Papers by Keyword: Superplasticity

Abstract: The superplastic performance of the dual-phase Ti-6242S titanium alloy makes it a good material for aerospace application to produce structural components using the advanced superplastic forming (SPF) process. The need to optimize the SPF process demands the understanding and quantifications of the influence of the different phase constituents - α and β on the global superplastic behavior. Numerical modelling has been useful to predict mechanical behavior for both one-level and multiscale approach. Multiscale approach: bottom-up (microscale to macroscale) has enabled to understand how the different microstructural parameters influence global material/structural mechanical response; which by large means the modelling approach depends on the material local properties. The identification of these local properties is non-trivial in polycrystal materials, particularly at superplastic (elevated) temperatures. We have developed a methodology that permit us to quantify the microstructural parameters of each of the constitutive phases of a polycrystal at a superplastic temperature using genetic algorithm optimization method on the data from in-situ high energy X-ray diffraction (synchrotron radiation), coupled with SEM (scanning electron microscope) and EBSD (electron backscattered diffraction). These identified local microstructural parameters were directly used in the finite strain crystal plasticity model to simulate the material global response. This approach enabled the quantification of the phase influences on global behavior with much accuracy. It was found that α phase planes have high critical resolved shearing stress (CRSS) at 730°C which is similar to its behaviours at room temperatures, while β phase slip planes have low CRSS that encourage slip shearing at low stress. However, more applied load is partitioned in β phase than in α phase, despite that β phase fraction is about 15% at 730°C. Keyword: Multiscale modelling, CPFE, optimization, HEXRD, dual-phase titanium alloy, superplasticity

Abstract: The superplastic deformation behavior, microstructure evolution in the volume and on the FIB-milled surface of the samples of fine-grained AA5083-type alloy with an initial grain size of ~5 µm were investigated, and the role of deformation mechanisms was discussed for two superplastic deformation regimes (1) a strain rate of 1×10^-3 s^-1 and a temperature of 0.87T_i.m. and (2) a strain rate of 5×10^-3 s^-1 and a temperature of 0.97T_i.m.. The m values were ~0.45-0.55 and elongations to failure were ~300% and ~600% for the first and second regimes, respectively. According to the shifts of the marker grid lines after straining to e=0.41, GBS contributed ~33% and ~23% to the total strain in the low-temperature and high-temperature deformation, respectively. The dislocation-induced intragranular deformation provided ~30% for the low temperature regime and ~20 % for the high temperature regime, and remaining 30-50% of strain was localized in the striated zones formed at the across grain boundaries due to both GBS and diffusion creep deformation mechanisms. Considering the strain induced by grain elongation for the low and high temperature deformation regimes, it was concluded that diffusion creep contributed 23% and 34% of the total deformation, and the recalculated GBS contribution, including both FIB grid shifts and a portion of the strain localized in the striated regions, was 43% and 38%, respectively.

Abstract: The effects of minor additions of alloying elements Fe/Ni/Co (0.5wt.%), B (0.01–0.1wt.%), and Y (0.2wt.%) on the superplastic behavior, microstructural evolution and mechanical properties of Ti-4 wt.%Al-3wt.%Mo-1wt.%V alloys are investigated. By increasing the high-angle grain boundary fraction and related facilitation of the grain boundary sliding, these elements reduce the flow stress values at the initial deformation stage and improve flow stability at a steady state. The most pronounced effect is found at low deformation temperatures when acceleration of recrystallization and globularization of the microstructure is critical. As a result, minor additions of the studied elements provide good superplasticity at relatively low temperatures of 625–775 °C (m≈0.50 and elongation to failure ≈ 500–1000%) and post-forming room-temperature strength (YS≈830 MPa and UTS≈990 MPa).

Abstract: In previous research conducted by the authors, a new vibration-damping steel sheet was developed using a new method employing friction stir forming (FSF) to create a laminated composite sheet in which steel sheets and superplastic alloys are laminated in layers. However, the details of the bonding mechanism have not yet been clarified. The present study investigates the relationship between the pressure during the process and the weld interface in the creation of the mentioned superplastic composite sheet. More specifically, a 0.5mm thick perforated steel plate is inserted between two Zn-22Al superplastic alloys and the FSF is applied to the top layer of Zn-22Al. The probe of the FSF tool passes directly above the perforated steel plate, the material stirred by the probe plastically flows into the hole and is joined to the underlying Zn-22Al interface by superplastic diffusion bonding (SPF/DB). It was revealed that the process parameters (rotational speed and tool feed rate) must be perfectly adequate to produce adequate heat input and pressure leading to a proper plastic flow of the material and the occurrence of superplasticity in Zn-22Al. In this study, as a first step to clarify the detailed joining mechanism, the amount of pressure applied to the specimen during the process is measured. While changing the process parameters, the pressure was measured at three points, under the probe of the friction stir process tool and, on the advancing, and retreating sides, to investigate the relationship between the parameters and the pressure at the joint interface.

Abstract: It is now well established that the grain size is the fundamental microstructural feature of all polycrystalline materials. In practice, a very wide range of grain sizes will be needed in order to fully evaluate the effect of grain size on the mechanical properties of metals. For many years this was a significant limitation because it was not possible to use conventional thermomechanical processing to produce materials with submicrometer or nanometer grain sizes. Recently, this problem has been addressed by developing alternative processing techniques based on the application of severe plastic deformation. This overview demonstrates that, although the flow stress increases with decreasing grain size at low temperatures and decreases with decreasing grain size at high temperatures, this clear dichotomy in behavior may be adequately explained by using a single theoretical flow mechanism based on the occurrence of grain boundary sliding.

Abstract: The assumption that stable deformation depends on the strain rate is verified. In case of stable deformation, local deformation in a weak cross-section leads to hardening of the metal. Its effect exceeds the effect of reducing the cross-sectional area, as a result of which the deformation affects other sections as well, while the deforming force increases. At low strain rates that are characteristic of superplasticity, the strain inside the grains cannot be really great. During hot deformation, there is resistance to intragrain deformation, intergrain sliding, and accommodation of grain boundaries. The phenomenon of superplasticity is investigated and fixed under stable and unstable deformation, which leads to contradictory results. It is shown numerically that the function of deformation resistance and stable deformation rate increases only when the deformation is carried out with acceleration. The effect of hardening of the surface layer of metal grains on the deformation parameters is described.

Abstract: Superplasticity denotes the ability of a limited number of materials to achieve exceptionally high tensile elongations of at least 400%. Experiments show that the Al-Mg-Sc alloys provide excellent capabilities for achieving superplastic flow and also they can be formed easily in biaxial superplastic forming operations. It is important, therefore, to examine the superplastic flow mechanism when the alloy is prepared using different procedures. This report examines the superplastic characteristics of these alloys after preparation without subjecting to any severe plastic deformation (SPD), after processing using the two SPD procedures of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) and after processing using the alternative procedure of friction stir processing (FSP). The results are compared using each technique and they are examined with reference to a theoretical model that was developed specifically for superplastic flow in conventional alloys.

Abstract: The manuscript considers microstructure, mechanical and processing properties (formability and solid state weldability) of sheet titanium alloy VT6(Ti-6Al-4V) with improved superplastic properties production of JSC «VSMPO-AVISMA Corporation" for the process of superplastic forming at low temperatures and new experimental cheaper sheet titanium alloy VST2k. Complex studies of microstructure, mechanical properties, formability and weldability in the solid state of these titanium alloys were carried out. Studies have shown that both alloys in the temperature range 750-850oC have good weldability in the solid state and exhibit good superplastic properties. Technological properties of the alloy VST2k almost as good as the properties of the alloy VT6. This makes it possible to recommend the sheet alloy VST2k along with the alloy VT6 for the manufacture of hollow structures by SPF/DB in low-temperature superplasticity.

Abstract: The paper is devoted to an experimental investigation of a high-temperature deformation in V-alloyed high-nitrogen austenitic Fe-19Cr-22Mn-1.5V-0.3C-0.6N steel processed via different thermo-mechanical treatments. Simple thermo-mechanical processing regimes (cold rolling or rolling with single post-deformation anneal) do not allow to realize a substantial elongation in high-nitrogen steel during high-temperature tensile tests. For fine-grained austenitic structure with an average grain size of 3 µm, the maximal value of elongation to failure of 150% was realized at temperature 950 °C. Using a multi-stage thermo-mechanical treatment included cold rolling and intermediate anneals, a heterophase grain/subgrain structure with high density of deformation-induced defects and precipitates was produced. When heated to a deformation temperature, this deformation-assisted microstructure recrystallizes into a stable fine-grained structure and demonstrates the attributes of superplastic flow (values of elongation to failure higher than 400%) in the temperature range of 850-1000 °C. The maximum elongation of 900% is achieved at temperature of 950 °C and an initial strain rate of 10^-4 s^-1.

Abstract: This article is devoted to the study of the mechanism of formation of dissimilar welded joints Ti-2Al-1.5Mn, pure titanium (Ti35A) and aluminum (Аl (pure), Аl-6Mg-0.5Mn) alloys obtained by friction stir welding (FSW). The investigated microstructure of the weld joint nugget (WN), zones of thermo mechanically affected zone (TMAZ) and heat affected zone (HAZ) formed at FSW between Ti-2Al-1.5Mn, Ti35A and aluminum (Аl (pure), Аl-6Mg-0.5Mn) alloys. Zones of welded joints at FSW are formed in the mode of structural superplasticity (SP) with a specific shear-band layered structure with alternating layers. The achievement of superplastic state (SPS) in the formation of WN, TMAZ, HAZ is provided by the step–by–step transformation of various mechanisms of plastic deformation in the mode of simple, collective, abnormal dynamic recrystallization, prepared by the processes of dynamic return, polygonization with the transition to post-dynamic recrystallization by the mechanisms of Bailey–Hirsch, Kahn-Burgers-Taylor. At FSW aluminum and titanium alloys with polymorphism, SPS is supported additionally due to recrystallization by twinning and as a result of phase transformations of alpha-gamma or alpha-beta phases.