Static Softening Research Articles

Post-dynamic transformation from α to β phase and reverse transformation in Ti-10V-2Fe-3Al alloy during interrupted hot deformation

Interrupted hot deformation tests were carried out to study the phase transformation behavior of Ti-10V-2Fe-3Al alloy during isothermal holding after deformation. The results show that post-dynamic transformation (post-DT) from α to β phase occurred first during isothermal holding and the meta-β phase retransformed to α phase after a long holding time. This phenomenon is quite different from α+β titanium alloys in which only reverse transformation (RT, βα) took place during isothermal holding. Microstructure evolution of β-matrix was also investigated and it was found that static recovery was the main softening mechanism. Furthermore, phase transformation has a great influence on the static softening and mechanical response of interrupted hot deformation. Due to the occurrence of post-DT, the yield strength of reloading pass reduced and became lower than the initial pass when the holding time exceeds 900 s. And significant static softening was achieved. The driving and opposing force for post-DT and RT were derived and compared based on thermodynamic analysis. Since the deformation and holding temperature is close to β-transus temperature, post-DT has a lower energy barrier which can be easily overcome by stored energy. This may explain why the phase transformation direction in Ti-10 V-2Fe-3Al alloy is opposite to that in α+β titanium alloys during isothermal holding.

Quantification of static softening process and its effects on work hardening characteristics of a typical high strength steel during multi-pass deformation

Heavy components of 300 M steel are usually manufactured by multi-pass forging. It is necessary to study the flow characteristics of 300 M steel during multi-pass deformation, which helps to regulate the flow behaviors during the actual forging process. In the study, multi-pass compression experiments are conducted on the Gleeble-3500 device to mimic the forging process of 300 M steel. Results show that the deformation parameters and inter-pass holding parameters can affect the work hardening rate significantly. It can be ascribed to coupling effects of dynamic softening and static softening behaviors. A unified static softening kinetics model is established to evaluate the coupling effects of static recovery, static recrystallization, and metadynamic recrystallization on the static softening behaviors. The established static softening kinetics model shows high prediction accuracy with a reliability of 0.99605. Furthermore, a new constitutive model is established to describe the effects of dynamic softening and static softening on the flow stress during multi-pass deformation. The prediction accuracy of the new constitutive model is 0.98897 with a mean absolute error of 4.075%, which demonstrates that the established constitutive model is reliable.

Experimental studies and modeling of strain rate- and temperature-dependent springback behavior of hot-deformed aluminum alloys

The unloading process of hot-deformed aluminum alloys exhibits obvious strain rate- and deformation temperature-dependent nonlinearity and the springback is generated by internal stress relaxation, which affects the forming accuracy of aluminum alloy components. The strain hardening and dynamic softening during deformation as well as static softening during unloading affect the springback of hot-deformed aluminum alloys. In this study, the homogeneous aluminum alloys samples were used to perform the single-pass isothermal hot compression and unloading tests with different deformation temperatures, loading strain rates and strains. The strain-, strain rate- and deformation temperature-dependent unloading and springback behavior of hot-deformed aluminum alloys were studied. The effects of strain hardening, dynamic and static softening on the springback behavior of alloys were revealed. The relationships between unloading stress, springback strain and instantaneous tangent modulus in unloading were analyzed. A polynomial-type equation was proposed according to the nonlinear unloading stress-strain curves. The microscopic mechanism of nonlinear unloading of hot-deformed aluminum alloys was also clarified based on the physical mechanism of internal stress relaxation and anelastic strain accumulation. A micro-mechanism-based model was developed to describe the springback and internal stress relaxation in unloading after hot deformation of aluminum alloys. The comparison of calculated unloading curves with experimental results indicates that the proposed polynomial-type equation and developed micro-mechanism-based model can describe accurately the nonlinear springback and stress decay in unloading of hot-deformed aluminum alloys, which provide a prerequisite for analyzing the final geometry and residual stress of precision hot-deformed alloy components.

Influence of pre-precipitation on the multi-stage hot deformation behavior of an Al-Cu-Mg-Zr alloy: Experiments and integrated modeling

Most industrial hot working operations of heat treatable Al alloys are frequently comprised of several successive deformation stages, and therefore the in-depth understanding and better prediction of flow stress evolution during multi-stage thermomechanical processing are required in modern aluminum industries. In this work, the effects of pre-precipitation microstructures on the complicated flow stress behaviors of an Al-Cu-Mg-Zr alloy during multi-stage hot deformation were first investigated. Further, a new integrated model was developed to predict the multi-stage flow stress behaviors. Concretely, by considering dynamic microstructural evolution, a microstructure-based constitutive KM + Avrami model in which the parameters could be well described by the empirical relationship with the Zener-Hollomon parameter was developed to predict the whole flow stress evolutions of single-stage deformation. Then, a novel approach was proposed to describe the whole flow behaviors during multi-stage hot deformation by integrating the microstructure-based constitutive approaches and a physically-based recovery model for static softening. A good modeling capability and agreement between the modeled and experimental flow stress results was found for the studied alloy with three types of pre-precipitation microstructures. In addition, the in-depth dynamic and static softening mechanisms with the influences of pre-precipitation were also discussed based on modeling analysis and microstructural observations.

Modeling the Double-Pass Flow Curve of Nb Micro-Alloyed Steel by Machine Learning and its Extrapolation to Static Softening Kinetics

Modeling the Double-Pass Flow Curve of Nb Micro-Alloyed Steel by Machine Learning and its Extrapolation to Static Softening Kinetics

Model for characterizing static softening behaviour of GH4500 superalloy during two-pass thermal compression

In this work, the static softening behaviour of GH4500 superalloy during the two-pass thermal deformation was investigated via thermal-simulation compression experiments at the temperature range of 1293 K to 1373 K, strain rate range of 0.01 s−1 to 1 s−1 and interval time range of 0 s to 180 s. The metallographic structure acquired from optical microscope (OM) indicated that both static recovery (SRV) and post dynamic recrystallization (PDRX) would occur during the holding stage. Meanwhile, the influence of interval time, deformation temperatures and strain rates on static softening behaviour were revealed. The results showed that the softening effects principally induced by PDRX strengthened with the increase of the interval time, deformation temperatures and strain rates. To quantitatively describe the PDRX kinetic process of GH4500 superalloy, a model on grounds of Avrami kinetics was established to predict the PDRX softening fraction via eliminating the effect of static recovery. The predicted data of the PDRX softening fraction were well consistent with the experimental data, manifesting that the model could precisely evaluate the PDRX softening behaviour in the stage of inter-pass holding.

Optimization of Thermomechanical Processing under Double-Pass Hot Compression Tests of a High Nb and N-Bearing Austenitic Stainless-Steel Biomaterial Using Artificial Neural Networks

Physical simulation is a useful tool for examining the events that occur during the multiple stages of thermomechanical processing, since it requires no industrial equipment. Instead, it involves hot deformation testing in the laboratory, similar to industrial-scale processes, such as controlled hot rolling and forging, but under different conditions of friction and heat transfer. Our purpose in this work was to develop an artificial neural network (ANN) to optimize the thermomechanical behavior of stainless-steel biomaterial in a double-pass hot compression test, adapted to the Arrhenius–Avrami constitutive model. The method consists of calculating the static softening fraction (Xs) and mean recrystallized grain size (ds), implementing an ANN based on data obtained from hot compression tests, using a vacuum chamber in a DIL 805A/D quenching dilatometer at temperatures of 1000, 1050, 1100 and 1200 °C, in passes (ε1 = ε2) of 0.15 and 0.30, a strain rate of 1.0 s−1 and time between passes (tp) of 1, 10, 100, 400, 800 and 1000 s. The constitutive analysis and the experimental and ANN-simulated results were in good agreement, indicating that ASTM F-1586 austenitic stainless steel used as a biomaterial undergoes up to Xs = 40% of softening due solely to static recovery (SRV) in less than 1.0 s interval between passes (tp), followed by metadynamic recrystallization (MDRX) at strains greater than 0.30. At T > 1050 °C, the behavior of the softening curves Xs vs. tp showed the formation of plateaus for long times between passes (tp), delaying the softening kinetics and modifying the profile of the curves produced by the moderate stacking fault energy, γsfe = 69 mJ/m2 and the strain-induced interaction between recrystallization and precipitation (Z-phase). Thus, the use of this ANN allows one to optimize the ideal thermomechanical parameters for distribution and refinement of grains with better mechanical properties.

Effect of Sample Geometry on Strain Uniformity and Double Hit Compression Tests for Softening Kinetics Determination

Static softening is an essential process during the hot rolling of steel to refine grain size and improve mechanical properties. The double hit test is used to measure the static softening volume fraction from the flow stress curves. Herein, the influence of different sample geometries on the determination of softening fraction from the double hit test for conditions where 0–99% softening is expected. The double hit tests are modeled in DEFORM for conventional sample geometries in both uniaxial compression and standard plane strain compression tests, and an additional simulation of a modified plane strain compression test sample geometry, designed to provide more uniform strain distributions, is also performed. A user routine is incorporated into the model to predict the localized softening volume fraction from localized strain based on known softening equations and set back the localized strain value to zero while the softening volume fraction reaches 85%. The standard plane strain compression test geometry shows a strong strain gradient resulting in inaccurate softening fraction volume predictions from flow stress, whereas the uniaxial compression test and modified plane strain compression test geometries show better predictions of softening volume fraction from the flow stress data.

The interrupted hot compression of an as-forged titanium matrix composite: flow softening behavior and deformation mechanism

In this paper, the deformation behavior and microstructure evolution of 2.5vol.% (TiBw + TiCp)-containing high-temperature titanium matrix composite (TMCs) was studied by the interrupted compression tests. The flow stress–strain curves of reloading of TMCs showed the flow softening behavior. The decrease of flow stress was associated with increasing holding time and deformation temperature or decreasing strain rate. The static softening mechanism was associated with the DRV, DRX and consume of dislocations during interrupted holding (SRV) reflected by the Q, n and microstructure. The fraction of αp phase decreased with increased holding time and deformation temperature or decreased strain rate. Due to the transformation of αp phase to β phase, the high-temperature deformation ability of the TMCs will be improved a lot, which is consistent with the decreased flow stress. For the reinforcements and αp phases, they played restricted role in the growth of α/β colonies. However, the decrease of αp phase made the grown up of α/β colonies in low strain rate. The deformation mechanism of the composites was cDRX caused by the transformation of LABs into HABs by progressive lattice rotation of α phase in (α+β) region and dDRX attributed to the nucleation and growth process of β grains in β region. Due to the addition of reinforcements, the weaken microtexture of composite can be obtained. After the interrupted compression, the optimal deformation parameter has been found for the TMCs, which can give guidance for the subsequent hot-working.

Seismic Performance Screening and Evaluation for Embankments on Liquefiable Foundation Soils

This paper proposes a framework for screening and evaluating seismic performance of river earth embankments on liquefiable foundation soils. The framework is executed in the order of simplest screening by soil liquefaction potential map to preliminary and detailed evaluation of seismic performance of embankment according to the sufficiency of data and the complexity and accuracy of the methods. The seismic performances of embankments are classified into four levels based on the seismic-induced crest settlement. The method used for preliminary evaluation is based only on the factors of safety of foundation soils against liquefaction and the embankment slope against sliding. The static softening method (SSM) and dynamic effective stress method (DESM) are suggested for detailed evaluation of seismic performance. SSM is a static FDM or FEM analysis for estimating liquefaction-induced settlement by weakening the strength of the liquefied foundation soil under the action of the self-weight of the embankment. However, DESM is a dynamic history analysis for estimating liquefaction-induced settlement by simulating the generation and dissipation of pore water pressure in the liquefaction process of foundation soils under the action of the self-weight of the soil embankment and its seismic inertial force. The damaged embankment of the Maoluo River in the 1999 Chi-Chi earthquake was used as a case to demonstrate the feasibility of this framework. The results showed that the simpler the adopted method is, the more conservative the estimated settlement.

Study on static softening behavior and hot working performance of Fe-0.2C-7Mn steel

Static softening behavior and hot working performance of medium Mn steel (Fe-0.2C-7Mn) are studied by 2-step and interrupt hot compression test with a Gleeble-3500 thermal-mechanical simulator. The results show that static softening behavior of the steel exhibits obvious sensitivity to the deformation temperature, inter-pass time and strain rate. But deformation temperature has a more significant effect on the static recrystallization (SRX) grain size than strain rate. 3D hot working processing maps are established, that low temperature and medium strain rate (850 °C ∼ 925 °C and 0.02 s−1 ∼ 0.3 s−1) are the preferred thermal deformation conditions for the test steel. Microstructure observations indicate that both static recovery (SRV) and SRX occur within inter-pass time, and SRV is the predominant mechanism in determining softening fraction. The microstructure is composed of film-like α at room temperature, and most of the nucleation of SRX preferably take place at triple junctions of grains or prior grain boundaries. Besides, high-angle grain boundaries (HAGBs) migration produce twin boundaries during grain growth and the main nucleation mechanism of the SRX is strain-induced boundary migration. The kinetics model of static softening is established and the experimental values are consistent with predicted ones, which proves the reliability of the model.

Static recovery of A5083 aluminum alloy after a small deformation through various measuring approaches

Static recovery was confirmed to be the dominant softening mechanism during annealing for the studied A5083 aluminum alloy. The kinetics of static recovery, described based on the activation parameters (activation energy and volume of static recovery) and the static softening fraction, were mainly studied through two thermomechanical tests, namely, double-pass compression and stress relaxation tests. A new approach was proposed to measure the static softening fraction in the stress relaxation test. In general, a higher temperature or strain rate accelerates static recovery, while interestingly, the effect of pre-strain on static recovery is opposite in cases with and without external stress, owing to the enhanced static recovery by external stress during annealing. In addition, microhardness tests and electron backscatter diffraction (EBSD) characterization were also conducted to verify the accuracy of the results. The grain average misorientation approach based on EBSD characterization was confirmed to be effective in distinguishing and quantifying static recovery. It is noteworthy that the special stress hardening phenomenon occurring at 400 °C and 0.1/s is caused by strain aging, showing the complexity of the material behaviors after deformation of the studied aluminum alloy.

Exploration of the static softening behavior and dislocation density evolution of TA15 titanium alloy during double-pass hot compression deformation

The static softening behavior and dislocation density evolution of TA15 titanium alloy during double-pass hot deformation were investigated under the deformation temperatures (910 °C–970 °C) and holding time (0s–2200s). The microstructure evolution of primary α phase was observed by optical micrograph (OM), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The evolution of dislocation density was investigated via X-ray diffraction (XRD) and W–H model. The results showed that the globularization behavior of primary α phase is emerged during the inter-pass of double-pass hot deformation. The globularization process is promoted by the extension of the inter-pass holding time and the increase of the deformation temperature. The groove of primary α phase is preferentially generated when the inter-pass holding time within 1000s. Subsequently, that groove of primary α phase is separated and then those α phases are dissolved when the holding time over 1000s, leading to the fraction of α phase reduction. Compared with deformation temperature of 910 °C and 940 °C, all of the primary α phases are completely separated after 1000s of the inter-pass holding time under 970 °C. Additionally, through the analysis of various crystal surface of α phase, the dislocation density in the inter-pass gradually decreases with the holding time and deformation temperature increasing.

Metadynamic recrystallization in A5083 aluminum alloy with homogenized and as-extruded initial microstructures

The static softening behavior of A5083 aluminum alloy with different initial microstructures was studied using various characterization techniques. To calculate the recrystallized fraction, in the compression test, a new method based on the offset yield stress method and the Hall–Petch relationship was proposed, which is more accurate than the conventional methods and applicable even when the initial microstructure is complex. And in the stress relaxation test, a method proposed in our previous study was applied, indicating that this method could be employed in materials with incomplete recrystallization or a complex initial microstructure. Although the as-extruded initial microstructure led to a more developed metadynamic recrystallization, the recrystallization became heterogeneous, compared with the homogenized initial microstructure. When the anvil was not unloaded during the interpass time, the resulting external stress accelerated static recovery but impeded metadynamic recrystallization. New insights were discussed that challenge the conventional view in metadynamic recrystallization. First, metadynamic recrystallization always required an incubation time, unless the experimental conditions were sufficiently favorable for recrystallization. Second, when recovery was active, not only the dynamic recrystallized grains but also the recovery-enhanced subgrains were the potential nuclei for the subsequent metadynamic recrystallization.

Effect of Plastic Anisotropy on the Kinetics of Static Softening in AA2024–T3 Aluminum Alloy

Effect of Plastic Anisotropy on the Kinetics of Static Softening in AA2024–T3 Aluminum Alloy

Multi-scale modeling and simulation for multi-pass processing of Ta-2.5 W alloy

Multi-pass deformation and annealing process is commonly utilized to prepare billets with homogeneous fine microstructure. Characterizing the deformation and microstructural evolution is not easy due to the complex process. This study proposed a multi-scale modeling and simulation method for the multi-pass processing and it was validated by experiments. Firstly, an integrate multi-scale model involving strain hardening and static softening equations was developed based on multi-pass cold deformation and annealing tests of Ta-2.5 W alloy. Of note, the model captures both deformation behavior and static softening and the model equations of each pass were integrated into a unified manner. Then, multi-scale simulation including finite element (FE) and cellular automatic (CA) methods was coupled with the developed model for static recrystallization (SRX) analysis. FE analysis was firstly performed to predict the distributions of strain, dislocation density as well as SRX. Simultaneously, the predicted data was used for generation of the initial conditions for mesoscopic model. Finally, a CA method was applied for further analysis on recrystallization nucleation and grain growth. The modeling and simulation method is examined by multi-pass cold deformation and annealing test and a reasonable consistency is obtained between experimental and predicted results.

Static Softening Behaviors of 7mo Super-Austenitic Stainless Steel with Sigma Pre-Precipitates During Double-Stage Deformation

Static Softening Behaviors of 7mo Super-Austenitic Stainless Steel with Sigma Pre-Precipitates During Double-Stage Deformation

Metadynamic recrystallization behavior of 5083 aluminum alloy under double-pass compression and stress relaxation tests

The static softening behavior of A5083 aluminum alloy was studied through two thermomechanical tests, namely double-pass compression test and stress relaxation test. Metadynamic recrystallization and the further conventional static recrystallization cannot achieve complete recrystallization, which means that in the stress relaxation test, the traditional method used to determine the recrystallized fraction is no longer applicable. Hence, a new method is proposed based on the sigmoidal relationship between the relaxation stress and the recrystallized fraction. The order of the effects of the forming parameters on the metadynamic recrystallization, from strong to weak, is as follows: strain rate, temperature, and first-pass strain. External stress plays a significant role in retarding metadynamic recrystallization, thereby amplifying the effects of the forming parameters on the metadynamic recrystallization. Interestingly, metadynamic recrystallization, which is considered to occur immediately, has an incubation time of approximately 1 s in the studied aluminum alloy. The evolution of the microhardness and microstructure studied through microhardness tests and electron backscatter diffraction confirmed these conclusions. In addition, the phenomena of stress hardening and yield stress drop, caused by strain aging, were observed only under special experimental conditions.

Dynamic and static softening mechanisms of commercial-purity Zr during double-stage hot compressive deformation

The dynamic and static softening behavior of R60702 commercial-purity Zr was studied through single-stage and double-stage uniaxial hot deformation tests in the range of 600oC–800 °C. The microstructural evolution to uniaxial compression loading for the different conditions was investigated using a combination of optical microscopy and electron back-scattering diffraction (EBSD). The results show that the static softening fraction increases with an increase in temperature as well as holding time. At 600 °C, the peak stress of the second stage was greater than that of the single-stage, and the sample did not undergo static softening. At 800 °C, the peak stress of the second stage was less than that of the single-stage, and the sample underwent rapid static softening during holding time. As the holding time increased at 700 °C, the peak stress of the second stage of the sample decreased successively. At 700 °C and 1s holding time, the sample did not undergo static softening, and dynamic recovery and recrystallization occurred in the subsequent second stage. At 700 °C and 60s holding time, the sample underwent static recovery and recrystallization; larger grain growth and deformation appeared in the subsequent second stage. This change in the microstructural morphology with the flow stress curve is discussed in the subsequent sections.

Research on static recrystallization behaviour of pure molybdenum rods during forging

In this work, thermal compression experiment of pure molybdenum rods with sintered density of 9.7–9.8 g cm−3 were performed on the Gleeble-1500D thermal simulator. The double-pass vacuum high-temperature compression test was carried out in a deformation temperature of 1200 °C–1350 °C, a strain rate of 3 s−1 and an interval time of 10–500 s. The stress-strain curve was obtained and analysed. The static softening rate was calculated by the 2% compensation method of yield stress, and the curve of that was fitted. The results show that the softening rate is proportional to the interval time and the deformation temperature. The static recrystallization activation energy and critical temperature of pure molybdenum rod is 583.79 kJ mol−1 and 1365 °C, respectively. Besides, the static recrystallization kinetics model and grain growth model of pure molybdenum rod was established and microstructure evolution of molybdenum was analysed during the deformation process.