Superplastic Materials Research Articles

Room Temperature Superplaticity in Fine/Ultrafine Grained Materials Subjected to Severe Plastic Deformation

Achieving superplasticity at high temperatures and at very low strain rates is considered to be the most important disadvange of superplastic forming processes. Therefore, it is crucial to achieve superplastic behavior at low temperatures and high strain rates. To do so, it is well known that, high amount of grain refinement in the superplastic material is required. Very recent advances in severe plastic deformation (SPD) techniques based on imposing very high strains to the material provide abnormal grain refinement, and ultrafine-grained (UFG) microstructures can be achived in metallic materials by this manner. Formation of UFG microstructures via SPD methods like equal channel angular pressing (ECAP), high pressure torsion (HPT) and friction stir processing (FSP) bring about superplastic behavior in some classes of alloys even at room temperature (RT) as an extreme example of low temperature superplasticity. This paper overviews the studies aiming to investigate the RT superplasticity in some specific metals and alloys after UFG formation by SPD methods. UFG formation or nanostructuring of the materials by SPD to attain RT superplasticity were analyzed in detail. Also, the parameters affecting the RT superplasticity in different classes of materials and superplastic deformation mechanisms operated at RT superplasticity were explained.

A possible stabilizing effect of work hardening on the tensile performance of superplastic materials

In general, the process of superplastic deformation is regarded as steady-state so that the flow stress is given as a function of the strain rate only, thereby emphasizing the significance of the strain rate sensitivity and its determining methods. In this work, in addition to the important role of the strain rate sensitivity, it is shown that it is necessary also to consider the stability criteria for real, stable superplastic deformation through other factors such as work hardening. A possible scenario is proposed to describe the process whereby the work hardening rate may stabilize the deformation process when a perturbation occurs in the cross-section of the sample. The assumption of a work hardening effect is confirmed by its application for interpretation of the systematic deviations observed between the strain rate sensitivities determined experimentally using different experimental methods.

Research Status and Prospect of Friction Stir Processing Technology

Friction stir processing (FSP) is a novel solid-phase processing technique that is derived from friction stir welding (FSW). The microstructure of the base metal can be modified with the friction heat and stir function during processing. It can be used to fabricate surface composites and in situ composites by adding reinforced particles into the metal matrix via FSP. Friction stir processing can significantly improve the hardness, wear resistance, ductility, etc., while preventing defects caused by material melting. It is an ideal material processing technology and has good prospects in the field of superplastic materials and for the preparation of metal matrix composites. This paper reviews research developments into the principle, process, and applications of FSP technology as well as its future research directions and development prospects.

On the Challenges and Prospects of the Superplastic Forming Process

Superplastic forming has already been proven as a practical solution for manufacturing lightweight components in niche applications such as the aerospace and luxury cars industries. The demand to produce such components will continue with the limited nature of the energy resources available today. Therefore, superplastic materials are expected to stay as potential candidates in such applications. In addition, superplastic forming offers many unique advantages over conventional forming techniques including greater design flexibility, relatively low tooling cost, and no spring back. However, the full potential of the process has not yet been fulfilled due to concerns about the nonuniformity of the produced parts thickness profiles and the need for heating to achieve the superplastic properties of the material. In this paper the authors address the main challenges that hinder the wide spread of the process. It is of great practical importance, for example, to develop accurate simulations of the superplastic forming process. Such simulations are required for identifying the optimum process parameters for high quality components. The results of any such simulations or experimental investigations should be translated into simple and clear industrial guidelines. In addition, they discuss the current trends and the prospects of this process.

A theory of steady state structural superplasticity in different classes of materials: A materials-agnostic analysis

A mechanism in which grain/interphase boundary sliding (GBS) that develops to a mesoscopic scale (defined to be of the order of a grain diameter or more) is suggested to be the rate controlling process that leads to steady state structural superplastic flow in different classes of materials, viz., metals and alloys, ceramics, composites, intermetallics, nanostructured materials, bulk metallic glasses, geological materials and ice/ice-mixture, of grain sizes varying from a few micrometres down to a few nanometres. The accommodation processes of dislocation emission from sliding boundaries, highly localized diffusion in and in the vicinity of grain boundaries and/or grain rotations due to unbalanced shear stresses resulting from sliding along grain/interphase boundaries oriented differently to the applied stress axis are assumed to be faster than GBS and so they do not enter the strain rate equation. The actual accommodation process will depend on the nature and strength of boundary obstacles, chemical composition of the alloy and phases present. The equations derived are shown to be accurate by analysing the experimental data concerning many superplastic materials of different classes and grain size ranges. In the present state of development of the model one is able to predict without carrying out any new experiments the steady state structural superplastic flow in any material, including those whose superplastic response was not considered while developing the analysis and regardless of their class and grain size range, with the help of four mesoscopic scale constants, whose values are given in this paper, and the Frost-Ashby equations for estimating the shear modulus of any material at any temperature. The greater accuracy in prediction obtainable by this model is demonstrated by comparing its predictions with those of other (diffusion-based) mechanisms, which also predict a grain-size dependent strain rate.

Formability Analysis on Superplastic Forming of AZ91 Magnesium Alloy Sheet

Superplastic sheet metal forming allows the production of complex parts that are not formable under normal conditions. Superplastic sheet metal forming processes are normally based on the same common principle: the sheet metal is firmly clamped between the die halves and is blow-formed by means of gas pressure. Generally superplastic forming can only be achieved in a very narrow range of strain rates and temperature. Superplastic materials are relatively stable when deformed; this behavior is related to the observation that the flow stress of a superplastic material is very sensitive to the rate of deformation. This paper aims to study the formability characteristic of Magnesium alloy by considering variable parameters, such as the sheet thickness, forming pressure and forming temperature. The forming time of 120 minutes is constant for the formability test. Keywards: Multi dome test, superplasticity, Mg – alloy, Thermomechanical processing, Formability.

Superplastic deformation behavior of Zn-22% Al alloy investigated by nanoindentation at elevated temperatures

Due to their high ductility, superplastic metallic alloys are promising candidates for microforming processes. However, it is not yet clear if the macroscopically observed superplastic material behavior also persists at micrometric length scales. For this reason, the superplastic deformation behavior of the eutectoid alloy Zn-22% Al was investigated at the microscale as a function of temperature, strain-rate and grain-size. Nanoindentation strain-rate jump tests at room temperature and elevated temperatures were correlated with the deformed microstructure. A sigmoidal relationship between the hardness and the strain-rate is evidenced, which is typical for superplastic alloys at the macroscale. In the central part of the sigmoidal curve strong indications for superplastic flow at the microscale are found. Microstructural investigations of the residual indents reveal noticeable intercrystalline deformation processes in combination with some intracrystalline dislocation activity regardless of the applied testing conditions. These experimental findings were further coupled with finite element analysis to determine the size of the plastic zone beneath the indents as a function of the penetration depth. Based on this, a critical cubic material volume of 8 grains over the edge length is found to be sufficient for superplastic flow at local length scales.

Optimization and selection of forming depth and pressure for box shaped Superplastic forming using grey based fuzzy logic

Superplastic forming (SPF) is the first choice of designers for manufacturing parts with complexity as used in aircraft and automobile industries, where the strength to weight ratio is the main criterion. Superplastic forming of a sheet metal has been extensively used to produce the parts with greater complexities that are much stronger at the same time lighter than with other methods. Superplastic forming of sheets invariably results in thickness variation. Minimum thickness results at the portion where sheet comes in to contact with the die last. Pressure, forming depth and complexity of the part affect this thinning. The present investigation aims for simultaneous optimisation of forming depth and pressure of box shaped Superplastic forming using grey based fuzzy logic. In the present study Sn-Pb chosen; which is a model material for SPF to carryout experiments, the same results could be applicable for any other Superplastic material. Results revealed that depth at level1 (D1) and pressure at level 3 (P3) parameter settings minimize the time of forming, and maximize the thinning ratio, simultaneously.

Superplasticity in Ultrafine-Grained Materials.

Abstract Superplasticity refers to the ability of a polycrystalline solid to exhibit a high elongation, of at least 400% or more, when testing in tension. The basic characteristics of superplastic flow are now understood and a theoretical model is available to describe the flow process both in conventional superplastic materials where the grain sizes are a few micrometers and in ultrafinegrained materials processed by severe plastic deformation where the grain sizes are in the submicrometer range. This report describes the basic characteristics of superplastic metals, gives examples of flow in ultrafine-grained materials, demonstrates the use of deformation mechanism mapping for providing a visual display of the flow processes and provides a direct comparison with the conventional model for superplastic flow. The report also describes the potential for using nanoindentation to obtain detailed information on the flow properties using only exceptionally small samples.

On the Common Origin of Structural Superplasticity in Different Classes of Materials

Abstract The present model starts with an assumption that grain/ interphase boundary sliding (GBS) that is dominant during optimal superplastic flow is slower than the accommodation processes of dislocation emission from sliding boundaries, highly localized diffusion in the boundary regions and/ or grain rotation that are present as a concomitant of the GBS process. When boundary sliding develops to a mesoscopic scale (of the order of a grain diameter or more), by the alignment of contiguous boundaries, plane interface formation/ mesoscopic boundary sliding is observed. Significant and simultaneous sliding along different plane interfaces and their interconnection can lead to large scale deformation and superplasticity. The accommodation steps, being faster than GBS, do not enter the strain rate equation. Mathematical development of these ideas using transition state theory results in a transcendental strain rate equation for steady state optimal superplastic flow, which when solved numerically helps one to describe the phenomenon quantitatively in terms of two constants, the activation energy for the rate controlling process, ΔF0and the threshold stress needed to be overcome for the commencement of mesoscopic boundary sliding, σ0.The analysis also explains quantitatively texture randomization as a function of superplastic strain. It is also pointed out, without going into details, that recently the problem has been reduced to FOUR “universal” constants, viz. the mean strain associated with a unit boundary sliding event g0, specific grain boundary energy γB, which is assumed to be isotropic, N the average number of boundaries that align to form a plane interface during mesoscopic boundary sliding and “a” a grain-size- and shape- dependent constant that obeys the condition 0 <a <0.5, in terms of which one can account for superplasticity in any material. In combination with the regression equations popularized by Frost and Ashby to predict the shear modulus of any material at any temperature, these four constants allow one to predict the steady state strain rate of any structurally superplastic material accurately, including those whose superplastic response is not considered for the analysis. The details of the last mentioned result, which are unpublished, will be presented elsewhere.

Finite Element Modeling of Superplastic Forming of Friction Stir Processed AZ31B Mg Alloy

Superplastic forming (SPF) is considered to be a near net shape manufacturing technique, mainly adopted to realize aircraft and automotive parts, which requires relatively high tooling and assembly costs. Furthermore the tuning of the process is a non trivial operation since very limited reliable models have been developed to predict the complex geometries obtained through SPF. In such context several researches, based on finite element method (FEM,) have been conducted on the numerical optimization of conventional SPF processes. Friction Stir Processing (FSP) can be used combined with conventional SPF to enhance the superplastic material behavior by means of grain refinement treatment locally performed. From this point of view very few models have been developed to simulate the different superplastic behavior distinguishing the materials after the application of FSP. In this work free bulge forming tests of AZ31B Mg alloy was experimentally performed by means of blow forming laboratory-scale equipment as well as FEM analysis were conducted to simulate the SPF in two different cases: unprocessed and friction stir processed (FSProcessed) condition. The most relevant parameters of the constitutive numerical model were optimized by numerical-experimental comparison. More specifically material strength factor (K) and strain rate sensitivity index (m) were considered during the parametric optimization. Strain and thickness distributions were compared to the experimental measurements in order to individuate the optimized constitutive equations governing the superplastic behavior in both case studies.

Dynamic reverse phase transformation induced high-strain-rate superplasticity in low carbon low alloy steels with commercial potential

Superplastic materials are capable of exhibiting large tensile elongation at elevated temperature, which is of great industrial significance because it forms the basis of a fabrication method to produce complex shapes. Superplasticity with elongation larger than 500% has been widely realized in many metals and alloys, but seldomly been succeeded in low carbon low alloy steel, even though it is commercially applied in the largest quantity. Here we report ultrahigh superplastic elongation of 900–1200% in the FeMnAl low carbon steels at high strain rate of 10−2–10−3 s−1. Such high-strain-rate superplasticity was attributed to dynamic austenite reverse phase transformation from a heavily cold rolled ferrite to fine-grained ferrite/austenite duplex microstructure and subsequent limited dynamic grain coarsening, under which a large fraction of high angle boundaries can be resulted for superplastic deformation. It is believed that this finding of the low carbon low alloy steel with ultrahigh superplasticity and relative low cost would remarkably promote the application of superplastic forming technique in automobile, aeronautical, astronautical and other fields.

Modeling of superplastic structural materials

The boundary problem in the mechanics of superplasticity is formulated on the basis of a standard superplasticity power law and the Perzyna viscoplasticity model. Methods of identifying these models on the basis of the same input data are proposed.

Research on Superplasticity of 5A90-O Aluminum Alloy Sheet

The influence of temperature and strain rate on the 5A90-O had been researched by costant strain-rate tensile method. The result shows that, although 5A90-O is not an ideal superplastic material, under certain conditions, with a large extension rate, the maximum elongation is193.6%. The strain rate has a significant impact on the flow stress and tensile strength under T=375°C∼500°C. With the decrease of strain rate, the load is reduced. Another important factor that affects the flow stress of 5A90 is temperature. Under the same strain rate, the flow stress of 5A90 is lower with the temperature increase. We choose the Backofen law as the constitutive equation of 5A90, the best condition of 5A90 surperplastic deformation is T=400°C, ε = 0.005s−1.

Evaluation of Strain and Strain rates at different stages of Superplastic Cone Forming

Superplastic forming (SPF) is a powerful tool to manufacture complex parts for industries like automobile and aerospace where strength to weight ratio of the part is the main criterion. Superplastic forming (SPF) of a sheet metal has been used to produce very complex shapes and integrated structures at much lighter and stronger than the assemblies they replace. Superplastic formed shapes like conical, hemispherical poses a problem of large thickness variation, minimum thickness results at the portion where die contact is made last. Proper understanding of strain and strain rates during forming and using the pressures accordingly can minimize this problem. The present investigation focused on the study of strain and strain rates during the forming, Experiments were carried on a model material Sn-Pb, which can be applicable for any other Superplastic materials. Results revealed that strain rates in the present study are within the range of superplastic regime and no failure of the formed part because of the thinning. Results also revealed that forming time can be reduced by increasing the pressure which in turn increase the strain rate.

Forming of a super plastic sheet metal made of MgAZ31 alloy

Metal forming industries are constantly looking for advanced innovation, economical and energy efficient techniques. Superplastic forming has a great potential to be one of those advanced forming methods. It is a near net shape forming process which uses a unique type of materials where elongation exceeds 200% during a controlled forming conditions, e.g. temperature, pressure, and strain rate. Most of superplastic materials are formed by gas technique at elevated temperature. The main objectives of the research work in this paper were: to study the effects of the forming schemes on the forming time and thickness distribution of the formed and device a method to improve the forming part thickness and its uniformity distribution and the forming time. In this paper, a hydraulic and heating system were designed and manufactured to facilitate the experimental investigation. The superplastic magnesium alloy AZ31, Mg AZ31, was formed at 350°C with different strain rates to investigate the effect of the forming pressure profiles on the thickness uniformity of the superplastic formed part. The pressure profiles were generated based on Dutta and Mukherjee analytical approach. Finally, a variable strain rate method is modified to improve the uniformity of the thickness distribution of the formed part and reduce the forming time; which is a major limitation of superplastic forming.

Friction Stir Processing as a Novel Technique to Achieve Superplasticity in Aluminum Alloys: Process Variables, Variants, and Applications

This article provides an overview of the potential for superplasticity of aluminum alloys using friction stir processing (FSP). FSP is a variant of friction stir welding (FSW), and FSP is an effective technique to alter the metallurgical and mechanical properties of the material, which results in superplastic properties at high strain rate and low temperature. This makes FSP as an attractive and cost-effective method to produce superplastic materials. A detailed summary of previously reported superplasticity in all aluminum alloys using FSP is tabulated in this review. It reveals the influence of tool design, machine variables, number of passes, active cooling, grain size, superplastic temperature, strain rate, and elongation on the superplastic properties of FSP aluminum alloys. Variants of FSP to achieve superplasticity at optimized conditions are proposed based on dual rotation of tool and additional cooing during the process. Applications of superplastic forming in aerospace and automotive are discussed. The direction of research in friction stir-processed superplasticity is covered in future scope.

Utilization of Superplastic Materials as a Shock Absorber and a Shielding Device against High Speed Fragments

In this paper, some of the failures and fractures caused by stress waves and shock waves resulting from impact and hypervelocity impact phenomena are presented and discussed. Furthermore, different designed and tested systems to protect the persons and the structural engineering members are also given and discussed. Finally, recent obtained experimental results are presented and discussed. The suggested design systems include alternative methods which include materials e.g. superplastic materials. The paper is expected to be of importance and value to military engineers, mining and design engineers and those involved in locomotive, automobile, aircraft and space vehicles industries.

Development of Superplastic Dieless Drawing Apparatus for 3Y-TZP Zirconia Ceramic Tube

A novel superplastic dieless drawing technique with local heating and tensile deformation has been focused on for fabrication of zirconia ceramic tube. In this study, 3Y-TZP zirconia ceramic tube with outer diameter of 6mm and inner diameter of 4mm as a superplastic material is used experimentally. An apparatus of superplastic dieless drawing with rotary stage for circumferential uniform heating and acetylene burner for high temperature of 1700°C is developed. The superplastiac dieless drawing experiment is carried out to investigate the effect of the ratio of drawing speed to feeding speed on the flexible controllability of diameter after drawing process. As a result, the reduction in area after drawing process can be controlled by the ratio of drawing speed to feeding speed. In addition, a maximum reduction in area of 79.2% can be realized in this process. Consequently, the validity of developed apparatus of superplastic dieless drawing for Zirconia ceramic tube can be demonstrated.

Superplasticity Effect Demonstrated by Gas Forming Hot Bent AA5XXX Sheets into V-Shaped Trough Containing Uneven Concavities

Using low gas pressure as a tool to form metal sheets is certainly feasible. However, the intrinsic flow properties of metal sheet are often a key factor in determining degree of formability, which has not been emphasized or explained well. Most metals, for examples aluminum alloys, can be either superplastic or non-superplastic depending on their original processing history or route (consequently, yielding suitable microstructure) and forming conditions, i.e. mostly temperature and strain rate. This superplasticity effect is clearly demonstrated by gas forming superplastic AA5083 and non-superplastic AA5052 into a V-shaped deep trough containing uneven concavities. The results show the superiority of superplastic material when harsh product standard is required.