Papers by Keyword: Dynamic Recrystallization (DRX)

Abstract: A two-dimensional modified cellular automaton (CA) model was developed to simulate the dynamic recrystallization (DRX) behaviour during thermo-mechanical processing. It provides a link for multiscale modeling to bridge the mesoscopic dislocation activities with the macroscopic mechanical properties. This model is applied to investigate the effect of initial grain sizes on DRX process in commercial pure copper. The simulated results indicate that the stable size of recrystallized grain is independent on initial grain sizes. However, the percentage of DRX is not only related to the thermo-mechanical parameters, but also influenced by the initial microstructure. It is concluded that larger initial grain sizes promote a delay in the DRX occur on commercial pure copper. The calculated results compare well with the limited number of experimental observations and theoretical conclusions.

Abstract: Isothermal constant strain rate compression tests on high-carbon steel are carried out under deformation temperation of 850°C, 950°C, 1050°C、1150°C and 1250°C respectively and deformation of 0.7 using the THERMECMASTOR-Z thermal simulator at a strain rate of 1s-1. The austenite grain morphology before and after deformation and the true stress-strain curves during the deformation process are analyzed. The experimental results show that increased deformation temperation can obviously contribute to the grain refinement. And under the same strain rate conditions, as the deformation temperation rises, peak stress will move toward the opposite direction of strain increment，thus the dynamic recrystallization is apt to occur.

Abstract: Using the compression tests on a Gleeble-1500 thermo-mechanical simulator to study the dynamic recrystallization behaviours of AZ80 magnesium alloy in the temperature range of 593-683K and strain rate range of 0.01-10s^-1. By the analysis of the dynamic recrystallization kinetics, the Avrami exponent (m) and the constant (k) have been determined, and they aren’t constant and depend on the dimensionless parameter(Z/A).

Abstract: Constrained groove pressing is a simple and effective method of grain refinement. Using the experimental data obtained by regression analysis, this paper analyzes the simulation of the four pass constrained groove pressing deformation of 7050 aluminum alloy. The simulation results show that the grain size of the billet is refined significantly after four pass constrained groove pressing deformation and decreases from the original 90 μm to a minimum of 14.0 μm. With the increase of the number of deformation passes, refinement effect becomes weakened gradually, the grain size tends to stabilize and the organization is more uniform.

Abstract: The paper focuses on the research of using the coupled thermo mechanical analysis method of rigid viscoplastic finite element and the evolution of dynamic recrystallization to establish the calculation model coupling factors of deformation, heat conduction, dynamic recrystallization in the DEFORM, and on the hot-forging simulation of AISI4140. Through the simulation, it makes a valid prediction of dynamic recrystallization volume fraction and the grain size of recrystallized during the metal hot-forging process.

Abstract: Al6063 powder was subjected to severe plastic deformation via high-energy mechanical milling to prepare ultrafine-grained (UFG) aluminium alloy. Uniaxial compression test at various temperatures between 300 and 450 °C and strain rates between 0.01 and 1 s-1 was carried out to evaluate hot workability of the material. Microstructural studies were performed by EBSD and TEM. The average activation energy and strain rate sensitivity of the hot deformation process were determined to be 280 kJ mol-1 and 0.05, respectively. The deformation temperature and applied strain rate significantly affected the grain structure of UFG Al alloy. A finer grain structure was obtained at lower temperatures and higher strain rates. The formation of highly misoriented and equiaxed grains also revealed that dynamic recrystallization occurred upon hot deformation. Furthermore, elongated grains with high dislocation density were observed that disclosed partial dynamic recrystallization of the aluminum matrix.

Abstract: Copper fragments are found to adhere on penetration channel wall after copper jets penetrate steel target, and the research on it is helpful to know microstructure evolution of jets in the process of penetration and cooling time. This paper is based on the observation of bore in steel target penetrated by copper jets, and uses numerical simulation to study the process of copper jets penetration, then the change of temperature and grain size of jets adhered on penetration channel wall can be gotten, and the results agree with the observation of penetration channel wall taken by SEM. From the observations of copper and steel, we can get the conclusion that copper jets are not melted but have dynamic recrystallization in the process of penetration, then copper grain size increases obviously in cooling time, and twins are formed at the same time.

Abstract: The Mg-5Zn-2.5Er matrix composite reinforced with the in-situ synthesized Mg2Si second phase particles was fabricated via repeated plastic working (RPW) process. The microstructures and the nanocrystals in the composite have been investigated using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HREM) and energy dispersive X-ray (EDX). Great deals of nanocrystals were found in the matrix, and they were around the in-situ synthesized Mg2Si. The HREM analysis showed that the size of nanocrystals was in the range of 5-10 nm, and the difference in their crystallographic orientation was bigger than 15°. It is suggested that the formation of nanocrystals in the matrix is attributed to the RPW deformation process and to the intensive stresse fields around the in-situ synthesized Mg2Si particles, which suppress the growth of nanocrystals by forming nonequilibrium grain boundaries containing disordered dislocation networks and junction disclinations.

Abstract: Strain-induced grain refinement in magnesium alloy AZ61 was studied by means of cyclic bending carried out at a temperature 623K. The deformed microstructures were investigated by optical and SEM/EBSD metallographic observation. The results show that the grain size of the surface layer can be significant refined to about 3 μm by dynamic recrystallization. The volume fraction of new grains increases with strain and reaches a value of 0.8 after 8 passes. Most of the new grains are separated by high angle boundaries. Hardness distribution through thickness for the cyclic bended sheet exhibits “V” shape with the marked difference between the surface and the inside, the hardness is critically increased in surface layer, i.e. ~82 Hv, which is about 1.39 times than the as-received AZ61.

Abstract: The deformation behavior and the microstructure evolution in a 304-type austenitic stainless steel were studied in multiple forging tests at temperature of 700°C. The flow stresses increased to its maximum value with straining to about 1 and, then, slightly decreased resulting in a steady state deformation behavior at strains above 3. The structural changes were characterized by the development of a spatial net of deformation subboundaries, the misorientations of which increased to the values typical of conventional grain boundaries. The number of ultrafine grains increased with straining, leading to development of submicrocrystalline structure. The fraction of submicrocrystalline structure composed of ultrafine grains with an average size of about 300 nm exceeded 0.7 after straining to 2.