Hot Working Conditions Research Articles

Simple constitutive analysis of AA 7075-T6 aluminium alloy deformed at low deformation temperatures

The tensile stress–strain behaviour of AA 7075-T6 aluminium alloy deformed at temperatures in the range of 123–248 K, at strain rates between 0·0023 and 0·21 s−1 has been analysed in order to develop a rational constitutive description of the material under such deformation conditions. The constitutive formulation has been derived on the basis of the mechanical threshold stress model developed at Los Alamos National Laboratory. The constitutive equation thus proposed is able to separate the contribution of the different components that give rise to the mechanical strength of the alloy. It has been determined that the flow stress of the material arises mainly from the contribution of three different components: athermal barriers to dislocation motion, solid solution and precipitation hardening and work hardening (dislocation–dislocation interaction). The flow stress in each strain increment during the course of plastic deformation is computed recursively from its previous value, before the updating of the deformation temperature and strain rate values. Such a computation is carried out by the numerical integration of the phenomenological work hardening law expressed in differential form. The constitutive description thus proposed provides an accurate prediction of the experimental values of both the flow stress and work hardening rate of the material. Moreover, its formulation in differential form provides a reasonable description of the stress changes that could take place when arbitrary temperatures and strain rate paths are imposed to the material during plastic deformation. It is shown that the formulation is general enough for describing also the constitutive behaviour and stress changes of a different aluminium alloy deformed under hot working conditions, where the material exhibits positive work hardening before achieving saturation at large strains.

Determination of flow stress under hot deformation conditions

In order to predict the flow stress curves up to peak under the hot working conditions two crucially modified models are presented. The modification is done by considering the effects of both temperature and strain rate. Besides, the ability of prediction at very low strains is added to them. Furthermore a new mathematical model has been developed to estimate the stress–strain curves beyond the peak via a secant hyperbolic function. The predicted results are found to be in accord with empirical values.

Hot-Working Behavior of 7075 Al/15% SiCp Composites

The hot-working behavior of 7075 aluminium alloy and 7075 Al alloy reinforced with 15% of SiC particles has been studied by using the processing maps. The material under investigation was deformed by compression in temperature and strain rate ranges of 300–500°C and 0.001–1.0 s−1, respectively. Domains for optimum hot working conditions were obtained using the processing maps and the instability zones were identified. Shear Band formation was noticed at higher strain rates and lower temperatures and particle fractures were noticed at higher strain rates and higher temperature thereby defining the domain of flow instability. The dynamic recrystallization (DRX) and instability zones were validated through micrographs. 7075 Al/15% SiCp composites is achieved better hot workability due to increase in strength and stiffness of the matrix by the addition of reinforcement.

Hot-Rolling of Advanced High-Manganese C-Mn-Si-Al Steels

The high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on a recrystallization behaviour was investigated. Flow stresses during the multistage compression test were measured using the Gleeble 3800 thermo-mechanical simulator. To describe the hot-working behaviour, the steel was compressed to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light microscopy. The flow stresses are much higher in comparison with austenitic Cr-Ni and Cr-Mn steels and slightly higher compared to Fe-(15-25) Mn alloys. Making use of dynamic and metadynamic recrystallization, it is possible to refine the microstructure and to decrease the flow stress during the last deformation at 850°C. Applying the true strains of 0.23 and 0.19 requires the microstructure refinement by static recrystallization. The obtained microstructure – hot-working relationships can be useful in the determination of powerful parameters of hot-rolling and to design a rolling schedule for high-manganese steel sheets with fine-grained austenitic structures.

Mechanical and microstructural analysis of 2205 duplex stainless steel under hot working condition

Hot deformation characteristics of 2205 duplex stainless steel were analyzed by performing hot compression tests at a temperature range of 950–1200 °C and a strain rate of 0.001–1 s−1. Flow stress was modeled by the constitutive equation of hyperbolic sine function. The constants of n, A, α, and the apparent activation energy were determined at different strains. They were then fitted by polynomial equations. Using the hyperbolic sine function and the relations derived between constants and strain flow curves were successfully modeled. Microstructural evolutions were characterized using optical microscopy and electron back scattered diffraction techniques. The results showed that dynamic recovery in ferrite is accelerated at higher temperatures followed by transformation to continuous dynamic recrystallization. Dynamic recrystallization in austenite was postponed by the accommodation of strain in ferrite and very few internal boundaries in austenite. At high strain rates, dynamic recovery in ferrite and dynamic recrystallization in austenite are very slow. Consequently, the total recrystallized fraction decreases. At low temperatures this situation may cause flow instabilities. At low strain rates, softening processes dominate in austenite and ferrite whereas at intermediate strain rates, the formation of substructures is observed in both phases.

Microstructure evolution of friction welded dissimilar joint during subsequent hot work

The present paper reports the influence of hot working conditions on the microstructure of Ti2AlNb/TC11 dissimilar joint. Linear friction welding technique was used to fabricate the joints. The microhardness and tensile properties of the joints have been tested. It was found that the fine structure of linear friction welds underwent abnormal grain growth and abnormal grain boundary phase growth in the post-weld solution heat treated condition. This phenomenon significantly deteriorated the ductility of the joint. After appropriate hot work, abnormal big grains/phases disappeared, and the joint exhibited good tensile properties due to its fine structures.

Processing Maps for Hot Working Behavior of a PM TiAl Alloy

The hot deformation behavior of a Ti-47Al-2Cr-2Nb-0.2W-0.15B (at.%) titanium aluminide alloy fabricated by pre-alloyed powder metallurgy has been investigated by using the hot compression tests in the temperature range from 950°C to 1300°C and at the strain rates between 10−3 s−1 and 10 s−1. The processing maps have been established to evaluate the optimum hot processing conditions and reveal the instability regions. It is found that the flow stress of the investigated alloy is a strong function of the temperature and the strain rate. The investigated alloy has the optimum hot-working condition at 950°C and 10−3 s−1, since the material undergoes dynamic recrystallization to produce a fine-grained microstructure. At 1250°C and 10−3 s−1, the alloy exhibits superplastic deformation. At 1300°C and 10−1 s−1 the cyclic dynamic recrystallization with high temperature grain coarsening takes place. The material undergoes flow instabilities at lower temperatures and higher strain rates, as predicted by the instability criterion. The processing maps demonstrate that the strain significantly affected the instability regions. The manifestations of the instabilities have been observed in the form of microvoids, wedge cracks, and surface fractures.

Chronic kidney disease among agricultural workers in El Salvador

ObjectivesChronic kidney disease (CKD) unrelated to conventional risk factors occurs in Central America, mostly in younger, male agricultural workers. We explored residence and occupation at different altitudes in El Salvador...

Research on Flow Stress During Hot Deformation Process and Processing Map for 316LN Austenitic Stainless Steel

In this study, the hot deformation behavior of austenitic stainless steel was investigated using Gleeble-3500 thermomechanical simulator at deformation temperatures in the range of 900-1200 °C and strain rates in the range of 0.001-10 s−1. The effects of initial austenitic grain size and deformation conditions on hot deformation behavior of 316LN were analyzed through true stress-strain curves under different deformation conditions. Both the constitutive equation and processing map for 316LN were obtained. The results show that, with the increase of the deformation temperature and the decrease of the strain rate, the peak stress decreases, and the initial austenitic grain size has a little influence on the peak stress. The relative error between the peak stress values calculated using the constitutive equation and the values measured is less than 10%. Using the processing map, the best hot-working condition for 316LN in the range of experimental deformation parameters appears when T = 1200 °C and \(\dot{\upvarepsilon } = 0.001\,{\text{s}}^{-1}.\)

Hot deformation behavior and microstructure evolution of spray formed GH738 superalloy

In order to evaluate the deformation characteristics of spray formed superalloy GH738, the hot compression test was conducted on Gleeble-3500 thermal mechanical simulator at a temperature range of 960–1120 °C, and a strain rates range of 0.1, 1.0 s−1 with engineering strain 50%. The flow stress data at various hot working conditions was obtained, and the influence of deformation temperature and strain rate on the microstructure evolution of spray formed GH738 was analyzed. The result shows that, the flow stress improves with the increasing of strain rate and the decreasing of temperature, the yield drop is observed on true stress-strain curves at strain rate of 1.0 s−1. The temperature and strain rate have a strongly effect on dynamic recrystallization. Higher temperature could efficiently promote the process of dynamic recrystallization, as the increment of temperature, the dynamic recrystallization grain size shows the tendency of growth. Higher strain rate is better for grain refinement.

Development of Processing Map for 7075 Al/20% SiCp Composite

The hot deformation behavior and microstructure evolution of 7075 Al/20% SiCp composite have been studied using the processing map. Compression tests were carried out in the temperature range of 300-500 °C and at the strain rate range of 0.001-1.0 s−1. The stable and unstable regions in the map were verified with the microstructural observations of the deformed compression specimens. The “stable” regions, i.e., dynamic recrystallization and “unstable” regions such as debonding of SiC particles, matrix crack, and adiabatic shear band formation were identified from the processing map and compared with the reported microstructural observations of the deformed compression specimens. The optimum hot working conditions for this composite were identified.

Hot working and crystallographic texture analysis of magnesium AZ alloys

The hot working behaviour of magnesium AZ (e.g. AZ31; Al: 3%, Zn: 1%) alloys and their associated crystallographic texture evolution is reviewed. Under hot working conditions, the stress–strain curves show flow softening at all the temperatures and strain rates indicating dynamic recrystallisation (DRX) is predominant. The mean size of the recrystallised grains in all the alloys decreases as the value of Zener–Hollomon parameter Z increases. The hot working range of the alloys dwell between 200 and 500°C and the strain rates between 10−3 and 5 s−1. The hot working of AZ series alloy shows discontinuous DRX as the main mechanism. Equal channel angular processing shows continuous DRX. The constitutive equation development shows a linear relationship between the stress and the Z parameter. The activation energy for the alloys ranges from 112 to 169 kJ mol−1 and Z values range from 10 to 10 s−1. Textural examinations show basal texture as the predominant orientation.

Construction of Processing Map for Biomedical Co-29Cr-6Mo-0.23C-0.14N Alloy by Using Compression Tests

The high-temperature deformation behavior of Co-29Cr-6Mo-0.23C-0.14N alloy was investigated by carrying out compression tests; the tests were carried out in the temperature range of 1000–1200°C and strain rates ranging from 0.01 to 30 s−1 with a height reduction of approximately 65%. The optimum hot-working conditions were determined from the processing map based on the dynamic materials model. Dynamic recrystallization was observed to occur over the entire temperature and strain rate range. However, uniformly sized grains were formed for strain rates higher than 1 s−1, which is considered to be the optimum hot-working condition. In addition, authors suggested that in extremely low stacking fault energy alloy, explosive and homogeneous formation of mechanical twinning should be considered to be stable state, and new processing map with more detailed description of deformation mechanism seems necessary in future.

Constitutive description of a low C-Mn steel deformed under hot-working conditions

The present investigation has been conducted in order to develop a constitutive description for a low C-Mn steel employed in structural applications, when this material is deformed under hot-working conditions. The set of equations that encompass the constitutive description of the steel are based on a simplified form of the Mechanical Threshold Stress Model. Accordingly, it has been shown that the flow stress of the material can be considered to arise from two different contributions: solid solution and precipitation hardening, which do not evolve in the course of plastic deformation and work-hardening, the only evolving component of the flow stress. The evolution of work-hardening has been introduced in the model by means of the work-hardening law proposed by Estrin and Mecking, in differential form. Also, the temperature and strain rate dependence of the contributions of solid solution and precipitation hardening to the flow stress, as well as that of the saturation stress in the work-hardening law, are described by means of the model earlier advanced by Kocks. The constitutive formulation encompasses seven material parameters which have been determined employing the experimental data corresponding to the strain interval where the material only exhibits positive work-hardening. It is able to predict accurately both work-hardening and work-softening transients when changes in deformation conditions occur. Furthermore, it can be easily implemented in any advanced computational tool employed for the analysis of hot-working processes of this material.

Investigation on hot deformation behavior of 00Cr23Ni4N duplex stainless steel under medium–high strain rates

The hot deformation behavior of 00Cr23Ni4N duplex stainless steel under medium–high strain rates (5–50 s − 1 ) has been analyzed using the Zener–Hollomon parameter and processing maps, which is based on compression tests made at temperatures ranging from 900 to 1150 °C. The results display the significant influence of high strain rate and high temperature on hot deformation behavior of 00Cr23Ni4N duplex stainless steel. A classical hyperbolic sine equation is applied to reveal the relations between the peak stress, strain rate and deformation temperature, in which the activation energy, Q and stress exponent, n are 263.4 kJ/mol and 2.6, respectively. The Zener–Hollomon parameters at low and high temperatures are calculated respectively to reflect the microstructural evolutions. Based on the processing map obtained, an ideal hot working condition for commercial processing is in the temperature range between 1075 and 1150 °C with a strain rate of 10 to 30 s − 1 . Under such condition, both ferrite and austenite dynamic re-crystallizations can be obtained and the corresponding Zener–Hollomon parameter is relatively low. Furthermore, the unstable domains are indicated by the processing map.

Hot deformation and processing map of 2519A aluminum alloy

The flow stress features of 2519A aluminum alloy have been studied by isothermal compression in the temperature range from 300 to 450°C with strain rates of 0.01–10s−1 on Gleeble-1500 machine. The results show that the deformation temperature and the strain rate have obvious effects on the flow characteristic, and the flow stress increases with increasing strain rate and decreasing temperature. The flow stress of 2519A aluminum alloy deformed in the strain rate range of 0.01–1s−1 shows a dynamic recovery character; while that deformed at the strain rates of 10s−1 and in the deformation temperature range of 350–450°C shows part dynamic recrystallization character. The processing map at the strain of 0.5 is obtained and exhibits two safe deformation domains (350–400°C at 0.001s−1 and 300–450°C at 1–10s−1). According to the processing map and microstructure observation, the optimum hot-working condition for 2519A aluminum alloy is determined to be 450°C and 10s−1.

Artificial Neural Network for Predicting the Flow Behaviors of Hot Compressed 2124-T851 Aluminum Alloy

The compressive deformation experiments of 2124-T851 aluminum alloy were carried out over a wide range of temperature and strain rate. An artificial neural network (ANN) model is developed for the analysis and simulation of the correlation between the flow behaviors of hot compressed 2124-T851 aluminum alloy and working conditions. The input parameters of the model consist of strain rate, forming temperature and deformation degree whereas flow stress is the output. A three layer feed-forward network with 15 neurons in a single hidden layer and back propagation (BP) learning algorithm has been employed. Good performance of the ANN model is achieved. The predicted results are consistent with what is expected from fundamental theory of hot compression deformation, which indicates that the excellent capability of the developed ANN model to predict the flow stress level, the strain hardening and flow softening stages is well evidenced.

Hot Deformation Behavior of 316LN Stainless Steel

Hot-compression experiments of 316LN stainless steel have been conducted on a Gleeble-1500D thermal-mechanical simulator. We have analyzed the flow stress-strain curve and acquired the constitutive equation of 316LN steel by calculating stress exponent, activation energy and Zemer-Hollomon parameter. Then, based on the material model theories and Prasad instability criterion, the iso-efficiency map at strain 0.6 of 316LN steel has been developed. The larger power dissipation rate is emerging at 1050~1200°C and lower strain rate. In addition, we have also analyzed the hot deformation microstructure mechanism of 316LN steel is discontinuous dynamic recrystallization by the observation of deformation microstructure. These results have been of great significance to understand microstructure evolution and to determine the optimum hot-working conditions in the production.

Critical issues in the simulation of hot forming operations

The paper deals with the correlations that can be established in the numerical simulation of manufacturing process chains among the process parameters, microstructure evolution, and properties of the final product. With the focus on process chains applied to bulk and sheet metal parts where deformation is carried out in hot working conditions, the paper illustrates through three application cases the role that the analysis of the microstructure evolution plays in the process chains simulations and, in particular, in enhancing the capabilities of the models that are currently implemented in commercial FE codes for predicting the properties of the final part.

Hot-Working Behaviour of Advanced High-Manganese C-Mn-Si-Al Steels

The high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on a recrystallization behaviour was investigated. Flow stresses during the multistage compression test were measured using the Gleeble 3800 thermo-mechanical simulator. To describe the hot-working behaviour, the steel was compressed to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light microscopy. The flow stresses are much higher in comparison with austenitic Cr-Ni and Cr-Mn steels and slightly higher compared to Fe-(15-25) Mn alloys. Making use of dynamic and metadynamic recrystallization, it is possible to refine the microstructure and to decrease the flow stress during the last deformation at 850°C. Applying the true strains of 0.23 and 0.19 requires the microstructure refinement by static recrystallization. The obtained microstructure – hot-working relationships can be useful in the determination of powerful parameters of hot-rolling and to design a rolling schedule for high-manganese steel sheets with fine-grained austenitic structures.