Papers by Keyword: Dynamic Recrystallization (DRX)

Abstract: The repeated plastic working deformation(RPW) process can reduce grain size of a Mg alloy from 50-100 um to 10-500 nm, but the mechanism responsible for it has not been clear up to now. In the present paper, the effect of RPW deformation process on the grain size of Mg-5Gd-1Er alloy has been studied, and a series microstructural evaluations were performed to investigate the possible mechanism for RPW deformation by using transmission electron microscopy. Although there are no twinning or fibrous microstructures, the dynamic recrystallization, which usually occurs in high stacking fault energy metals, has been found in the alloy deformed by RPW process. The results indicated that the vacancy diffusion played an important role in this kind of dynamic recrystallization. According to the evolution of microstructure, a mechanism was proposed to explain the dynamic recrystallization for RPW deformation process.

Abstract: Sheets of commercial purity aluminum and super saturated solid solution of 0.3% Sc were accumulatively roll bonded to form 64 alternating layers in a 0.5 mm thickness. The rolling was done at 350 °C to ensure dynamic recovery/recrystallization in Al layers and precipitation hardening of Al(Sc) layers during the rolling. The sheet crystallographic texture was distributed along β fibre orientations. The recrystallization texture of aluminum layers after annealed at 250-350 °C was randomly distributed. The tensile property of this novel composite has achieved a small improvement over the commercially available grades and delamination between the layers was identified as the key issue to improve in this fabrication technique.

Abstract: Mechanical behavior and microstructure evolution of commercial pure titanium during successive compressions of samples along three orthogonal directions (or so-called “abc” deformation) at 400°C and strain rate 10-3s-1 were studied. The cumulative S- curve demonstrates a steady state flow stage following the intensive strengthening. The microstructure evolution of titanium during first increments of “abc” deformation is associated with twinning and shear deformation. Further deformation results in microstructure refinement due to transformation of coincidence site lattice twin boundaries to high-angle arbitrary ones and formation of high-angle deformation induced boundaries. Another mechanism of new grains formation is continuous dynamic recrystallization.

Abstract: In this study, the microstructure evolution as well as mechanical properties of T2 copper tube during severe hot rolling has been investigated. Owing to the severe hot rolling deformation and high rolling temperature, the microstructure of T2 copper tube evolved from initial cast structure into relatively homogenous and equiaxed grains during the process, which was testified through the results of optical microscope observation. Besides, the microhardness measurements were taken, from which the results gained were in correspondence to the microstructure transformation.

Abstract: The high temperature deformation behaviors of Ti-47Al-2Cr-2Nb-0.2W alloy were investigated by isothermal compressive tests, performed at temperatures between 1000 and 1150 , strain rates between 0.001s-1 and 1s-1. The stress-strain curves of IM alloy exhibited an obvious work hardening peak followed by a broad flow softening at high strain rates(≥0.1s-1) while work hardening could hardly be seen from the stress-strain curves at low strain rates(≤0.01s-1). Also, the constitutive equation of the alloy had been established to describe the flow behavior. The apparent activation energy of hot deformation was calculated to be 351.61kJ/mol. The size of the recrystallized grain increased with increasing temperature and with decreasing strain rate, namely with decreasing the Zener-Hollomon parameter Z. It is necessary to select proper Z, that is, to strictly control the deformation parameters in order to obtain a homogeneous and fine microstructure.

Abstract: The hot compression behavior of ZK60 magnesium alloy was investigated at the temperatures from 523 to 673K and strain rates from 0.001 to 1s-1 on Gleeble-1500 thermal simulator. The results show that flow stress of ZK60 magnesium alloy decreases with the increase of deformation temperature and the decrease of strain rate. The flow stress curves obtained from experiments can be described in four different stages, i.e., work hardening stage, transition stage, softening stage and steady stage. For higher temperature and lower strain rate, the transition and softening stage are less obvious. The onset of dynamic recrystallization (DRX) occurred before the stress peak in true stress-true strain curves. The critical stress characterizing the onset of DRX rises with the increase of strain rate and/or the decrease of deformation temperature. The constitutive equation of ZK60 magnesium alloy during hot compression was constructed allowing for the effect of true strain on materials constants. The predicted stress-strain curves according to the constitutive equation are in good agreement with experimental results.

Abstract: Both the critical conditions and evolution of dynamic recrystallization behavior for a Nb-Ti microalloyed 3.5Ni steel were carried out by isothermal compression deformation on a Gleeble-3500 thermal simulator. Results show that the critical conditions of dynamic recrystallization behavior deformation of 30 % at 1050 °C, and the accumulative deformation for full dynamic recrystallization is 50 % at a strain rate of 1 /s. Accordingly, proposals on how to roll 3.5Ni steel were made.

Abstract: The prediction of dynamic recrystallization (DRX) plays an important role in design of hot forging process. In this work, a simulation model coupled cellular automaton (CA) with FEM was proposed to quantitatively simulate the microstructure evolution and flow stress during 7050 aluminium alloy wheel die forging. The model of dislocation density was established by isothermal compression test on Gleeble-1500. The physical fields, such as temperature, strain rate and strain, were obtained by DEFORM-3D for microstructure simulation. To refine grain size and improve uniformity, the isothermal forging parameters for 7050 aluminium alloy wheel were optimized by CA simulation. The simulation results showed that DRX percentage increased from 9.9% to 50% by using isothermal forging instead of hot die forging, while the flow stress decreased from 84MPa to 40MPa. Uniform density and small grain related with high mechanical properties were achieved via isothermal die forging process. The experiments of scaled wheel die forging were carried out on a hydraulic press with nominal working pressure 3150KN to verify simulation results. The good agreement between the simulation results and the experimental results indicate that the simulation model presented in this work can be used to predict the microstructure evolution of forging and optimization of forging process.

Abstract: Microstructure evolution during dynamic recrystallization (DRX) of hot deformed GH625 superalloy was investigated by optical microscope (OP) and transmission electron microscope (TEM). Hot compression tests were conducted using Gleeble-1500 simulator. It was found that the nucleation mechanism of DRX for the alloy deformed at 1150°C is composed of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) in the vicinity of the serrated grain boundaries. With the increasing strain, the fraction of the DRX grains increases, while the size of the DRX grains almost remains in the same range. As the deformation temperature increasing, the size and fraction of the DRX grains increase, and no precipitation of intergranular carbides are found when the deformation temperature increases to 1150°C. At lower strain rate, the size and volume fraction of DRX grains decrease with the increasing strain rates. However, the size and volume fraction of DRX grains increase at higher strain rates due to the deformation thermal effect.

Abstract: The flow stress behavior of Cu-2.0Ni-0.5Si alloy during hot compression deformation was studied by isothermal compression test at Gleeble-1500D thermal-mechanical simulator.Dynamic recrystallization and dynamic recovery during high temperature deformation have been investigated by means of compression tests in the temperature and strain rate ranges of 873 to 1073 K and 0.01 to 5s-1 under maxium strain of 60%. The results show the flow stress was controlled by both strain rate and deforming temperature,the flow stress decreases with the increase of deforming temperature,while increases with the increase of strain rate.The hot deformation equation is e& = e28.47[sinh(0.013s )]5.52 exp(-245.4´103 / RT ) .