Superplastic Forming Research Articles

Microstructure of commercial sheet out of Ti-6Al-4V alloy after superplastic forming at 700°C

The possibility of superplastic forming of commercial sheet out of Ti-6Al-4V alloy at rather low temperature T = 700°C was shown for the first time in the present study. Experimental hemisphere with a radius of 35 mm was successfully formed at 700°C with a constant argon gas pressure p = 2.5 MPa in 4352 seconds. Unlike to previous studies, preheating of the preform, superplastic forming process and subsequent cooling were carried out not in air but in vacuum, in order to avoid the formation of the surface oxide layer that limits the ductility resource of the material. Initial microstructure of Ti-6Al-4V alloy commercial sheet was found to be very non-homogeneous: along with ultrafine grains of α and β phases, the coarse elongated α grains with developed substructure were observed. After superplastic forming at 700°C a substantial improvement of microstructure homogeneity of the commercial sheet took place. Microstructure of the cross-section of the hemisphere did not contain coarse elongated grains. Although the average grain size remained almost unchanged (d =1.4 –1.5 μm), the coefficient of variation decreased significantly, from 1.3 to 0.6. Microtexture analysis revealed randomization of the rolling texture in Ti-6Al-4V commercial sheet and decrease in the texture maximums after superplastic forming at 700°C.

Роль противосварочного покрытия в трехслойных гофрированных конструкциях из титановых сплавов

In the manufacture of three-layer corrugated structures made of titanium alloys by diffusion bonding followed by superplastic forming, stop-off materials are used. The use of stop-off material makes it possible to selectively connect sheet blanks with each other and, as a result, get a hollow structure with an internal set of stiffeners in the form of a corrugation. The effect of stop-off materials based on boron nitride and yttrium oxide on the structure and mechanical properties of the titanium alloy Ti-6Al-4V is studied. Special attention is paid to the influence of stop-off materials on the quality of solid-phase joints in a corrugated structure. It is shown that the stop-off material based on yttrium oxide leads to the least degradation of the mechanical properties of the titanium alloy. It is established that the quality of the design depends not only on the composition of the stop-off materials, but also on the procedure of its processing before diffusion bonding of blanks. The composition of stop-off materials as binders includes organic components that contain light elements: nitrogen, carbon, oxygen and hydrogen. The latter are known to be harmful impurities for titanium and its alloys, and their chemical interaction with titanium is sharply intensified at high temperatures. The traditional temperatures of diffusion bonding and superplastic forming of the Ti-6Al-4V alloy are in the range of 900 – 950°C. Therefore, it is necessary to control and prevent the process of adsorption of hydrogen, nitrogen, oxygen and carbon atoms on the connected surfaces of titanium blanks, since this can have a noticeable effect on the quality of diffusion joints, will lead to defects, and the fatigue strength of the entire structure already depends on this.

Modeling of the process of superplastic forming of hemispherical shells from blanks of different profiles

In the present work, the process of superplastic forming of hemispherical shells from blanks of different profiles on the basis of computer modeling is considered. A relevant task in superplastic forming of shells is to reduce the thickness difference, which will significantly improve the quality of their geometric characteristics with a general reduction in the cost of production. It is shown that the forming of a hemispherical shell from a billet of constant thickness with a fixed flange leads to an inevitable appearance of thickness differenses. It has been shown in a number of works that, in the case of superplastic forming of hemispherical shells from blanks of constant thickness made of titanium alloys, the difference in thickness exceeds 50 %. Various methods have been tried to solve this problem. From the analysis of published works, it follows that the solution to this problem lies in the use of a profiled blank. Traditionally, it is suggested to use a blank with a spherical profile. In the present work, a blank with a conical profile was used, since such a shape of the blank is easier both in manufacture and in calculations compared to a spherical one. The superplastic forming process was simulated using the ANSYS 10ED software package. Using the example of the formation of a hemispherical shell made of titanium alloy Ti-6Al-4V, it was found that the use of a blank with a conical profile makes it possible to reduce the thickness difference down to 7 %.

Superplastic formability of the developed Zr40Hf10Ti5Al10Cu25Ni10 high entropy bulk metallic glass with enhanced thermal stability

The Zr55Cu30Al10Ni5 BMG as a base alloy was alloyed with Hf and Ti to develop a high entropy bulk metallic glass. The aim of the present work was to investigate the superplastic formability of HE-BMG, which can be used for micro-forming applications. Therefore, thermal stability and crystallization kinetics of the newly developed HE-BMG was first studied and then the results were used for the designed hot deformation experiments. It was demonstrated that HE-BMG has higher thermal stability than the base BMG, supposedly due to the sluggish diffusion and high entropy effect. Hot deformation behavior of the developed HE-BMG was investigated and it was found that at temperature of 753 K and the strain rate of 2 × 10–5 s-1, the alloy shows a good superplastic deformation with a plastic strain of at least 50% at compression stress around 500 MPa that is good enough for near net shape forming.

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.

Experimental and numerical analysis of innovative processes for producing a resorbable cheekbone prosthesis

Magnesium alloys have gained interest in biomedical applications due to the mechanical properties (very similar to the human bones) and the capability to be absorbed (additional surgery steps after the implantation not necessary). But suitable manufacturing techniques are required for these alloys, since the standard ones (machining, casting) or even the newest (additive) cannot be successfully adopted in every situation or cannot guarantee the requirements in terms of precision or cost. In the present paper two sheet forming processes (the Super Plastic Forming and the Incremental Sheet Forming) have been investigated for manufacturing a resorbable cheekbone implant using the Mg alloy AZ31B. An experimental/numerical approach was adopted: specific tests were conducted for determining the material behaviour to be implemented in the process simulations used for designing both the manufacturing processes; finally some prototypes of the investigated case study were produced and the post forming characteristics (thickness, roughness, accuracy) evaluated. Both the processes allowed to successfully and optimally produce the resorbable prosthesis adopted as case study, which revealed to be characterised by good enough surface quality (especially in the case of the Super Plastic Forming) and uniform thickness distribution (especially in the case of the Incremental Sheet Forming).

Low-temperature superplasticity of ultrafine-grained near β titanium alloy

Studies of the low-temperature superplasticity (SP) of ultrafine-grained (UFG) near β alloy Ti-5Al-5V-5Mo-1Cr-1Fe at a temperature of 823 K (∼0.42 Tm) in the range of strain rates (2.0–6.9)·10−3 s−1 have been carried out. It is shown that for an UFG alloy with an average grain size of d ∼ 0.17 µm, obtained by the method of multi-axial pressing, the value of the relative elongation to failure exceeds 950% at a strain rate of 2·10−3 s−1. Annealing of the UFG alloy at a temperature of 873 K for 1 h does not cause a noticeable increase in the average grain size (d ∼ 0.23 µm), but leads to the transition of a part of grain boundaries to a more equilibrium state and a decrease in elongation to failure by more than 3 times in the range of strain rates. The study of the evolution of the microstructure under tension at a rate of 2·10−3 s−1 showed that in both states of the alloy up to an elongation of 150%, the average size of the elements of the grain-subgrain structure practically does not change, which is due, among other things, to the appearance of new grains less than 100 nm in size during the SP deformation. It is shown that the significantly lower elongation to failure of the UFG Ti-5Al-5V-5Mo-1Cr-1Fe alloy subjected to annealing may be due to the hindered accommodation of grain boundary sliding during the SP deformation by intragranular dislocation slip because of the transition of individual grain boundaries to an equilibrium state. The preservation of the UFG structure of the alloy with high strength characteristics under conditions of low-temperature superplasticity creates good prerequisites for obtaining high-strength products of complex shape in the regime of SP forming.

Optimization of Superplastic Forming Process of AA7075 Alloy for the Best Wall Thickness Distribution

This work aims to optimize the process parameters for improving the wall thickness distribution of the sheet superplastic forming process of AA7075 alloy. The considered factors include forming pressure p (MPa), deformation temperature T (°C), and forming time t (minutes), while the responses are the thinning degree of the wall thickness ε (%) and the relative height of the product h*. First, a series of experiments are conducted in conjunction with response surface method (RSM) to render the relationship between inputs and outputs. Subsequently, an analysis of variance (ANOVA) is conducted to verify the response significance and parameter effects. Finally, a numerical optimization algorithm is used to determine the best forming conditions. The results indicate that the thinning degree of 13.121% is achieved at the forming pressure of 0.7 MPa, the deformation temperature of 500°C, and the forming time of 31 minutes.

Pressure Prediction by Programme Logic Control Approach on Superplastic Forming in 7075 Al Alloy

A widespread problem in a superplastic forming process is to obtain a uniform thickness distribution in multiple geometry components. Hence, nowadays, the variable pressure control approach has been implementing in order to obtain uniform thickness variation in complex profiles. This paper observed the outcome of various stages in the superplastic forming process for multistage hemispherical die cavity in a newly developed pressure prediction method. Superplastic behaviour such as forming pressure, forming temperature, forming time and thickness distribution are analysed and optimised all parameters in multi-stage profile by using a new pressure prediction approach. Experimentally and numerically evaluated the superplastic parameters and the values are obtained from finite element methods agrees well with the experimental results.

Effect of oxidation on spectral and integrated emissivity of Ti-6Al-4V alloy at high temperatures

Thermomechanical simulation is challenging for the optimization and virtual monitoring of high temperature shape processes, such as SuperPlastic Forming (SPF) of Titanium alloys alike Ti-6Al-4V (TA6V). Part of this challenge is the accurate and process-representative knowledge of emissivity, which controls radiative heat transfer during the process. In this paper, the characterization of TA6V emissivity, with regards to its oxidation stage, is performed within the [600–1000 °C] thermal range. The main contribution of this work is the use of an in-house characterization bench, enabling the fast radiative heating of samples, and the control of oxide layers thicknesses ranging from 250 nm to 120 µm. An oxidation law is established thanks to post-mortem mass gain and oxide thicknesses measurement, and the various oxide phases are identified. Oxide thicknesses versus heating time were then calculated and implemented in the emissivity analysis. Two patterns are observed for oxide thickness versus temperature: below 900 °C, emissivity values are quasi-continuously increasing with oxide thickness, exhibiting an oxide layer mainly composed by TiO2, above 900 °C, emissivity values are also continuously increasing but with a different trend, due to the formation of alumina, confirmed by X-Ray Diffraction (XRD) measurements. Finally, the dependence of emissivity to temperature and oxide thickness emphasized in this paper constitutes a non-trivial mandatory input to increase the radiative heat transfer simulations codes accuracy.

α/β phase transformation and dynamic recrystallization induced microstructure development in fine-grained Ti-6Al-4V friction stir weld

Fine-grained materials possess great industrial significance as they have exceptional strength, ductility, and superplastic formability. However, the poor thermal stability of these materials leads to an excessive grain growth and property deterioration during welding. In this regard, friction stir welding (FSW) is considered as a potential candidate for the welding of fine-grained materials as it is a solid-state welding process with a lower process temperature. In this work, the microstructure development in the stir zone of a fine-grained Ti-6Al-4V alloy during FSW was investigated. The microstructure, misorientation distribution and crystallography of the base metal and stir zone were characterized using the electron backscatter diffraction technique to explain the microstructural evolution. The microstructure of different layers of stir zone composed of either fine α + β lamellar structure or fine equiaxed α and β grains or the mixture of both. The combination of heating/cooling induced α/β phase transformations and plastic deformation induced continuous dynamic recrystallization (CDRX) was responsible for such microstructural developments in the stir zone. Homogeneous misorientation increase in low angle boundaries was found as the dominant mechanism of CDRX in the stir zone of fine-grained Ti-6Al-4V during FSW. The hardness measurement confirmed the absence any plausible softening though there was a little grain coarsening in the top layer of the stir zone. • Quenched and hot rolled fine-grained (1.21 μm) Ti-6Al-4V alloy was friction stir welded. • Stir zone composed of a mixture of α + β lamellar structure and equiaxed α and β grains. • Homogeneous misorientation increase in low angle boundaries was dominant mechanism of continuous dynamic recrystallization. • β➔α phase transformation was governed by the Burgers orientation relationship. • No softening in the stir zone though a little grain coarsening occurred.

Superplasticity and Superplastic Forming

In both academic and industrial research endeavours, driving forces are essential to keep the interest alive for a specific topic [...]

Superplastic deformation mechanical behavior and constitutive modelling of a near-α titanium alloy TNW700 sheet

TNW700 is a newly developed near-α high temperature titanium alloy and has great application potential in the aerospace industry owing to excellent creep strength and short-term service temperature up to 700 °C. The superplastic tensile deformation behavior and constitutive model of TNW700 alloy in the temperature range of 900–975 °C and strain rate range of 0.0005–0.01s−1 were investigated. Results indicated that TNW700 alloy exhibits significant work hardening behavior, which may be related to dynamic growth of β grains, dislocation evolution, and silicide precipitates. In addition, work hardening coefficient (n) as well as the critical strain between flow hardening and softening increases with decreasing strain rate, and first increases and then decreases with increasing deformation temperature. TNW700 alloy exhibits excellent superplasticity at 900 °C–950 °C corresponding to strain rate sensitivity exponent (m) greater than 0.3, and expresses poor superplasticity at 975 °C with lower m value. The m value decreases monotonously with strain owing to decrease of grain boundary sliding contribution caused by dynamic grain growth of β grains. The deformation activation energy increases with decreasing strain rate due to the change of deformation mechanism. A phenomenological constitutive model considering strain hardening, strain rate hardening and temperature softening was proposed and material constants were calibrated by true stress-strain data. The constitutive model was implemented into Abaqus code by UHARD subroutine to simulate the superplastic forming of cone parts, and thickness distribution and bulging heights under different forming time were used to verify the validity of constitutive model.

Superior low-temperature superplasticity in fine-grained ZK60 Mg alloy sheet produced by a combination of repeated upsetting process and sheet extrusion

Fine-grained magnesium alloy sheets are potential candidates for superplastic forming applications. In the present study, the ZK60 Mg alloy sheet was obtained by a combination of repeated upsetting (RU) process, as a severe plastic deformation method, and subsequent forward sheet extrusion. Performing extrusion on the specimens processed by 1 and 5 passes of RU resulted in sheets with average grain sizes smaller than 10 μm, and a basal texture component. The sheet produced by 5 passes of RU and subsequent extrusion (denoted as A5RE) showed low-temperature superplasticity with the strain rate sensitivity index (m-value) of 0.57 and 0.62 at 523 K and 573 K, respectively. The microstructural characterization revealed that a fully recrystallized (VDRX = 96%) fine-grained microstructure containing a large volume fraction of high-angle grain boundaries (HAGBs) of 80.5%, together with the presence of thermally stable secondary phase particles, are the main reasons for achieving the superplasticity in the A5RE condition. The m-value greater than 0.5 and activation energy of 106 kJ/mol calculated for this condition indicate the grain boundary sliding (GBS), accommodated by grain boundary diffusion, as the dominant deformation mechanism during superplastic forming.

The Process Design and Rapid Superplastic Forming of Industrial AA5083 for a Fender with a Negative Angle in a Small Batch

A front automobile fender with a negative angle was trial produced via rapid superplastic forming (SPF) technology. The tensile test of industrial AA5083 was carried out at elevated temperatures, and the results showed that the maximum elongation was 242% at 480 °C/0.001 s−1. A rigid-plastic constitutive model of the SPF process was established. Initial dies of preforming and final forming were designed. The finite element method (FEM) was used to simulate the forming process and predict the thickness distribution of different areas. Furthermore, the dies were optimized to make the thickness distribution uniform. In the final structure, the maximum thinning ratio decreased from 83.2% to 63% due to the optimized design of the forming dies. The front automobile fender was then successfully fabricated by the preforming process and final forming process at 480 °C. A thickness measurement was carried out, and the minimum thickness of the preforming structure was 2.17 mm at the transverse tank, while that of the final structure was 2.49 mm near the edge of the lamp orifice. The average grain size grew from 20 to 35 μm. The grain growth led to the reduction of mechanical properties. Compared with the mechanical properties of the initial material, the maximum decrease in tensile strength for the material after superplastic forming was 5.78%, and that of elongation was 18.5%.

Microstructure and mechanical properties of Ti-Ti2AlNb interface

Diffusion bonding of Ti2AlNb alloy using pure titanium (Ti) foil as an interlayer was carried out on superplastic forming and diffusion bonding special equipment by gas pressure loading method. The microstructure of Ti-Ti2AlNb interface was observed using scanning electron microscope and energy-dispersive spectrometer while the mechanical properties of the joints were evaluated by shear test. The results show that the thickness of Ti foil interlayer has a great influence on the microstructure and shear strength of the interface diffusion region. When the thickness of the intermediate layer is thin (25 µm), Ti, aluminum (Al), and niobium (Nb) elements are fully diffused with uniform element distribution through the diffusion region. The diffusion layer region presents uniform, fine, and disordered lamellar α-Ti + β-Ti dual-phase structure with high shear strength. When the thickness of Ti foil interlayer is thick (50 µm), the distribution of Al elements is relatively uniform through the diffusion region due to its smaller radius and faster diffusion speed, and Ti and Nb elements present gradient distribution from the middle to both sides. The diffusion layer region presents a coarse and long strip shape α-Ti + β-Ti dual-phase structure in the middle part and a fine needle-like or irregular α-Ti + β-Ti dual-phase structure in both side parts, with slightly lower shear strength. Temperature has a great influence on the microstructure and mechanical properties of the diffusion bonding joints. The diffusion region presents a black α-Ti strip area in the middle part with the width of about 10 µm at lower temperature (910°C) with poorer property, due to the grain growth of the parent metal, the property is slightly poorer when the temperature is too high (960°C), and the optimal temperature is 930°C with a higher shear strength.

Numerical simulation of superplastic bulge forming test

Superplastic blow forming is a technology of shell parts production. The development of these processes requires computer simulation which cannot be realized without accurate parameters of the applied material. This characterization can be based on results of free bulging tests. Characterization techniques utilize the models of the dome growth during the bulging test. This study is devoted to the assessing of friction coefficient effect on the linear behavior of normalized thickness - normalized height relation.

Linear analysis of parameters in Arrhenius model for Ti-6Al-4V at superplastic forming

PurposeWith the discussion on the linear relationship of determined material parameters, this study aims to propose a new method to analyze the deformation mechanism.Design/methodology/approachA modified constitutive model based on the hyperbolic sine Arrhenius equation has been established, which is applied to describe the flow behavior of Ti-6Al-4V alloy during the superplastic forming (SPF).FindingsThe modified constitutive model in this work has a good ability to describe the flow behavior for Ti-6Al-4V in SPF. Besides, a deformation map of titanium material is obtained based on the parameters. As the supplement, finite element models of high-temperature tensile tests are carried out as the application of the constitutive model.Originality/valueThe relationship between constitutive model parameters and forming mechanism is established, which is a new angle in rheological behavior research and constitutive model analysis.

Initial microstructure influence on Ti–Al–Mo–V alloy's superplastic deformation behavior and deformation mechanisms

Superplastic forming is an effective way to manufacture complex-shaped parts of titanium-based alloys. This paper studies the influence of the initial microstructure and its strain-induced evolution on superplastic deformation behavior and the formability of a titanium-based alloy. Two types of Ti–Al–V–Mo alloy samples having a different fraction of recrystallized grains before the start of the superplastic deformation were studied. The deformation behavior, including strain hardening and strain rate sensitivity of the flow stress, was analyzed in a temperature range of 775 °C–900 °C and a strain rate range of 10−5 to 10−2 s−1. Strain-induced changes of the microstructure within the bulk of the samples and on the surface of the pre-polished samples were studied during superplastic deformation with a constant strain rate. The dynamic recrystallization and dynamic grain growth in the volume of the samples and the multiple slip bands on the samples' surface were revealed after superplastic deformation. The grain structure evolution and slip bands localization depended on the samples’ initial microstructure. The results showed that the samples with an increased fraction of recrystallized grains exhibited better superplasticity and higher quality of the formed parts with a more uniform thickness distribution across the section than the samples with a lower initial recrystallized fraction.

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.