Papers by Keyword: Recrystallization

Abstract: The microstructure evolution during the extrusion process of AA6XXX aluminum alloys is getting a significant interest from extruders and researchers because of the effect of the grain structure on the extruded component properties. Several process and material parameters such as chemical composition, homogenization, temperature evolution, extrusion speed, geometries and quenching have a direct impact on the final grain size of extruded profiles. Because there are so many affecting elements, it is extremely challenging to forecast the microstructure evolution and, as a result, research activities are still required to understand and control the aluminum alloy recrystallization behaviour. In this work, a methodology for the microstructural characterization of AA6XXX aluminum alloys is proposed. The methodology involves the experimental investigation of the profile grain evolution during the extrusion process, the development of a AA6XXX recrystallization model optimized to describe the AA6063 recrystallization behaviour and the simulation by means of finite element method of the final microstructure of the extruded profile.

Abstract: In order to increase strength while maintaining the ductility of material, pure titanium was improved through the thermomechanical treatment that combines rolling and heat treatment. The tensile properties of pure titanium treated by rolling and heating were investigated. Test material was JIS Grade 2. This material has a higher corrosion resistance. However, the strength of JIS Grade 2 is lower than that of JIS Grade 3. JIS Grade 2 with high strength while maintaining corrosion resistance is being developed. Techniques for improving the properties of materials with simple compositions are important. Thermomechanical treatment is used as a method for improving material properties. In the present study, the effect of thermomechanical treatment on the material properties of JIS Grade 2 was investigated. Rolling was performed at room temperature and the reduction ratio ranged from 70 to 90 %. The heating temperature was in the range of 300 to 700 °C. Heat treatment from 400 to 500 °C showed an increase in tensile strength while maintaining ductility. When the heat treatment temperature was 450 °C, the strength and elongation were approximately 600 MPa and 25 %. Tensile stress of JIS Grade 4 and the tensile strain of JIS Grade 1 were exhibited.

Abstract: Three main rheological laws are found in the literature to describe the strain hardening of materials at high temperatures. The choice of the most suited law to describe a flow stress curve is often discussed as a function of the nature of the material; but it still remains difficult to choose the most appropriate one. These semi-empirical laws systematically comprise two main terms linked either to the dislocations generation or their annihilation.The objective of this paper is to determine by an inverse method which law appears to be the most suited. It is finally demonstrated that the application of one law is mostly equivalent to another. The various laws are overall equivalent and do not help to describe some peculiar physical mechanism of plasticity.

Abstract: EUROFER97 martensitic steel is recognized in EU as the reference material for the test blanket module in ITER reactor and for structural sections subject to high radiation doses in DEMO reactor. An extended experimental campaign has been carried out with the scope of improving strength without loss of ductility. The main idea behind the present study is to reach the goal through grain refinement achieved by cold rolling and heat treatments for inducing recrystallization of the work hardened structure. A combination of five cold rolling reduction ratios (CR) (20%, 40%, 50%, 60%, 80%) and eight heat treatments in the temperature range 400-750°C (steps of 50 °C) with soaking time of 1 hour has been examined to describe the evolution of microstructure and mechanical properties. The strength of deformed samples decreases as the heat treatment temperature increases and the change is more pronounced in the samples cold-rolled with higher CR ratios. The results showed that cold rolling with CR of 80% followed by a treatment at 650 °C produces a fully recrystallized structure with sub-micrometric grains which guarantees improved yield stress and hardness than standard EUROFER97 steel, with a comparable total elongation. In conclusion, this work demonstrated the feasibility to strengthen EUROFER97 without compromising its ductility.

Abstract: The heterogeneity of recrystallized subgrains is a substantial parameter which ought to be consider when controlling recrystallization fraction within a microstructure is needed. The statistical descriptor pair-correlation function is employed as stereological reference for reconstructing the nucleation sites spatial distribution within given volumes based on 2D data. The aim of the current study is to implement the 3D reconstructed particles distribution into a Monte Carlo approach to explore the evolution of microstructure in 7xxx Al alloy during homogenization process. Once the stored energy around the coarse particles is consumed recrystallizing grains the recrystallization is fulfilled. The simulated grain structures are qualitatively evolved in manner of the recrystallization of experimental data, and this verify that recrystallization process is mainly controlled by PSN mechanism.

Abstract: In this study, activation energies required for the static recrystallization behavior during the annealing process after cold deformation of Al-Mg-Si alloy to which zirconium was added in various proportions were investigated. Depending on the zirconium content, the activation energies of the alloys were found and compared both experimentally and by calculation. For this purpose, alloys containing 0.1, 0.2 and 0.3 wt-% Zr were cold rolled after taking into solution and quenching. And then, the alloys were annealed at 375 °C and 500 °C for different annealing times. After the alloys were prepared metallographically, their grain structures were examined microscopically. Depending on the temperature, recrystallization-% was found by image analysis and experimental recrystallization-% curves were drawn. The time taken for recrystallization-50% to experimentally find the activation energy required for recrystallization to occur was found from the curves. These values were replaced in the relevant formulations and the required activation energy was experimentally found from the slope of the Arrhenius equation and the ln t_50% and 1/T graph. In order to find the recrystallization-% by calculation, the nucleation rate and growth rate of the new recrystallized grains were found by image analysis. By substituting these values in Johnson-Mehl-Avrami equation, the calculated recrystallization-% curves of the alloys were found. From here, using the relevant equations, Arrhenius equation was passed and the activation energy was calculated from the slope of ln k and 1/T graph. The results showed that the activation energy increased with the increase of the zirconium ratio, and even the most effective zirconium ratio was between 0.1-0.2% by weight in increasing the activation energy. Therefore, this ratio should be considered in processes where recrystallization, which also affects other properties of the alloy, is not desired.

Abstract: Numerical modelling tools provide valuable means to quantitatively control thermomechanical processing. Several modelling tools have been applied and developed at University of Oulu during previous years, such as finite element models for hot rolling, recrystallization models, heat transfer and conduction model, coupled with phase transformation, as well as cellular automata and phase field models for simulating phase transformation during cooling. This article describes the overall development and recent progress of the developed numerical modeling tools.

Abstract: The recrystallization and inhibitor precipitation behaviors of rare earths under water cooling after holding for different times of oriented silicon steel hot rolled at 1200°C in the ferrite zone with 30% deformation were analyzed using Gleeble 1500D, SEM,TEM and ICP. The results showed that: during the hot rolling of oriented silicon steel in the high temperature ferrite zone, only dynamic reversion occurred, no dynamic recrystallization occurred, and the amount of precipitates did not increase significantly; after deformation at 1200°C, static recrystallization occurred after holding for about 20s, and the inhibitor started to precipitate and grow. The higher the recrystallization rate, the faster the volume fraction of precipitates. After the deformation, the amount of Cu₂S and MnS in the precipitates was similar. At 64% recrystallization rate, MnS increased by about 8% and Cu₂S increased by about 23%. At the same time, most of the precipitates were precipitated in the crystal, and gradually nucleated and grew at the grain boundary when the holding time was extended. After rare earth lanthanum and cerium were added, the precipitation amount of inhibitor was reduced. The higher the degree of static recrystallization, the more obvious the effect of rare earth on the precipitation of inhibitor.

Abstract: SiC sputtered and e-beam evaporated layers have been deposited on 4H-SiC substrates. High temperature annealing with two plateaus at 1400°C and 1700°C is performed to recrystallize the layers. The crystallinity was investigated by Raman spectroscopy with laser lines of 785, 405 and 325nm. To determine the electrical conductivity of the layers, electrical measurements are made. Only the electron beam evaporated layers presents a recrystallization close to homoepitaxial quality but, contrary to sputtered layers, they don’t have an electrical conductivity.

Abstract: Recrystallization often occurs in the production of aero engine turbine blades of single crystal superalloys. It is believed that annealing treatment can reduce the recrystallization tendency, as some of the casting residual stress is released. To evaluate the annealing effect on recrystallization, tensile test is carried out in present study for as-cast specimens of a second generation single crystal superalloy to induce stress. Two different annealing treatments are applied to the prestrained specimens, followed by the solution treatment at 1300^°C for 2h. In order to compare the effect of the annealing treatments, the specimens are investigated by x-ray Laue diffraction technique and metallographic observation. According to the comparison, the more effective annealing treatment is recommended.