Isothermal Uniaxial Tensile Tests Research Articles

Assessment of aluminium alloys’ formability by elevated temperature testing

Abstract This work presents an investigation of the warm formability of AA7075-T6 aluminium alloy sheets using both experimental and numerical methods. Forming limit diagrams (FLDs) can be created by plotting the failure criteria on a graph with two axes representing major and minor strains. Sheets with a grid can be used to quantify both large and minor strains. General techniques available for the grid involve regular patterns of circles in this experiment. Experiments were conducted at various temperature levels, including room temperature, 100, 120, 140, 160, and 180 °C, as well as different strain rates. The investigation has revealed a significant impact of temperature on formability, while the effect of forming speed is quite insignificant across the whole range studied. At an ageing temperature of 140 °C, the distribution of precipitates is evenly balanced, resulting in improved mechanical strength. To offer accurate numerical models for warm-forming processes, a proposed constitutive model is introduced for aluminium alloy sheets. The present model incorporates plastic anisotropy and the effects of temperature and strain rate dependence. An investigation was conducted on the stress-strain behaviour of thin sheets composed of AA7075-T6 aluminium alloy under elevated temperatures and different strain rates. The objective was to ascertain the appropriate fitting parameters. It was associated with ABAQUS finite element modelling. The recommended material model was utilized to build finite element models, which were subsequently compared to the experimental data. The findings obtained from simulated isothermal uniaxial tensile tests were compared to the empirical measurements. The models used in the tensile test accurately predicted the material’s mechanical response. The predicted failure location, punch forces, and strain distributions for all three forming processes were found to be in close agreement with the experimental results. During deep draw operation, the warm forming limit curves were utilized to precisely predict both the pierce depth at which failure occurs and the precise location where the failure initiates.

Predicting Flow Stress Behavior of an AA7075 Alloy Using Machine Learning Methods

The present work focuses on the prediction of the hot deformation behavior of thermo-mechanically processed precipitation hardenable aluminum alloy AA7075. The data considered focus on a novel hot forming process at different tool temperatures ranging from 24∘C to 350∘C to set different cooling rates after solution heat-treatment. Isothermal uniaxial tensile tests in the temperature range of 200∘C to 400∘C and at strain rates ranging from 0.001 s−1 to 0.1 s−1 were carried out on four different material conditions. The present paper mainly focuses on a comparative study of modeling techniques based on Machine Learning (ML) and the Zerilli–Armstrong model (Z–A) as reference. Related work focuses on predicting single data points of the curves that the model was trained on. Due to the way data were split with respect to training and testing data, it is possible to predict entire stress–strain curves. The model allows to decrease the number of required laboratory experiments, eventually saving costs and time in future experiments. While all investigated ML methods showed a higher performance than the Z–A model, the extreme Gradient Boosting model (XGB) showed superior results, i.e., the highest error reduction of 91% with respect to the Mean Squared Error.

High-Temperature Behavior and Constitutive Modeling of MS1180 High-Strength Steel

The flow behavior analysis of MS1180 steel at high-temperature was carried out by the stress–strain data which were obtained through isothermal uniaxial tensile tests with Gleeble-3800 thermal simulator. The temperature range is 600 °C~900 °C and the strain rate is varied from 0.005 s−1 to 0.02 s−1. A modified Johnson-Cook (JC) constitutive model of MS1180 steel considering the mutual effect of deformation parameters was proposed applying multiple nonlinear regression method. The calculated stress values obtained by the modified constitutive equations and the experimental ones were compared with each other. The average relative error is less than 5% and correlation coefficient is greater than 0.95. The results verified that the modified constitutive equations has good prediction accuracy for the flow stress of MS1180.

Rheological behavior and dynamic softening mechanism of AA7075 sheet under isothermal tensile deformation

Rheological behavior and dynamic softening mechanism of high-strength AA7075 sheet were investigated by performing isothermal uniaxial tensile tests at deformation temperatures of 573−723 K and strain rates of 0.01–10 s−1. On the basis of the obtained flow stress, response surface models for the ultimate tensile stress (UTS) and fracture strain were developed as a function of temperature and strain rate, respectively. Error analysis certified the reliability of the models. Besides, a constitutive model coupling with dislocation evolution was built to predict the deformation behavior of the alloy, and the dislocation evolution law was revealed based on the established constitutive model. In addition, the dynamic softening mechanism of the alloy during hot deformation was exposed according to the Zener-Hollomon parameter map, microstructural observation and apparent activation energy. Owing to precipitates pinning effect on the motion of dislocation and migration of subgrain boundaries, the dominant softening mechanism is concluded to be dynamic recovery (DRV), relating to cross-slip of screw dislocation.

Performance of Thermo-Mechanically Processed AA7075 Alloy at Elevated Temperatures—From Microstructure to Mechanical Properties

This study focuses on the high temperature characteristics of thermo-mechanically processed AA7075 alloy. An integrated die forming process that combines solution heat treatment and hot forming at different temperatures was employed to process the AA7075 alloy. Low die temperature resulted in the fabrication of parts with higher strength, similar to that of T6 condition, while forming this alloy in the hot die led to the fabrication of more ductile parts. Isothermal uniaxial tensile tests in the temperature range of 200–400 °C and at strain rates ranging from 0.001–0.1 s−1 were performed on the as-received material, and on both the solution heat-treated and the thermo-mechanically processed parts to explore the impacts of deformation parameters on the mechanical behavior at elevated temperatures. Flow stress levels of AA7075 alloy in all processing states were shown to be strongly temperature- and strain-rate dependent. Results imply that thermo-mechanical parameters are very influential on the mechanical properties of the AA7075 alloy formed at elevated temperatures. Microstructural studies were conducted by utilizing optical microscopy and a scanning electron microscope to reveal the dominant softening mechanism and the level of grain growth at elevated temperatures.

Modelling approach for predicting the superplastic deformation behaviour of titanium alloys with strain hardening/softening characterizations

This paper introduces an approach for modelling the flow behaviour of different titanium alloys (VT6, OT4-1 and VT14 alloys by Russian specifications) in superplastic deformation temperature and strain rate ranges. The initial microstructure parameters () before starting the deformation test were included in the constructed model for each alloy. The investigated alloys have different initial microstructures and flow behaviour characteristics. The isothermal uniaxial tensile deformation tests were performed at the superplastic deformation temperature and strain rate ranges of each alloy. The VT6, OT4-1 alloys were characterized by strain hardening effect during the deformation test, while VT14 alloy was characterized by strain softening effect. A comparison study between the experimental and modelled data was performed. The general equation of the constructed models was affected by the flow behaviour of the investigated alloys. The comparison results proved the good capability of the constructed models to predict the flow behaviour of the studied alloys. The correlation coefficient R was 0.98, 0.95 and 0.97 for VT6, OT4-1 and VT14, respectively. The predictability of the constructed model was assessed by the cross-validation technique, which ascertained the quality of the constructed model.

High-temperature deformation behavior and constitutive relationship of press-hardening steel 38MnB5

With the rapid development of global economy, problems in energy production and environmental protection are becoming severe, and the automotive industry is under increasing pressure to reduce the weight of vehicles and improve crash performance. Due to the demand for reduced vehicle weight as well as improved safety and crashworthiness, hot-stamped components from ultra-high strength steels have been utilized for automobile manufacturing. Currently, the most widely used hot-stamped steel plate is 22MnB5. Its tensile strength is 1500 MPa and yield strength is 1200 MPa. In contrast, as the demands for steel strength have increased, the demand for high strength grades of steel has been quickly put on the production agenda. In recent years, a novel hot-stamped steel, 38MnB5 has been developed, with a tensile strength exceeding 2000 MPa. The high temperature deformation behavior of 38MnB5 steel was investigated by the Gleeble-3500 thermal-mechanical simulator. The isothermal uniaxial tensile tests of the steel were performed within deformation temperature range of 650-950℃ under strain rates of 0. 01, 0. 1, 1, and 10 s-1, and the typical true stress-strain curves of 38MnB5 at relative conditions were analyzed. The experimental results show that the flow stress rises with decreasing deformation temperature under the same strain rate, and with an increasing strain rate. When the strain rate gradually increased, dynamic recovery and dynamical recrystallization exhibited an apparent effect on the hot deformation process, while the inconspicuous impact receded with rising temperature. In consideration of the multiple influences on deformation temperature, strain rate and strain, a phenomenological, constitutive relationship was developed to depict the hot deformation process of 38MnB5. In the established equation, the material constants dependent on the deformation temperature, strain rate, and strain were obtained using regression analysis of the experimental data for flow stress, strain, strain rate, etc. The comparison between the calculated data and the experimental data show that the calculated data derived from the constitutive models are found to be in satisfactory agreement with the experimental results.

Thermal forming limit diagram (TFLD) of AA7075 aluminum alloy based on a modified continuum damage model: Experimental and theoretical investigations

The formability of high strength Al-Zn-Mg-Cu (7000 series) aluminum alloys can be significantly improved at elevated temperatures, which has been paid more attention in recent decades. The formability of high strength aluminum alloys at elevated temperatures is essentially governed by thermal-damage evolution. In this paper, the main purpose is to propose a modified continuum damage model to describe the damage evolution and predict the fracture behavior of AA7075 at elevated temperatures (300–400 °C). Firstly, the thermal-mechanical behavior and forming limit of AA7075 alloy sheet were experimentally investigated using a series of isothermal uniaxial tensile tests and Nakajima tests at different temperatures and strain rates. A set of uniaxial continuum damage constitutive equations (CDCEs) coupling continuum damage mechanics (CDM) with unified viscoplastic theory was proposed to describe the uniaxial tensile behavior of AA7075. Subsequently, the uniaxial equations were extended into a set of multi-axial CDCEs by introducing a multi-axial damage correction formula to predict the TFLD of AA7075. Besides, the forward Euler method was employed to integrate the proposed CDCEs, and the corresponding material constants of CDCEs were further calibrated by the non-dominated sorting genetic algorithmⅡ(NSGA-Ⅱ). The results illustrate that the thermal flow behavior and the TFLD of AA7075 alloy can be predicted successfully by the proposed damage model. Detailed discussions about the effects of corresponding parameters on the computed TFLD indicate the established multi-axial CDCEs are flexible and beneficial to the application potential in numerical simulation of hot sheet metal forming.

Influence Of Temperatures And Strain Rates On Tensile Deformation Behaviour Of DP 590 Steel

Abstract In this study, tensile deformation behaviour and material properties has been investigated at different temperatures and strain rates. The isothermal uniaxial tensile tests were conducted from room temperature to 700°C at an interval of 100°C at different quasi-static strain rates (0.01, 0.001, 0.0001 s-1). The flow stress is significantly influenced by temperature and strain rate change. Various important material properties namely: yield strength (σy), ultimate tensile strength (σut), % elongation and strain hardening exponent (n) are determined at different temperatures and strain rates. In order to understand the fracture behaviour of a DP steel, microstructural characteristics were studied using Scanning Electron Microscope (SEM). Predominately ductile failure has been reported at higher temperatures. The individual phases (ferrite + martensite) play an important role in defining the ultimate failure mode in tensile testing.

Thermal deformation behavior and processing maps of 7075 aluminum alloy sheet based on isothermal uniaxial tensile tests

Process parameters and their effects on thermal-mechanical properties of aluminum alloys are vitally important for the successful manufacture of aluminum alloy panel components under hot forming condition. In this study, firstly, thermal-mechanical properties of 7075 aluminum alloy sheets under hot forming conditions were comprehensively investigated using a series of isothermal uniaxial tensile tests at different temperatures and strain rates on a Gleeble 3500 thermal mechanical simulator. Based on the experimental results, the temperature rise ΔT generated from plastic deformation was further calculated considering the fraction of plastic work converted into heat, β, and the fraction of deformation heat appearing as a temperature rise, δ. ΔT increases with an increase in strain rate and a decrease in deformation temperature, with a maximum temperature rise ΔTmax reaching 33.5 K. Furthermore, taking ΔT into consideration, the relationship model between the work hardening rate, temperature and strain rate was constructed according to the Zener-Hollmon parameter. The variation of the work hardening rate with strain rate and temperature can be interpreted using this model. Finally, to determine the optimal processing parameters, the processing maps of 7075 aluminum alloy were established subsequently under the experimental conditions. The optimal processing parameters for 7075 aluminum alloy are located within the windows: (1) temperature, 573–680 K and strain rate, 0.368–8.1 s−1; (2) temperature, 695–723 K and strain rate, 0.05–1 s−1. According to the microstructure observations, it can be concluded that the main softening mechanism of “safe” domains is dynamic recovery (DRV) and the continuous dynamic recrystallization (DRX) can be effectively retarded due to the presence of precipitated particles. The work performed in this research, for the first time, provides quantitative evaluations of process parameters on the hot deformation of aluminum alloy sheet forming using processing maps considering corresponding microstructural evolutions, which enables to provide useful guides for process designers of sheet metal forming.

High temperature characteristics of Al2024/SiC metal matrix composite fabricated by friction stir processing

In the present study, friction stir processing (FSP) is used for the synthesis of an aluminum metal matrix composite (MMC) reinforced by SiC particles. MMC specimens with reinforced microstructures exhibited significant improvement in hardness (near 50%). Isothermal uniaxial tensile tests were employed for the as-received, friction stir processed and composite microstructures at ambient and high temperatures under strain rates ranging from 10−2 to 10−4 s−1 to investigate the effect of deformation rate on the mechanical behavior. At ambient temperature, notable improvement of the yield strength was observed reaching about 240% of the as-received samples while the ductility was reduced near to 4%. Elevated temperature flow curves were perceptibly sensitive to strain rate, especially for FSPed and MMC samples. Fracture surface observations hinted at the distribution of second phase particles along with possible damage mechanisms.

A Modified Constitutive Model for Tensile Flow Behaviors of BR1500HS Ultra-High-Strength Steel at Medium and Low Temperature Regions

AbstractConstitutive model of materials is one of the most requisite mathematical model in the finite element analysis, which describes the relationships of flow behaviors with strain, strain rate and temperature. In order to construct such constitutive relationships of ultra-high-strength BR1500HS steel at medium and low temperature regions, the true stress-strain data over a wide temperature range of 293–873 K and strain rate range of 0.01–10 s−1 were collected from a series of isothermal uniaxial tensile tests. The experimental results show that stress-strain relationships are highly non-linear and susceptible to three parameters involving temperature, strain and strain rate. By considering the impacts of strain rate and temperature on strain hardening, a modified constitutive model based on Johnson-Cook model was proposed to characterize flow behaviors in medium and low temperature ranges. The predictability of the improved model was also evaluated by the relative error ($W(\%)$), correlation coefficient (R) and average absolute relative error (AARE). The R-value and AARE-value for modified constitutive model at medium and low temperature regions are 0.9915 & 1.56 % and 0.9570 & 5.39 %, respectively, which indicates that the modified constitutive model can precisely estimate the flow behaviors for BR1500HS steel in the medium and low temperature regions.

Influence of thermal deformation conditions on the microstructure and mechanical properties of boron steel

The influence of thermal deformation conditions on the microstructure and mechanical properties of B1500HS boron steel was investigated based on a series of isothermal uniaxial tensile tests. The relationship model between work hardening rate and temperature and strain rate was established on the basis of Johnson Cook constitutive model. Moreover, the equation of the temperature rise caused by plastic deformation was modified by introducing the conversion efficiency of deformation work to heat. Next, the effects of deformation conditions (temperature and strain rate) on the volume fraction of martensite and ferrite were studied by metallographic observation. It was found that a higher strain rate brought out the martensite lath with a shorter length and thus a better ductility, and the ferrite transformation was restrained at the higher strain rate. Finally, the hardness and dislocation densities of the boron steel were detected by Vickers micro-hardness and X-ray diffraction (XRD) tests, respectively. The dislocation densities of the boron steel were quantitatively characterized by analyzing the XRD peak profiles according to the Williamson–Hall (WH) method. The results show that deformation temperature and strain rate have a similar influence on the dislocation density and micro-hardness, and hence the relationship of dislocation density and micro-hardness was deduced.

Thermomechanical response of a TWIP steel during monotonic and non-monotonic uniaxial loading

The tensile properties of a Fe-18%Mn-0.6%C-1.5%Al Twinning-Induced Plasticity (TWIP) steel were investigated at different strain rates in three loading modes, i.e. uniaxial monotonic loading, stress relaxation and loading-unloading-reloading. Infrared thermography was used to investigate the effect of the dynamic strain aging, the strain rate and the temperature on the flow stress. In addition to the standard, i.e., non-isothermal tensile tests, isothermal uniaxial tensile tests were performed at 25°C, 45°C and 65°C. While the non-monotonic loading modes resulted in an increase of the total elongation at a low strain rate of 10−3s−1, no increase was observed for strain rates higher than 6×10−3s−1. The temperature gradients observed during non-isothermal tests were reduced when non-monotonic loading conditions were used. Temperature changes were found to influence the hardening behavior, and consequently the ductility, of the TWIP steel. Deformation twinning also had a significant influence on the results as its kinetics in TWIP steel are determined by the temperature dependence of the stacking fault energy.

On the mechanical behavior of cold deformed aluminum 7075 alloy at elevated temperatures

In the present study, elevated temperature deformation behavior and microstructural evolution of 7075 aluminum alloy at annealed and cold rolled conditions were examined. Isothermal uniaxial tensile tests at a temperature range of 200–350°C and at a strain rate range of 0.001–0.1s−1 were conducted to investigate the effects of deformation parameters on the mechanical behavior. High temperature flow was noticeably strain rate sensitive especially for the rolled condition. Cold work was shown to have a remarkable influence on increasing the peak stress up to 250°C. At and above this temperature the rolled microstructure enabled higher ductility reaching over 50% accompanied by a stress plateau. The ductility drop at 350°C at the slowest deformation rate was attributed to impurity rich regions with possible formation of secondary phases due to dynamic precipitation.

Constitutive Modeling of High-Temperature Flow Behavior of an Nb Micro-alloyed Hot Stamping Steel

The thermal deformation behavior and constitutive models of an Nb micro-alloyed 22MnB5 steel were investigated by conducting isothermal uniaxial tensile tests at the temperature range of 873-1223 K with strain rates of 0.1-10 s−1. The results indicated that the investigated steel showed typical work hardening and dynamic recovery behavior during hot deformation, and the flow stress decreased with a decrease in strain rate and/or an increase in temperature. On the basis of the experimental data, the modified Johnson-Cook (modified JC), modified Norton-Hoff (modified NH), and Arrhenius-type (AT) constitutive models were established for the subject steel. However, the flow stress values predicted by these three models revealed some remarkable deviations from the experimental values for certain experimental conditions. Therefore, a new combined modified Norton-Hoff and Arrhenius-type constitutive model (combined modified NH-AT model), which accurately reflected both the work hardening and dynamic recovery behavior of the subject steel, was developed by introducing the modified parameter ke. Furthermore, the accuracy of these constitutive models was assessed by the correlation coefficient, the average absolute relative error, and the root mean square error, which indicated that the flow stress values computed by the combined modified NH-AT model were highly consistent with the experimental values (R = 0.998, AARE = 1.63%, RMSE = 3.85 MPa). The result confirmed that the combined modified NH-AT model was suitable for the studied Nb micro-alloyed hot stamping steel. Additionally, the practicability of the new model was also verified using finite element simulations in ANSYS/LS-DYNA, and the results confirmed that the new model was practical and highly accurate.

A ductile fracture criterion with Zener-Hollomon parameter of pure molybdenum sheet in thermal forming

Formability of pure molybdenum in thermal forming process has been greatly improved, but it is still hard to avoid the generation of rupture and other quality defects. In this paper, a ductile fracture criterion of pure molybdenum sheet in thermal forming was established by considering the plastic deformation capacity of material and stress states, which can be used to describe fracture behaviour and critical rupture prediction of pure molybdenum sheet during hot forming process. Based on the isothermal uniaxial tensile tests which performed at 993 to 1143 K with strain rate range from 0.0005 to 0.2 s−1, the material parameters are calculated by the combination method of experiment with FEsimulation. Based on the observation, new fracture criteria can be expressed as a function of Zener-Hollomon parameter. The critical fracture value that calculated by Oyane-Sato criterion increases with increasing temperature and decreasing strain rate. The ductile fracture criterion with Zener-Hollomon parameter of pure molybdenum in thermal forming is proposed.

Flow Stress Prediction of Ti-6Al-4V Alloy at Elevated Temperature Using Artificial Neural Network

Flow stress during hot deformation depends mainly on the strain, strain rate and temperature, and shows a complex nonlinear relationship with them. In this work, experimental flow stress have been predicted for Ti-6Al-4V alloy using isothermal uniaxial tensile tests ranging from 323K to 673K at an interval of 50K and strain rates 10-5, 10-4, 10-3 and 10-2 s-1. Based on the input variables strain, strain rate and temperature, a back propagation neural network model has been developed to predict the flow stress as output. The whole experimental data is randomly divided in two parts: 90% data as training data and 10% data as testing data. The artificial neural network enhanced with differential evolution algorithm is successfully trained based on the training data and employed to predict the flow stress values for the testing data, which were compared with the experimental values. Correlation coefficient for training and testing data is found to be 0.9997 and 0.9985 respectively. Based on the correlation coefficient, it indicates that predicted flow stress by using artificial neural network is in good agreement with experimental results.

Constitutive flow stress formulation, model validation and FE cutting simulation for AA7075-T6 aluminum alloy

A consistent and reasonable description of the constitutive behavior of material under the coupled effect of strain, strain rate and temperature on the flow stress of the material is highly essential in current industrial practice to design and optimize the process parameters in manufacturing operations. In order to formulate a suitable constitutive model to predict the elevated-temperature deformation behavior in high strength aluminum alloy, AA7075-T6, isothermal uniaxial tensile tests over a practical range of deformation temperatures (300–623K) and strain rates (10−4–10−1s−1) were conducted. Two constitutive models, modified-Johnson Cook (m-JC) and modified-Zerilli–Armstrong (m-ZA) models are formulated considering the combined effects of strain, strain rate and temperature on flow stress. The prediction capability of these models is assessed in terms of statistical parameters, correlation coefficient and average absolute error between experimental and predicted flow stress data. In addition, finite element (FE) simulations have been carried out in order to validate the formulated model during orthogonal cutting processes. It was observed that both the models show a very high degree of linear dependence of fit as the value above 0.98. However, the m-ZA model can offer an accurate and precise estimate of the flow behavior for studied workmaterial over m-JC model. Further, the performance of developed model in FE cutting simulations is the most obvious for all moderate machining conditions and can provide relatively good prediction results for the AA7075-T6 machining process.

Comparative study of constitutive modeling for Ti–6Al–4V alloy at low strain rates and elevated temperatures

An accurate prediction of flow behavior of metals considering the combined effects of strain, strain rate and temperature is essential for understanding flow response of metals. Isothermal uniaxial tensile tests have been performed from 323K to 673K at an interval of 50K and strain rates 10−5, 10−4, 10−3 and 10−2s−1. In this study prediction of flow behavior of Ti–6Al–4V alloy sheet is done using four constitutive models namely; Johnson–Cook (JC), Fields–Backofen (FB), Khan–Huang–Liang (KHL) and Mechanical Threshold Stress (MTS). The predictions of these constitutive models are compared using statistical measures like correlation coefficient (R), average absolute error (Δ) and its standard deviation (δ). Analysis of statistical measures revealed that FB model has more deviation from the experimental values. Whereas, the predictions of all other models (JC, KHL, and MTS) are very close to the experimental results. JC and KHL are better models for predicting the flow stress. However, considering the fact that MTS model is a physical based model, MTS model is preferred over other models.