Difference In Flow Stress Research Articles

Comparison of Thermal Deformation Behavior and Characteristics of Mg-Gd-Y-Zn Alloys with and without Bulk LPSO Phase.

Alloys Mg-8Gd-4Y-0.6Zn-0.5Zr (referred to as 0.6Zn) without the bulk long-period stacking ordered (LPSO) phase and Mg-8Gd-4Y-1.1Zn-0.5Zr (referred to as 1.1Zn) containing the bulk LPSO phase were prepared and a series of hot compression tests were conducted to examine and evaluate the influence of the bulk LPSO phase on the thermal deformation behavior and characteristics of the Mg-Gd-Y-Zn-Zr alloy. The bulk LPSO phase affects the dynamic recrystallization behavior, resulting in differences in flow stress between two alloys under different conditions. Specifically, in the temperature range of 380~460 °C, compression at lower strain rates is beneficial for the LPSO phase to promote dynamic recrystallization, while compression at a high strain rate inhibits the dynamic recrystallization due to the severe deformation of the bulk LPSO phase to release the stress concentration instead. The increase in temperature helps the LPSO promote dynamic recrystallization. As a result, the LPSO phase promotes dynamic recrystallization at all experimental strain rates at 500 °C. Furthermore, the thermal processing maps of the 0.6Zn and 1.1Zn alloys are established, and their optimal processing windows are located at 500 °C/0.001~0.01 s-1 and 500 °C/0.01 s-1, respectively. In addition, the instability zones for the 1.1Zn alloy are much larger than that for the 0.6Zn alloy, which corresponds to the microcracks generated at the interfaces between α-Mg and bulk LPSO phases.

Direct simulation of viscoelastic-viscoelastic emulsions in sliding bi-periodic frames using Cahn–Hilliard formulation

In this study, a direct numerical simulation technique is proposed for 2-D droplet emulsions in simple shear flow with viscoelastic-viscoelastic liquid systems. To treat the multi-drop problem without wall effects, the sliding bi-periodic frame is combined with the Cahn–Hilliard equation in the DEVSS/DG (Discrete Elastic-Viscous Split Stress/Discontinuous Galerkin method) finite element framework. We employ the Galerkin weak formulation for flows of an Oldroyd-B fluid in a creeping regime with the Cahn–Hilliard equations. Sliding bi-periodic frame constraints for velocity, phase variable, and chemical potential are implemented by Lagrangian multipliers. We discuss the bulk rheology, morphological development, and effect of the viscoelasticity of emulsions through numerical analysis of single-, two-, and multi-drop problems. We report that increasing Weissenberg number orients the droplet in the flow direction, and the breakup time exhibits non-monotonic behavior similar to drop deformation. The coalescence of two drops is observed to be accelerated with the presence of viscoelasticity due to an extensional flow and normal stress difference. The bulk rheology is found to be consistent with prior experimental data from multi-drop problems. For the first time, the sliding bi-periodic frame has been implemented with the combination of the DEVSS/DG finite element method and the Cahn–Hilliard equation in the absence of wall effects, which is essential for understanding industrially important droplet emulsions.

On the Uniaxial Compression Testing of Metallic Alloys at High Strain Rates: An Assessment of DEFORM-3D Simulation

In this study, the challenges associated with conducting high-strain rate uniaxial compression testing experiments are highlighted. To address these challenges, DEFORM-3D simulation was explored as an alternative approach to experimental testing. Previously established constitutive constants obtained from experimental low strain rate uniaxial compression testing of three titanium (α + β) alloys were used as input codes. From the results, the peak flow stress values obtained from the DEFORM-3D simulation were close to the values obtained experimentally at low (0.1 to 10/s) and high (20 and 50/s) strain rates. For the alloys considered in this study, a discrepancy of ~20% in the peak flow stress was obtained at a 10/s strain rate. The difference in peak flow stress for strain rates less than 10/s or higher (20 and 50/s) is within acceptable limits. The limitations of using DEFORM-3D simulations for high strain rate uniaxial compression testing are highlighted.

Hot workability and microstructure evolution of wire arc additive manufactured 300M steel

Wire arc additive manufacturing (WAAM) technology offers a material-saving and efficient method of manufacturing the 300M steel components, but the obtained microstructure and properties hardly reach the level of wrought materials. By combining WAAM technology with forging process, a novel forming process is proposed. The preferred shape pre-forgings are easily prepared, and the number of forging steps significantly decreases. The WAAMed as-cast microstructure also can be transformed into the wrought state by the proposed forming process. The aim of this study is to investigate the hot deformation behavior and microstructure evolution of WAAMed 300M steel under various deformation temperatures and strain rates. The results show that the WAAMed 300M steel exhibits a higher plastic deformation resistance than the wrought 300M steel. With the increase of deformation temperature and strain rate, the difference of flow stress between the WAAMed and wrought 300M steel decreases. The hot deformation activation energy of WAAMed 300M steel is calculated as 374.1 kJ/mol, which is much higher than that of wrought 300M steel (332.3 kJ/mol). The possible processing windows obtained from the hot deformation activation energy maps are 1040-1120 °C/0.01-10 s−1 for the WAAMed 300M steel and 1010-1130 °C/ 0.1-10 s−1 for the wrought 300M steel. The constitutive model considering strain compensation is established to describe the hot deformation behaviors of WAAMed and wrought 300M steel. The high correlation coefficient and low average absolute relative error confirm the suitability of the developed constitutive models for predicting the flow stresses of WAAMed and wrought 300M steel.

Hot Deformation Behavior Coordination and Processing Maps of 40Cr/Q345B Bimetallic Blank by Centrifugal Casting

The compact cast-rolling compound forming of bimetallic ring is an efficient process for manufacturing large bimetallic rings. The difference in hot deformation behavior of the two metals directly affects the coordinated deformation of bimetals during hot rolling. In this paper, hot compression tests of 40Cr/Q345B bimetallic blank produced by centrifugal casting were carried out at temperatures of 950–1200 °C and strain rates of 0.005–5 s−1. Based on the comparisons of flow stress behavior, activation energy, and activation volume, hot deformation behavior coordination of 40Cr/Q345B bimetallic blank was investigated. Processing maps were established to study the optimum processing windows for 40Cr/Q345B bimetallic blank. Under the same deformation conditions, 40Cr shows lower overall flow stress than Q345B, which facilitate the coordinated deformation of the bimetallic blank during rolling, and the flow stress difference between 40Cr and Q345B decreases with the increase of temperature. It was connected with the increase in the second phase particles pinning effect for higher MnS content in Q345B. The common rate-controlling mechanism for 40Cr and Q345B is the thermal breaking of attractive junctions and movement of jogged screw dislocations. The bimetallic blanks exhibit good deformation coordination in the optimum processing window (1030–1100 °C and 0.5–1 s−1 with a strain of 0.6).

Introduction to Heterostructured Materials: A Fast Emerging Field

Strong and tough materials are desired for lightweight, energy efficient applications such as electric cars and aerospace applications. Recently, heterostructures are found to produce unprecedented strength and ductility that are considered impossible based on the materials science in our textbooks. Such superior mechanical properties are enabled by a new scientific principle: hetero-deformation-induced (HDI) strengthening and work hardening. Heterostructured (HS) materials consist of heterogeneous zones with dramatic difference (> 100 pct) in flow stresses. The inter-zone interaction produces back stress in the soft zones and forward stress in the hard zones, which collectively produces the HDI stress. HS materials possess a significant synergistic effect where the integrated property exceeds the prediction by the rule of mixtures. Importantly, HS materials can be produced by current industrial facilities at large scale and low cost. The new materials sciences and promising applications are driving the fast development of the HS materials as an emerging field. There are many fundamental issues that need to be probed so as to effectively design HS materials for superior properties. To solve these issues, it requires collaborative efforts by the communities of experimental materials science and computational material science and mechanics.

Friction Stir Welding of Similar Aluminum Alloys Thick Plates: An Understanding of the Material Flow, Microstructure Evolution, and Mechanical Properties

A systematic study of friction stir welding in three precipitation hardened wrought Al alloys (2024-T351, 6061-T651, and 7075-T735) has been conducted. The material flow, microstructure evolution, defects and precipitates formation mechanisms, and mechanical properties for different tool rotation and traverse speeds have been systematically investigated for 15 mm-thick butt-welds of similar alloys plates. The nugget zones in all welds were determined to be formed by two material flows – shoulder-driven and pin-driven. The shoulder-driven flow at the top of the weld corresponds to bulk material transfer (i.e., bulk material flow), while the pin-driven flow occurs through a combination of layer-by-layer material transfer (i.e., layered extrusion flow, due to the pin’s extrusion effect) and bulk material flow. The relative volumes of the layered and bulk material flows are dependent on the material and processing parameters. Weld defects are formed when significant differences in flow stress between shoulder-driven and pin-driven flows exist, due to the inhomogeneous heat distribution across the large weld thickness. For materials with higher thermal conductivity, lower flow stresses and temperature gradients, as well as reduced heat inputs result in reduced defect formation. Different techniques, including TEM and DSC characterization, have been used to study the precipitation behavior in friction stir welds. Heterogeneity between the top and bottom regions of the nugget zone was observed. Increasing traverse speed improves the tensile strength and ductility of most studied alloys, and discussions on optimizing the resulting weld quality and mechanical properties using an integrated material flow-microstructure evolution understanding will be presented.

Strain energy method to determine static recrystallization volume fraction of microalloyed steel

Double compression method is an effective method to determine static recrystallization volume fraction of materials because there is a marked difference in flow stress between two passes, which is caused by static recrystallization. And now offset method that is based on some specific flow stress is widely used. However, this method is not suitable to determine static recrystallization volume fraction for all the materials, especially for those alloyed steels. As the measurement of yield or flow stress is not very sensitive to a subtle change in precipitation, strain energy is the integration of the flow stress and so it is more sensitive to the subtle change and so this method could be better. This study aims to present strain energy method to determine static recrystallization volume fraction in details by performing uniaxial compression test on a microalloyed steel. And it was found that there were two points worth to note while performing this method: one is that if work hardening plays a dominant role during compression process, static recrystallization volume fraction can be determined by analyzing double compression curve and its extrapolation, on the contrary if dynamic softening plays the dominant role, the static recrystallization volume fraction has to be determined with the assistance of curve from single pass compression test.

Atomistic simulation of deformation behaviors polycrystalline CoCrFeMnNi high-entropy alloy under uniaxial loading

Molecular dynamics is used to examine the evolution of dislocations and FCC → HCP phase transformation, as well as deformation twinning for a typical polycrystalline CoCrFeMnNi high-entropy alloy. It is found the discrepancy of the stress-strain curve for tension and compression owing to the strength mechanism during deformation. The grain refinement behavior is only observed in samples of compression, which leads to the difference of flow stress compared with the tension. At the early plastic stage, FCC → HCP phase transformation is the major deformation behavior. Three paths of FCC → HCP phase transformation for inducing plasticity are analyzed, and the mechanism of spontaneous nucleation of stacking faults is the mainly approach to the transformation. For large strain, the pattern of twinning induced plasticity plays a critical role on the plastic deformation of the HEA. The results are qualitatively consistent with experiments and provide a fundamental understanding of plastic deformation in FCC polycrystalline CoCrFeMnNi HEA.

Plasticity and size effects of micro-forming sheet processed by electropulsing

ABSTRACT Commercial sheets need effective pretreatment to eliminate work hardening and anisotropy, and improve their forming properties to meet the requirement of micro-forming process. Compared with the traditional heat treatment, the electropulsing treatment (EPT) process has some advantages of performance improvement and consumes less energy and is environmentally friendly. It is a potential pretreatment method or process for sheets of micro-forming. In this paper, the effect of EPT on copper sheets with varying thicknesses and different grain sizes is studied. Electropulsing can significantly improve the plasticity of cold-rolled sheet, and plasticity improvement effect was better than that of heat treatment. The mechanical properties of copper sheets with different thicknesses which undergo electropulsing show the same size effect as that of samples without electric pulse. After EPT of copper sheets with different grain sizes, the flow stress difference decreased, which weakened Hall-Petch effect. As the thickness of copper sheets decreased, the threshold of current density for plasticizing effect increased. Based on the analysis of the dislocation motion induced by electropulsing, the mechanism of electropulsing to improve the plasticity of copper sheet was presented, which provides a reference for future research on the pretreatment process for micro-forming materials.

Low cycle fatigue behavior and microstructural evolution of nickel-based superalloy M951G at elevated temperatures

Low cycle fatigue (LCF) tests of the newly developed nickel-based superalloy M951G have been conducted at 900 and 1000 °C under different total strain amplitudes. Results show that the fatigue properties, fracture mechanisms as well as coarsening of γ′ precipitates are dependent on testing temperatures and strain amplitudes. Fatigue life and cyclic stress response under the same total strain amplitude at 1000 °C are lower than that at 900 °C, which is due to the degradation of microstructures, shearing of γ′ precipitates by dislocations and serious oxidation. Fracture modes change from intergranular cracking to the mixed mode cracking as the strain amplitude increases. At low strain amplitudes, M951G alloy fails in the form of intergranular cracking owing to the oxidation of surface carbides and the relatively low deformation rate. At higher strain amplitudes, the strain localization in grain interior, the distribution of broken carbides and eutectics as well as the relatively higher strain rate are the main reasons for the formation of transgranular microcracks. Ultimately, the effects of fatigue conditions on coarsening of cubic γ′ precipitates are also analyzed from the aspect of γ′ volume fraction, fatigue life and flow stress difference between the γ/γ′ interfaces.

Hot working characteristics of S32760 super duplex stainless steel

Hot deformation behavior of S32760 super austenitic stainless steel was studied in the temperature range of 950 ∼ 1250 °C and strain rate range of 0.1∼10 s-1 employing Gleeble 3800 equipment. The flow stress was modeled using Arrhenius equation and Zener-Hollomon parameter(Z). The microstructures of the specimens under various conditions were investigated, and dynamic recrystallization of austenite, dynamic recovery of ferrite and phase contents varied with temperature and strain rate. The difference of flow stress between experiment and constitutive equation was explained by the phase contents. Finite element analysis was performed to calculate forging load under the same conditions of compression test using modeled flow stress. It was validated that forging load can be predicted precisely with Arrhenius equation.

마그네슘 합금 판재(AZ31B)의 온도 및 성형속도에 따른 2D-draw 굽힘 특성 분석

Recently magnesium alloy sheet has been used as a lightweight material in transportation area. Warm forming is a forming method that improves formability and reduces springback. The magnesium alloy sheet has a characteristic that large difference of flow stress increases depending on strain rate at high temperature. These characteristics cause low dimensional accuracy of formed products. In this study, experiments were performed on the 2D-draw bending with respect to the temperature and forming speed in order to investigate the effects of strain rate and temperature. It was found that as the temperature increases, spinrgback of 2D-draw bending decreased and formability of AZ31B increased. Additionally, the effect of the punch speed was investigated. At 250°C, as the punch speed increased, the springback of 2D-draw bending decreased.

Effect of refinement of grains and icosahedral phase on hot compressive deformation and processing maps of Mg-Zn-Y magnesium alloys with different volume fractions of icosahedral phase

The effect of the volume fraction of I-phase on the hot compressive behavior and processing maps of the extruded Mg-Zn-Y alloys was examined, and the obtained results were compared with those of the cast alloys in a previous work. The average grain sizes, fractions of dynamically recrystallized (DRXed) grains, and sizes of DRXed grains of the extruded alloys after compressive deformation were significantly smaller, higher and smaller, respectively, than those of the cast alloys after compressive deformation under the same experimental conditions. This was because the microstructures of the extruded alloys, having much more grain boundaries and more refined I-phase particles than the cast alloys, provided a larger number of nucleation sites for dynamic recrystallization than those of the cast alloys. The constitutive equations for high-temperature deformation of the extruded and cast alloys could be derived using the same activation energy for plastic flow, which was close to the activation energy for lattice diffusion in magnesium. Compared with the cast alloys, the onset of the power law breakdown (PLB) occurred at larger Zener-Holloman (Z) parameter values in the extruded alloys. This was because the extruded alloys had finer initial grain sizes and higher fractions of finer DRXed grains compared to the cast alloys, such that the onset of PLB caused by creation of excessive concentrations of deformation-induced vacancies was delayed to a higher strain rate and a lower temperature. The flow-stress difference between the extruded alloys and the cast alloys could be attributed to the difference in the fraction of DRXed grains. According to the processing maps, the extruded alloys exhibited higher power dissipation efficiency and flow stability than the cast alloys. This agreed with the microstructural observations.

High accuracy springback simulation by using material model considering the SD effect

In order to enhance the accuracy of springback simulation, the strength differential (SD) effect, i.e., the difference in flow stress between tension and compression, of a high strength cold-rolled steel sheet with a tensile strength of 980 MPa is measured by means of in-plane tension-compression test apparatus. Bi-axial stress tests are also performed to measure the contour of plastic work of the test material. From those experimental results, the material model which can consider the SD effect is determined. Furthermore, this material model is implemented into commercial FEM code by using user-subroutine function. To check the validity of this model and established FEM analysis system, curvature-hat crush forming experiment is performed. By comparing the experimental result and forming simulation result, the accuracy of the material model which can consider the SD effect is validated. Consequently it is concluded that the use of material model which is capable of reproducing the SD effect is a must to enhance the accuracy of springback simulation.

Hot stamping of titanium alloy sheets using partial contact heating

Hot stamping of titanium alloy sheets using partial contact heating by contact heating was developed, and cylindrical deep drawing was performed. An outer portion of a 6Al-4V titanium alloy disc blank was heated with a hot stainless-steel ring by thermal conduction. The partially heated blank was drawn just after contact heating. The limiting drawing ratio in partial contact heating was higher than that of whole heating with a furnace. The concentration in deformation at a corner was prevented by the difference in flow stress between the outer portion and punch portion of the blank. A small distortion of the bottom of the cup was observed for the result of partial contact heating because of the non-uniform thermal shrinking.

Improvement of formability using partial cooling during transfer in hot stamping of ultra-high strength steel parts

The formability in hot stamping was improved by partial cooling of a heated sheet during the transfer from a furnace to dies. Using the transfer device having grips on the heated sheet, the temperature of the portions undergoing large deformation during forming is decreased to increase the flow stress. The cooling pressure and the contacting width in transfer device were changed to control the temperature distribution in the sheet. In W-bending, a temperature difference of 250 °C and a flow stress difference of 300 MPa were obtained, and the depth of the W-bent sheet was increased by 26.1%.

Correlation between softening mechanisms and deformation non-uniformity of laser-welded titanium alloy tube during gas bulging process

In this paper, post-weld annealing was carried out to adjust the flow stress difference between weld seam and base material (BM). Deformation non-uniformity of laser-welded titanium alloy tubes before and after annealing was evaluated by gas bulging at 800°C. Results show that after double annealing (950°C/2h, air cooling+600°C/2h, air cooling), weld seam and BM had similar peak stress at first during tensile test at 800°C, 0.001s−1, then the flow stress difference between them changed dynamically due to different softening rates. The maximum flow stress difference ratio was reduced from 36% to 17% after annealing. Deformation uniformity of the bulged tube was improved by 24.6% after annealing, but the higher softening rate of BM during gas bulging confined its further improvement. At early stage of gas bulging of the annealed tube, the main softening mechanism for weld seam was dynamic recovery and that for BM was globularization of secondary α and phase transformation of α to β. At middle-late stage of gas bulging, the main softening mechanism for weld seam was partial globularization of lamella α and that for BM was wide dynamic recrystallization.

Effect of plastic anisotropy on the deformation behavior of bicrystalline aluminum films – Experiments and modeling

We investigated the effect of texture-induced plastic anisotropy on the deformation behavior of ultrafine-grained aluminum films with a bicrystalline texture (two grain variants). The films were uniaxially loaded along two different directions such that the heterogeneity in the plastic behavior of the two grain variants due to plastic anisotropy was minimized along one direction and maximized along the other. The bicrystalline films show smaller strain rate sensitivity and hysteresis of stress-strain response when they are deformed along the direction that minimizes plastic heterogeneity compared to the direction that maximizes it. Notable differences in flow stress and residual hardening were also found for the two loading directions. To quantitatively understand the effect of plastic anisotropy, we simulated the response of the films using three-dimensional finite elements with a microstructurally explicit model built from TEM automated crystal orientation mapping of the samples that includes a grain boundary region, along with crystal plasticity and anisotropic elasticity. The simulations reveal markedly different distribution of stresses and strains in the two grain variants when loading is performed along the two directions, which can be directly related to the ratio of Schmid factors of their most active slip systems.

Effects of fine particle peening on fatigue strength of a TRIP-aided martensitic steel

The effects of fine particle peening on the fatigue properties of a transformation-induced plasticity-aided martensitic steel with a chemical composition of 0.2% C, 1.5% Si, 1.5% Mn, 1.0% Cr, and 0.05% Nb (mass%) were investigated for applications in precision gears. When this steel was subjected to fine particle peening after heat treatment, higher fatigue limits and lower notch sensitivity were achieved compared with those of SNCM420 martensitic steel. The increased fatigue limits are principally associated with higher hardness and compressive residual stress on and just below the surface, as well as low surface roughness. Fine particle peening also increased the threshold stress intensity factor range in stage I, and decreased the crack propagation rate in stage II in this steel. The crack propagation behavior was mainly suppressed by (1) high hardness and high compressive residual stress, and (2) the strain-induced martensite transformation of retained austenite and high internal stress resulting from a difference in flow stress between the soft martensite matrix and the finely dispersed hard MA-like phase that developed during fatigue deformation.