regarding the prediction of the forming times of a free-forming process, was reduced from 20% to 8%. The development of the proposed procedure, as well as the comparison of the results with a conventional procedure, required the development of an experimental activity. This activity consists of free-forming tests conducted at a constant pressure (the pressures employed vary from 0.2 to 0.4 MPa), at a temperature of 753 K, and on circular sheets (thickness 1.0 mm and radius 40 mm) in superplastic magnesium alloy AZ31.

Strain Rate Sensitivity of Low‐Temperature Superplastic Heterogeneous Medium‐Mn Steel Fabricated by a Novel High‐Ratio Differential Speed Rolling

In the present article, insights into the high‐temperature deformation behavior of medium‐Mn steel (MMS), which is prepared via high‐ratio differential speed rolling (HRDSR), are provided. Moreover, through innovative bidirectional jump experiments, variations in the strain rate sensitivity index m under various conditions are obtained. In the research findings, it is indicated that an increase in strain rate (SR) leads to a hardening of the alloy. During high‐temperature deformation, the value of m decreases with the increase in SR, but the rate of decrease gradually slows down. Furthermore, the higher the temperature (T), the greater the impact of changes in SR on m. In addition, the change in deformation mechanism during deformation leads to microstructural changes, and under the main deformation mechanism of grain‐boundary sliding, m generally increases with strain. Interestingly, at a T of 760 °C, the material exhibits a strong texture with high orientation, resulting in larger m values and superior superplasticity (≈691%). This study not only enriches the research content of HRDSR but also has significant implications for the superplasticity research of MMS. Moreover, a reference is provided for the research of other superplastic materials.

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.

The rheological behavior of brittle BMGs of Cu44.25Ag14.75Zr36Ti5 and Ti32.8Zr30.2Cu9Ni5.3Be22.7 in the supercooled liquid region

Bulk metallic glasses (BMGs) show limited plasticity at room temperature. However, BMGs usually exhibit superplasticity and high plastic forming ability in their supercooled liquid region (SLR). The rheological behavior of BMGs in SLR is vitally important to their thermoplastic forming process. In contrast to the ductile BMGs, thermoplastic deformation behavior of brittle BMGs is rarely reported. In present work, the rheological behavior of two brittle BMGs with high glass forming ability Cu44.25Ag14.75Zr36Ti5 and Ti32.8Zr30.2Cu9Ni5.3Be22.7 was investigated on Gleeble3500. The two BMGs can deform homogeneously depending on the temperature and strain rate. According to the high value of m (strain rate sensitivity index), which is the most important mechanical characteristic of a superplastic material, the two BMGs show superplasticity in their SLR with m ≥ 0.3. Based on the free volume model, their activation volumes are calculated as 0.263∼0.486 nm3 and 1.261∼1.650 nm3, indicating the minimum displacement clusters with average 26∼48 and 91∼127 atoms, for Cu44.25Ag14.75Zr36Ti5 and Ti32.8Zr30.2Cu9Ni5.3Be22.7, respectively. Thus, the two investigated brittle BMGs can be thermoplastic processed in the SLR and the deformation maps are given. BMG Cu44.25Ag14.75Zr36Ti5 shows better machinable property than Ti32.8Zr30.2Cu9Ni5.3Be22.7. Compared to the ductile BMGs, no Newtonian flow is found for the two investigated brittle BMGs.

Finite Element Formulation and Computation of Superplastic Metal Forming Processes with Optimized Rate of Deformation Control

Superplastic forming (SPF) is a material forming technique that uses superplastic exceptional elongations and deformation characteristics to form superplastic materials into certain shapes. The combination of superplastic forming with diffusion bonding (SPF/DB) gives rise to an almost unlimited extension of superplastic forming since more integral lightweight cellular structural components can be manufactured. This paper discusses numerical modelling of the mechanism of superplasticity in metallic materials. The SPF computational method based on the finite element technique augmented with the controlling rate of deformations is developed to examine a range of design or operating conditions leading to more economical forming processes. The non-Newtonian ‘viscous flow’ material is used to model the constitutive of superplastic material during the forming period. The contact mechanics between the sheet material and the mold surface and the intersheet material contact mechanics are imposed using the penalty control method, in which the sticking contact boundary conditions are employed. The space discretization is carried out using the membrane element under plane strain and axisymmetric flow stress conditions, while the implicit time integration technique is utilized to follow the shape changes of the formed sheet material. The validation of the SPF finite element formulation was performed by comparing it with the available analytical solution of Hydraulic Free Bulging of Thin strips. The SPF of a hemispherical dome made of 7475 aluminum sheet alloy was performed to demonstrate the forming process as well as to validate the results obtained between the SPF finite element numerical simulation and the experimental results. The SPF/DB of the multicell component section is considered in the final part.

Superplasticity of a hot rolled Mg–3Zn–0.5RE–0.5Zr (ZEK300) alloy

Superplasticity of a hot rolled fine-grained Mg–3Zn–0.5RE–0.5Zr (ZEK300) alloy was assessed via shear punch testing (SPT). The deformation behavior of the alloy having a grain size of 4.5 μm exhibited regions I, II and III, typical of superplastic materials. In region II, the strain rate sensitivity indices of ZEK300 alloy were obtained as 0.51, 0.48 and 0.41 at 350, 400 and 450 °C, respectively. The average activation energy of 87.6 kJ mol–1 implies that the principal mechanism of deformation is grain boundary sliding (GBS) facilitated by grain boundary diffusion.

Overview: Using Severe Plastic Deformation in the Processing of Superplastic Materials

In tensile testing, polycrystalline materials generally fail at relatively low total elongations but under some limited conditions it is possible to achieve exceptionally high elongations of up to and exceeding 400%. This superplastic condition is important scientifically but also it has important uses through the industrial development of superplastic forming operations. This overview traces the historical development of this superplastic effect and it provides a summary of the main characteristics of the superplastic flow process. Conventional thermomechanical processing is not able to produce exceptionally small grain sizes within the submicrometer or nanometer range but this limitation was effectively overcome through processing using severe plastic deformation (SPD). The advantages of SPD processing are discussed and examples are presented. Finally it is demonstrated that the experimental data may be easily and effectively displayed through the construction of deformation mechanism maps based on combinations of stress, grain size and temperature.

Characterization of the Superplastic Magnesium Alloy AZ31 through Free-Forming Tests and Inverse Analysis

This work proposes a simple procedure to characterize 1.0 mm thick sheets of superplastic magnesium alloy AZ31. The simplest mathematical function that models the behavior of a superplastic material is a power law between stress and strain rate with two parameters connected to the material: K and m. First, the parameter m (variable with the strain) was defined directly by carrying out free-forming experimental tests at constant pressure and using a simple expression taken from the analytical modeling of the free-forming process. In the second step, an inverse analysis was carried out through a finite element model (FEM) and based on a numerical–experimental comparison between the results of the dimensionless height–time (H–t) curve, which made it possible to identify the variation of the parameter K in the same strain range. Once the m and K parameters were evaluated, it was possible to simulate the free-forming tests at constant pressure in the pressure range used to characterize the material. The proposed procedure to estimate m and K parameters made it possible to best match the numerical with the experimental results in terms of the dimensionless height–time curve. The difference between the forming time estimated by FEM and that measured experimentally along the H–t curve was found to be less than 9%.

Efficient Coarse-Grained Superplasticity of a Gigapascal Lightweight Refractory Medium Entropy Alloy.

Superplastic metals that exhibit exceptional ductility (>300%) are appealing for use in high-quality engineering components with complex shapes. However, the wide application of most superplastic alloys has been constrained due to their poor strength, the relatively long superplastic deformation period, and the complex and high-cost grain refinement processes. Here these issues are addressed by the coarse-grained superplasticity of high-strength lightweight medium entropy alloy (Ti43.3 V28 Zr14 Nb14 Mo0.7 , at.%) with a microstructure of ultrafine particles embedded in the body-centered-cubic matrix. The results demonstrate that the alloy reached a high coarse-grained superplasticity greater than ≈440% at a high strain rate of 10-2 s-1 at 1173 K and with a gigapascal residual strength. A consecutively triggered deformation mechanism that sequences of dislocation slip, dynamic recrystallization, and grain boundary sliding in such alloy differs from conventional grain-boundary sliding in fine-grained materials. The present results open a pathway for highly efficient superplastic forming, broaden superplastic materials to the high-strength field, and guide the development of new alloys.

On a unique constitutive equation for steady state isotropic optimal structural superplastic deformation in all classes of materials

Structural Superplasticity in materials has been reported in so many different classes of materials that there is a case to state that this phenomenon is (near)-ubiquitous. Yet, many authors have proposed different rate controlling processes for different superplastic materials. Such an approach goes against Newton’s (Principia, Part 3) axiom that “to the same natural effects we must, so far as possible, assign the same causes”. In contrast, a viewpoint also exists that steady state, isotropic, optimal structural superplastic deformation in different classes of materials can be attributed to a grain-boundary-sliding-rate-controlled process that develops to a mesoscopic scale (defined to be of the order of a grain diameter or more). If this were the case, it should be possible to generate in properly normalized spaces material-independent “universal” curves (2D) and surfaces (3D) for the relationships among the different experimental variables/parameters like stress, strain rate, strain rate sensitivity index, temperature, real activation energy for the rate controlling process and viscosity. In this paper, by a careful analysis of experimental data concerning 175 states of superplastic materials of different classes it is demonstrated that such universal curves and surfaces indeed exist. The existence of such universal curves and surfaces that describe the phenomenology of steady state, isotropic, optimal structural superplastic deformation in different classes of materials in terms of unique equations reinforces the view experimentally arrived at that a unique physical mechanism of deformation is responsible for the near-ubiquitous phenomenon of steady state, isotropic, optimal structural superplasticity.

Experimental evaluation of normalized thickness variation of superplastic cone forming

Superplastic Forming (SPF) is the designers first choice for producing the products that have complexity as used in automobiles and aircrafts, in which the ratio of strength to weight is the major concern. SPF of sheet widely used in the manufacture of the products with more significant complexities simultaneously lighter and stronger than with any other manufacturing process. Superplastic formed products having shapes like hemispherical, conical and box results significant variation in thickness. Lowest thickness will be at the locations where the sheet contacts the die at the last. Complexity of the product, forming depth, and pressure affect the thinning. Unmitigated understanding of stress, strain rates, and strains during the forming process and the use of suitable pressures for forming can reduce this problem. In this study Lead & Tin alloy has chosen, as an exemplar material for Superplastic forming to carry out investigations. The results obtained can be applied for any other superplastic materials. The present investigation focused on studying suitable strain rates and Normalized Thickness Variation during the superplastic cone forming. Results revealed that Normalized Thickness Variation (NTV) is lower i.e., 2.27 when formed at the pressure of 3 bar, which provides an equivalent strain rate of 1.1 × 10−4 s−1. Therefore 3 bar pressure is desirable for cone forming. Results also disclosed that strain rates obtained in this investigation are in the limits of superplastic forming zone and hence no fracture happened on the conical dome due to the thinning.

Effects of Variable Punch Speed and Blank Holder Force in Warm Superplastic Deep Drawing Process

A cylindrical deep drawing test was conducted for the purpose of improving the drawability, product accuracy, and quality in warm deep drawing using a superplastic material with large strain rate dependence. Then, the effects of blank holding force (BHF) and punch speed (SPD) on the flange wrinkle behavior and wall thickness distribution were investigated by experiments and theoretical analysis. A Zn-22Al-0.5Cu-0.01Mg alloy superplastic material SPZ2 with a sheet thickness of 1 mm was employed as the experimental material, and a cylindrical deep drawing experiment with the drawing ratio (DR) of 3.1 and 5 was performed at 250 °C. A good agreement was qualitatively obtained between the elementary theory on the flange wrinkle limit, the fracture limit, and the experimental results. In addition, the authors examined each variable BHF and SPD method obtained from the theory and experimentally demonstrated that the variable BHF method has a great effect on uniform wall thickness distribution and that variable SPD has a great effect on shortening the processing time for superplastic materials. Furthermore, the authors demonstrated the effectiveness of the variable BHF/SPD deep drawing method that varies both BHF and SPD simultaneously.

A method for estimating superplastic material parameters via free–inflation tests

In this work, a new methodology for evaluating the constitutive parameters of superplastic materials is presented. The proposed methodology provides the characterization of the material by means of a variable called apparent viscosity. This variable is calculated for three different materials through data collected by free inflation tests made at different temperatures and pressure values. The apparent viscosity was then used to calculate some material parameters by which the experimental tests were reproduced numerically in a finite element commercial code. The results obtained by numerical simulations were compared both with the experimental ones and with ones deriving by simulations run with material parameters calculated by a commonly used methodology. The proposed approach revealed to have a good prediction capability with deviations lower than the one found by the approach taken as reference. A second validation step was then performed by comparing the stress and strain-rate values found through the proposed methodology with the curves constructed by applying uniaxial tests results from literature. This latter comparison showed that results fit well with the behaviour shown using the standardised uniaxial tests.

Computer simulation of superplastic tensile test

Superplasticity is an ability of polycrystalline materials to archive extremely large deformations, which is utilized in advanced forming technologies demanded mainly in the aerospace industry. Design of such technologies needs an information of the material flow behaviour, which describes the relation of the effective stress on the strain and strain rate taking place during deformation. The most popular experimental method for investigation of the flow behaviour of superplastic materials is tensile testing. The procedure of superplastic tensile testing and interpretation of its results is described by several international standards. At the same time, it is known that the due to flow inhomogeneity in the specimen volume the accuracy of such tests may be violated. Moreover, different standards provide different ratio between the width and the height of the gauge area of a specimen. This work provides the numerical analysis aimed to study how the initial specimen geometry affects the results of tensile tests. A computer program implementing finite element method (FEM) was developed to predict the specimen deformation during the test. A flat specimen is discretized using prismatic elements with specific geometrical constraints reducing the degree of freedom to the order of a plane stressed task. The output stress and strain values were calculated as specified in the ASTM E2448 standard. The effect of the gauge length was studied focusing on the output stress strain curves. The results were compared with the experimental results available in the literature.

Re-entrant Formation on Superplastic Forming Process by Mechatronics Approach in 5083 Al Alloy

Superplastic materials are attributed viscous in behavior which exhibit very narrow and steady grain formation at half of the melting point temperature of given components. Superplastic deformation characteristics are carried out in different pressure control with constant strain rate conditions. In different alloy, the maximum (optimum) strain rate changes from 0.001 to 0.00001s-1. The objective of this research work is to predict the pressure requirements (optimize) to obtain smooth (uniform) profile in a re-entrant shape formation of 5083 Aluminium alloy by using mechatronics approach of Programming Logic Control method. The effect of various forming parameter such as, forming pressure, bulge forming time and thickness distribution of the sheet in a re- entrant shape product. Finite Element analysis is a powerful tool to evaluate the superplastic forming processes (SPF) with accurate prediction of the deformation characteristics.

Dimensional Analysis of Superplastic Processes with the Buckingham Π Theorem

This work applies the Buckingham Π theorem from dimensional analysis on superplastic processes in order to obtain laws of behaviour in a simple way. For this reason, a mathematical background is developed. The particular behaviour of superplastic materials makes it necessary to adapt the way in which these are treated, modelling them by a viscosity function of the strain-rate. Then, dimensional analysis is applied on a set of free-inflation tests in order to obtain a formula that defines the forming time as single function of geometric and material variables. Dimensional analysis allows us to reduce the number of variables to analyse from six to only three. Finally, two different forming time estimators are compared to measure the accuracy of our method, showing a significant improvement over previous methods.

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.

Superplastic Materials Characterisation Based on Free-Bulge Tests

A new characterisation method based on free-bulge tests is proposed to find the characteristic parameters of the strainstress relation in superplastic alloys. The method is applied to experimental tests found in the literature and to new adhoc tests performed on aluminium, magnesium and titanium alloys. The parameters are then compared to independent values obtained from tensile test and computer-aided simulations. The characterisation of the material is evaluated as an apparent viscosity function of the strain-rate and is extracted from the height evolution at the apex dome during the forming process. Results show a good agreement between parameter estimation using this method and independent values. Moreover, this characterisation method exposes the non-newtonian fluid behaviour of these materials during superplastic processes known as shear-thinning or pseudoplasticity.

Investigation on Al2024 with Si3N4 and AlN composites using friction stir processing

Friction stir processing (FSP) is developed for the usage aluminum alloys and it is metal processing method that can provide grain structure of microstructures. Due to the frictional processing heat and stir processing the microstructure of the foundation metal is changed. The Silicon Nitride (Si3N4) and Aluminium Nitride (AlN) are used as reinforcement with the base metal Al2024 by using friction stir processing. Hardness, corrosion resistance, micro structure formation wear resistance and durability of Al2024 have been increased in this friction stir processing by reducing the defects due to the melting of the metal. It is an excellent material processing technology, and there is a high possibility for the preparation of super plastic materials and metal matrix composites. This work is mainly to improve the mechanical properties of Al2024 while it is reinforced by Si3N4 and AlN. Thus the results from this investigation is explained and discussed.

A Lifetime of Research in Creep, Superplasticity, and Ultrafine‐Grained Materials

A long‐term career at the University of Southern California, followed by an appointment at the University of Southampton, provided an opportunity to conduct extensive research into the flow behavior of polycrystalline metals. Initially, research is conducted on creep properties at elevated temperatures and it is shown that solid solution metallic alloys exhibit transitions in creep behavior with dislocation climb and viscous glide as the dominant rate‐controlling mechanisms. There are transitions between climb and glide with increasing stress and also a breakaway from the glide process at high stresses. These transitions are predicted theoretically and the results are in excellent agreement with the experimental data for a wide range of alloys. Attention is directed to the process of superplasticity and it is shown that the flow occurs by grain boundary sliding with accommodation by a limited amount of intragranular slip. Separate rate equations are developed for sliding in coarse‐grained materials and in superplastic materials where the grain sizes are generally <10 μm so that flow occurs without the development of any subgrains. Finally, attention is directed to the properties of ultrafine‐grained materials having submicrometer or nanometer grain sizes produced through the application of severe plastic deformation.