Sharif Digital Repository

In this work, static strain aging behavior of an alloy steel containing high amounts of silicon and manganese was examined while the influences of initial microstructure and pre-strain on the aging kinetics were evaluated as well. The rate of strain aging in a low carbon steel was also determined and compared with that occurred in the alloy steel. The rates of static strain aging in the steels were defined at room temperature and at 95 °C by means of double-hit tensile testing and hardness measurements. In addition, three-stage aging experiments at 80 °C were carried out to estimate aging behavior under multi-pass deformation processing. The results showed that in-solution manganese and... 

The influence of equal channel angular pressing (ECAP) on microstructure and mechanical behavior, as well as, strain aging during ECAP and post-ECAP of Mg-1.8Zn-0.7Si-0.4Ca biomedical alloy was examined. The alloy in solid solution heat-treated condition was processed by ECAP with route BC at 350 and 400 °C. The specimens ECAPed at 350 °C were aged at 125 and 150 °C for different times. The hardness and shear punch test results indicated that performing ECAP at 350 °C for 4 passes increases the hardness and shear strength of the alloy from 48 HV and 110 MPa to 71 HV and 188 MPa, respectively. It was revealed that the average grain size of the alloy decreases from 78 µm to about 3 µm after 4... 

Flow behavior of two grades of steel including a high silicon (HS) steel and a plain low carbon steel as the reference were considered in this work. Tensile testing at temperatures varying between 25 and 550 °C and different strain rates in the range of 4×10-5 to 0.1 s-1 were conducted and the mechanical properties, such as elongation at fracture point and strain rate sensitivity were then determined. It is observed that for both steels, dynamic strain aging occurs in the employed deformation conditions, however, the region of serrated flow and the type of the serration were somehow different. For the case of the HS steel, the serrated flow region is shifted to the higher temperatures and... 

An integrated model based on finite-element method has been proposed to examine the mechanical and thermal responses of strips and work-rolls in tandem and reverse cold rolling operations. The model has been developed such that the influence of various process parameters, such as lubrication, rolling speed, frictional state and back-up rolls, can be examined. Thermal behaviors of the rolled material and the work-rolls have been analyzed using stream-line upwind Petrov-Galerkin approach, in order to make the model applicable to high-speed rolling processes, as well. The results have been compared to the actual on-line measurements and shown to be of acceptable accuracy. Such modeling approach... 

In this study, the kinetics of aging in AA6061 and AA6061 with 5% volume fraction SiCp were studied and compared. The composite was first produced and homogenized using stir casting technique followed by hot extrusion with the ratio of 18:1. Then, both AA6061 and the composite were aged at three different temperatures including room temperature, 170 °C and 240 °C, while mechanical properties during aging were evaluated employing hardness measurements and tensile testing. Moreover, in order to assess the effect of plastic deformation on the kinetics of aging, a series of samples were first deformed by equal-channel angular pressing immediately after solution treatment and then aged in the... 

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  

In this work, the flow stress behavior of a metal matrix composite AA2017-10% SiCp was studied by means of the uni-axial compression test. The composite was first produced by stir casting technique and then, hot extrusion with the ratio of 18:1 was carried out to achieve a microstructure with a homogeneous distribution of SiC particles. In the next stage, the isothermal compression tests were conducted on the cylindrical specimens up to the true strain of 0.6. The experiments were performed at temperatures between room temperature to 400°C and strain rates of 0.003, 0.03 and 0.3s-1. Negative strain rate sensitivity was observed in the temperatures less than 250°C indicating the occurrence of... 

The microstructure of cold rolled Al–6Mg alloy is investigated after two steps annealing at different coupled temperatures of 250–320 °C and 320–400 °C for various times. Dynamic strain aging behavior in terms of serrated flow and strain rate sensitivity is investigated. The effect of three microstructural features, cell structure, recovered and recrystallized microstructures, on the strain rate sensitivity is elucidated. Two steps annealing process is utilized to capture the effect of recovery and precipitation phenomena on recrystallization and dynamic strain aging behaviors. The results show that the negative strain rate sensitivity of cold rolled specimen increases to positive values in... 

Artificial neural network (ANN) and genetic algorithm were used in this study to obtain a relatively high flow stress in compression tests for 304 stainless steel. Cold and warm compression were carried out in a temperature range from 20 to 600 °C, strain-rate from 0.001 to 100 S-1 and a strain range from 0.1 to 0.5. Optimum conditions for each case were obtained experimentally and were evaluated by the ANN model. The ANN model was used as fitness function for genetic algorithm. The results indicated that this combined algorithm offers an effective condition for 304 stainless steel, which avoids flow localization, dynamic strain aging, adiabatic shear deformation and void generation. © 2005... 

In this study, a multistage strain aging method that used rolling pre-strain (compression) was developed to study the effects of temperature, and inter-pass time on static strain aging behavior of low carbon steel. An increase in hardness and strength caused by work hardening due to the forming process and aging at every stage of aging that is calculated separately. To comparing the effects of multistage aging against typical strain aging, the samples were rolled and subjected to the aging process both exist in typical one-stage aging that setting a 20% rolling pre-strain and in multiple stages pre-strain by setting a 5% rolling ratio in four stages. The mechanical properties of aged samples... 

AISI 4340 steel bars were heated at 900 °C for one hour, annealed at 738 °C for different durations and oil-quenched in order to obtain dual phase steels with different ferrite volume fractions. A 3% prestrain at the temperature range of 150-450 °C was then imposed to the samples, and room temperature tensile tests were carried out, afterwards. Results indicate that the maximum values for both yield and ultimate tensile strength would exist for the samples pre-strained at the temperature range of 250-300 °C. Also, a sudden drop of the ductility was observed at the mentioned temperature range. The observed behavior might be attributed to the occurrence of dynamic strain aging taken place at... 

In this paper, dynamic strain aging (DSA) behavior in a temperature range of (25-235°C) and strain rate range of (10-4-5×10-2s-1) was investigated using a supersaturated age hardenable aluminum alloy. It was found that two mechanisms consisted of pinning of solute atoms to mobile dislocations and dynamic precipitation, were responsible for DSA in the testing conditions. The effects of both mechanisms on the macroscopic flow curve were studied using experimental and improved physically based material modeling approaches. It was shown that both phenomena lead to a negative strain rate hardening in the alloy. Dynamic precipitation acting at high temperature results in considerable work... 

An investigation was performed on the static strain aging behavior of warm-rolled low carbon steel during a nearly 1-year aging period, from the view point concerning with influence of changing the deformation speed and cooling media. Mechanical response of the examined material during aging period was evaluated through variations occurred in strength and hardness of the warm-deformed steel. It was shown that changing the rolling speed as well as cooling rate, may result in the occurrence of different metallurgical phenomena, consequently altering the aging kinetics of the material. It was also found that by increasing rolling speed, an increase in the value of hardness and UTS takes place,... 

A new integrated physically based constitutive model was developed for an age hardenable Al- Mg-Si alloy. The kinetics of precipitation during various stages of aging was modelled. The precipitate features consisted of particle radius and volume fraction obtained from the kinetics model, which was used to compute the alloy yield strength/hardness. A published multiinternal variable workhardening model was improved to take into account the effects of solute solution and precipitates on the alloy hardening capacity after performing different cycles of aging treatment. The flow curves and hardness predicted by the model were in good agreement with the experimental results. The model is able to... 

In this work, the 6061 aluminum alloy tubes are severely deformed through a novel method called Tube Channel Pressing (TCP). The ability of this process in improving mechanical properties, grain refinement and microstructural changes of the alloy with different heat treatments before and after TCP process is investigated. Results show that TCP has notable effect on grain refinement and decreases crystallite size of solid solution treated aluminum 6061 material to 52. nm after equivalent strain of 3.09 which is comparable with the measured data from other SPD processes. The strength of the specimens aged before TCP is higher than that of those aged after TCP. The specimens artificially aged...