Superplastic Conditions Research Articles

Improving the superplastic properties of a two-phase Mg–8% Li alloy through processing by ECAP

Significant grain refinement was achieved in a cast Mg–8 mass% Li alloy through processing by equal-channel angular pressing (ECAP) using a die having an internal channel angle of 110° and a pressing temperature of 473 K. Following extrusion and subsequent ECAP through two passes, the alloy exhibited excellent superplastic properties including a maximum elongation of ∼970% at 473 K when using an initial strain rate of 1.0 × 10 −4 s −1. The strain rate sensitivities under the optimum superplastic conditions were measured as m ≈ 0.4–0.6. The maximum elongation achieved in this investigation is very high by comparison with other Mg alloys tested in tension at similar temperatures and strain rates.

An experimental investigation of phase transformation superplastic diffusion bonding of titanium alloy to stainless steel

The solid-state direct diffusion bonding of a near α-phase titanium alloy to an austenitic stainless steel by means of the phase transformation superplasticity (PTSP) caused by the cycles of heating and cooling has been carried out. The test results showed that, under the conditions of Tmax = 890°C, Tmin = 800°C, cyclic number of heating and cooling N = 10 cycles, specific pressure P = 5 MPa, heating rate Vh = 30°C/s and cooling rate Vc = 10°C/s, the ultimate tensile strength of the joint reached its maximum value (307 MPa), and the bonding time was only 120 s. In the phase transformation superplastic state, the deformation of titanium alloy has a character of ratcheting effect and it accumulates with the cycles of heating and cooling. The observations of tensile fracture interface showed that both the brittle intermetallic compound (FeTi) and the solid solution based on β-Ti were formed on the interface, and the more in quantity and the smaller in size the solid solutions are, the higher the ultimate tensile strength is.

An OIM Analysis on the Deformation Mechanism in Hot Compressed AZ31 Magnesium Alloy

Orientation mapping based on EBSD technique was applied to analyze the rules of orientation evolution of grains in AZ31 magnesium alloy. Results show that not only under deformation strain rate of 1×10-2s-1, but under 4×10-4s-1(the superplastic deformation condition), grains in all samples with initial textures rotate gradually to near basal orientation ({0002} || compression plane) at different ways, and basal texture becomes stronger with increasing strain, which indicates plastic slip plays an important role during hot deformation. Otherwise, no evident non-basal pyramidal slip of <a+c> as some studies mentioned was observed in the sample with the initial basal texture, and the basal orientation is kept unchanged during the deformation process, which suggests that basal slip is the uppermost plastic slip mechanism in this sample. In addition, the phenomenon of viscous laminar flow was observed in the sample with initial basal texture.

Application of Superplastic Flow to Manufacturing of Microcellular Aluminum Foams

A new application of superplasticity was proposed in the manufacturing process of metal foams. Preform sheets were manufactured using superplastic 5083 aluminum alloy sheets through accumulative roll-bonding (ARB) process. Microcellular aluminum foam plates with 50% porosity were produced through solid-state foaming under the superplastic condition. The cell shape was oblate spheroid, which is effective to reduce the thermal conductivity. The present aluminum foam plates have a potential as an excellent heat insulator.

High Temperature Deformation Behavior of AZ31 Mg Alloy

High temperature deformation behavior of AZ31 Mg alloy was investigated in this study on the basis of a processing map (e » 0.6). To construct a processing map, compression tests were carried out at various temperatures and strain rates. Two regions of high deformation efficiency (h) were identified as: (1) a dynamic recrystalization (DRX) domain at 250°C and 1/s and (2) a superplasticity domain at 450°C and 10-4/s. The average grain size observed in the DRX region was considerably smaller (2.9µm) than in any other region. In the superplastic condition, tensile elongation to failure approached to 1040%. At the high Z regions, flow softening occurred resulting from the dynamic recrystallization but below 1010 of Z value, flow hardening occurred due to the grain growth. Possible deformation mechanisms operating at high temperature were discussed in relation to the activation energy. A two-stage deformation method was found to be effective in enhancing the superplasticity of AZ31 Mg alloy.

Influence of Stress State on Superplastic Deformation Behavior in a Zn-Al Eutectoid Alloy

A Zn-Al eutectoid alloy known as a fine-grained superplastic material is tested both in tension and compression under superplastic conditions, and the influence of the stress state on superplastic deformation behavior, such as flow stress, is investigated. In the compression test, deformation is interrupted and turning is performed to remove a barreled portion so that the gauge section becomes uniform, and then the deformation is continued. Microstructural change during the deformation is also investigated. As a result, flow stress in the compression becomes higher than that in tension even after the barreled portion is removed. After deformation, grain growth is observed both in the tension and compression, and growth rate in the compression is larger than in the tension.

Friction welding of R6M5 steel in the superplastic condition. Changes in the structure, phase composition and properties of welded joints during welding

Friction welding of R6M5 steel in the superplastic condition. Changes in the structure, phase composition and properties of welded joints during welding

Pressure welding in the fabrication of spherical vessels of titanium alloys in the superplastic condition

Pressure welding in the fabrication of spherical vessels of titanium alloys in the superplastic condition

Structure and phase composition of an Al-Mg-Li-Zr alloy under high-rate superplasticity conditions

Uniaxial-tensile tests are performed on samples of a commercial aluminum-lithium alloy subjected to equal-channel angular extrusion. It is found that the material under study has a highly fine-grain structure and exhibits superplasticity under tension. The microstructure of the samples is studied during their plastic deformation.

Friction welding R6M5 steel in the superplastic condition: the mechanism of formation of structural imperfections and methods of removing them

Friction welding R6M5 steel in the superplastic condition: the mechanism of formation of structural imperfections and methods of removing them

Effect of thermal history on the superplastic expansion of argon-filled pores in titanium: Part I kinetics and microstructure

Titanium was foamed at high temperature by expanding pressurized argon bubbles which had been trapped in the metal during prior hot-isostatic pressing. During foaming, transformation superplasticity was induced in the matrix by thermal cycling around the α/β allotropic temperature of titanium, thus accelerating pore growth and delaying cell wall fracture and pore opening to the surface, as compared to foaming performed exclusively under isothermal creep conditions in the β-Ti field. The relative importance of creep and superplastic deformation upon foaming rate, rate of porosity opening and maximum porosity was studied by changing the thermal cycling frequency and maximum temperature. Under superplastic conditions, porosity ceased to increase when reaching levels between 29% and 44%, principally because of the decrease of internal gas pressure due to pore expansion and, to a lesser extent, because of premature gas escape from the pores. Continued thermal cycling after the maximal porosity had been reached lead to pore coalescing and opening to the specimen surface by localized pore wall rupture, which resulted in large open pores with a jagged morphology.

Secondary Processing of AZ31 Magnesium Alloy Concomitant with Grain Growth or Dynamic Recrystallization

Plastic formability in the secondary processing of an AZ31 magnesium alloy was investigated using fine-grained and relatively coarse-grained materials, respectively. To produce components for mobile electric appliances, boss formability was examined at a relatively low temperature of 523 K. The boss with a height of 14 mm was successfully formed in fine-grained material under a nominal forming pressure of 582 MPa in 5 s. Analysis of the boss forming revealed that the forming occurred under the high-strain-rate superplastic condition. The grain size in the boss region was slightly coarsened, probably as a result of strain-induced grain growth, which is a widespread property of superplastic flow. On the other hand, even in the coarse-grained material, a relatively high boss height of 5 mm was obtained under the same forming condition. After the boss forming, grain refinement was observed in the coarse-grained material. It was suggested that the coarse-grained materials exhibited relatively high formability because of dynamic recrystallization. It was concluded that plastic forming concomitant with grain growth was more effective than that concomitant with grain refinement in AZ31 magnesium alloy. An empirical equation to perform plastic forming under the condition concomitant with grain growth was developed.

Achieving Superplasticity of Al-1%Mg-0.2%Sc Alloy in Plate Samples Processed by Equal-channel Angular Pressing

Equal-channel angular pressing (ECAP) was successfully applied to samples of an Al-1% Mg-0.2% Sc alloy in the form of plates. Pressings were conducted at room temperature using route BCZ in which the plates are rotated in the same direction by 90 � around the vertical axis between each pass. Following ECAP, the grain size was measured as � 0:5 mm. Tensile specimens were cut from the plates and pulled to failure at a temperature of 673 K. Superplastic elongations were achieved and the measured ductilities were essentially independent of the orientation within the plate. These results provide the first demonstration of the potential for directly producing a plate in a superplastic condition through the application of ECAP. Thus, this new procedure removes the necessity of rolling conventional as-pressed bars or rods into sheets or plates for subsequent superplastic forming operations.

Movement of alloying elements in Mg -8.5 wt-%Li and AZ91 alloys during tensile tests for superplasticity

Magnesium - 8.5 wt-% lithium and AZ91 alloys, processed using rotary die equal channel angular pressing, exhibit superplasticity. Deformed specimens have been used to investigate the movement of alloying elements during superplastic deformation. Optical microscopy and electron probe microanalysis were used for the investigation of microstructure changes in Mg - 8.5 wt-%Li and AZ91 alloys. In a two phase Mg - 8.5 wt-%Li alloy, the average content of β phase (lithium richer than α) decreased during superplastic deformation. No β phase layer was found at all in the surface area of the deformed specimens. The higher the test temperature, the thicker the layer without β phase is. In addition, the higher the test temperature, the lower the average β phase content is. The results indicate that lithium is lost during superplastic deformation. At too high a temperature of 673 K, lithium oxidises were obtained in the tested alloy. At the optimum conditions of superplasticity, no oxidation was obtained in the specimen. In the AZ91 alloy, zinc was almost lost at the superplastic deformed area. Both Mg - 8.5 wt-%Li and AZ91 alloys show low activation energies derived from the stress - strain rate results, connecting to the microstructure changes during tensile tests. The mechanism of superplasticity of these alloys seems to be grain boundary sliding assisted by the movement of the relatively low activation energy elements during tensile tests.

Effect of the Type of Loading on the Evolution of Microstructure and Crystallographic Texture in VT9 Titanium Alloy

Formation of crystallographic texture due to plastic deformation of cylindrical specimens of VT9 titanium alloy by uniaxial stretching, twisting, and simultaneous twisting and stretching is considered under the conditions of superplasticity. The dependence of the kind of loading on the type of texture formed and the grain size is considered. The uniformity of the variation of the texture in the cross section of specimens is studied as a function of the loading type.

Effect of Composite-Like Microstructure on Superplastic Flow and Concurrent Microstructural Evolution in AA8090 AL-Li Alloy

The SPF grade sheets of AA8090 Al-Li alloy were observed to possess a composite-like layered microstructure with different microstructures and microtextures near the surface region and in the mid-thickness region in the longitudinal and transverse directions. Tensile specimens representing the microstructures near the surface in the mid-thickness and full thickness of a sheet were deformed under the optimum superplastic condition of strain rate (∈) = 1 × 10−3 s−1 and T = 803 K. The microstructural and microtextural gradient along the sheet thickness were found to result in the highest flow stress in the mid-thickness material which was dominated (60 v%) by brass (110) [12] texture and elongated grains. Grain growth, transformation of elongated grains into equiaxed grains, texture weakening and cavitation occurred during superplastic deformation. Such microstructural evolution in different zones across the thickness direction was found to influence the nature of stress-strain curves to a varying extent in the near surface, mid-thickness and full thickness samples. The mechanical properties and deformation behaviour could be explained by treating the full thickness sheet as a composite of surface and mid-thickness materials.

Anisotropy in flow and microstructural evolution during superplastic deformation of a layered-microstructured AA8090 Al–Li alloy

The superplastic forming grade sheets of AA8090 Al–Li alloy were observed to contain layers of different microstructure and microtexture across their cross-section along the normal to the rolling direction (RD). The surface layer (SL) material contained coarse equiaxed grains and the dominance of S {123}[634] texture whereas the center layer (CL) material contained fine elongated grains and the dominance of Bs {110}[112] texture. Tensile specimens, machined to represent the SL of 0.6 mm thickness from the surface towards center (SL), the CL of 0.6 mm thickness, obtained by removing the material of 0.6 mm thickness from each surface towards center (CL), and full thickness (FL) material of 1.8 mm thick, in a sheet of AA8090 Al–Li alloy, were deformed at optimum superplastic condition of strain rate=1×10−3 s−1 and temperature=803 K to investigate the effect of loading direction. In SL material, the specimen parallel to RD exhibited maximum and the specimen perpendicular to RD exhibited minimum flow stresses. This trend was reversed in CL material. The anisotropy in flow stress could be explained on the basis of texture in the SL material, but the contribution of grain directionality became important in the CL material. The flow behavior of FL material was found to consist of the composite-like contributions of SL and CL materials.

Phase Transformation and Domain Switching Behaviors during Superplastic Deformation in 3Y-TZP

Test pieces of a tetragonal zirconia polycrystal containing 3mol% yttria were deformed in tension under superplastic conditions in air to pre-determined nominal strains ranging from 0.0 to 200%. The X-ray diffraction experiments were conducted on surface regions of gauge sections of the deformed test pieces to investigate phase transformation and domain switching behaviors that might occur during the superplastic deformation. The measured X-ray patterns were analyzed using a Rietveld refinement method. As a result: (i) Reflections from monoclinic (m) phase were not observed in the X-ray patterns, indicating that tetragonal (t) to m phase transformation might not occur during superplastic deformation under the above condition. Meanwhile, the fraction of cubic (c) phase seemed to decrease with increasing strain. (ii) Traces of ferroelastic domain switching were observed in the X-ray patterns of the deformed specimens. The switching amount was found to depend on flow stress and deformation amount. The tetragonal axis ratio (c/a) along the scattering vector used in this study was found to decrease with increasing the switching amount.

The use of the self-consistent field method to take into account the anisotropy caused by the orientation and shape of the grains of metals

In the process of manufacturing parts by plastic deformation, the shape and orientation of the grains of the metallic material changes. The emerging mechanical texture causes anisotropy of the mechanical properties of the material. On the basis of the analysis of the interaction of grains at the microlevel, equations of state are obtained that describe the mechanical properties of the material in a wide temperature-speed range under the action of alternating loads, the superplasticity state, and the Bauschinger effect. The knowledge of the anisotropy introduced by the technology of manufacturing the part is necessary for a numerical analysis of the residual mechanical properties of the parts.

Rheological Model of Flow of a Material with a Variable Structure under Superplastic Deformation

Versions of rheological models describing the phenomenological behavior of materials in the superplasticity state with allowance for the elastic component and without allowance for it have been presented. Consideration has been given to the basic indications of manifestation of superplastic properties from the viewpoint of the influence of the dimensions of the structural components on them. Analytical dependences of the governing relations of the mechanics of a deformable rigid body, which allow for the influence of the applied stresses, the structure, and the temperature on the rate of superplastic deformation (SPD) and enable one to calculate the shear viscosity and the rheological dependences of SPD for deformable aluminum alloys AMg4 and AMg6 with a prepared ultrafine‐grained structure, have been given.