Sharif Digital Repository

In this work, the effects of dynamic strain aging and dynamic recovery on metal flow during warm working are studied. Compression experiments are utilized to assess the flow behavior of a low carbon steel under warm deformation conditions. Then, a two dimensional finite element routine is coupled with dynamic recovery and dynamic strain aging models. In this way, the temperature and the velocity fields are predicted during warm working operations with regard to the effects of dynamic recovery and dynamic strain aging. Warm rolling tests are performed in order to verify the modelling results. Comparison between the predicted and measured roll forces shows reliability of the employed model  

A mathematical model has been developed to determine flow stress at hot deformation condition. The proposed model is capable of including work softening due to dynamic phase transformations as well as the effect of temperature and strain rate variation on flow stress utilizing the additivity rule for strain. To verify the model, hot compression tests for two grades of steels has been carried out. The comparison between the experimental and theoretical results confirms the validity of the model  

A novel method is proposed in order to find the kinetics of dynamic recovery in the form of Avrami relation using hot flow curves. The relation is derived on the basis of variation of dislocation density with strain. The model is applied on an Al-Mg alloy  

A comprehensive analysis on flow stress of a wrought Al-Mg alloy is performed to examine the effect of strain. For this study, hot compression tests were carried at different temperatures and strain rates. Corrections of friction and adiabatic heating effects lead to the true stress-true strain curves in the form of dynamic recovery, which reach to a steady-state condition. After correction, constitutive analysis at a constant strain is carried out using hyperbolic-sine equation. The effect of strain on each constitutive parameter is studied to derive a strain-dependent constitutive equation based on hyperbolic-sine equation. Some of constitutive parameters reach to the constant values at... 

An analytical model is developed to study the effect of cooling rate on the final grain size of stirred zone of the Al5083 subjected to friction stir processing. The effect of cooling rate on the grain size of the stirred zone was investigated experimentally and numerically. A new microstructural evolution model was also suggested illustrating the mechanisms contributed in refining the microstructure. A new mechanism termed meta-dynamic recovery (MDRV) is introduced here in this regard. The simulation results also show that the rapid cooling rate resulted in superior mechanical properties through refining the microstructure of the stirred zone. However, decreasing the rotational speed and... 

A unified model composed of the flow function model, dislocation model and Taylor theory is used to investigate the evolution of dislocation density, cell size, and strength of tantalum during ECAP process. From the flow function model, strain and strain rate distributions are achieved and then using a modified version of three-dimensional ETMB model, the dislocation density, cell size, and strength are predicted. The predicted dislocation density, cell size, and strength are compared with the experimental data and a remarkable agreement is obtained. In addition, the effect of dynamic recovery on the strength of the processed tantalum is modeled and compared with other materials. © 2008... 

A two-dimensional finite element analysis is coupled with dynamic recovery and dynamic strain aging models to achieve metal flow during warm deformation of a low carbon steel. In order to assess metal behavior, compression experiments are utilized under warm deformation conditions and the results are used in the model and temperature and velocity fields are predicted with regard to the effects of dynamic recovery and dynamic strain aging. In addition, warm rolling tests are performed to verify the modelling results. Comparison between the predicted and measured roll forces shows reliability of the employed model. © 2005 Elsevier Ltd. All rights reserved  

The behavior of hot extruded Al-6Mg during hot deformation has been studied using hot compression test. The tests were performed at temperatures of 350, 450, and 550°C and strain rates of 0.001, 0.1, and 1s -1. Due to the effects of friction and adiabatic heating generated during compression, the attained true stress-true strain curves have been corrected. After correction of friction effect, the virtual work hardening has been removed from stress-strain curves. The flow stress increases and reaches a steady state after correction of adiabatic heating effect. Corrected curves and microstructural examinations exhibit the occurrence of dynamic recovery during hot compression of the alloy. The... 

True stress vs. true strain responses of Cu-6 wt.% Al and Cu-12 wt.% Mn alloys are presented. While Cu-6 wt.% Al alloy shows the typical mechanical response of low stacking fault energy alloys, the Cu-12 wt.% Mn alloy behaved similarly to medium to high stacking fault energy alloys. These findings clearly show that while short-range ordering triggers slip planarity, it has a minor effect on total dynamic recovery of these copper alloys  

The effects of severe plastic deformation on properties of AA1100 and AA2024 including the ultrasound velocity and the shear strength during multi-axial compression (MAC) are studied. Additionally, optical microscopy, scanning electron microscopy, and X-ray energy-dispersive spectroscopy are utilized. For both AA1100 and AA2024, an opposite trend is reported in the shear strength and the sound velocity versus deformation strain of MAC. At high strain range, AA1100 and AA2024 samples reach a plateau in the strength due to the occurrence of dynamic recovery. On the other hand, the sound velocity is shown to decrease 11% and 15% for AA1100 and AA2024, respectively. Furthermore, the... 

Compression test was used to study the deformation behaviour of sintered (12% porosity) and powder extruded (1% porosity) Al6061 alloy at high temperatures ranging from 350 to 500°C. The flow curves of the materials were determined at strain rates of 0.001, 0.01 and 0.1 s-1. The extruded alloy exhibited plastic instability in the form of bulging and formation of shear bands. The deformation activation energy of the extruded material was determined to be QI=77±16 kJ mol-1 (T<450°C) and OII=136±12 kJ mol-1 (T≥450°C). The values of activation energy and appearance of strain softening in the stress-strain curves revealed that thermally activated restoration processes, i.e. dynamic recovery and... 

The accumulative roll bonding process was carried out on an AA3003 aluminum alloy sheet up to eight cycles. The electron backscattering diffraction (EBSD) method was employed to investigate the microtextural development in the ARB processed sheets. The results indicate that with increasing the number of cycles, the overall texture intensity increases even up to the eighth rolling pass and a strong texture develops. The main textural components are the copper and Dillamore components of which the intensities increase with increasing number of cycles. Measurement of microhardness and lamellar spacing of grains in the processed sheets revealed that the presence of second phase particles in this... 

The deformation behavior of a 49.8 Ni-50.2 Ti (at pct) alloy was investigated using the hot compression test in the temperature range of 700 °C-1100 °C, and strain rate of 0.001 s-1 to 1 s-1. The hot tensile test of the alloy was also considered to assist explaining the related deformation mechanism within the same temperature range and the strain rate of 0.1 s-1. The processing map of the alloy was developed to evaluate the efficiency of hot deformation and to identify the instability regions of the flow. The peak efficiency of 24-28% was achieved at temperature range of 900 °C-1000 °C, and strain rates higher than 0.01 s -1 in the processing map. The hot ductility and the deformation... 

The hot workability of a wrought 49.8 Ni-50.2 Ti (at pct) alloy was assessed using the hot compression tests in temperature range of 700-1000°C, strain rate of 0.001-1s-1, and the total strain of 0.7. The constitutive equations of Arrhenius-type hyperbolic-sine function was used to describe the flow stress as a function of strain rate and temperature. The preferable regions for hot workability of the alloy were achieved at Z (Zener-Holloman parameter) values of about 109-1013 corresponding to the peak efficiency of 20-30% in the processing map. However, a narrow area in the processing map including the deformation temperature of 1000°C and strain rate of 1s-1 is inconsistent with the related... 

Uniaxial compression test at different temperatures [573 K to 723 K (300 °C to 450 °C)] and strain rates (0.01 to 1 s−1) was employed to study the hot deformation behavior of an ultrafine-grained (UFG) Al6063 alloy prepared by the powder metallurgy route. The UFG alloy with an average grain size of ~0.3 µm was prepared by mechanical milling of a gas-atomized aluminum alloy powder for 20 hours followed by hot powder extrusion at 723 K (450 °C). To elaborate the effect of grain size, the aluminum alloy powder was extruded without mechanical milling to attain a coarse-grained (CG) structure with an average grain size of about 2.2 µm. By employing the dynamic materials model, processing maps for... 

The influence of Cu addition on the hot deformation behavior of NiTi shape memory alloys was investigated using hot compression test. A series of alloys with different Cu contents of Ti50.4Ni49.6-xCux (x = 0, 3, 5, 7 at.%) were deformed under compression to a true strain of 0.7 at the temperature range of 700-1000 °C with 100 °C intervals and constant strain rate of 0.1 s-1. The stress-strain curves showed that the addition of Cu to NiTi alloy made the flow curves shift upward. This was confirmed by the calculated critical stress, σc, obtained from inflections in θ-σ plots, which is attributed to the formation of high strength Cu containing precipitates and solid solution hardening caused by... 

Hot deformation behavior of Al6063 alloy produced by direct powder extrusion was studied by means of uniaxial compression test in the temperature range between 300 and 450°C and strain rate range between 0.01 and 1s -1. Electron backscatter diffraction (EBSD) technique and transmission electron microscopy (TEM) were utilized to study the microstructure of the material before and after the hot deformation. The microstructure of the extruded alloy consisted of elongated grains within a subgrain structure and small grains free of low angle grain boundaries (LAGBs). An equiaxed duplex microstructure consisting of large substructured grains and fine grains separated by high angle grain boundaries...