Effect of layered microstructure on superplastic forming property of AA8090 Al–Li alloy
TL;DR: In this paper, a superplastic bulge forming of commercial grade AA8090 Al-Li alloy sheet was done, where three layers of distinct microstructural features along thickness direction were obtained from the as-received sheet.
Abstract: Superplastic bulge forming of commercial grade AA8090 Al–Li alloy sheet was done. The sheet contained three layers of distinct microstructural features along thickness direction. In order to understand the effect of such microstructure on the forming parameters, the blanks of three different layers, viz. surface layer, middle layer and composite layer, comprising of different microstructures, were obtained from the as-received sheet. Three different forming pressures—low, intermediate and high, corresponding to initial strain rates of 4 × 10 −4 , 1 × 10 −3 and 5 × 10 −3 s −1 , respectively, were employed for forming of each layer. The superplastic forming characteristics, including thickness and bulge profile, of surface layer were found to be superior to middle layer. It is interesting to find that the composite layer also exhibited better bulge profile and more uniform thickness distribution than middle layer. The difference in forming characteristics among different layers can be attributed to the presence of favorable equiaxed microstructure in the surface layer and unfavorable elongated grains in the middle layer.
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20 Jan 2013-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, microstructures required for superplasticity were fabricated by intermittent multipass friction stir processing (FSP) in a 6mm 5086 aluminium alloy plate.
Abstract: Microstructures required for superplasticity were fabricated by intermittent multipass friction stir processing (FSP) in a 6 mm 5086 aluminium alloy plate. Two processing parameters corresponding to two different heat inputs were used. Multipass FSP created a gradient microstructure with fine and coarse grain-depth features on the processed plates. Three sheets of 1.5 mm thickness with different microstructural features were extracted for deformation testing under superplastic conditions: a layer with only fine grains from the nugget layer (NL), a layer with a thermomechanical-/heat-affected layer containing coarse grains (TL), and a composite layer (CL) having both fine and coarse grains in equal proportions. High temperature tensile testing was conducted for different layers between 450–550 °C with strain rates ranging from 5×10−4 s−1 to 1×10−2 s−1 to determine the optimum superplastic conditions. The NL and CL were comparable in terms of ductility with a high m value of 0.44. The maximum ductility values were 325% for NL, 355% for CL and 230% for TL. The high ductility of the composite layers, despite their microstructural inhomogeneity, establishes multipass FSP as an effective bulk processing technique.
38 citations
16 May 2018-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: Multipass friction stir processing (FSP) was performed in as-cast AZ91 Mg alloy (AC) to generate layered microstructure through the thickness and study its effect on superplasticity as discussed by the authors.
Abstract: Multipass friction stir processing (FSP) was performed in as-cast AZ91 Mg alloy (AC) to generate layered microstructure through the thickness and study its effect on superplasticity. The FSP tools with different tool pin length were used to develop three kinds of layered microstructured materials. A full thickness fine grained microstructure (FFG), a half thickness fine grained with remaining half in as-cast condition (HFG). The last variation was one third thickness modified into fine grain from both the surfaces and the middle section having as-cast microstructure (SFG). FSP was performed at a tool rotational speed of 720 rpm and at a transverse speed of 150 mm/min. The coarse α-Mg dendrites with large plate like interconnected β-Mg17Al12 interdendrites were the characteristics microstructure of as-cast AZ91 alloy. Fine grains and uniformly distributed precipitates were the characteristics of FSPed microstructure. High temperature tensile tests were carried out at 350 °C using three different initial strain rates i.e. 5 × 10−3 s−1, 1 × 10−3 s−1 and 5 × 10−4 s−1. The FFG material showed superplasticity at all strain rates and highest ductility of 680% was achieved at the strain rate of 5 × 10−4 s−1. The AC and HFG material displayed very low elongation while SFG material exhibited superplasticity of 388%. The superplastic behaviour in SFG was due to increase in the fraction of fine grained microstructure and modification of as-cast microstructure on both the surfaces. Microstructure and texture studies revealed that grain boundary sliding accommodated by grain boundary migration and grain rotation was responsible for superplasticity in FSPed region.
32 citations
TL;DR: Multipass friction stir processing (FSP) was performed on as-cast (AC) AZ91 magnesium alloy with different tool probe lengths to introduce layered microstructure through the thickness.
Abstract: Multipass friction stir processing (FSP) was performed on as-cast (AC) AZ91 magnesium alloy with different tool probe lengths to introduce layered microstructure through the thickness. Three microstructural variations were developed. They were, half thickness fine grain microstructure (HFG), surface modified-fine grain microstructure (SFG) and full thickness fine grain microstructure (FFG). FSP was performed at tool rotation rate of 720 rpm and transverse speed of 150 mm/min. The coarse α-Mg dendrites of 100 μm were refined to approximately 2 μm. Network of β-Mg 17 Al 12 interdendritic particles were broken and distributed uniformly after multipass FSP. Tensile test and notch fracture toughness test were conducted to understand the effect of layered microstructure on mechanical properties. The tensile properties, namely, yield strength, tensile strength and percentage elongation of AC material were found to be 92 MPa, 100 MPa and 0.8% respectively and the corresponding values for FFG were improved to 242 MPa, 327 MPa and 4.7%. For HFG and SFG, these values were found to follow the rule of mixture. Similarly, apparent fracture toughness (K Q ) values of single edge notch bend (SENB) specimen without precrack were compared and the results showed improvement from 6.2 MPa√m in AC to 12.3 MPa√m in FFG.
27 citations
01 Jan 2014
TL;DR: In this paper, the authors discuss the processes and mechanisms that give rise to significant plasticity/Superplasticity in aluminum-lithium alloys and their applications in forming.
Abstract: Since the late 1950s, lithium is being used as an alloying element in aluminum. Their excellent ductility, which allows superplastic forming, along with their ability to decrease the density, makes aluminum-lithium alloys a primary choice for many structural applications in aerospace industry. This chapter initially discusses the processes and mechanisms that give rise to significant plasticity/Superplasticity in these alloy systems. The superplastic behavior of Al-Li alloys and their applications in forming are then discussed. The thickness variation during superplastic forming strongly depends on the peak strain rate sensitivity index (m) of the material, and an increase in the m value reduces the thickness variation in the formed component. The factors that help increase the value of m are enumerated. Superplastic forming is carried out at temperatures ≥ 0.5 Tm (Tm is the absolute melting temperature), often closer to 0.7 − 0.8 Tm. Significant cavitation and grain growth are present at such high temperatures. This has ushered in low temperature superplastic forming by the production of ultra-fine grain sizes via dispersion strengthening and severe plastic deformation (SPD). The promise of superplastic forming of Al-Li alloys for the future and the factors including cost that limit the use of Al-Li alloys are also discussed.
11 citations
TL;DR: In this article, multipass friction stir processing (FSP) of AA5086 Al-Mg alloy was carried out to obtain bulk fine grain material for superplastic forming.
Abstract: Multipass friction stir processing (FSP) of AA5086 Al-Mg alloy was carried out to obtain bulk fine grain material for superplastic forming. FSP produced inhomogeneous microstructure in the thickness direction. The aim of the present work was to understand superplastic forming behavior of distinct microstructural layers, i.e., nugget layer (NL) containing microstructure from nugget zone, thermo-mechanically affected/heat-affected layer (TL) containing microstructure from thermo-mechanically affected/heat-affected (TMAZ/HAZ) zone, and composite layer (CL) containing microstructure from both the above zones (nugget and TMAZ/HAZ). Superplastic forming of NL, TL, and CL blanks was carried out at constant gas pressure. Three different forming gas pressures of 0.75, 1.15, and 1.5 MPa corresponding to strain rates of 5 × 10−4 s−1, 1 × 10−3 s−1 , and 5 × 10−3 s−1, respectively, were used. Forming characteristics of CL were found to be comparable to that of NL and even better at higher forming pressures. Concomitant microstructural evolution during bulging of CL and NL plays an important role here.
10 citations
References
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TL;DR: A review of superplasticity in polycrystalline materials can be found in this article, where the authors present an overview of these new developments using the established behavior of conventional metallic alloys as a standard for comparison with the mechanical properties of new materials.
Abstract: The ability to achieve a high tensile ductility in a polycrystalline material is of interest both from a scientific point of view and also because of potential applications in the materials forming industry. The superplasticity of conventional metallic alloys is now well-documented and understood reasonably well. However, the field of superplasticity has expanded recently beyond the traditional metallic alloys to include evidence of superplastic-like behavior in a very wide range of new and advanced materials. To date, superplasticity has been reported in mechanically alloyed metals, metal matrix composites, ceramics, ceramic matrix composites and intermetallic compounds. This review presents an overview of these new developments using the established behavior of conventional metallic alloys as a standard for comparison with the mechanical properties of these new materials. As well be demonstrated, the new materials often exhibit significant differences in their flow characteristics in comparison with the traditional superplastic metallic alloys. The successful utilization of superplastic materials in forming applications requires an understanding of the failure processes occurring in the materials in terms of both the localization of external flow and the accumulation of internal damage through the nucleation and growth of cavities. These problems are also addressed in this review.
427 citations
"Effect of layered microstructure on..." refers background in this paper
...Unlike the usual assumptions of the models for superplasticty [4] the strain rate sensitivity index does not remain constant ven over optimum strain rate range, but varies with strain rate 5]....
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TL;DR: In this paper, an approximate analysis of the viscous deformation of a thin circular diaphragm clamped at the periphery and subjected to one-sided hydrostatic pressure is presented.
Abstract: An approximate analysis is presented of the viscous deformation of a thin circular diaphragm clamped at the periphery and subjected to one-sided hydrostatic pressure. Equations relating pressure, stress, strain-rate, time and deformation are given for diaphragms of materials whose representative stress, strain-rate curves can be fitted by an empirical relation of the type \ gs = K \ g3 m . The analysis indicates that for deformation under constant pressure the strain-rate may vary by as much as some orders of magnitude and that if a constant strain-rate is to be achieved, the pressure must vary continuously during the process. A method is presented whereby a stepwise loading path can be derived so that the strain-rate is maintained within a predetermined range throughout the process. The analysis has particular application to the pressure forming of superplastic materials as these can only be formed within a particular strain-rate range. Non-dimensional pressure-time loading paths are presented for these materials for different values of m and for different strain-rate ranges. The results of experiments in forming superplastic diaphragms are given and these are in good agreement with the theoretical predictions.
168 citations
"Effect of layered microstructure on..." refers background in this paper
...Superplastic forming (SPF) is usually carried out by applying nert gas pressure on one side of blank, while holding the sheet t its periphery into a die [1]....
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...During free bulging by gas pressure, the shape of the bulge is enerally assumed to be a part of spherical geometry [1] though, m...
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TL;DR: In this article, the superplastic deformation of thin circular diaphragms subjected to onesided hydrostatic pressure is examined both theoretically and experimentally both for simple geometry (domes) and more complex shapes related to those encountered in practical forming operations.
Abstract: The superplastic deformation of thin circular diaphragms subjected to onesided hydrostatic pressure is examined both theoretically and experimentally. Special attention is given to the thickness variations in bulged shapes of simple geometry (domes) and also in more complex shapes related to those encountered in practical forming operations.
141 citations
TL;DR: In this paper, the applicability of uniaxial superplastic properties on the BIAxial deformation behavior as in the super-plastic forming process is addressed, including a relationship of such properties to the selection of forming parameters.
Abstract: The strain rate sensitivity of flow stress has long been recognized as an important factor in determining superplastic ductility, and its relationship to high tensile elongations is well understood from the mechanics point of view. However, the measurements of this parameter and other properties of superplastic materials are challenging, and quite varied results are observed from different test procedures used. In this paper a discussion of the various characterization methods is presented, and the relationship between the superplastic characteristics and the microstructure is brought forth. The applicability of the uniaxial superplastic properties on the biaxial deformation behavior as in the superplastic forming process is addressed, including a relationship of such properties to the selection of forming parameters.
129 citations
15 May 1991-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the major experimental observations in superplastic metals are summarized and the physical mechanisms of flow are discussed with reference to the behavior at high, intermediate and low strain rates, respectively.
Abstract: Superplasticity is an important mode of deformation in metallic alloys with very small grain sizes (usually less than 10 μm). In general, high elongations are observed over a rather limited range of intermediate strain rates and there is a decrease in the superplastic effect at both high and low strain rates. The major experimental observations in superplastic metals are summarized and the physical mechanisms of flow are discussed with reference to the behavior at high, intermediate and low strain rates, respectively. Superplastic-like behavior has been reported recently in some ceramics but the experimental evidence suggests that the mechanism of flow in these materials in not the as in metals.
128 citations
"Effect of layered microstructure on..." refers background in this paper
...The strain rate sensitivity index is reported to be maximum at ntermediate strain rates and drops substantially at low and high train rates [6]....
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