Papers by Keyword: Dynamic Recovery

Abstract: Recovery usually softens strain hardened coarse grained metallic materials but it can increase the strength of ultrafine grained materials. The present work shows evidence that dynamic recovery can produce strain-hardening behavior in ultrafine grained aluminum processed by high pressure torsion. Mechanical testing reveals an increase in flow stress during low strain rate tensile tests and a decrease in strain rate during creep tests. No significant change is observed in the grain size. It is shown that this effect can be used to increase the uniform elongation of these materials.

Abstract: In order to get the insights about microstructural changes that occurs under the thermo-mechanical processing conditions, the physics based modelling approach is very useful. Therefore, the flow curves of alloy 718 are theoretical simulated using a dislocation density dependent constitutive model for different conditions. Presented model considers the microstructural ingredients that are immobile dislocation density, effective grain size and dislocation cell size as the variables to address the creep. The simulated flow curves show a good agreement with the experimental flow curves. The magnitude of immobile dislocation density and dislocation cell size in between 3.87× 10¹⁴ - 3.87× 10^14­ m^-2 and 8.29-8.45 μm, respectively, at the completion of the simulation. Furthermore, this approach also provides the possibility to quantify and depict the variation in each strengthening contributions.

Abstract: Regenerative brake control creates an optimal synergy between mechanical and electrical braking. Based on the study of vehicle dynamics under braking conditions propose a new control mode that ensures the best braking performance and maximum braking energy recovery. The implementation of the above control mode requires a combination of the traction control model and the brake control system. The HEV power distribution model is built using Matlab/ Simulink and the simulation results have shown a significant improvement in fuel consumption when using the regenerative braking system.

Abstract: Aluminum alloys are widely used in automotive industry due to their low density and good corrosion resistance. This category includes alloys based on AlSiMg which are suitable for load bearing parts operating under higher temperatures. This paper deals with analysis of influence of deformation parameters and heat treatment on structure and mechanical properties of EN AW-6082 (AlSi1MgMn) alloy manufactured by horizontal cast module technology. Casted rods were used as a billet, which was formed to defined height by hot open-die forging. Subsequently the precipitation hardening was used as heat treatment. Changes in microstructure were evaluated based on the metallographic analyzes performed by light optical microscopy and scanning electron microscopy using an energy dispersive X-ray spectrometry and electron backscatter diffraction. Mechanical properties were determined by uniaxial tensile test and hardness testing. The results showed, that due to the process parameters, no significant structural changes were observed in the surface layer of forging. However, microstructure is significantly inhomogeneous in the core due to the dynamic softening processes. Mechanical properties are increasing which is significantly influenced by the type and distribution of precipitates emerging during the artificial aging.

Abstract: Relationships between macroscopic and microscopic constitutive parameters associated with steady state DDRX are derived for a material in which strain-hardening and dynamic recovery are described by the Yoshie-Laasraoui-Jonas equation. First examples are given for illustration.

Abstract: This work deals with the analysis and modelling of the microstructural evolution of the metastable titanium alloy Ti-5Al-5V-5Mo-3Cr during hot deformation up to moderate and large strains. Experimental flow curves and deformed samples are obtained by hot compression and hot torsion tests using a Gleeble ® 3800 device. The samples are deformed above and below the beta transus temperature and in a wide range of strain rates. Microstructures are characterized after deformation and in-situ water quenching using light optical and scanning electron microscopy and electron back scattered diffraction (EBSD). Dynamic recovery of the beta phase is found to be the main deformation mechanism up to moderated strains. By increasing the strain, continuous dynamic recrystallization (cDRX) is confirmed by the progressive conversion of low angle boundaries into high-angle boundaries. Alpha phase plays a secondary role in the deformation of the material by pinning the movement of beta high angle grain boundaries (HAGB). The evolution of the microstructure is modelled using dislocation density as internal variable in the single β field.

Abstract: Grain refinement is attracting attention as a strengthening method which does not depend on the alloying elements added to steels. Many reports have described the manufacturing methods and mechanical properties of ultra-fine grained steels. In ultra-fine grained steels, it is well known that yielding stress drastically increases in accordance with the Hall-Petch relationship, while uniform elongation significantly decreases. These tendencies imply that grain size affects not only yielding but also work-hardening behavior. However, the influence of grain size on work-hardening behavior has not been clearly understood. Therefore, in this study, we investigated the work-hardening behavior during tensile deformation of 12Cr stainless steel with various grain sizes. Grain refining was conducted by cold-rolling of annealed and quenched steel specimens, followed by recrystallization annealing. The grain size of the specimens decreased as the cold-rolling reduction rate increased. The minimum grain size obtained by this method was approximately 5 μm. With decreasing grain size, 0.2% proof stress increased and the strain which reached the plastic instability condition decreased. In the session, we report the dislocation accumulation behavior estimated by grain hardness and XRD and the dynamic recovery behavior assessed by the Kocks-Mecking model.

Abstract: To determine the relations between rolling passes, mechanical behaviours and microstructure evolution of AA7050 aluminum alloys, finite element modeling of a multipass hot rolling process is developed and employed to investigate thermo-mechanical evolution during this processing. Through parametric studies, the distribution of local strain and temperature across thickness during the hot rolling process are numerically determined. These results are used to determine the subgrain size and thus the microstructure evolution during the hot rolling process are estimated.

Abstract: It has been reported that the elongation of the AA1050-H26 sheet annealed at 200°C is lower than that of the AA1050-H18 sheet. In this study, the effect of the annealing time at 250°C on the elongation of AA1200 alloy sheets was investigated, and the cause of the low elongation was discussed by observing the change in the microstructures before and after tensile deformation. The elongation of the samples annealed at 250°C for less than 50 min was below 1%, and this elongation was lower than the elongation of the as-rolled and annealed ones at 250°C for more than 150 min. In the samples annealed for a short time, the subgrains with diameters of 0.5~2μm formed, and Fe and Si, which were a solid solution, became segregated at the sub-boundaries. These samples were locally deformed in a stress concentrated area during the tensile deformation, and there was no significant increase in the dislocation density near the fracture part after the deformation. The cause of the low elongation was considered to be due to the dynamic recovery that locally occurred in a stress concentrated area during the tensile deformation, because dislocations introduced into the subgrains by the deformation easily moved to the sub-boundaries due to the low solute levels within the subgrains.

Abstract: The elevated temperature flow stress behavior of Mg-9Gd-2.5Y-1Nd-0.5Zr magnesium alloy was carried out by Gleeble-1500 thermal mechanical simulator in the temperature range of 460-520°C and in strain rates of 0.0005~1s-1 at a strain of 0.6. The optical microscopy was used for microstructure characterization. The results showed that the flow stress increases with increasing strain rates and decreasing temperature. All the deformed magnesium alloy specimens show a dynamic recovery characters in the temperature range from 460~500°C, and show dynamic recrystallization characters at 520°C. The flow stress of this alloy can be represented by Zener-Hollomon parameter function, and values of related parameters A, α and n, are 2.24×1013s-1、0.027MPa-1 and 2.93, respectively. Its activation energy for hot deformation Q is 212.6kJ/mol.